1 00:00:06,550 --> 00:00:04,150 well i see everybody got their clock set 2 00:00:10,070 --> 00:00:06,560 appropriately so uh no one showed up in 3 00:00:12,470 --> 00:00:10,080 her an hour uh earlier late this is carl 4 00:00:15,190 --> 00:00:12,480 pilcher broadcasting to you here from 5 00:00:17,349 --> 00:00:15,200 the temporary video con room at the car 6 00:00:19,510 --> 00:00:17,359 talk plaza excuse me at the nasa astro 7 00:00:21,429 --> 00:00:19,520 biology institute 8 00:00:23,830 --> 00:00:21,439 um 9 00:00:26,550 --> 00:00:23,840 and i am really really pleased to be 10 00:00:28,390 --> 00:00:26,560 able to introduce ariel lanbar uh to 11 00:00:30,550 --> 00:00:28,400 present the second 12 00:00:32,950 --> 00:00:30,560 nai director seminar 13 00:00:35,350 --> 00:00:32,960 uh of the new series 14 00:00:37,590 --> 00:00:35,360 uh ariel has been a member of the nai 15 00:00:40,229 --> 00:00:37,600 and a good friend since uh the nai's 16 00:00:42,310 --> 00:00:40,239 earliest days he has done a great deal 17 00:00:44,310 --> 00:00:42,320 of service to the nai and the 18 00:00:46,790 --> 00:00:44,320 astrobiology community as a whole in 19 00:00:49,190 --> 00:00:46,800 addition to his great research and most 20 00:00:50,549 --> 00:00:49,200 recently that service has taken the form 21 00:00:53,189 --> 00:00:50,559 of ariel 22 00:00:56,630 --> 00:00:53,199 taking the hopefully not too thankless 23 00:00:58,310 --> 00:00:56,640 job of being the chair of the 24 00:01:00,389 --> 00:00:58,320 science steering committee for the next 25 00:01:03,189 --> 00:01:00,399 astrobiology science conference and he's 26 00:01:06,070 --> 00:01:03,199 been doing a fabulous job 27 00:01:08,070 --> 00:01:06,080 ariel's work focuses on stable isotope 28 00:01:09,830 --> 00:01:08,080 geochemistry of non-traditional 29 00:01:11,109 --> 00:01:09,840 geochemical systems like iron and 30 00:01:12,950 --> 00:01:11,119 molybdenum 31 00:01:14,550 --> 00:01:12,960 and what he works on is understanding 32 00:01:16,630 --> 00:01:14,560 how earths and how earth and its 33 00:01:18,310 --> 00:01:16,640 biosphere have evolved through time 34 00:01:21,190 --> 00:01:18,320 particularly in the archan and 35 00:01:23,030 --> 00:01:21,200 proterozoic and a few years ago he and 36 00:01:25,109 --> 00:01:23,040 andy noel wrote a particularly 37 00:01:28,070 --> 00:01:25,119 influential article that was published 38 00:01:30,710 --> 00:01:28,080 in science that focused attention on how 39 00:01:32,390 --> 00:01:30,720 unusual uh the chemistry of earth 40 00:01:35,109 --> 00:01:32,400 particularly earth's oceans may have 41 00:01:37,190 --> 00:01:35,119 been during earth's middle age 42 00:01:39,429 --> 00:01:37,200 about a billion to two billion years ago 43 00:01:41,429 --> 00:01:39,439 and if you haven't read that article i 44 00:01:44,469 --> 00:01:41,439 recommend it to you 45 00:01:47,429 --> 00:01:44,479 ariel got his bachelor's from harvard 46 00:01:49,429 --> 00:01:47,439 college in 1989 in his phd 47 00:01:51,910 --> 00:01:49,439 in geological and planetary sciences 48 00:01:53,590 --> 00:01:51,920 from caltech in 1996. 49 00:01:56,789 --> 00:01:53,600 he was on the faculty of the university 50 00:01:59,510 --> 00:01:56,799 of rochester for about six years before 51 00:02:02,310 --> 00:01:59,520 joining arizona state university 52 00:02:04,069 --> 00:02:02,320 where he is speaking to us uh from today 53 00:02:07,350 --> 00:02:04,079 and he's going to tell us 54 00:02:09,749 --> 00:02:07,360 about something that occurred just at 55 00:02:12,550 --> 00:02:09,759 the interface between the arcane and the 56 00:02:15,589 --> 00:02:12,560 paleo proterozoic a whiff of oxygen 57 00:02:17,830 --> 00:02:15,599 before the great oxidation event and 58 00:02:19,830 --> 00:02:17,840 ariel take it away 59 00:02:21,910 --> 00:02:19,840 thanks very much carl um i appreciate 60 00:02:22,790 --> 00:02:21,920 the generous introduction after all that 61 00:02:24,470 --> 00:02:22,800 um 62 00:02:26,150 --> 00:02:24,480 although most of my work has focused on 63 00:02:27,270 --> 00:02:26,160 iron and molybdenum isotopes at least a 64 00:02:29,190 --> 00:02:27,280 great deal of it you're not going to see 65 00:02:30,630 --> 00:02:29,200 any of that in this talk here 66 00:02:32,309 --> 00:02:30,640 in fact it's going to be almost an 67 00:02:34,150 --> 00:02:32,319 isotope-free zone except for a little 68 00:02:36,309 --> 00:02:34,160 bit of sulfur isotopes 69 00:02:38,949 --> 00:02:36,319 that we simply can't resist putting in 70 00:02:40,390 --> 00:02:38,959 um i want to uh stress at the outset of 71 00:02:41,350 --> 00:02:40,400 this talk although i'm giving it and 72 00:02:43,509 --> 00:02:41,360 although i'm going to be featuring 73 00:02:45,750 --> 00:02:43,519 primarily research results uh generated 74 00:02:47,670 --> 00:02:45,760 by by my research group um this has been 75 00:02:49,750 --> 00:02:47,680 part of a larger collaborative project 76 00:02:51,190 --> 00:02:49,760 involving a number of institutions um 77 00:02:53,830 --> 00:02:51,200 university of washington university of 78 00:02:55,830 --> 00:02:53,840 maryland uh uc riverside 79 00:02:57,670 --> 00:02:55,840 university of alberta and also mit and 80 00:02:59,430 --> 00:02:57,680 harvard have had a role in this and the 81 00:03:00,710 --> 00:02:59,440 geological survey of western australia 82 00:03:02,630 --> 00:03:00,720 so this is truly a collaborative 83 00:03:04,790 --> 00:03:02,640 international collaborative project 84 00:03:05,830 --> 00:03:04,800 that i'm going to be uh discussing i'm 85 00:03:09,830 --> 00:03:05,840 going to be presenting essentially some 86 00:03:12,070 --> 00:03:09,840 of the first results from that project 87 00:03:15,110 --> 00:03:12,080 uh what this research is all about 88 00:03:16,949 --> 00:03:15,120 is trying to better understand the rise 89 00:03:19,190 --> 00:03:16,959 of oxygen through time in the earth's 90 00:03:21,430 --> 00:03:19,200 atmosphere so what i'm showing you here 91 00:03:22,869 --> 00:03:21,440 and my apologies for the delivery here 92 00:03:24,390 --> 00:03:22,879 it's very awkward giving a talk at a 93 00:03:26,550 --> 00:03:24,400 computer screen so hopefully this will 94 00:03:27,750 --> 00:03:26,560 come off reasonably well um hopefully 95 00:03:30,149 --> 00:03:27,760 you can all see the pointer here what's 96 00:03:31,589 --> 00:03:30,159 shown here is uh it's a cartoon from a 97 00:03:33,830 --> 00:03:31,599 scientific american article by jim 98 00:03:36,630 --> 00:03:33,840 casting a number of years ago 99 00:03:38,550 --> 00:03:36,640 showing uh a sort of a general consensus 100 00:03:39,750 --> 00:03:38,560 concept of change in oxygen in the 101 00:03:41,990 --> 00:03:39,760 environment through time in the 102 00:03:44,710 --> 00:03:42,000 atmosphere through time 103 00:03:46,229 --> 00:03:44,720 the figure is notably vague on the 104 00:03:47,830 --> 00:03:46,239 y-axis where there are no units 105 00:03:50,229 --> 00:03:47,840 whatsoever 106 00:03:52,390 --> 00:03:50,239 which is somewhat reflective actually 107 00:03:53,990 --> 00:03:52,400 of the state of knowledge about oxygen 108 00:03:55,670 --> 00:03:54,000 through time in the earth's environment 109 00:03:57,830 --> 00:03:55,680 we know that today we have about 21 110 00:03:58,630 --> 00:03:57,840 percent but as we go back in time 111 00:04:00,309 --> 00:03:58,640 there's 112 00:04:02,149 --> 00:04:00,319 plenty of room for controversy even 113 00:04:03,990 --> 00:04:02,159 amongst those of us who are pretty 114 00:04:05,350 --> 00:04:04,000 convinced that oxygen was very low 115 00:04:07,350 --> 00:04:05,360 during the first half of history just 116 00:04:09,190 --> 00:04:07,360 what is low mean does low mean anoxic 117 00:04:10,789 --> 00:04:09,200 does low mean slightly oxic 118 00:04:12,229 --> 00:04:10,799 there's a very good reason that we don't 119 00:04:13,509 --> 00:04:12,239 have units here 120 00:04:15,990 --> 00:04:13,519 what we're going to focus on here is 121 00:04:18,629 --> 00:04:16,000 this major initial transition from this 122 00:04:19,990 --> 00:04:18,639 world of uh very low oxygen running 123 00:04:21,590 --> 00:04:20,000 through the first half of earth history 124 00:04:23,990 --> 00:04:21,600 roughly and then there was a sharp 125 00:04:25,990 --> 00:04:24,000 change uh this is this this is unitless 126 00:04:28,629 --> 00:04:26,000 but it's essentially a log axis making 127 00:04:31,350 --> 00:04:28,639 this change look uh very dramatic it was 128 00:04:33,110 --> 00:04:31,360 actually um in in 129 00:04:34,790 --> 00:04:33,120 linear terms a relatively small change 130 00:04:36,310 --> 00:04:34,800 but it was a change from a world with 131 00:04:37,830 --> 00:04:36,320 very little oxygen to a world with some 132 00:04:39,590 --> 00:04:37,840 appreciable fraction of the modern 133 00:04:41,030 --> 00:04:39,600 amount we believe 134 00:04:42,950 --> 00:04:41,040 so this is often referred to as the 135 00:04:44,150 --> 00:04:42,960 great oxidation event so we want to 136 00:04:45,590 --> 00:04:44,160 understand more about this event and 137 00:04:47,110 --> 00:04:45,600 i'll go into some more details about 138 00:04:48,390 --> 00:04:47,120 what we want to understand as we move 139 00:04:49,990 --> 00:04:48,400 along but that's the context of this 140 00:04:51,189 --> 00:04:50,000 talk we're trying to understand issues 141 00:04:52,070 --> 00:04:51,199 associated with the great oxidation 142 00:04:54,070 --> 00:04:52,080 event 143 00:04:55,909 --> 00:04:54,080 now there's been a lot of recent 144 00:04:57,270 --> 00:04:55,919 evolution on the topic of oxygen 145 00:04:59,510 --> 00:04:57,280 evolution 146 00:05:00,870 --> 00:04:59,520 and i list here a series of papers 147 00:05:02,629 --> 00:05:00,880 that 148 00:05:04,629 --> 00:05:02,639 have come out in the past year and a 149 00:05:07,189 --> 00:05:04,639 half or so um 150 00:05:08,550 --> 00:05:07,199 2006 and 2007 a number of them quite 151 00:05:11,110 --> 00:05:08,560 prominent uh 152 00:05:12,629 --> 00:05:11,120 in nature um and this is by way of 153 00:05:13,990 --> 00:05:12,639 illustrating this is a very vibrant area 154 00:05:15,990 --> 00:05:14,000 of research with a good deal of 155 00:05:17,430 --> 00:05:16,000 controversy particularly if uh you're 156 00:05:19,029 --> 00:05:17,440 interested in going into more detail 157 00:05:21,110 --> 00:05:19,039 than you'll see in this talk these two 158 00:05:21,990 --> 00:05:21,120 papers here by omoto at all and farquaad 159 00:05:23,909 --> 00:05:22,000 all 160 00:05:27,189 --> 00:05:23,919 battle back and forth 161 00:05:28,550 --> 00:05:27,199 about the earliest oxygen record 162 00:05:30,550 --> 00:05:28,560 um 163 00:05:32,070 --> 00:05:30,560 arguing about what was the archaean 164 00:05:34,150 --> 00:05:32,080 anoxic or not and what does that really 165 00:05:35,990 --> 00:05:34,160 mean 166 00:05:38,790 --> 00:05:36,000 so this work follows on the heels of 167 00:05:40,390 --> 00:05:38,800 much of these studies and what i'm going 168 00:05:42,230 --> 00:05:40,400 to focus on are two additional papers 169 00:05:44,469 --> 00:05:42,240 that aren't on that list 170 00:05:46,710 --> 00:05:44,479 that were published at the end of 171 00:05:48,310 --> 00:05:46,720 september in science 172 00:05:49,430 --> 00:05:48,320 one is titled the same as this talk a 173 00:05:51,110 --> 00:05:49,440 whiff of oxygen before the great 174 00:05:53,270 --> 00:05:51,120 oxidation event and the other a 175 00:05:55,270 --> 00:05:53,280 companion paper uh late arcane 176 00:05:58,310 --> 00:05:55,280 biospheric oxygenation and atmospheric 177 00:05:59,510 --> 00:05:58,320 evolution but led by jake kaufman uh and 178 00:06:00,550 --> 00:05:59,520 his colleagues at the university of 179 00:06:03,350 --> 00:06:00,560 maryland 180 00:06:04,790 --> 00:06:03,360 both of these studies uh uh 181 00:06:06,950 --> 00:06:04,800 the reason they're paired together is 182 00:06:09,430 --> 00:06:06,960 because they tell a complimentary story 183 00:06:11,350 --> 00:06:09,440 about a change in environmental 184 00:06:13,830 --> 00:06:11,360 oxygenation just before the great 185 00:06:14,870 --> 00:06:13,840 oxidation event 186 00:06:15,990 --> 00:06:14,880 and 187 00:06:18,150 --> 00:06:16,000 it's particularly appropriate to be 188 00:06:19,510 --> 00:06:18,160 talking about this at an nai director's 189 00:06:21,350 --> 00:06:19,520 seminar because 190 00:06:23,430 --> 00:06:21,360 this work is the outcome of the nasa 191 00:06:25,270 --> 00:06:23,440 astrobiology drilling program 192 00:06:27,510 --> 00:06:25,280 um some of you may not be aware that 193 00:06:30,469 --> 00:06:27,520 nasa has and the ass astrology institute 194 00:06:31,430 --> 00:06:30,479 has a drilling program but it does 195 00:06:32,150 --> 00:06:31,440 and 196 00:06:35,189 --> 00:06:32,160 the 197 00:06:37,189 --> 00:06:35,199 drilling rig that we're working with and 198 00:06:39,110 --> 00:06:37,199 the sort of uh samples that we are 199 00:06:42,710 --> 00:06:39,120 obtaining from this drilling program and 200 00:06:45,909 --> 00:06:42,720 this program over the in 2003 and 2004 201 00:06:48,309 --> 00:06:45,919 um generated uh 10 drill cores from 202 00:06:50,230 --> 00:06:48,319 western australia all focusing on the 203 00:06:52,230 --> 00:06:50,240 period of time before the rise of oxygen 204 00:06:53,590 --> 00:06:52,240 and various periods of various spans of 205 00:06:54,710 --> 00:06:53,600 time before that 206 00:06:56,469 --> 00:06:54,720 trying to get at various questions 207 00:06:58,150 --> 00:06:56,479 associated with that first great 208 00:06:59,270 --> 00:06:58,160 oxidation event 209 00:07:01,510 --> 00:06:59,280 the two papers i'm going to be 210 00:07:04,390 --> 00:07:01,520 discussing here in particular the i'm 211 00:07:06,230 --> 00:07:04,400 focusing on the whiff of oxygen paper um 212 00:07:08,309 --> 00:07:06,240 uh both are the represent the first 213 00:07:10,390 --> 00:07:08,319 results from the long core the one 214 00:07:13,029 --> 00:07:10,400 kilometer deep core that was drilled in 215 00:07:14,550 --> 00:07:13,039 2004 in the hammersley basin so the 10 216 00:07:16,469 --> 00:07:14,560 cores one of them in particular is what 217 00:07:17,830 --> 00:07:16,479 we're highlighting here in this study 218 00:07:20,070 --> 00:07:17,840 now 219 00:07:21,990 --> 00:07:20,080 by way of context you might ask well why 220 00:07:23,749 --> 00:07:22,000 is the nasa astrobiology institute in 221 00:07:26,469 --> 00:07:23,759 nasa more generally interested in 222 00:07:27,990 --> 00:07:26,479 drilling old rocks in western australia 223 00:07:29,430 --> 00:07:28,000 and why really there's interest in this 224 00:07:31,110 --> 00:07:29,440 oxygen question and for many of you this 225 00:07:32,790 --> 00:07:31,120 is this is obvious but for some this may 226 00:07:35,110 --> 00:07:32,800 not be especially for students so it's 227 00:07:36,629 --> 00:07:35,120 worth belaboring for just a moment so 228 00:07:38,629 --> 00:07:36,639 what you're seeing here is a cartoon of 229 00:07:41,990 --> 00:07:38,639 an extrasolar planet one of these hot 230 00:07:43,189 --> 00:07:42,000 jupiters these giant uh planets um 231 00:07:45,670 --> 00:07:43,199 close to their stars which are being 232 00:07:46,710 --> 00:07:45,680 discovered with regularity these days 233 00:07:48,390 --> 00:07:46,720 um 234 00:07:50,870 --> 00:07:48,400 amazingly we can actually sit here and 235 00:07:52,390 --> 00:07:50,880 say that uh to imagine a world like this 236 00:07:54,550 --> 00:07:52,400 is no longer an extraordinary thing 237 00:07:56,230 --> 00:07:54,560 because we know of many of them what is 238 00:07:58,230 --> 00:07:56,240 still extraordinary is this data here 239 00:08:00,309 --> 00:07:58,240 this is from a paper in nature this past 240 00:08:01,350 --> 00:08:00,319 year showing what i believe are the 241 00:08:03,029 --> 00:08:01,360 first 242 00:08:05,670 --> 00:08:03,039 uh spectroscopic data from the 243 00:08:07,909 --> 00:08:05,680 atmosphere of such an extrasolar planet 244 00:08:10,950 --> 00:08:07,919 and although these planets are very very 245 00:08:13,189 --> 00:08:10,960 unlikely to be hospitable to life 246 00:08:15,909 --> 00:08:13,199 this sort of data spectroscopic data 247 00:08:18,469 --> 00:08:15,919 from the atmospheres of uh extrasolar 248 00:08:20,629 --> 00:08:18,479 planets are likely in the next couple of 249 00:08:22,070 --> 00:08:20,639 decades to be obtained from planets that 250 00:08:23,670 --> 00:08:22,080 are much more like earth and hence 251 00:08:25,670 --> 00:08:23,680 planets that are more likely to support 252 00:08:26,790 --> 00:08:25,680 life and so the question before us is 253 00:08:29,029 --> 00:08:26,800 how would we go about looking for 254 00:08:30,790 --> 00:08:29,039 evidence of life on such planets what 255 00:08:33,589 --> 00:08:30,800 would what would constitute a planetary 256 00:08:36,310 --> 00:08:33,599 biosignature on an exoplanet and such a 257 00:08:38,230 --> 00:08:36,320 biosignature would be uh uh reflect 258 00:08:39,430 --> 00:08:38,240 would be something we can measure in the 259 00:08:41,750 --> 00:08:39,440 spectrum of the atmosphere of that 260 00:08:43,269 --> 00:08:41,760 planet that's the most uh 261 00:08:44,389 --> 00:08:43,279 first thing we're going to be able to do 262 00:08:46,630 --> 00:08:44,399 we hope 263 00:08:47,829 --> 00:08:46,640 and um oxygen features very prominently 264 00:08:50,870 --> 00:08:47,839 in that sort of discussion so what 265 00:08:55,509 --> 00:08:50,880 you're seeing here are uh a spectra in 266 00:08:57,350 --> 00:08:55,519 the infrared of venus earth and mars 267 00:08:58,630 --> 00:08:57,360 showing you uh the two things should 268 00:09:01,750 --> 00:08:58,640 jump out at you here from this figure 269 00:09:03,990 --> 00:09:01,760 one is that all these atmospheres show a 270 00:09:06,470 --> 00:09:04,000 pronounced carbon dioxide 271 00:09:07,750 --> 00:09:06,480 uh band which is which is a result of 272 00:09:09,910 --> 00:09:07,760 the fact that all these atmospheres have 273 00:09:11,509 --> 00:09:09,920 significant amounts of carbon dioxide 274 00:09:12,790 --> 00:09:11,519 but only earth shows that it shows this 275 00:09:14,949 --> 00:09:12,800 ozone feature 276 00:09:17,269 --> 00:09:14,959 an ozone is a byproduct of the 277 00:09:19,829 --> 00:09:17,279 photolysis of oxygen in a planetary 278 00:09:21,750 --> 00:09:19,839 atmosphere and to have ozone in amounts 279 00:09:23,990 --> 00:09:21,760 that you can detect like this from space 280 00:09:26,630 --> 00:09:24,000 is a reflection of the presence of large 281 00:09:29,030 --> 00:09:26,640 amounts of oxygen in 282 00:09:31,190 --> 00:09:29,040 the atmosphere of a planet and 283 00:09:32,870 --> 00:09:31,200 uh commonly it's it's believed and i'm 284 00:09:34,470 --> 00:09:32,880 not going to dispute this at all that if 285 00:09:36,389 --> 00:09:34,480 you have such large amounts of oxygen in 286 00:09:39,269 --> 00:09:36,399 the atmosphere of a planet you are 287 00:09:43,030 --> 00:09:39,279 seeing evidence of life on that planet 288 00:09:45,590 --> 00:09:43,040 and so as a result it if we want to uh 289 00:09:47,910 --> 00:09:45,600 uh develop such a concept of how we 290 00:09:50,710 --> 00:09:47,920 might look for life in uh 291 00:09:52,550 --> 00:09:50,720 beyond our solar system it behooves us 292 00:09:53,910 --> 00:09:52,560 to try to understand what controls the 293 00:09:56,389 --> 00:09:53,920 amount of oxygen in a planetary 294 00:09:58,230 --> 00:09:56,399 atmosphere if a planet has oxygenic 295 00:10:00,150 --> 00:09:58,240 photosynthesis will it 296 00:10:02,870 --> 00:10:00,160 will it inevitably evolve an oxygenated 297 00:10:04,550 --> 00:10:02,880 atmosphere that's a good question 298 00:10:06,470 --> 00:10:04,560 if um 299 00:10:08,310 --> 00:10:06,480 we look at lots of planets and don't see 300 00:10:09,110 --> 00:10:08,320 any oxygen what does that tell us about 301 00:10:10,870 --> 00:10:09,120 life 302 00:10:12,550 --> 00:10:10,880 um what are the odds that the planet 303 00:10:14,630 --> 00:10:12,560 that has oxygenic photosynthesis will 304 00:10:15,910 --> 00:10:14,640 evolve a 20 oxygen atmosphere like the 305 00:10:17,990 --> 00:10:15,920 earth these are all questions we don't 306 00:10:19,269 --> 00:10:18,000 really have answers to and if we want to 307 00:10:21,430 --> 00:10:19,279 understand them 308 00:10:23,670 --> 00:10:21,440 what we need to do is study the earth's 309 00:10:25,430 --> 00:10:23,680 past um given that this is the only 310 00:10:26,710 --> 00:10:25,440 planet that we really the only living 311 00:10:28,710 --> 00:10:26,720 planet that we really have to work with 312 00:10:29,910 --> 00:10:28,720 so uh this is what this is our natural 313 00:10:31,030 --> 00:10:29,920 laboratory 314 00:10:33,829 --> 00:10:31,040 so the question that we're going to 315 00:10:36,870 --> 00:10:33,839 focus on here is why why is there this 316 00:10:38,230 --> 00:10:36,880 rise of oxygen uh about 2.3 billion 317 00:10:40,310 --> 00:10:38,240 years ago 318 00:10:41,509 --> 00:10:40,320 in earth history 319 00:10:43,030 --> 00:10:41,519 we're not going to resolve that question 320 00:10:44,590 --> 00:10:43,040 but we're going to point the way towards 321 00:10:47,110 --> 00:10:44,600 the resolution 322 00:10:49,750 --> 00:10:47,120 hopefully so the simplest answer to that 323 00:10:51,509 --> 00:10:49,760 question that you might uh imagine 324 00:10:53,829 --> 00:10:51,519 is a biological explanation an 325 00:10:57,430 --> 00:10:53,839 evolutionary explanation if oxygenic 326 00:10:58,630 --> 00:10:57,440 photosynthesis is required uh to 327 00:11:01,269 --> 00:10:58,640 have large amounts of oxygen in the 328 00:11:03,269 --> 00:11:01,279 atmosphere then perhaps at 2.3 billion 329 00:11:05,670 --> 00:11:03,279 years ago or two point just a bit before 330 00:11:07,829 --> 00:11:05,680 that 2.4 billion years ago perhaps then 331 00:11:09,110 --> 00:11:07,839 that is the time when photosystem 2 332 00:11:11,509 --> 00:11:09,120 evolves and here's a picture of the 333 00:11:14,310 --> 00:11:11,519 oxygen evolving center of photosystem ii 334 00:11:16,150 --> 00:11:14,320 um and so perhaps this uh biochemistry 335 00:11:17,430 --> 00:11:16,160 evolves that point and rapidly proceeds 336 00:11:19,430 --> 00:11:17,440 to take over the world and make the 337 00:11:20,389 --> 00:11:19,440 planet an oxygenated and oxygenated 338 00:11:21,750 --> 00:11:20,399 world 339 00:11:23,829 --> 00:11:21,760 that is a view that you will certainly 340 00:11:26,310 --> 00:11:23,839 find in many textbooks it's a view that 341 00:11:29,350 --> 00:11:26,320 is strongly espoused by researchers like 342 00:11:31,350 --> 00:11:29,360 joe kershwink uh today um at least as of 343 00:11:32,790 --> 00:11:31,360 last week when i saw him at gsa 344 00:11:36,550 --> 00:11:32,800 um 345 00:11:38,150 --> 00:11:36,560 you can advance and it's and it's a 346 00:11:39,990 --> 00:11:38,160 reasonable hypothesis it's in fact a 347 00:11:41,509 --> 00:11:40,000 very difficult hypothesis to refute 348 00:11:43,750 --> 00:11:41,519 without going into the geologic record 349 00:11:44,949 --> 00:11:43,760 as we're going to try to do 350 00:11:46,310 --> 00:11:44,959 um 351 00:11:48,150 --> 00:11:46,320 most of the community though thinks it's 352 00:11:50,150 --> 00:11:48,160 probably not so simple 353 00:11:52,470 --> 00:11:50,160 and the reason for that is as follows i 354 00:11:56,069 --> 00:11:52,480 want to give you get across for you a at 355 00:11:57,590 --> 00:11:56,079 least a cartoon idea of why uh it's 356 00:12:00,150 --> 00:11:57,600 probably not as simple as just oxygen 357 00:12:02,550 --> 00:12:00,160 photosynthesis evolves and uh the planet 358 00:12:03,829 --> 00:12:02,560 becomes oxygenated 359 00:12:05,910 --> 00:12:03,839 so we're going to start just by thinking 360 00:12:08,629 --> 00:12:05,920 very simply the way a geochemist does 361 00:12:10,310 --> 00:12:08,639 about oxygenic photosynthesis what is it 362 00:12:11,670 --> 00:12:10,320 so uh if there are any biologists in the 363 00:12:13,269 --> 00:12:11,680 audience you'll be horrified by this but 364 00:12:14,710 --> 00:12:13,279 the way a geochemist thinks about oxygen 365 00:12:16,230 --> 00:12:14,720 with photosynthesis is that one takes 366 00:12:19,269 --> 00:12:16,240 co2 and water 367 00:12:22,470 --> 00:12:19,279 and reacts them together and makes o2 368 00:12:24,069 --> 00:12:22,480 and um here even if you're not a a 369 00:12:25,350 --> 00:12:24,079 life scientist this is just horrifying 370 00:12:27,110 --> 00:12:25,360 if you're an organic geochemist it's a 371 00:12:28,870 --> 00:12:27,120 horrifying thing to say that all organic 372 00:12:30,710 --> 00:12:28,880 carbon is ch2o 373 00:12:33,110 --> 00:12:30,720 that's of course a gross simplification 374 00:12:34,710 --> 00:12:33,120 but to um 375 00:12:36,230 --> 00:12:34,720 to a first approximation if we're just 376 00:12:38,150 --> 00:12:36,240 thinking stoichiometrically of the ratio 377 00:12:41,190 --> 00:12:38,160 of carbon to hydrogen to oxygen this is 378 00:12:43,269 --> 00:12:41,200 the product of photosynthesis o2 and 379 00:12:46,710 --> 00:12:43,279 quote unquote organic carbon 380 00:12:51,030 --> 00:12:46,720 in the stoichiometry ch2o 381 00:12:54,710 --> 00:12:52,790 but there's also of course a back 382 00:12:56,629 --> 00:12:54,720 reaction which is very fortunate um it's 383 00:12:58,870 --> 00:12:56,639 what is allowing all of us to sit in our 384 00:13:01,750 --> 00:12:58,880 various rooms across the country and and 385 00:13:04,550 --> 00:13:01,760 watch this this uh uh seminar for better 386 00:13:06,470 --> 00:13:04,560 or worse um it's aerobic respiration the 387 00:13:08,310 --> 00:13:06,480 back reaction of o2 with organic carbon 388 00:13:09,910 --> 00:13:08,320 to give you back co2 and water we are 389 00:13:11,509 --> 00:13:09,920 all all of us sitting in the room in the 390 00:13:12,870 --> 00:13:11,519 room in our rooms are doing this right 391 00:13:14,470 --> 00:13:12,880 now 392 00:13:16,069 --> 00:13:14,480 and this is a very efficient process 393 00:13:17,750 --> 00:13:16,079 such that almost all the oxygen that's 394 00:13:19,509 --> 00:13:17,760 produced per year in the biosphere is 395 00:13:22,069 --> 00:13:19,519 reconsumed by oxidation with organic 396 00:13:23,990 --> 00:13:22,079 carbon to give you back co2 and water 397 00:13:25,670 --> 00:13:24,000 so in a sense this is 398 00:13:27,190 --> 00:13:25,680 although obviously this is important 399 00:13:28,870 --> 00:13:27,200 because it's producing lots of oxygen 400 00:13:31,030 --> 00:13:28,880 this is this is very rapidly all 401 00:13:32,790 --> 00:13:31,040 consumed so it's not so simple on a 402 00:13:34,310 --> 00:13:32,800 geologic time scale it's not so simple 403 00:13:36,150 --> 00:13:34,320 to just look at oxygen photosynthesis 404 00:13:38,710 --> 00:13:36,160 and say aha if we have it then we have 405 00:13:40,550 --> 00:13:38,720 lots of oxygen in the atmosphere 406 00:13:43,990 --> 00:13:40,560 to it 407 00:13:48,470 --> 00:13:46,069 to a geochemist thinking about this 408 00:13:49,750 --> 00:13:48,480 requires thinking outside this fast box 409 00:13:51,670 --> 00:13:49,760 a little bit 410 00:13:52,629 --> 00:13:51,680 to the geochemist what uh needs to be 411 00:13:53,750 --> 00:13:52,639 considered 412 00:13:55,350 --> 00:13:53,760 is that 413 00:13:57,590 --> 00:13:55,360 on geologically relevant time scales 414 00:14:00,150 --> 00:13:57,600 there's a slow leakage out of this loop 415 00:14:01,750 --> 00:14:00,160 in the form of organic carbon being 416 00:14:03,110 --> 00:14:01,760 buried in sediments 417 00:14:04,470 --> 00:14:03,120 actually most of the organic carbon that 418 00:14:05,910 --> 00:14:04,480 makes it uh 419 00:14:07,670 --> 00:14:05,920 to be buried in sediments also gets 420 00:14:09,910 --> 00:14:07,680 re-oxidized by processes happening in 421 00:14:11,670 --> 00:14:09,920 sediments but a small fraction of that 422 00:14:13,350 --> 00:14:11,680 actually gets buried it's sequestered 423 00:14:15,189 --> 00:14:13,360 for geologically relevant periods of 424 00:14:17,750 --> 00:14:15,199 time um 425 00:14:19,350 --> 00:14:17,760 uh in in sediments particularly in the 426 00:14:21,189 --> 00:14:19,360 deep ocean 427 00:14:22,790 --> 00:14:21,199 um a consequence of this is that for 428 00:14:24,949 --> 00:14:22,800 every mole of organic carbon that one 429 00:14:27,509 --> 00:14:24,959 varies in sediments there is a mole of 430 00:14:29,590 --> 00:14:27,519 oxygen left behind in the atmosphere 431 00:14:31,430 --> 00:14:29,600 and to a geochemist thinking in long 432 00:14:33,509 --> 00:14:31,440 time scales this is the geologically 433 00:14:35,269 --> 00:14:33,519 relevant source quote unquote of oxygen 434 00:14:36,470 --> 00:14:35,279 to the atmosphere you need to have this 435 00:14:38,230 --> 00:14:36,480 going on in here you need to have the 436 00:14:40,310 --> 00:14:38,240 oxygen footage the synthesis to produce 437 00:14:41,670 --> 00:14:40,320 the oxygen but without some process like 438 00:14:43,750 --> 00:14:41,680 this bearing organic carbon you're not 439 00:14:46,230 --> 00:14:43,760 going to have build up of of oxygen in 440 00:14:47,590 --> 00:14:46,240 the atmosphere over time scales that are 441 00:14:49,509 --> 00:14:47,600 relevant to us when we think in terms of 442 00:14:52,710 --> 00:14:49,519 hundreds of millions of years 443 00:14:54,389 --> 00:14:52,720 of course that this is all a cycle and 444 00:14:57,269 --> 00:14:54,399 so there is a back reaction of that 445 00:14:59,030 --> 00:14:57,279 oxygen with reduced gases from volcanoes 446 00:15:01,189 --> 00:14:59,040 with dissolved reductance in seawater 447 00:15:03,509 --> 00:15:01,199 with reduced compounds iron ii plus and 448 00:15:05,269 --> 00:15:03,519 other materials in igneous rocks and and 449 00:15:08,069 --> 00:15:05,279 sediments when these when these buried 450 00:15:10,150 --> 00:15:08,079 organic sediments um uh are exhumed 451 00:15:11,670 --> 00:15:10,160 which in some cases they are 452 00:15:13,590 --> 00:15:11,680 you have then the opportunity for those 453 00:15:15,269 --> 00:15:13,600 sediments to re-react with oxygen but 454 00:15:16,710 --> 00:15:15,279 all these processes happen slowly there 455 00:15:19,590 --> 00:15:16,720 are they operate on geological time 456 00:15:21,590 --> 00:15:19,600 scales but to a geoscientist then the 457 00:15:23,750 --> 00:15:21,600 the rise of oxygen and the controls on 458 00:15:25,269 --> 00:15:23,760 oxygen are in part about oxygen 459 00:15:27,829 --> 00:15:25,279 photosynthesis but they're just as much 460 00:15:29,430 --> 00:15:27,839 about the balance between this flux 461 00:15:31,910 --> 00:15:29,440 which is really about this flux down 462 00:15:34,470 --> 00:15:31,920 here versus the rate of consumption of 463 00:15:36,470 --> 00:15:34,480 oxygen by reductance that that become 464 00:15:38,550 --> 00:15:36,480 exposed on geologically meaningful time 465 00:15:40,870 --> 00:15:38,560 scales so it's this balance that becomes 466 00:15:42,870 --> 00:15:40,880 very important if you are a 467 00:15:45,509 --> 00:15:42,880 geoscientist and one way you can think 468 00:15:47,509 --> 00:15:45,519 about this um i apologize for the typo 469 00:15:48,790 --> 00:15:47,519 up here there shouldn't be a b up there 470 00:15:50,550 --> 00:15:48,800 is you could sort of divide these two 471 00:15:53,030 --> 00:15:50,560 views into a supply side view and a 472 00:15:54,870 --> 00:15:53,040 demand side view so the supply side view 473 00:15:57,030 --> 00:15:54,880 is that oxygenic photosynthesis evolves 474 00:15:59,350 --> 00:15:57,040 at 2.4 or 2.3 billion years ago and 475 00:16:00,870 --> 00:15:59,360 rapidly takes over the planet 476 00:16:02,310 --> 00:16:00,880 the demand side view is that oxygen 477 00:16:04,389 --> 00:16:02,320 photosynthesis 478 00:16:06,310 --> 00:16:04,399 has to evolve by that time but may have 479 00:16:08,470 --> 00:16:06,320 evolved much much earlier and that 480 00:16:10,230 --> 00:16:08,480 oxygen takes over only when the oxic 481 00:16:12,470 --> 00:16:10,240 sources this production of oxygen the 482 00:16:13,910 --> 00:16:12,480 net production of oxygen overwhelms the 483 00:16:15,749 --> 00:16:13,920 reducing sinks 484 00:16:19,030 --> 00:16:15,759 which may be as much because of a change 485 00:16:22,069 --> 00:16:19,040 in the reflux of reductance um as it may 486 00:16:23,269 --> 00:16:22,079 be because of a rise in oxygen 487 00:16:25,189 --> 00:16:23,279 production 488 00:16:26,949 --> 00:16:25,199 so this is a more complicated picture 489 00:16:29,509 --> 00:16:26,959 and in particular from an evolutionary 490 00:16:31,269 --> 00:16:29,519 standpoint this demand side picture uh 491 00:16:33,269 --> 00:16:31,279 allows for the possibility that oxygenic 492 00:16:34,949 --> 00:16:33,279 photosynthesis is extremely ancient it 493 00:16:37,030 --> 00:16:34,959 could you know hypothetically could date 494 00:16:37,829 --> 00:16:37,040 back to very shortly after the origin of 495 00:16:40,710 --> 00:16:37,839 life 496 00:16:42,870 --> 00:16:40,720 which could be 3.8 billion years ago 497 00:16:44,389 --> 00:16:42,880 so these are very these two views have 498 00:16:47,110 --> 00:16:44,399 very different consequences for how 499 00:16:48,389 --> 00:16:47,120 early oxygen photosynthesis might exist 500 00:16:50,230 --> 00:16:48,399 as i said before it's very hard to 501 00:16:51,430 --> 00:16:50,240 differentiate between these hypotheses 502 00:16:52,870 --> 00:16:51,440 but one of the simplest ways you can 503 00:16:55,030 --> 00:16:52,880 think of to differentiate between them 504 00:16:57,430 --> 00:16:55,040 is to search for evidence of oxygen in 505 00:16:59,030 --> 00:16:57,440 the record before 2.4 or 2.3 billion 506 00:17:01,269 --> 00:16:59,040 years ago before the great rise of 507 00:17:04,069 --> 00:17:01,279 oxygen if you find evidence of oxygen in 508 00:17:05,510 --> 00:17:04,079 that earlier record then you have a if 509 00:17:07,590 --> 00:17:05,520 at least if it's in amounts that are 510 00:17:09,750 --> 00:17:07,600 larger than what can easily be made by 511 00:17:10,870 --> 00:17:09,760 non-biological processes then you have 512 00:17:14,309 --> 00:17:10,880 an argument 513 00:17:16,069 --> 00:17:14,319 for this demand side point of view 514 00:17:18,150 --> 00:17:16,079 now of course there have been many uh 515 00:17:19,350 --> 00:17:18,160 studies over the years that have 516 00:17:21,590 --> 00:17:19,360 provided data that people have 517 00:17:24,309 --> 00:17:21,600 interpreted in terms as indicating 518 00:17:25,750 --> 00:17:24,319 evidence of oxygen uh in the fir during 519 00:17:27,350 --> 00:17:25,760 the first half of earth history at least 520 00:17:29,830 --> 00:17:27,360 in in some amounts 521 00:17:31,190 --> 00:17:29,840 um so the most classic ex 522 00:17:32,789 --> 00:17:31,200 line of evidence that you'll find is the 523 00:17:34,710 --> 00:17:32,799 presence of banded iron formations in 524 00:17:36,070 --> 00:17:34,720 the archaean and paleo-protozoa but 525 00:17:37,990 --> 00:17:36,080 particularly here in the archaean 526 00:17:40,150 --> 00:17:38,000 geological record so abandon iron 527 00:17:41,750 --> 00:17:40,160 formations are these massive deposits of 528 00:17:43,110 --> 00:17:41,760 iron oxide 529 00:17:44,630 --> 00:17:43,120 another line of evidence that has become 530 00:17:45,430 --> 00:17:44,640 very much in vogue in the last decade or 531 00:17:48,230 --> 00:17:45,440 so 532 00:17:49,990 --> 00:17:48,240 are particularly organic biomarkers 533 00:17:52,630 --> 00:17:50,000 that for example those biomarkers that 534 00:17:54,549 --> 00:17:52,640 are indicative of the 535 00:17:56,230 --> 00:17:54,559 argued by many people and i think agreed 536 00:17:58,630 --> 00:17:56,240 to by most to be indicative of the 537 00:18:01,430 --> 00:17:58,640 presence of eukaryotes which require 538 00:18:02,870 --> 00:18:01,440 oxygen for some of their biochemistry 539 00:18:04,310 --> 00:18:02,880 in the early record 540 00:18:06,870 --> 00:18:04,320 the trouble with these and these are 541 00:18:08,230 --> 00:18:06,880 both fine lines of of evidence 542 00:18:10,390 --> 00:18:08,240 and i'm not trying to argue against any 543 00:18:12,150 --> 00:18:10,400 of them is that they have they have 544 00:18:13,750 --> 00:18:12,160 they're controversial so in the case of 545 00:18:15,350 --> 00:18:13,760 organic biomarkers as i just said while 546 00:18:17,190 --> 00:18:15,360 most of the community i think accepts 547 00:18:19,510 --> 00:18:17,200 the validity of the of the data that's 548 00:18:21,190 --> 00:18:19,520 been produced to date um there is great 549 00:18:22,870 --> 00:18:21,200 concern including by some of the authors 550 00:18:24,630 --> 00:18:22,880 of some of the most notable studies 551 00:18:26,710 --> 00:18:24,640 about the possibility of contamination 552 00:18:28,870 --> 00:18:26,720 of ancient sediments um from which some 553 00:18:31,669 --> 00:18:28,880 of these spectacular molecules have been 554 00:18:33,510 --> 00:18:31,679 extracted um so although this is this is 555 00:18:36,549 --> 00:18:33,520 a great line of evidence it's not 556 00:18:38,070 --> 00:18:36,559 without its its criticisms and flaws um 557 00:18:39,990 --> 00:18:38,080 or at least challenges and so you'd like 558 00:18:42,549 --> 00:18:40,000 to have other lines of evidence banded 559 00:18:44,470 --> 00:18:42,559 our information similarly uh you can 560 00:18:46,310 --> 00:18:44,480 produce banana formations of course by 561 00:18:48,390 --> 00:18:46,320 taking iron two plus in the oceans and 562 00:18:50,549 --> 00:18:48,400 oxidizing it with o2 to make iron three 563 00:18:53,430 --> 00:18:50,559 plus which precipitates out to make iron 564 00:18:54,950 --> 00:18:53,440 hydroxides so they you can take these as 565 00:18:56,310 --> 00:18:54,960 evidence of oxygen but there are at 566 00:18:59,270 --> 00:18:56,320 least two other mechanisms which have 567 00:19:01,430 --> 00:18:59,280 not been uh refuted despite some efforts 568 00:19:02,630 --> 00:19:01,440 to do so um some experimental efforts to 569 00:19:05,029 --> 00:19:02,640 do so 570 00:19:06,710 --> 00:19:05,039 which can alternatively produce 571 00:19:08,549 --> 00:19:06,720 massive deposit of iron oxide without 572 00:19:11,110 --> 00:19:08,559 having oxygen around one of these 573 00:19:12,870 --> 00:19:11,120 methods is by photo oxidation so 574 00:19:15,029 --> 00:19:12,880 ultraviolet light can react with iron 575 00:19:16,390 --> 00:19:15,039 two plus to form iron three plus and 576 00:19:18,310 --> 00:19:16,400 laboratory experiments show that that's 577 00:19:21,029 --> 00:19:18,320 rapid enough that it can account for at 578 00:19:22,549 --> 00:19:21,039 least some banded iron formations um and 579 00:19:25,190 --> 00:19:22,559 more in vogue in recent years is the 580 00:19:27,270 --> 00:19:25,200 notion of an oxygenic photosynthesis 581 00:19:29,029 --> 00:19:27,280 photosynthesis by bacteria that that 582 00:19:31,190 --> 00:19:29,039 instead of using water as an electron 583 00:19:33,350 --> 00:19:31,200 donor use iron two plus as an electron 584 00:19:36,230 --> 00:19:33,360 donor and such bacteria instead of 585 00:19:38,390 --> 00:19:36,240 producing oxygen produce iron three plus 586 00:19:42,310 --> 00:19:38,400 as the oxidized waste product 587 00:19:44,630 --> 00:19:42,320 and such bacteria were were a concept uh 588 00:19:46,150 --> 00:19:44,640 uh when i was first studying about this 589 00:19:47,830 --> 00:19:46,160 20 years ago or so 590 00:19:49,909 --> 00:19:47,840 um but in the last decade they've been 591 00:19:52,390 --> 00:19:49,919 discovered to actually exist and they 592 00:19:54,950 --> 00:19:52,400 oxidize iron at a rate fast enough to 593 00:19:56,630 --> 00:19:54,960 also account for banded iron formations 594 00:19:58,150 --> 00:19:56,640 so although both these lines of evidence 595 00:19:59,669 --> 00:19:58,160 are are quite reasonable neither of them 596 00:20:01,510 --> 00:19:59,679 is incontrovertible so you'd like to 597 00:20:03,350 --> 00:20:01,520 have additional lines of evidence to 598 00:20:06,470 --> 00:20:03,360 work with and this is typically the case 599 00:20:08,870 --> 00:20:06,480 when you study earth history that uh you 600 00:20:10,390 --> 00:20:08,880 are in a deductive mode of reasoning and 601 00:20:11,270 --> 00:20:10,400 you know you rarely find a smoking 602 00:20:16,230 --> 00:20:11,280 bullet 603 00:20:17,909 --> 00:20:16,240 smoking gun 604 00:20:20,070 --> 00:20:17,919 and you're left trying to accumulate 605 00:20:21,270 --> 00:20:20,080 lines of evidence and hope that if you 606 00:20:22,390 --> 00:20:21,280 find multiple lines of evidence that 607 00:20:23,830 --> 00:20:22,400 point in the same direction that the 608 00:20:26,870 --> 00:20:23,840 cumulative weight of those lines even 609 00:20:28,230 --> 00:20:26,880 though none is perfectly solid is um 610 00:20:30,470 --> 00:20:28,240 convincing to 611 00:20:32,470 --> 00:20:30,480 what one hopes is two sigma confidence 612 00:20:34,870 --> 00:20:32,480 so what we're going to talk about today 613 00:20:37,270 --> 00:20:34,880 is uh the use of redox sensitive 614 00:20:39,430 --> 00:20:37,280 transition metals as a tool to look at 615 00:20:40,950 --> 00:20:39,440 changes in oxygenation through time and 616 00:20:42,310 --> 00:20:40,960 this is not a new idea i learned about 617 00:20:45,029 --> 00:20:42,320 it uh from a book that dick holland 618 00:20:47,110 --> 00:20:45,039 wrote in 1984 and i don't know that the 619 00:20:48,789 --> 00:20:47,120 idea even was unique to dick at that 620 00:20:50,230 --> 00:20:48,799 time so this is an old idea but it 621 00:20:51,750 --> 00:20:50,240 hasn't really been applied all that much 622 00:20:53,590 --> 00:20:51,760 and not at the level of detail and 623 00:20:55,350 --> 00:20:53,600 systematic investigation that we've been 624 00:20:56,789 --> 00:20:55,360 able to do with this astrobiology drill 625 00:20:58,390 --> 00:20:56,799 core 626 00:21:00,630 --> 00:20:58,400 um this figure is meant to get across 627 00:21:02,470 --> 00:21:00,640 the basic concept though so why why what 628 00:21:04,950 --> 00:21:02,480 elements we're focusing on we're going 629 00:21:06,549 --> 00:21:04,960 to focus a lot on the element molybdenum 630 00:21:08,149 --> 00:21:06,559 which i don't pick just because it's a 631 00:21:11,270 --> 00:21:08,159 little bit of a tongue twister 632 00:21:12,789 --> 00:21:11,280 i also don't pick it because uh 633 00:21:14,549 --> 00:21:12,799 the joke i like to tell is that for 634 00:21:15,750 --> 00:21:14,559 those of you who are douglas adams fans 635 00:21:17,669 --> 00:21:15,760 if you've read the hitchhiker's guide to 636 00:21:18,950 --> 00:21:17,679 the galaxy um you know that the answer 637 00:21:20,230 --> 00:21:18,960 to life the universe and everything is 638 00:21:22,549 --> 00:21:20,240 42 639 00:21:24,950 --> 00:21:22,559 and uh the atomic number of molybdenum 640 00:21:26,070 --> 00:21:24,960 is indeed 42. so it makes good sense to 641 00:21:27,110 --> 00:21:26,080 be obsessed with molybdenum for that 642 00:21:28,950 --> 00:21:27,120 reason but that's actually not the 643 00:21:31,110 --> 00:21:28,960 reason we're focusing on it the reason 644 00:21:32,950 --> 00:21:31,120 geochemically that molybdenum is a 645 00:21:34,310 --> 00:21:32,960 entertaining element to work with or a 646 00:21:35,830 --> 00:21:34,320 uh 647 00:21:37,669 --> 00:21:35,840 a good element to work with is because 648 00:21:39,190 --> 00:21:37,679 as a very contrasting behavior as a 649 00:21:42,230 --> 00:21:39,200 function of environmental redox 650 00:21:44,149 --> 00:21:42,240 chemistry it has a switch-like behavior 651 00:21:46,230 --> 00:21:44,159 as it has been uh described in the 652 00:21:47,830 --> 00:21:46,240 literature so this this figure is meant 653 00:21:49,190 --> 00:21:47,840 to get this concept across so what 654 00:21:51,590 --> 00:21:49,200 you're seeing here is a profile of 655 00:21:53,190 --> 00:21:51,600 molybdenum concentration in the black 656 00:21:54,710 --> 00:21:53,200 sea which is the type locality on the 657 00:21:57,110 --> 00:21:54,720 modern planet that you go to if you want 658 00:21:58,870 --> 00:21:57,120 to look at strongly contrasting redox 659 00:22:01,029 --> 00:21:58,880 behavior so the surface waters of the 660 00:22:03,110 --> 00:22:01,039 black sea are oxygenated the deep waters 661 00:22:04,630 --> 00:22:03,120 of the black sea have very little oxygen 662 00:22:06,310 --> 00:22:04,640 and have in fact a large amount of 663 00:22:07,909 --> 00:22:06,320 hydrogen sulfide very reducing 664 00:22:09,830 --> 00:22:07,919 environment 665 00:22:11,590 --> 00:22:09,840 and molybdenum shows a very sharply 666 00:22:13,190 --> 00:22:11,600 contrasting behavior as a go you go 667 00:22:15,909 --> 00:22:13,200 across this transition the so-called 668 00:22:16,950 --> 00:22:15,919 redox cline so why is that 669 00:22:19,350 --> 00:22:16,960 the reason for that is that in 670 00:22:21,110 --> 00:22:19,360 oxygenated waters molybdenum is highly 671 00:22:23,110 --> 00:22:21,120 unreactive it's present as the molybdate 672 00:22:26,549 --> 00:22:23,120 anion 673 00:22:28,070 --> 00:22:26,559 which is a highly unreactive uh uh ion 674 00:22:29,510 --> 00:22:28,080 such that molybdenum has a very long 675 00:22:31,110 --> 00:22:29,520 residence time in the modern oceans 676 00:22:32,310 --> 00:22:31,120 about eight hundred thousand years and 677 00:22:34,149 --> 00:22:32,320 in fact it's the most abundant 678 00:22:35,990 --> 00:22:34,159 transition metal in the modern oceans 679 00:22:38,230 --> 00:22:36,000 because this molybdate ion is so 680 00:22:39,669 --> 00:22:38,240 unreactive so even aluminum is by far 681 00:22:40,870 --> 00:22:39,679 not an abundant element in the crust it 682 00:22:42,149 --> 00:22:40,880 turns out to be the most abundant 683 00:22:43,350 --> 00:22:42,159 transition metal in the oceans because 684 00:22:45,590 --> 00:22:43,360 of its 685 00:22:47,270 --> 00:22:45,600 great stability uh chemical stability 686 00:22:49,510 --> 00:22:47,280 when in the when 687 00:22:50,710 --> 00:22:49,520 fully oxygenated 688 00:22:52,470 --> 00:22:50,720 however when molybdenum gets into a 689 00:22:54,789 --> 00:22:52,480 reducing environment the chemistry 690 00:22:56,470 --> 00:22:54,799 changes very sharply what happens is 691 00:22:59,430 --> 00:22:56,480 actually not a redox reaction to first 692 00:23:02,070 --> 00:22:59,440 order but a substitution of sulfurs for 693 00:23:04,470 --> 00:23:02,080 oxygens around this molybdenum six plus 694 00:23:05,590 --> 00:23:04,480 uh uh 695 00:23:07,190 --> 00:23:05,600 center 696 00:23:09,270 --> 00:23:07,200 and to form these guys which are 697 00:23:10,630 --> 00:23:09,280 referred to as oxy thiomalipdates and 698 00:23:12,149 --> 00:23:10,640 these are very particle reactive and 699 00:23:13,190 --> 00:23:12,159 hence gets scavenged out of the water 700 00:23:15,270 --> 00:23:13,200 column 701 00:23:17,029 --> 00:23:15,280 and in uh 702 00:23:19,270 --> 00:23:17,039 ocean basins the 703 00:23:20,630 --> 00:23:19,280 um dominant form of particulate matter 704 00:23:23,110 --> 00:23:20,640 that's raining down for this stuff to 705 00:23:24,230 --> 00:23:23,120 stick to are organic carbon particles 706 00:23:26,149 --> 00:23:24,240 and hence you have an affinity of 707 00:23:27,430 --> 00:23:26,159 molybdenum for organic carbon in these 708 00:23:28,549 --> 00:23:27,440 kind of environments so the molybdenum 709 00:23:30,549 --> 00:23:28,559 gets removed from the water and it's 710 00:23:33,590 --> 00:23:30,559 getting removed by being scrubbed out 711 00:23:36,149 --> 00:23:33,600 onto organic carbon particles 712 00:23:39,190 --> 00:23:36,159 um so the upshot is that molybdenum uh 713 00:23:40,549 --> 00:23:39,200 is abundant in oxygenated seawater it it 714 00:23:42,710 --> 00:23:40,559 is not going to be so abundant in sea 715 00:23:44,710 --> 00:23:42,720 water if that's not oxygenated and as 716 00:23:46,470 --> 00:23:44,720 we'll talk about also aluminum will tend 717 00:23:48,070 --> 00:23:46,480 to accumulate in sedimentary 718 00:23:49,270 --> 00:23:48,080 environments that are full of hydrogen 719 00:23:50,549 --> 00:23:49,280 sulfide 720 00:23:51,830 --> 00:23:50,559 that accumulated waters that are full of 721 00:23:54,789 --> 00:23:51,840 hydrogen sulfide those will tend to 722 00:23:56,470 --> 00:23:54,799 concentrate molybdenum 723 00:23:57,750 --> 00:23:56,480 in addition molybdenum rhenium is 724 00:23:59,909 --> 00:23:57,760 another element with very similar 725 00:24:01,510 --> 00:23:59,919 behavior and to some degree uranium 726 00:24:02,830 --> 00:24:01,520 although as we'll talk about uranium is 727 00:24:05,350 --> 00:24:02,840 not quite 728 00:24:07,750 --> 00:24:05,360 identical so the concept underlying this 729 00:24:09,350 --> 00:24:07,760 application is isn't very simple 730 00:24:11,990 --> 00:24:09,360 the idea here is that on a world where 731 00:24:13,430 --> 00:24:12,000 you have oxygen if oxygen is present 732 00:24:15,430 --> 00:24:13,440 then molybdenum can accumulate in the 733 00:24:16,710 --> 00:24:15,440 oceans and it does so in through several 734 00:24:19,110 --> 00:24:16,720 steps that are oxygen sensitive the 735 00:24:20,390 --> 00:24:19,120 first step is weathering of molybdenum 736 00:24:22,630 --> 00:24:20,400 carrying minerals on the continents 737 00:24:24,870 --> 00:24:22,640 which are mostly sulfide minerals so 738 00:24:26,789 --> 00:24:24,880 those are very reactive with oxygen so 739 00:24:28,230 --> 00:24:26,799 molybdenum sulfides and igneous rocks 740 00:24:29,430 --> 00:24:28,240 will weather in the presence of oxygen 741 00:24:31,269 --> 00:24:29,440 releasing their molybdenum to river 742 00:24:32,789 --> 00:24:31,279 waters if there's a lot of oxygen around 743 00:24:35,190 --> 00:24:32,799 then that molybdenum becomes malibate 744 00:24:36,470 --> 00:24:35,200 and is highly soluble and unreactive and 745 00:24:38,390 --> 00:24:36,480 it gets delivered to the oceans where it 746 00:24:40,470 --> 00:24:38,400 can accumulate and that can happen on 747 00:24:42,310 --> 00:24:40,480 land it can also happen uh 748 00:24:44,149 --> 00:24:42,320 in a submarine setting where you have 749 00:24:46,070 --> 00:24:44,159 submarine basalts where again you have 750 00:24:47,510 --> 00:24:46,080 sulfides that contain molybdenum and 751 00:24:49,190 --> 00:24:47,520 rhenium 752 00:24:51,029 --> 00:24:49,200 and the chemistry of those is going to 753 00:24:52,310 --> 00:24:51,039 be sensitive to oxygen it will release 754 00:24:54,149 --> 00:24:52,320 their molybdenum into this into the 755 00:24:55,590 --> 00:24:54,159 water in the presence of oxygen 756 00:24:56,710 --> 00:24:55,600 on the other hand if oxygen is absent 757 00:24:58,870 --> 00:24:56,720 then you have a very different story 758 00:25:00,070 --> 00:24:58,880 then this flux is turned off essentially 759 00:25:01,909 --> 00:25:00,080 because the weathering rates of the 760 00:25:04,149 --> 00:25:01,919 sulfide minerals carrying these elements 761 00:25:06,310 --> 00:25:04,159 on land are very very slow 762 00:25:07,909 --> 00:25:06,320 um and the um 763 00:25:09,669 --> 00:25:07,919 the minerals will be much more robust 764 00:25:11,590 --> 00:25:09,679 against weathering the solubility of 765 00:25:12,630 --> 00:25:11,600 molybdenum will be lower in in waters 766 00:25:14,149 --> 00:25:12,640 they won't be able to accumulate in the 767 00:25:15,590 --> 00:25:14,159 oceans as much and so you'll have a 768 00:25:18,390 --> 00:25:15,600 world with much less molybdenum in the 769 00:25:20,070 --> 00:25:18,400 oceans so the the amount of molybdenum 770 00:25:22,070 --> 00:25:20,080 and rhenium and to some degree uranium 771 00:25:26,070 --> 00:25:22,080 in the oceans in these two scenarios is 772 00:25:30,630 --> 00:25:27,590 and where you would go to look for that 773 00:25:32,710 --> 00:25:30,640 um let me skip this slide um is in 774 00:25:34,710 --> 00:25:32,720 anoxic environments like the sediments 775 00:25:37,510 --> 00:25:34,720 beneath the modern black sea 776 00:25:39,110 --> 00:25:37,520 um those are the places where um 777 00:25:40,549 --> 00:25:39,120 molybdenum is going to get is going to 778 00:25:42,390 --> 00:25:40,559 accumulate from sea water if there is 779 00:25:44,710 --> 00:25:42,400 molybdenum in the sea water to 780 00:25:45,990 --> 00:25:44,720 uh to accumulate so if you go to the 781 00:25:47,029 --> 00:25:46,000 modern black sea you'll find there's a 782 00:25:50,230 --> 00:25:47,039 good amount of molybdenum in the 783 00:25:51,750 --> 00:25:50,240 sediments beneath that basin uh because 784 00:25:54,070 --> 00:25:51,760 although this is an anoxic base and it's 785 00:25:56,310 --> 00:25:54,080 in contact with a very large oxygenated 786 00:25:57,990 --> 00:25:56,320 ocean which is chock full of molybdenum 787 00:25:59,029 --> 00:25:58,000 if such a basin however formed in the 788 00:26:00,549 --> 00:25:59,039 archaean 789 00:26:01,909 --> 00:26:00,559 um 790 00:26:03,269 --> 00:26:01,919 and if in the archaean there was very 791 00:26:04,549 --> 00:26:03,279 little oxygen in the environ in the 792 00:26:05,990 --> 00:26:04,559 environment you would expect there to be 793 00:26:07,750 --> 00:26:06,000 very little molybdenum in the oceans of 794 00:26:09,430 --> 00:26:07,760 that time and the sediments accumulating 795 00:26:11,830 --> 00:26:09,440 in such a basin in the archaean would 796 00:26:14,630 --> 00:26:11,840 probably be relatively low in molybdenum 797 00:26:16,230 --> 00:26:14,640 that's the basic concept here 798 00:26:17,669 --> 00:26:16,240 and in fact if we go to a number of 799 00:26:20,230 --> 00:26:17,679 modern basins what you're seeing here is 800 00:26:22,870 --> 00:26:20,240 the molybdenum concentration versus a 801 00:26:24,310 --> 00:26:22,880 parameter called toc which is just an 802 00:26:26,310 --> 00:26:24,320 abbreviation to confuse you this is 803 00:26:27,430 --> 00:26:26,320 total organic carbon remember i said 804 00:26:28,950 --> 00:26:27,440 there should be an affinity between 805 00:26:30,549 --> 00:26:28,960 molybdenum and organic carbon and 806 00:26:32,390 --> 00:26:30,559 sediments and so you can go to number of 807 00:26:33,190 --> 00:26:32,400 modern basins here's the black sea the 808 00:26:35,990 --> 00:26:33,200 from the 809 00:26:37,990 --> 00:26:36,000 barn fjord karyako basin the sonic inlet 810 00:26:40,310 --> 00:26:38,000 these are these are very well studied 811 00:26:41,750 --> 00:26:40,320 modern anoxic basins and you can see 812 00:26:43,830 --> 00:26:41,760 that they all each one of them shows 813 00:26:45,909 --> 00:26:43,840 this trend each one is a different slope 814 00:26:48,070 --> 00:26:45,919 but each one shows a trend of molybdenum 815 00:26:50,149 --> 00:26:48,080 versus organic carbon and the amount of 816 00:26:51,430 --> 00:26:50,159 molybdenum in these sediments and these 817 00:26:53,029 --> 00:26:51,440 are in the sediments underneath these 818 00:26:54,470 --> 00:26:53,039 the waters in these basins and the 819 00:26:56,390 --> 00:26:54,480 amount of molybdenum in these sediments 820 00:26:58,230 --> 00:26:56,400 is very high compared to average crustal 821 00:27:00,230 --> 00:26:58,240 abundances so molybdenum the average 822 00:27:02,149 --> 00:27:00,240 crust is around a part per million by 823 00:27:03,830 --> 00:27:02,159 weight um and the sediments in these 824 00:27:05,350 --> 00:27:03,840 kind of basins you're talking about tens 825 00:27:07,430 --> 00:27:05,360 to even hundreds of parts per million of 826 00:27:08,950 --> 00:27:07,440 molybdenum and the reason for that again 827 00:27:10,310 --> 00:27:08,960 just to stress this is that these kind 828 00:27:11,990 --> 00:27:10,320 of environments these anoxic 829 00:27:14,710 --> 00:27:12,000 environments are environments in which 830 00:27:16,789 --> 00:27:14,720 molybdenum that is abundant in sea water 831 00:27:18,149 --> 00:27:16,799 um uh there are environments in which 832 00:27:20,070 --> 00:27:18,159 molybdenum which is abundant in seawater 833 00:27:21,269 --> 00:27:20,080 becomes destabilized and gets scrubbed 834 00:27:23,350 --> 00:27:21,279 out into sediment so these are the kind 835 00:27:24,789 --> 00:27:23,360 of places you go to look for evidence 836 00:27:26,870 --> 00:27:24,799 that you had oceans with lots of 837 00:27:28,070 --> 00:27:26,880 molybdenum in them so today we have an 838 00:27:29,350 --> 00:27:28,080 ocean with lots of molybdenum and 839 00:27:31,510 --> 00:27:29,360 sediments in these environments are then 840 00:27:32,789 --> 00:27:31,520 very rich in molybdenum 841 00:27:34,470 --> 00:27:32,799 um 842 00:27:37,029 --> 00:27:34,480 now you can go back so these kind of 843 00:27:38,549 --> 00:27:37,039 sediments then if you if you let them 844 00:27:40,230 --> 00:27:38,559 get buried and altered through time turn 845 00:27:41,750 --> 00:27:40,240 into the kind of rocks that refer to as 846 00:27:46,470 --> 00:27:41,760 black shales 847 00:27:48,230 --> 00:27:46,480 the geologic past for example in the 848 00:27:49,830 --> 00:27:48,240 devonian where these have been very well 849 00:27:51,350 --> 00:27:49,840 studied and look at the trend of 850 00:27:54,389 --> 00:27:51,360 molybdenum versus organic carbon in 851 00:27:57,110 --> 00:27:54,399 devonian black shales you find that that 852 00:27:58,549 --> 00:27:57,120 that trend is very similar to the range 853 00:28:00,310 --> 00:27:58,559 of 854 00:28:02,870 --> 00:28:00,320 values that you find in these modern 855 00:28:04,710 --> 00:28:02,880 sediments so this is telling us first of 856 00:28:05,830 --> 00:28:04,720 all that the devonian the bulk oceans 857 00:28:06,870 --> 00:28:05,840 probably had a lot of molybdenum and 858 00:28:08,389 --> 00:28:06,880 hence there was probably a fair amount 859 00:28:10,389 --> 00:28:08,399 of oxygen around even though there are 860 00:28:11,830 --> 00:28:10,399 lots of black shales from that time 861 00:28:13,669 --> 00:28:11,840 but the other thing it's the main point 862 00:28:15,510 --> 00:28:13,679 i want to drive home is that we see in 863 00:28:17,110 --> 00:28:15,520 modern sedimentary systems this 864 00:28:19,110 --> 00:28:17,120 relationship between molybdenum organic 865 00:28:21,590 --> 00:28:19,120 carbon and these molybdenum enrichments 866 00:28:23,269 --> 00:28:21,600 and we see these also in devonian um 867 00:28:24,950 --> 00:28:23,279 black shale so we can go back to back in 868 00:28:25,990 --> 00:28:24,960 time to rocks that are hundreds of 869 00:28:27,830 --> 00:28:26,000 millions or 870 00:28:30,310 --> 00:28:27,840 millions of years old and see something 871 00:28:31,909 --> 00:28:30,320 very similar to what we find in modern 872 00:28:33,590 --> 00:28:31,919 bases that gives us confidence that we 873 00:28:35,430 --> 00:28:33,600 are kind of on to what's going on here 874 00:28:37,350 --> 00:28:35,440 that we have a handle on it 875 00:28:39,190 --> 00:28:37,360 ariel would you just say how long ago 876 00:28:41,190 --> 00:28:39,200 the devonian was 877 00:28:44,230 --> 00:28:41,200 uh it's a few well i'm not sure exactly 878 00:28:49,029 --> 00:28:44,240 these these samples it's about on the 879 00:28:51,350 --> 00:28:49,830 so 880 00:28:53,590 --> 00:28:51,360 um to 881 00:28:54,870 --> 00:28:53,600 assess this a little bit um so it's 882 00:28:56,549 --> 00:28:54,880 nowhere near as old as what we're go 883 00:28:57,750 --> 00:28:56,559 where we're going but it's a period of 884 00:28:59,990 --> 00:28:57,760 time has been very well studied with 885 00:29:01,269 --> 00:29:00,000 respect to the chemistry of of black 886 00:29:03,830 --> 00:29:01,279 shales so 887 00:29:04,950 --> 00:29:03,840 it's sort of the ancient analog of the 888 00:29:06,630 --> 00:29:04,960 modern black sea which has been very 889 00:29:09,190 --> 00:29:06,640 well studied for the chemistry of these 890 00:29:11,510 --> 00:29:09,200 redox sensitive metals 891 00:29:14,950 --> 00:29:11,520 so um we've actually used this sort of 892 00:29:17,909 --> 00:29:14,960 tool in a in a gross sense already 893 00:29:19,750 --> 00:29:17,919 in studies shown here this is a paper by 894 00:29:21,430 --> 00:29:19,760 um clint scott who's a student of tim 895 00:29:23,269 --> 00:29:21,440 lines at uc riverside who's been 896 00:29:25,190 --> 00:29:23,279 compiling data both from the literature 897 00:29:27,110 --> 00:29:25,200 and published already previously 898 00:29:28,789 --> 00:29:27,120 published data on the lived in black 899 00:29:30,389 --> 00:29:28,799 shales through time 900 00:29:32,230 --> 00:29:30,399 and there are complexities here that i i 901 00:29:34,710 --> 00:29:32,240 don't want to go into but the basic 902 00:29:36,389 --> 00:29:34,720 concept here is that um if you look at 903 00:29:37,830 --> 00:29:36,399 molybdenum and black shales through time 904 00:29:39,190 --> 00:29:37,840 you indeed see 905 00:29:40,950 --> 00:29:39,200 a uh 906 00:29:42,149 --> 00:29:40,960 sort of secular changes in the archaean 907 00:29:45,430 --> 00:29:42,159 you generally see very low 908 00:29:46,389 --> 00:29:45,440 concentrations of molybdenum um uh you 909 00:29:48,389 --> 00:29:46,399 know 910 00:29:50,630 --> 00:29:48,399 one to ten ppm sort of values are are 911 00:29:52,789 --> 00:29:50,640 typical then you go into the proteozoic 912 00:29:54,789 --> 00:29:52,799 and you see higher you see some olympic 913 00:29:56,470 --> 00:29:54,799 enrichments but there's nowhere near as 914 00:29:58,950 --> 00:29:56,480 spectacular as what you see in the 915 00:30:00,470 --> 00:29:58,960 phanerozoic and the more modern record 916 00:30:03,350 --> 00:30:00,480 so if you recall that cartoon i showed 917 00:30:04,630 --> 00:30:03,360 you the three stages of oxygen evolution 918 00:30:06,230 --> 00:30:04,640 and that figure the axis ran the other 919 00:30:08,470 --> 00:30:06,240 way my apologies that these axes flip 920 00:30:09,830 --> 00:30:08,480 around in that figure 921 00:30:11,510 --> 00:30:09,840 back in time was there and the modern 922 00:30:13,269 --> 00:30:11,520 was over here and this figure we flipped 923 00:30:15,510 --> 00:30:13,279 it the other way here's the archaean and 924 00:30:17,830 --> 00:30:15,520 here's the modern but you can see that 925 00:30:19,750 --> 00:30:17,840 as in that cartoon there's a three sort 926 00:30:21,190 --> 00:30:19,760 of suggestion of a three-stage story 927 00:30:23,029 --> 00:30:21,200 here which is what you would expect that 928 00:30:25,110 --> 00:30:23,039 if oxygen really went from very low to 929 00:30:26,630 --> 00:30:25,120 intermediate to high then molybdenum and 930 00:30:27,990 --> 00:30:26,640 sediments would go from very low to 931 00:30:29,269 --> 00:30:28,000 immediate to high and indeed we see 932 00:30:30,470 --> 00:30:29,279 something like that with with some 933 00:30:32,549 --> 00:30:30,480 complexities that we're not going to 934 00:30:34,070 --> 00:30:32,559 discuss in this talk here but this sort 935 00:30:35,830 --> 00:30:34,080 of the fact that we can see this sort of 936 00:30:37,190 --> 00:30:35,840 thing in the ancient record gives us 937 00:30:38,870 --> 00:30:37,200 good confidence that in again that 938 00:30:41,110 --> 00:30:38,880 indeed we are on to something when it 939 00:30:43,510 --> 00:30:41,120 comes to using molybdenum and sediments 940 00:30:44,950 --> 00:30:43,520 through time as a proxy for oxygen in 941 00:30:48,149 --> 00:30:44,960 the environment 942 00:30:49,750 --> 00:30:48,159 as we're going to talk about uh in in uh 943 00:30:51,669 --> 00:30:49,760 in a couple minutes with the data that 944 00:30:53,190 --> 00:30:51,679 we're showing that we're obtaining now 945 00:30:55,750 --> 00:30:53,200 we're finding evidence of 946 00:30:57,430 --> 00:30:55,760 short-lived variations um in molybdenum 947 00:31:00,470 --> 00:30:57,440 uh possibly short-lived variations of 948 00:31:03,350 --> 00:31:00,480 molybdenum even in the archaean um which 949 00:31:04,630 --> 00:31:03,360 is the big story here so as we'll get to 950 00:31:05,830 --> 00:31:04,640 so if you want to study this sort of 951 00:31:07,669 --> 00:31:05,840 thing you need to find the right kind of 952 00:31:09,510 --> 00:31:07,679 sediments we've said you want these so 953 00:31:11,190 --> 00:31:09,520 called black shales these sediments are 954 00:31:12,230 --> 00:31:11,200 black because they're full of organic 955 00:31:14,310 --> 00:31:12,240 carbon 956 00:31:16,070 --> 00:31:14,320 and here's a nice picture of one of the 957 00:31:17,590 --> 00:31:16,080 drill core samples that we've worked on 958 00:31:19,909 --> 00:31:17,600 this is the kind of thing that you want 959 00:31:21,190 --> 00:31:19,919 nice as we say in the business juicy 960 00:31:23,510 --> 00:31:21,200 black shales 961 00:31:25,590 --> 00:31:23,520 also they contain these beautiful pyrite 962 00:31:27,029 --> 00:31:25,600 nodules and laminations these are iron 963 00:31:28,470 --> 00:31:27,039 sulfides 964 00:31:29,830 --> 00:31:28,480 that form in reduced environments this 965 00:31:31,590 --> 00:31:29,840 is what you want 966 00:31:33,190 --> 00:31:31,600 this kind of material 967 00:31:35,190 --> 00:31:33,200 the reason that drilling becomes very 968 00:31:36,549 --> 00:31:35,200 important uh that and the reason 969 00:31:38,549 --> 00:31:36,559 something like astrobiology drilling 970 00:31:40,070 --> 00:31:38,559 program has been so critical 971 00:31:41,830 --> 00:31:40,080 is because what you find at the surface 972 00:31:43,190 --> 00:31:41,840 is very typically altered it's usually 973 00:31:44,710 --> 00:31:43,200 not quite as bad as this picture but 974 00:31:46,710 --> 00:31:44,720 this is a fun picture to show the 975 00:31:48,389 --> 00:31:46,720 extreme of what can happen so in western 976 00:31:50,630 --> 00:31:48,399 australia where you have rocks of the 977 00:31:51,909 --> 00:31:50,640 appropriate antiquity and of the 978 00:31:53,590 --> 00:31:51,919 appropriate very low grade of 979 00:31:55,110 --> 00:31:53,600 metamorphism which is critical for this 980 00:31:57,029 --> 00:31:55,120 kind of work 981 00:31:58,630 --> 00:31:57,039 you unfortunately also have pervasive 982 00:31:59,669 --> 00:31:58,640 oxidative weathering of the surface 983 00:32:01,750 --> 00:31:59,679 environment 984 00:32:03,830 --> 00:32:01,760 water over time flows through rocks if 985 00:32:05,830 --> 00:32:03,840 that water has oxygen in it 986 00:32:07,830 --> 00:32:05,840 then you start to oxidize whatever's in 987 00:32:10,710 --> 00:32:07,840 the rocks so what you're seeing here is 988 00:32:13,110 --> 00:32:10,720 a picture of what uh in a drill core is 989 00:32:14,549 --> 00:32:13,120 a nice black shale but an outcrop or 990 00:32:16,470 --> 00:32:14,559 it's been exposed for we don't know how 991 00:32:19,509 --> 00:32:16,480 long um 992 00:32:21,669 --> 00:32:19,519 uh in outcrop it's weathered to pieces 993 00:32:22,789 --> 00:32:21,679 uh the it's not black at all it's white 994 00:32:24,230 --> 00:32:22,799 because the organic carbon has 995 00:32:27,029 --> 00:32:24,240 essentially been oxidized it's been 996 00:32:28,230 --> 00:32:27,039 ashed effectively chemically slowly um 997 00:32:30,870 --> 00:32:28,240 but it's analogous to what would happen 998 00:32:32,710 --> 00:32:30,880 if you stuck that sample in in a furnace 999 00:32:34,630 --> 00:32:32,720 and burned all the organic carbon it's 1000 00:32:36,630 --> 00:32:34,640 just been a very slow burn 1001 00:32:37,990 --> 00:32:36,640 so naturally this kind of rock is not 1002 00:32:39,909 --> 00:32:38,000 all that useful for the sort of thing 1003 00:32:41,590 --> 00:32:39,919 that we're after uh because the redox 1004 00:32:44,149 --> 00:32:41,600 sensitive metals like molybdenum that 1005 00:32:45,509 --> 00:32:44,159 were in here will no longer be in their 1006 00:32:47,750 --> 00:32:45,519 primary 1007 00:32:48,870 --> 00:32:47,760 abundances so drilling becomes very 1008 00:32:50,630 --> 00:32:48,880 important 1009 00:32:52,789 --> 00:32:50,640 so the astrobiology drilling program as 1010 00:32:54,950 --> 00:32:52,799 i said drilled 10 drill cores of various 1011 00:32:57,669 --> 00:32:54,960 depths at various periods of time in 1012 00:32:59,350 --> 00:32:57,679 western australia in 2003 and 2004 this 1013 00:33:01,350 --> 00:32:59,360 is a picture here of roger buick who was 1014 00:33:03,990 --> 00:33:01,360 the mastermind behind this hammersley 1015 00:33:06,470 --> 00:33:04,000 basin drill corps um he's pointing both 1016 00:33:08,230 --> 00:33:06,480 to western australia which is where the 1017 00:33:11,029 --> 00:33:08,240 hammersley basin and the the pildora 1018 00:33:13,830 --> 00:33:11,039 craton is and to our drill core so this 1019 00:33:15,990 --> 00:33:13,840 particular cord this is number nine uh 1020 00:33:18,149 --> 00:33:16,000 of the ten cores and was drilled in this 1021 00:33:19,830 --> 00:33:18,159 location here 1022 00:33:22,310 --> 00:33:19,840 now this project in some ways was rather 1023 00:33:23,990 --> 00:33:22,320 star-crossed initially um 1024 00:33:25,750 --> 00:33:24,000 first of all took quite a long time for 1025 00:33:27,350 --> 00:33:25,760 it to evolve it evolved initially as an 1026 00:33:28,789 --> 00:33:27,360 outgrowth of the 1027 00:33:31,909 --> 00:33:28,799 early earth focus group which was 1028 00:33:35,190 --> 00:33:31,919 created when barry blumberg uh the 1029 00:33:37,590 --> 00:33:35,200 former director of the nai um asked for 1030 00:33:40,070 --> 00:33:37,600 the nai teams to propose ideas for 1031 00:33:41,750 --> 00:33:40,080 astrobiology missions and he had in mind 1032 00:33:43,269 --> 00:33:41,760 of course space missions but a few of us 1033 00:33:44,870 --> 00:33:43,279 got together and proposed the idea of a 1034 00:33:46,549 --> 00:33:44,880 mission to the early earth 1035 00:33:47,990 --> 00:33:46,559 and so was born the focus group and so 1036 00:33:48,950 --> 00:33:48,000 eventually was born this particular 1037 00:33:50,070 --> 00:33:48,960 project 1038 00:33:52,230 --> 00:33:50,080 and as you can see there was a long 1039 00:33:54,470 --> 00:33:52,240 gestation and finally we began to be 1040 00:33:55,509 --> 00:33:54,480 able to analyze things in 2006. but as i 1041 00:33:56,710 --> 00:33:55,519 said this project was somewhat 1042 00:33:58,149 --> 00:33:56,720 star-crossed 1043 00:33:59,669 --> 00:33:58,159 and i'll explain why because it's kind 1044 00:34:01,669 --> 00:33:59,679 of an amusing story of how science 1045 00:34:03,190 --> 00:34:01,679 really works 1046 00:34:05,269 --> 00:34:03,200 so here's the stratigraphy that we were 1047 00:34:08,950 --> 00:34:05,279 pursuing we were going after the last 1048 00:34:11,510 --> 00:34:08,960 250 million years of the archaean 1049 00:34:14,149 --> 00:34:11,520 and in particular the big focus was to 1050 00:34:15,750 --> 00:34:14,159 study these sediments down here these 1051 00:34:17,909 --> 00:34:15,760 very old black shales that date back to 1052 00:34:19,669 --> 00:34:17,919 around 2.7 billion years ago which are 1053 00:34:21,909 --> 00:34:19,679 particular interest because these are 1054 00:34:23,589 --> 00:34:21,919 the sediments from which 1055 00:34:25,349 --> 00:34:23,599 biomarker evidence 1056 00:34:28,069 --> 00:34:25,359 which constitutes the earliest evidence 1057 00:34:30,069 --> 00:34:28,079 of eukaryotes comes from so these are 1058 00:34:32,470 --> 00:34:30,079 very important rocks and there's great 1059 00:34:33,589 --> 00:34:32,480 desire to sample them again and the need 1060 00:34:36,230 --> 00:34:33,599 reason to say that they need to be 1061 00:34:38,310 --> 00:34:36,240 sampled again is because the biomarker 1062 00:34:40,149 --> 00:34:38,320 evidence that had what has been uh that 1063 00:34:42,389 --> 00:34:40,159 was produced um in a landmark paper in 1064 00:34:44,710 --> 00:34:42,399 1999 by jochen brox and roger summons 1065 00:34:45,829 --> 00:34:44,720 and roger buick um those samples had 1066 00:34:47,349 --> 00:34:45,839 been um 1067 00:34:49,270 --> 00:34:47,359 had not been obtained in a pristine 1068 00:34:51,030 --> 00:34:49,280 manner with respect to contamination had 1069 00:34:52,950 --> 00:34:51,040 not been stored in a manner to avoid 1070 00:34:54,950 --> 00:34:52,960 contamination and so there's been 1071 00:34:57,670 --> 00:34:54,960 controversy uh ever since the results 1072 00:34:59,990 --> 00:34:57,680 were were first announced as to whether 1073 00:35:01,430 --> 00:35:00,000 or not the biomarkers extracted from 1074 00:35:02,550 --> 00:35:01,440 those rocks were really indigenous to 1075 00:35:04,390 --> 00:35:02,560 the rocks 1076 00:35:05,910 --> 00:35:04,400 great care was taken to try to confirm 1077 00:35:07,990 --> 00:35:05,920 that that was indeed the case but there 1078 00:35:10,069 --> 00:35:08,000 remains lingering controversy about it 1079 00:35:11,750 --> 00:35:10,079 so it was great desire to to resample 1080 00:35:13,270 --> 00:35:11,760 those rocks to sample them fresh and to 1081 00:35:15,109 --> 00:35:13,280 drill in a way that would be free of 1082 00:35:16,630 --> 00:35:15,119 contamination 1083 00:35:18,230 --> 00:35:16,640 so that was the primary goal of this 1084 00:35:19,510 --> 00:35:18,240 project to go after those biomarkers and 1085 00:35:21,430 --> 00:35:19,520 to also look at the environment at that 1086 00:35:22,870 --> 00:35:21,440 time and along the way to also look at 1087 00:35:25,030 --> 00:35:22,880 the environment and biomarkers 1088 00:35:26,950 --> 00:35:25,040 throughout this last 250 million years 1089 00:35:28,950 --> 00:35:26,960 of the archaean leading leading up to 1090 00:35:31,910 --> 00:35:28,960 the rise of oxygen the great oxidation 1091 00:35:33,510 --> 00:35:31,920 event as it turns out we had two major 1092 00:35:36,150 --> 00:35:33,520 problems um 1093 00:35:37,829 --> 00:35:36,160 the first problem is that uh in the end 1094 00:35:39,750 --> 00:35:37,839 the the drillers on this project 1095 00:35:42,630 --> 00:35:39,760 insisted on using a hydrocarbon-based 1096 00:35:44,230 --> 00:35:42,640 drilling fluid despite our protestations 1097 00:35:45,750 --> 00:35:44,240 the flu that they chose to use was 1098 00:35:48,150 --> 00:35:45,760 something that should have been fairly 1099 00:35:49,990 --> 00:35:48,160 molecularly well characterized so that 1100 00:35:51,589 --> 00:35:50,000 we could uh cope with it but it turns 1101 00:35:54,630 --> 00:35:51,599 out to be a much messier material than 1102 00:35:55,990 --> 00:35:54,640 had been hoped um roger summons is still 1103 00:35:58,390 --> 00:35:56,000 fairly optimistic that he'll be able to 1104 00:35:59,910 --> 00:35:58,400 pull biomarker data out of here but it's 1105 00:36:02,150 --> 00:35:59,920 much proven to be much harder and more 1106 00:36:03,589 --> 00:36:02,160 complicated than expected so that was 1107 00:36:06,069 --> 00:36:03,599 problem number one 1108 00:36:07,990 --> 00:36:06,079 problem number two is that um 1109 00:36:09,430 --> 00:36:08,000 uh we didn't actually get down to the 1110 00:36:11,270 --> 00:36:09,440 stratigraphy in the end 1111 00:36:12,470 --> 00:36:11,280 because the location where we were 1112 00:36:13,990 --> 00:36:12,480 forced to drill 1113 00:36:16,150 --> 00:36:14,000 was turned out not to be the ideal 1114 00:36:17,430 --> 00:36:16,160 location and i say force because 1115 00:36:19,990 --> 00:36:17,440 you don't have free reign to drill 1116 00:36:22,390 --> 00:36:20,000 wherever you want to in the pilbara 1117 00:36:23,910 --> 00:36:22,400 aboriginal groups have their claims over 1118 00:36:26,470 --> 00:36:23,920 this land and you have to negotiate with 1119 00:36:28,390 --> 00:36:26,480 them to find the place that is uh where 1120 00:36:29,990 --> 00:36:28,400 they will permit you to drill and so the 1121 00:36:31,750 --> 00:36:30,000 ideal location for this drilling turned 1122 00:36:33,990 --> 00:36:31,760 out not to be the ideal location from 1123 00:36:35,109 --> 00:36:34,000 their perspective um and we were hopeful 1124 00:36:36,470 --> 00:36:35,119 that the location that we finally 1125 00:36:37,829 --> 00:36:36,480 settled on would work but as it turns 1126 00:36:39,750 --> 00:36:37,839 out what happened is that in that 1127 00:36:41,510 --> 00:36:39,760 location this unit here the whitney 1128 00:36:43,430 --> 00:36:41,520 formation was stratigraphically 1129 00:36:44,790 --> 00:36:43,440 thickened by about a factor of two and 1130 00:36:46,069 --> 00:36:44,800 so that although we drilled about a 1131 00:36:47,750 --> 00:36:46,079 kilometer we never actually got through 1132 00:36:49,750 --> 00:36:47,760 this stratigraphy here 1133 00:36:52,550 --> 00:36:49,760 so our initial take on this project was 1134 00:36:56,790 --> 00:36:52,560 oh my god what a what a colossal 1135 00:36:58,870 --> 00:36:56,800 failure um and we decided to to uh to 1136 00:37:00,790 --> 00:36:58,880 forge ahead nonetheless and our our 1137 00:37:02,470 --> 00:37:00,800 notion was to focus all our attention on 1138 00:37:05,510 --> 00:37:02,480 this last period of time on this unit 1139 00:37:07,670 --> 00:37:05,520 here called the mount mcrae shale um 1140 00:37:09,349 --> 00:37:07,680 uh and to look at the environment in 1141 00:37:10,390 --> 00:37:09,359 biology at that time 1142 00:37:12,069 --> 00:37:10,400 so that's what we're going to do here as 1143 00:37:14,310 --> 00:37:12,079 i walk you through the data so this 1144 00:37:15,670 --> 00:37:14,320 mount mcrae shale is a classic archaean 1145 00:37:18,550 --> 00:37:15,680 black shale 1146 00:37:19,990 --> 00:37:18,560 archaean 1147 00:37:21,910 --> 00:37:20,000 um 1148 00:37:23,190 --> 00:37:21,920 and uh it's been known for years and 1149 00:37:24,550 --> 00:37:23,200 studied in various ways for years but 1150 00:37:27,670 --> 00:37:24,560 never with the detail we're going to go 1151 00:37:28,710 --> 00:37:27,680 into here and so here in in detail is is 1152 00:37:30,069 --> 00:37:28,720 the piece of the core that we're looking 1153 00:37:31,670 --> 00:37:30,079 at here it's about 100 meters worth of 1154 00:37:33,670 --> 00:37:31,680 drill core and when you look at it in 1155 00:37:35,670 --> 00:37:33,680 detail it's not all black shale there 1156 00:37:38,310 --> 00:37:35,680 are actually two black shale units 1157 00:37:40,310 --> 00:37:38,320 within here the so-called s1 unit and s2 1158 00:37:42,069 --> 00:37:40,320 unit and we further subdivide the s1 1159 00:37:43,829 --> 00:37:42,079 unit into two subunits we'll mention to 1160 00:37:45,109 --> 00:37:43,839 talk about that a little bit more 1161 00:37:47,030 --> 00:37:45,119 so we decided to focus a lot of 1162 00:37:48,950 --> 00:37:47,040 attention on these black shale units to 1163 00:37:51,589 --> 00:37:48,960 see what we could learn in particular 1164 00:37:53,990 --> 00:37:51,599 with respect to paleo redox 1165 00:37:55,829 --> 00:37:54,000 um so i apologize the masking here 1166 00:37:57,589 --> 00:37:55,839 doesn't seem to have translated to webex 1167 00:37:58,950 --> 00:37:57,599 all that perfectly but hopefully this 1168 00:38:00,470 --> 00:37:58,960 will come out okay 1169 00:38:02,230 --> 00:38:00,480 um so what you're seeing here is that 1170 00:38:04,390 --> 00:38:02,240 drill core the depths and the 1171 00:38:05,750 --> 00:38:04,400 lithologies the kind of rocks and here 1172 00:38:07,589 --> 00:38:05,760 you see going to see them you're going 1173 00:38:09,430 --> 00:38:07,599 to see the molybdenum concentration in 1174 00:38:11,510 --> 00:38:09,440 parts per million and then here and here 1175 00:38:13,030 --> 00:38:11,520 the molybdenum enrichment factor which 1176 00:38:14,230 --> 00:38:13,040 is essentially a way of comparing the 1177 00:38:16,390 --> 00:38:14,240 molybdenum 1178 00:38:17,589 --> 00:38:16,400 and and here the rhenium concentrations 1179 00:38:19,109 --> 00:38:17,599 in the sediment 1180 00:38:21,109 --> 00:38:19,119 to those in the average continental 1181 00:38:22,630 --> 00:38:21,119 crust so a value of one is the same as 1182 00:38:24,230 --> 00:38:22,640 average continental crust and an 1183 00:38:25,510 --> 00:38:24,240 enhancement from that is a deviation 1184 00:38:27,670 --> 00:38:25,520 away from 1185 00:38:29,190 --> 00:38:27,680 above average crust 1186 00:38:32,870 --> 00:38:29,200 so when we look at the lower part here 1187 00:38:35,270 --> 00:38:32,880 this s2 unit what do we find in s2 we 1188 00:38:36,870 --> 00:38:35,280 find very low values of molybdenum very 1189 00:38:39,670 --> 00:38:36,880 low molybdenum enrichment factors and 1190 00:38:41,349 --> 00:38:39,680 very low rhenium enrichment factors 1191 00:38:43,030 --> 00:38:41,359 it looks very much like these black 1192 00:38:44,950 --> 00:38:43,040 shales although as we'll see they have a 1193 00:38:47,829 --> 00:38:44,960 good amount of organic carbon 1194 00:38:49,030 --> 00:38:47,839 enough that in uh the phanerozoic if you 1195 00:38:51,270 --> 00:38:49,040 found such rocks you'd find them 1196 00:38:52,790 --> 00:38:51,280 spectacularly enriched in aluminum um in 1197 00:38:54,550 --> 00:38:52,800 this case they're not at all enriched in 1198 00:38:56,470 --> 00:38:54,560 molybdenum um and the simplest 1199 00:38:58,710 --> 00:38:56,480 interpretation this as we'll come back 1200 00:39:00,390 --> 00:38:58,720 to again is that the oceans in which 1201 00:39:02,390 --> 00:39:00,400 these sediments accumulated probably had 1202 00:39:03,990 --> 00:39:02,400 relatively little organic i probably had 1203 00:39:05,190 --> 00:39:04,000 relatively little molybdenum dissolved 1204 00:39:06,950 --> 00:39:05,200 in them 1205 00:39:09,430 --> 00:39:06,960 as we move up though into the upper part 1206 00:39:11,109 --> 00:39:09,440 of the unit we see the beginning of an 1207 00:39:12,710 --> 00:39:11,119 enrichment in molybdenum concentrations 1208 00:39:14,550 --> 00:39:12,720 and enrichment factors and rhenium 1209 00:39:16,470 --> 00:39:14,560 enrichment factors as well getting up to 1210 00:39:17,990 --> 00:39:16,480 molybdenum concentrations of around 40 1211 00:39:19,430 --> 00:39:18,000 parts per million 1212 00:39:21,109 --> 00:39:19,440 that's a 1213 00:39:24,150 --> 00:39:21,119 a respectable amount of molybdenum it's 1214 00:39:25,990 --> 00:39:24,160 it's lower than um equivalent black 1215 00:39:27,109 --> 00:39:26,000 shale that you might find in the younger 1216 00:39:28,550 --> 00:39:27,119 record 1217 00:39:30,710 --> 00:39:28,560 but it is 1218 00:39:33,109 --> 00:39:30,720 still quite uh 1219 00:39:34,550 --> 00:39:33,119 it's quite unusual for the archaean 1220 00:39:35,750 --> 00:39:34,560 what we see here is more typical of 1221 00:39:37,750 --> 00:39:35,760 what's been seen in our keen black 1222 00:39:39,349 --> 00:39:37,760 shales this is really quite unusual 1223 00:39:41,030 --> 00:39:39,359 and as we go further up we see that 1224 00:39:42,870 --> 00:39:41,040 these concentrations die away the 1225 00:39:44,390 --> 00:39:42,880 enrichment factors die away but they 1226 00:39:46,550 --> 00:39:44,400 never quite get back to the original 1227 00:39:47,990 --> 00:39:46,560 values right you have to look at in some 1228 00:39:49,829 --> 00:39:48,000 detail but the enrichment factors here 1229 00:39:51,349 --> 00:39:49,839 are all substantially higher than one 1230 00:39:52,710 --> 00:39:51,359 whereas down here they're barely higher 1231 00:39:54,950 --> 00:39:52,720 than one if at all 1232 00:39:56,870 --> 00:39:54,960 um so there's some change in the system 1233 00:39:59,589 --> 00:39:56,880 when you compare the region down below 1234 00:40:01,270 --> 00:39:59,599 to the region up above 1235 00:40:02,870 --> 00:40:01,280 now before we get too excited we might 1236 00:40:04,069 --> 00:40:02,880 want to ask ourselves how do we know 1237 00:40:06,310 --> 00:40:04,079 that what we're seeing here in terms of 1238 00:40:08,310 --> 00:40:06,320 variations is actually primary how do we 1239 00:40:09,829 --> 00:40:08,320 know it's indigenous to the rocks 1240 00:40:11,829 --> 00:40:09,839 of course as i mentioned before fluids 1241 00:40:13,589 --> 00:40:11,839 passed through rocks these 1242 00:40:15,589 --> 00:40:13,599 rocks have had around uh two and a half 1243 00:40:18,310 --> 00:40:15,599 billion years to um 1244 00:40:19,910 --> 00:40:18,320 to be altered um and so how do you know 1245 00:40:21,829 --> 00:40:19,920 this molybdenum enrichment is not the 1246 00:40:23,750 --> 00:40:21,839 result of metals coming in much later 1247 00:40:25,109 --> 00:40:23,760 after deposition 1248 00:40:27,109 --> 00:40:25,119 and the way we go after that is by 1249 00:40:29,829 --> 00:40:27,119 looking at the rhenium osmium 1250 00:40:31,190 --> 00:40:29,839 geochronological system 1251 00:40:33,589 --> 00:40:31,200 and the idea here is that this is the 1252 00:40:35,910 --> 00:40:33,599 geochronometer the rhenium 187 decays to 1253 00:40:37,109 --> 00:40:35,920 osmium 187 with a half life of around 42 1254 00:40:38,390 --> 00:40:37,119 billion years 1255 00:40:39,750 --> 00:40:38,400 so we can do is we can take these 1256 00:40:41,750 --> 00:40:39,760 sediments particularly the ones that are 1257 00:40:44,230 --> 00:40:41,760 rich in rhenium 1258 00:40:46,390 --> 00:40:44,240 and we can see if they fall along a nice 1259 00:40:48,710 --> 00:40:46,400 isochron we can see if we get a 1260 00:40:49,990 --> 00:40:48,720 geologically meaningful age and the idea 1261 00:40:51,670 --> 00:40:50,000 here is that if we come up with a nice 1262 00:40:53,430 --> 00:40:51,680 isochron and a good age we've got we've 1263 00:40:54,550 --> 00:40:53,440 done two things one is that we've got a 1264 00:40:56,390 --> 00:40:54,560 better constraint on the age of these 1265 00:40:57,990 --> 00:40:56,400 rocks than we had before which is 1266 00:41:00,230 --> 00:40:58,000 valuable as we'll see 1267 00:41:01,430 --> 00:41:00,240 and in addition it provides a test for 1268 00:41:03,109 --> 00:41:01,440 alteration 1269 00:41:05,750 --> 00:41:03,119 because if you get a nice isochrone and 1270 00:41:07,349 --> 00:41:05,760 a geologically meaningful age 1271 00:41:08,630 --> 00:41:07,359 and by geologically meaningful i mean an 1272 00:41:10,790 --> 00:41:08,640 age that's consistent with what we think 1273 00:41:13,589 --> 00:41:10,800 the age of this rock is a priori from 1274 00:41:14,950 --> 00:41:13,599 other constraints um then it's likely 1275 00:41:17,109 --> 00:41:14,960 that the rhenium enrichment that we're 1276 00:41:19,670 --> 00:41:17,119 seeing here is indeed primary if on the 1277 00:41:21,829 --> 00:41:19,680 other hand that rhenium came in you know 1278 00:41:24,309 --> 00:41:21,839 a hundred hundreds of millions of years 1279 00:41:26,150 --> 00:41:24,319 later when this uh region was subject to 1280 00:41:27,910 --> 00:41:26,160 some alteration or another then you 1281 00:41:30,950 --> 00:41:27,920 would expect to get a very unreasonable 1282 00:41:35,109 --> 00:41:30,960 age if if you get a a nice isochron at 1283 00:41:37,910 --> 00:41:35,119 all so this is a great um test of the 1284 00:41:39,750 --> 00:41:37,920 primacy of this signal 1285 00:41:41,510 --> 00:41:39,760 and this work was done by brian kendall 1286 00:41:43,270 --> 00:41:41,520 and rob kreiser at university of alberta 1287 00:41:45,030 --> 00:41:43,280 brian was actually a visiting student at 1288 00:41:46,710 --> 00:41:45,040 asu learning how to work with molybdenum 1289 00:41:48,309 --> 00:41:46,720 isotopes when we were studying these 1290 00:41:50,309 --> 00:41:48,319 rocks and he asked if he could take some 1291 00:41:51,349 --> 00:41:50,319 samples back to geranium osmium on 1292 00:41:53,589 --> 00:41:51,359 because that's what he'd been doing for 1293 00:41:55,349 --> 00:41:53,599 the bulk of his phd thesis and this is 1294 00:41:56,470 --> 00:41:55,359 the data that he produced which 1295 00:41:58,230 --> 00:41:56,480 none of us suspected we could get 1296 00:41:59,990 --> 00:41:58,240 something so nice so here you're seeing 1297 00:42:03,270 --> 00:42:00,000 the osmium isotopes versus rhenium 1298 00:42:05,910 --> 00:42:03,280 osmium this is a classic uh iso uh way 1299 00:42:07,430 --> 00:42:05,920 of plotting a geochronologic data 1300 00:42:09,750 --> 00:42:07,440 and you're seeing and here you see the 1301 00:42:11,510 --> 00:42:09,760 various data points they form an array 1302 00:42:13,270 --> 00:42:11,520 because we've sampled what branded is 1303 00:42:15,190 --> 00:42:13,280 sampled 1304 00:42:16,950 --> 00:42:15,200 at different distances along the core in 1305 00:42:18,390 --> 00:42:16,960 order to get an array of 1306 00:42:19,910 --> 00:42:18,400 parent-to-daughter ratios which is 1307 00:42:21,430 --> 00:42:19,920 essential for building an isochrone this 1308 00:42:23,190 --> 00:42:21,440 way so these numbers correspond to 1309 00:42:25,829 --> 00:42:23,200 different depths in the core 1310 00:42:28,630 --> 00:42:25,839 um across about 1311 00:42:30,470 --> 00:42:28,640 15 meters or so of depth 1312 00:42:32,710 --> 00:42:30,480 which which spans that rhenium 1313 00:42:34,309 --> 00:42:32,720 enrichment interval and what you see is 1314 00:42:36,230 --> 00:42:34,319 they follow along a very nice array they 1315 00:42:37,430 --> 00:42:36,240 define a very tight isocon corresponding 1316 00:42:40,550 --> 00:42:37,440 to an age 1317 00:42:44,870 --> 00:42:40,560 of uh 2501 million years plus or minus 1318 00:42:46,390 --> 00:42:44,880 8.2 very very nice and tight isocron 1319 00:42:47,829 --> 00:42:46,400 and this falls very nicely within the 1320 00:42:49,190 --> 00:42:47,839 other constraints that exist for the age 1321 00:42:50,470 --> 00:42:49,200 of this section we knew it was around 1322 00:42:52,550 --> 00:42:50,480 two and a half billion years ago we knew 1323 00:42:54,390 --> 00:42:52,560 it was before the great oxidation event 1324 00:42:57,030 --> 00:42:54,400 and now we know quite clearly it's it's 1325 00:42:58,470 --> 00:42:57,040 almost smack dab at the archaean 1326 00:43:00,710 --> 00:42:58,480 protozoa boundary that's commonly 1327 00:43:02,390 --> 00:43:00,720 defined at 2.5 billion years ago 1328 00:43:04,630 --> 00:43:02,400 um so this is a spectacular conformation 1329 00:43:06,550 --> 00:43:04,640 both the age of this of this 1330 00:43:07,430 --> 00:43:06,560 sequence of rocks and of the fact that 1331 00:43:12,390 --> 00:43:07,440 um 1332 00:43:13,990 --> 00:43:12,400 metal variations we're seeing here are 1333 00:43:15,589 --> 00:43:14,000 primary to the rock they're not some 1334 00:43:17,589 --> 00:43:15,599 sort of alteration 1335 00:43:18,870 --> 00:43:17,599 product 1336 00:43:20,550 --> 00:43:18,880 now to get a little more sophisticated 1337 00:43:21,829 --> 00:43:20,560 in how we interpret the data 1338 00:43:23,589 --> 00:43:21,839 if you recall before we looked at a 1339 00:43:25,589 --> 00:43:23,599 bunch of modern sedimentary environments 1340 00:43:27,349 --> 00:43:25,599 i plotted molybdenum concentration 1341 00:43:29,910 --> 00:43:27,359 versus organic carbon content in 1342 00:43:31,349 --> 00:43:29,920 sediments so we can do the same here 1343 00:43:33,190 --> 00:43:31,359 and the reason to do this is because the 1344 00:43:34,790 --> 00:43:33,200 absolute molybdenum concentration may 1345 00:43:36,630 --> 00:43:34,800 vary as a function of the organic carbon 1346 00:43:38,550 --> 00:43:36,640 content in the rock so just looking at 1347 00:43:39,829 --> 00:43:38,560 the molybdenum concentration alone is 1348 00:43:41,829 --> 00:43:39,839 only seeing part of the story and you 1349 00:43:44,150 --> 00:43:41,839 could be fooled um looking at these 1350 00:43:45,589 --> 00:43:44,160 trends is is a bit more solid 1351 00:43:47,190 --> 00:43:45,599 so here's molybdenum versus total 1352 00:43:48,710 --> 00:43:47,200 organic carbon 1353 00:43:50,630 --> 00:43:48,720 this is the field defined by the modern 1354 00:43:52,710 --> 00:43:50,640 anoxic basins and also this devonian 1355 00:43:55,270 --> 00:43:52,720 trend these uh uh 1356 00:43:56,950 --> 00:43:55,280 paleozoic black shales 1357 00:43:58,870 --> 00:43:56,960 and what you see here here's the data 1358 00:44:01,510 --> 00:43:58,880 from this s2 unit the lower unit the 1359 00:44:03,109 --> 00:44:01,520 unit has very little molybdenum and it 1360 00:44:04,950 --> 00:44:03,119 it has actually although it's much less 1361 00:44:06,390 --> 00:44:04,960 organic carbon than the upper unit so 1362 00:44:07,829 --> 00:44:06,400 the axis we're down here around values 1363 00:44:09,670 --> 00:44:07,839 of five this is actually a quite high 1364 00:44:11,829 --> 00:44:09,680 amount of organic carbon for uh for a 1365 00:44:13,990 --> 00:44:11,839 sedimentary rock this is a black shale 1366 00:44:15,510 --> 00:44:14,000 and in the younger record if you saw a 1367 00:44:16,950 --> 00:44:15,520 rock with five percent organic carbon 1368 00:44:18,710 --> 00:44:16,960 you'd have molybdenum concentrations 1369 00:44:20,390 --> 00:44:18,720 that are way up here 1370 00:44:22,069 --> 00:44:20,400 um however that's not what we have in 1371 00:44:23,829 --> 00:44:22,079 this s2 unit we have a very shallow 1372 00:44:25,190 --> 00:44:23,839 slope here in other words even as the 1373 00:44:27,750 --> 00:44:25,200 organic carbon content was varying 1374 00:44:29,990 --> 00:44:27,760 around from from uh two to two percent 1375 00:44:32,069 --> 00:44:30,000 to six or seven percent the aluminum 1376 00:44:34,150 --> 00:44:32,079 concentration was staying very low 1377 00:44:36,150 --> 00:44:34,160 and this says to us that that during 1378 00:44:37,270 --> 00:44:36,160 this period of time these sedum the 1379 00:44:38,710 --> 00:44:37,280 water column that these sediments 1380 00:44:41,030 --> 00:44:38,720 accumulated in look very different from 1381 00:44:43,190 --> 00:44:41,040 the water column of younger sediments 1382 00:44:45,270 --> 00:44:43,200 younger analogous sediments in the sense 1383 00:44:46,710 --> 00:44:45,280 of having much less molybdenum content 1384 00:44:48,470 --> 00:44:46,720 in the water column 1385 00:44:50,550 --> 00:44:48,480 as you go into the s1 unit as i 1386 00:44:51,910 --> 00:44:50,560 mentioned we subdivided in we split it 1387 00:44:53,990 --> 00:44:51,920 in half and each half has a slightly 1388 00:44:55,990 --> 00:44:54,000 different trend but each half has a 1389 00:44:58,550 --> 00:44:56,000 quite respectable trend considering the 1390 00:44:59,990 --> 00:44:58,560 antiquity of these rocks 1391 00:45:01,109 --> 00:45:00,000 the correlation coefficients are quite 1392 00:45:02,150 --> 00:45:01,119 reasonable 1393 00:45:04,790 --> 00:45:02,160 and you can see that these trends 1394 00:45:06,710 --> 00:45:04,800 actually approach those of modern anoxic 1395 00:45:08,790 --> 00:45:06,720 basins they're lower 1396 00:45:10,150 --> 00:45:08,800 but they are they do approach them 1397 00:45:11,510 --> 00:45:10,160 so what we're seeing here it looks like 1398 00:45:13,829 --> 00:45:11,520 is a transition from a world in which 1399 00:45:15,829 --> 00:45:13,839 the oceans had very little molybdenum to 1400 00:45:17,990 --> 00:45:15,839 a world in which uh it's very hard to 1401 00:45:19,270 --> 00:45:18,000 explain these data unless the water 1402 00:45:22,150 --> 00:45:19,280 column in which these sediments 1403 00:45:23,670 --> 00:45:22,160 accumulated had a appreciable amount of 1404 00:45:25,589 --> 00:45:23,680 molybdenum dissolved in it that's the 1405 00:45:27,589 --> 00:45:25,599 only way you can get these relationships 1406 00:45:29,030 --> 00:45:27,599 as far as we understand right now so it 1407 00:45:31,030 --> 00:45:29,040 looks like we have a transition from one 1408 00:45:33,430 --> 00:45:31,040 world to another world as we go from 1409 00:45:34,870 --> 00:45:33,440 this array to either of these arrays 1410 00:45:36,150 --> 00:45:34,880 there may be a slight change between 1411 00:45:37,589 --> 00:45:36,160 these two 1412 00:45:39,270 --> 00:45:37,599 and we could discuss that a bit i don't 1413 00:45:40,870 --> 00:45:39,280 want to pin too much on that 1414 00:45:41,990 --> 00:45:40,880 but clearly this world was different 1415 00:45:44,630 --> 00:45:42,000 from this world with respect to the 1416 00:45:46,630 --> 00:45:44,640 molybdenum content of the oceans 1417 00:45:48,309 --> 00:45:46,640 and arguably it's certainly a reasonable 1418 00:45:50,390 --> 00:45:48,319 interpretation to say that that change 1419 00:45:51,910 --> 00:45:50,400 in aluminum concentrations reflects a 1420 00:45:53,910 --> 00:45:51,920 change in the oxidation state of the 1421 00:45:55,430 --> 00:45:53,920 environment which could be well be due 1422 00:45:56,550 --> 00:45:55,440 to change the amount of oxygen in the 1423 00:45:58,550 --> 00:45:56,560 environment 1424 00:46:00,069 --> 00:45:58,560 we can argue nuances of that but that's 1425 00:46:02,150 --> 00:46:00,079 certainly a reasonable story and so the 1426 00:46:04,630 --> 00:46:02,160 notion would be that we we have captured 1427 00:46:07,349 --> 00:46:04,640 here at 2.5 billion years ago a 1428 00:46:09,190 --> 00:46:07,359 transition from a situation in which the 1429 00:46:10,470 --> 00:46:09,200 oceans were very low in molybdenum 1430 00:46:12,309 --> 00:46:10,480 because the environment was very low in 1431 00:46:14,790 --> 00:46:12,319 oxygen to a world in which there was 1432 00:46:16,790 --> 00:46:14,800 perhaps more oxygen available 1433 00:46:19,589 --> 00:46:16,800 now i should stress that i've presented 1434 00:46:20,790 --> 00:46:19,599 this all so far in terms of a world and 1435 00:46:23,190 --> 00:46:20,800 we really don't know that we're looking 1436 00:46:24,470 --> 00:46:23,200 at one location at one period of time 1437 00:46:26,870 --> 00:46:24,480 it's quite possible that what we're 1438 00:46:28,710 --> 00:46:26,880 seeing is a very local phenomenon you 1439 00:46:29,670 --> 00:46:28,720 could imagine a a 1440 00:47:22,550 --> 00:46:29,680 a 1441 00:47:24,470 --> 00:47:22,560 oxygen present in the environment even 1442 00:47:27,349 --> 00:47:24,480 if only locally 1443 00:47:29,349 --> 00:47:27,359 2.5 billion years ago which is 100 1444 00:47:31,349 --> 00:47:29,359 million years or so before 1445 00:47:32,870 --> 00:47:31,359 the great oxidation event really takes 1446 00:47:34,470 --> 00:47:32,880 off 1447 00:47:35,829 --> 00:47:34,480 now you might ask how much oxygen are we 1448 00:47:36,630 --> 00:47:35,839 talking about how much do we need to do 1449 00:47:37,910 --> 00:47:36,640 this 1450 00:47:39,510 --> 00:47:37,920 and there they're 1451 00:47:40,710 --> 00:47:39,520 that's that's a very hard thing to get a 1452 00:47:42,470 --> 00:47:40,720 handle on 1453 00:47:44,390 --> 00:47:42,480 um but we go we've gone about it in this 1454 00:47:45,670 --> 00:47:44,400 way um this it's the simplest thing 1455 00:47:47,190 --> 00:47:45,680 about the only thing that we can do 1456 00:47:49,430 --> 00:47:47,200 easily right now 1457 00:47:51,349 --> 00:47:49,440 we can ask ourselves if if molybdenum 1458 00:47:55,510 --> 00:47:51,359 and rhenium are carried in sulfide 1459 00:47:57,430 --> 00:47:55,520 minerals like like iron pyrite here 1460 00:47:58,790 --> 00:47:57,440 we can ask ourselves how much oxygen do 1461 00:48:00,870 --> 00:47:58,800 we need to start to change the 1462 00:48:02,309 --> 00:48:00,880 weathering behavior of these minerals 1463 00:48:03,910 --> 00:48:02,319 how much oxygen 1464 00:48:06,470 --> 00:48:03,920 do we need to start liberating lots of 1465 00:48:08,470 --> 00:48:06,480 molybdenum uranium from igneous rocks if 1466 00:48:09,510 --> 00:48:08,480 the molybdenum are present in the form 1467 00:48:11,990 --> 00:48:09,520 of 1468 00:48:15,109 --> 00:48:12,000 molybdenum sulfides or molybdenum and 1469 00:48:16,950 --> 00:48:15,119 rhenium associated with pyrite or other 1470 00:48:18,470 --> 00:48:16,960 sulfide minerals 1471 00:48:19,750 --> 00:48:18,480 and this is fool's gold so i suppose we 1472 00:48:21,349 --> 00:48:19,760 should be careful that we aren't on a 1473 00:48:22,710 --> 00:48:21,359 fool's errand than doing this but it's 1474 00:48:24,870 --> 00:48:22,720 something we can at least try to do to 1475 00:48:26,470 --> 00:48:24,880 get a handle on how much oxygen 1476 00:48:28,870 --> 00:48:26,480 it turns out there are very few studies 1477 00:48:30,549 --> 00:48:28,880 of the rate of weathering of sulfides as 1478 00:48:33,109 --> 00:48:30,559 a function of the amount of oxygen but 1479 00:48:34,470 --> 00:48:33,119 there is one study from the early 1990s 1480 00:48:35,829 --> 00:48:34,480 by williamson and rinstead that looks 1481 00:48:38,390 --> 00:48:35,839 pretty good that other workers in the 1482 00:48:39,750 --> 00:48:38,400 field have also made use of 1483 00:48:41,670 --> 00:48:39,760 so they looked at the weathering rate of 1484 00:48:43,670 --> 00:48:41,680 pyrite as a function of oxygen and from 1485 00:48:46,309 --> 00:48:43,680 from the the function that they derived 1486 00:48:47,430 --> 00:48:46,319 you can make a plot like this 1487 00:48:48,710 --> 00:48:47,440 which is which is answering the 1488 00:48:50,470 --> 00:48:48,720 following question 1489 00:48:52,390 --> 00:48:50,480 let's imagine that i have a pyrite 1490 00:48:55,750 --> 00:48:52,400 nodule that's a pyrite a grain that's 1491 00:48:57,270 --> 00:48:55,760 about 100 microns in size a pyrite cube 1492 00:48:59,270 --> 00:48:57,280 that's sort of a typical size you might 1493 00:49:02,230 --> 00:48:59,280 find in a igneous rock 1494 00:49:04,150 --> 00:49:02,240 and let's ask how long in years will 1495 00:49:06,790 --> 00:49:04,160 that pyrite grain last 1496 00:49:08,470 --> 00:49:06,800 um when exposed to oxygen 1497 00:49:09,589 --> 00:49:08,480 to oxygenated waters where the oxygen in 1498 00:49:11,430 --> 00:49:09,599 the atmosphere is at these different 1499 00:49:13,109 --> 00:49:11,440 partial pressures 1500 00:49:14,309 --> 00:49:13,119 and what you do if you use if you look 1501 00:49:16,390 --> 00:49:14,319 at their paper and use their functions 1502 00:49:18,230 --> 00:49:16,400 you obtain a line like this 1503 00:49:19,910 --> 00:49:18,240 and the message here is that even with 1504 00:49:21,910 --> 00:49:19,920 very low amounts of oxygen 10 to the 1505 00:49:24,230 --> 00:49:21,920 minus 5 or even 10 to the minus 6 of the 1506 00:49:26,390 --> 00:49:24,240 present atmospheric level 1507 00:49:27,829 --> 00:49:26,400 that pyrite grain is going to fall apart 1508 00:49:30,790 --> 00:49:27,839 is going to dissolve to nothing within 1509 00:49:32,390 --> 00:49:30,800 10 to 100 000 years which geologically 1510 00:49:35,349 --> 00:49:32,400 speaking in a weathering context is 1511 00:49:37,270 --> 00:49:35,359 fairly fast fairly fast so the idea here 1512 00:49:39,510 --> 00:49:37,280 is that you don't need very much oxygen 1513 00:49:41,190 --> 00:49:39,520 to cause pyrite grains of this size to 1514 00:49:42,710 --> 00:49:41,200 weather very rapidly much more rapidly 1515 00:49:44,870 --> 00:49:42,720 than they did before 1516 00:49:46,150 --> 00:49:44,880 and so this is why we suspect um that 1517 00:49:47,430 --> 00:49:46,160 the amount of oxygen that we need to 1518 00:49:48,710 --> 00:49:47,440 explain this is actually very small 1519 00:49:50,950 --> 00:49:48,720 that's why we talk about this in terms 1520 00:49:53,270 --> 00:49:50,960 of a width of oxygen we're not talking 1521 00:49:55,270 --> 00:49:53,280 about a 2.5 billion years of shift to an 1522 00:49:57,510 --> 00:49:55,280 oxygenated planet we're not talking 1523 00:49:59,430 --> 00:49:57,520 about a percent of the modern oxygen 1524 00:50:00,470 --> 00:49:59,440 level let alone something uh close to 1525 00:50:04,069 --> 00:50:00,480 the modern oxygen level we're talking 1526 00:50:05,829 --> 00:50:04,079 about something very small um but um but 1527 00:50:08,069 --> 00:50:05,839 more than what was there before because 1528 00:50:09,750 --> 00:50:08,079 before this this excursion to metal 1529 00:50:13,030 --> 00:50:09,760 concentration it looks like there was 1530 00:50:14,710 --> 00:50:13,040 little or no uh uh even even less oxygen 1531 00:50:17,430 --> 00:50:14,720 around if we can infer from this 1532 00:50:18,470 --> 00:50:17,440 molybdenum proxy 1533 00:50:20,470 --> 00:50:18,480 and a little bit of strengthening of 1534 00:50:21,990 --> 00:50:20,480 this idea comes from looking at the 1535 00:50:23,829 --> 00:50:22,000 uranium as well as the rhenium and 1536 00:50:25,510 --> 00:50:23,839 molybdenum so i've mentioned uranium 1537 00:50:27,589 --> 00:50:25,520 several times as we've gone along here 1538 00:50:28,870 --> 00:50:27,599 i've said it's similar to molybdenum but 1539 00:50:29,910 --> 00:50:28,880 i've always hedged that and said it's a 1540 00:50:31,190 --> 00:50:29,920 little different 1541 00:50:32,470 --> 00:50:31,200 and and 1542 00:50:34,069 --> 00:50:32,480 as you can see here when we actually 1543 00:50:35,750 --> 00:50:34,079 look at the uranium 1544 00:50:37,510 --> 00:50:35,760 through this drill core 1545 00:50:38,950 --> 00:50:37,520 at high resolution 1546 00:50:40,150 --> 00:50:38,960 we don't see anything like the feature 1547 00:50:42,150 --> 00:50:40,160 that we see in molybdenum and rhenium 1548 00:50:43,670 --> 00:50:42,160 abundances somehow the uranium is 1549 00:50:46,069 --> 00:50:43,680 behaving differently 1550 00:50:47,510 --> 00:50:46,079 and we're we think we hypothesize anyway 1551 00:50:49,510 --> 00:50:47,520 that the story here has to do with the 1552 00:50:50,950 --> 00:50:49,520 fact that the oxygen level is very low 1553 00:50:52,309 --> 00:50:50,960 and the fact that uranium rheum and 1554 00:50:53,589 --> 00:50:52,319 molybdenum actually have some 1555 00:50:55,109 --> 00:50:53,599 differences in their chemistry when 1556 00:50:56,870 --> 00:50:55,119 oxygen is very low 1557 00:50:59,109 --> 00:50:56,880 one of the primary differences is the 1558 00:51:01,430 --> 00:50:59,119 carrier mineral of these metals in 1559 00:51:02,950 --> 00:51:01,440 igneous rocks so as i said molybdenum 1560 00:51:04,950 --> 00:51:02,960 and rhenium are found in sulfide 1561 00:51:07,349 --> 00:51:04,960 minerals which are very susceptible to 1562 00:51:08,790 --> 00:51:07,359 oxidative weathering uranium though in 1563 00:51:10,710 --> 00:51:08,800 igneous rocks is primarily found in 1564 00:51:12,069 --> 00:51:10,720 feldspars and zircons and various 1565 00:51:14,549 --> 00:51:12,079 minerals that are not nearly as 1566 00:51:16,069 --> 00:51:14,559 susceptible to oxygen so if you imagine 1567 00:51:18,230 --> 00:51:16,079 a world where there's very little oxygen 1568 00:51:20,390 --> 00:51:18,240 around essentially anoxic 1569 00:51:22,710 --> 00:51:20,400 and then you introduce a whiff of oxygen 1570 00:51:24,870 --> 00:51:22,720 um quote unquote 10 to the minus 5 or of 1571 00:51:26,950 --> 00:51:24,880 the modern level something like that 1572 00:51:28,549 --> 00:51:26,960 you will start to oxidize sulfides you 1573 00:51:29,990 --> 00:51:28,559 will significantly perturb the 1574 00:51:31,750 --> 00:51:30,000 molybdenum and rhenium budgets of the 1575 00:51:33,910 --> 00:51:31,760 ocean but you won't significantly 1576 00:51:36,710 --> 00:51:33,920 perturb the uranium budget of the ocean 1577 00:51:37,829 --> 00:51:36,720 um either locally or globally and that's 1578 00:51:39,030 --> 00:51:37,839 one of the ways that uranium is 1579 00:51:40,870 --> 00:51:39,040 different from living and uranium there 1580 00:51:41,990 --> 00:51:40,880 are other ways that also factor in in 1581 00:51:43,829 --> 00:51:42,000 the same way 1582 00:51:45,510 --> 00:51:43,839 that all point in the direction of of 1583 00:51:47,589 --> 00:51:45,520 arguing that we have evidence of a small 1584 00:51:49,109 --> 00:51:47,599 amount of oxygen enough to perturb these 1585 00:51:50,950 --> 00:51:49,119 elements budgets but not enough to 1586 00:51:51,990 --> 00:51:50,960 perturb the uranium budget and somewhere 1587 00:51:53,030 --> 00:51:52,000 in here if we could get more 1588 00:51:54,230 --> 00:51:53,040 quantitative about these various 1589 00:51:55,430 --> 00:51:54,240 weathering rates of minerals we could 1590 00:51:57,190 --> 00:51:55,440 probably actually put a much tighter 1591 00:51:58,390 --> 00:51:57,200 constraint than what i've done so far on 1592 00:52:00,630 --> 00:51:58,400 the amount of oxygen that might have 1593 00:52:02,309 --> 00:52:00,640 been around 1594 00:52:04,069 --> 00:52:02,319 and i'd be remiss if i didn't stress 1595 00:52:06,150 --> 00:52:04,079 that what's it the message from the 1596 00:52:07,349 --> 00:52:06,160 companion paper kaufman at all as i 1597 00:52:09,430 --> 00:52:07,359 mentioned there's two papers published 1598 00:52:10,549 --> 00:52:09,440 in science at the end of september one 1599 00:52:12,630 --> 00:52:10,559 focus on the metals that's what i've 1600 00:52:14,390 --> 00:52:12,640 dwelt on here the other paper focuses on 1601 00:52:15,829 --> 00:52:14,400 mass independent sulfur isotopes which 1602 00:52:17,829 --> 00:52:15,839 are a 1603 00:52:19,910 --> 00:52:17,839 an hour plus lecture unto themselves so 1604 00:52:21,270 --> 00:52:19,920 i can't do this justice 1605 00:52:23,109 --> 00:52:21,280 but for those who are familiar with this 1606 00:52:25,589 --> 00:52:23,119 i want to point out that the sulfur 1607 00:52:28,230 --> 00:52:25,599 isotopes also show 1608 00:52:30,390 --> 00:52:28,240 distinct changes that couple very nicely 1609 00:52:32,790 --> 00:52:30,400 to what we're seeing with the metals 1610 00:52:35,750 --> 00:52:32,800 so here's plotted this two parameters 1611 00:52:38,230 --> 00:52:35,760 the cap delta 33 sulfur versus the uh 1612 00:52:39,270 --> 00:52:38,240 delta 34 sulfur again i'm not going to 1613 00:52:41,990 --> 00:52:39,280 explain this because it would take an 1614 00:52:44,710 --> 00:52:42,000 hour to do so but notice that the down 1615 00:52:47,190 --> 00:52:44,720 here the data fall along a well-defined 1616 00:52:49,349 --> 00:52:47,200 array here this is in the lower unit 1617 00:52:50,710 --> 00:52:49,359 this lower black shell unit 1618 00:52:52,230 --> 00:52:50,720 the data follow along this this 1619 00:52:53,910 --> 00:52:52,240 well-defined array 1620 00:52:56,549 --> 00:52:53,920 but when you go up to the upper black 1621 00:52:58,309 --> 00:52:56,559 shell units something that that that 1622 00:53:01,030 --> 00:52:58,319 the data 1623 00:53:03,109 --> 00:53:01,040 array changes markedly the array you see 1624 00:53:05,750 --> 00:53:03,119 down here is typical of what's seen in 1625 00:53:07,990 --> 00:53:05,760 archaean uh sediments this is very 1626 00:53:09,910 --> 00:53:08,000 unusual and the simplest explanation if 1627 00:53:12,390 --> 00:53:09,920 you follow through all the the details 1628 00:53:14,309 --> 00:53:12,400 of logic um the simple explanation for 1629 00:53:16,790 --> 00:53:14,319 this transition is that there is a turn 1630 00:53:18,309 --> 00:53:16,800 on from here to here of an oxidative 1631 00:53:19,750 --> 00:53:18,319 part of the sulfur cycle that we start 1632 00:53:21,430 --> 00:53:19,760 to oxidize 1633 00:53:22,630 --> 00:53:21,440 reduce sulfur compounds in the surface 1634 00:53:24,069 --> 00:53:22,640 ocean 1635 00:53:26,069 --> 00:53:24,079 to make 1636 00:53:28,069 --> 00:53:26,079 sulfate which can then again be 1637 00:53:30,230 --> 00:53:28,079 re-reduced in a reducing basin to give 1638 00:53:33,270 --> 00:53:30,240 us this particular isotopic signal it's 1639 00:53:35,030 --> 00:53:33,280 a slightly complicated story um but uh 1640 00:53:36,790 --> 00:53:35,040 and so i'm not i can't possibly go into 1641 00:53:38,470 --> 00:53:36,800 it here but suffice to say there's a big 1642 00:53:39,750 --> 00:53:38,480 change here and this the most 1643 00:53:42,390 --> 00:53:39,760 straightforward interpretation although 1644 00:53:43,990 --> 00:53:42,400 it is not a simple interpretation 1645 00:53:46,069 --> 00:53:44,000 in and of itself is that this is 1646 00:53:48,950 --> 00:53:46,079 consistent with this change in oxidation 1647 00:53:51,829 --> 00:53:48,960 state in the environment 1648 00:53:53,829 --> 00:53:51,839 you can also plot the capital delta 36 1649 00:53:55,270 --> 00:53:53,839 against capital delta 33 these are two 1650 00:53:56,470 --> 00:53:55,280 mass independent signatures plotted 1651 00:53:58,230 --> 00:53:56,480 against each other 1652 00:54:00,309 --> 00:53:58,240 and you see that in the lower unit the 1653 00:54:01,670 --> 00:54:00,319 data fall along this array here 1654 00:54:02,950 --> 00:54:01,680 and the upper unit they follow along at 1655 00:54:03,910 --> 00:54:02,960 this array with a somewhat different 1656 00:54:05,510 --> 00:54:03,920 slope 1657 00:54:07,030 --> 00:54:05,520 and this is indicative of some sort of 1658 00:54:09,510 --> 00:54:07,040 change in the atmospheric chemistry of 1659 00:54:11,829 --> 00:54:09,520 sulfur that we don't fully understand um 1660 00:54:13,510 --> 00:54:11,839 but it is it is certainly suggestive it 1661 00:54:16,470 --> 00:54:13,520 well it's indicative of some sort of 1662 00:54:18,710 --> 00:54:16,480 change in in atmospheric chemistry um 1663 00:54:19,910 --> 00:54:18,720 which may well uh couple with the notion 1664 00:54:22,470 --> 00:54:19,920 of there being a little bit of oxygen 1665 00:54:24,549 --> 00:54:22,480 around changing the the speciation of 1666 00:54:27,109 --> 00:54:24,559 gases in the atmosphere the final thing 1667 00:54:29,030 --> 00:54:27,119 to take away from this figure is the the 1668 00:54:30,390 --> 00:54:29,040 axis over here so so over here we're 1669 00:54:32,390 --> 00:54:30,400 looking at the lower and upper unit in 1670 00:54:34,630 --> 00:54:32,400 the mount mcrae shale this western 1671 00:54:36,069 --> 00:54:34,640 australian black shale 1672 00:54:37,670 --> 00:54:36,079 but the data that i've shown you here 1673 00:54:39,990 --> 00:54:37,680 are actually a compilation that includes 1674 00:54:44,470 --> 00:54:40,000 data from our drill core as well as data 1675 00:54:47,510 --> 00:54:45,910 and um 1676 00:54:49,990 --> 00:54:47,520 what you see is that the two data sets 1677 00:54:51,510 --> 00:54:50,000 on both these basins both 1678 00:54:53,510 --> 00:54:51,520 both the south african basin and the 1679 00:54:55,589 --> 00:54:53,520 western australian basin show the same 1680 00:54:58,069 --> 00:54:55,599 sort of trends and that is at least 1681 00:54:59,430 --> 00:54:58,079 suggestive of um 1682 00:55:01,510 --> 00:54:59,440 the effects we're seeing not being 1683 00:55:03,430 --> 00:55:01,520 simply in one small isolated basin but 1684 00:55:04,790 --> 00:55:03,440 perhaps being global now there's debate 1685 00:55:06,069 --> 00:55:04,800 about this because it's not entirely 1686 00:55:07,910 --> 00:55:06,079 clear that these two basins one in 1687 00:55:10,390 --> 00:55:07,920 western australia one in south africa 1688 00:55:11,589 --> 00:55:10,400 were actually uh very far apart two and 1689 00:55:13,349 --> 00:55:11,599 a half billion years ago they may have 1690 00:55:15,430 --> 00:55:13,359 been part of one large basin at that 1691 00:55:17,349 --> 00:55:15,440 time um but it's what you got to work 1692 00:55:19,430 --> 00:55:17,359 with so it's it's where it's it's what 1693 00:55:20,790 --> 00:55:19,440 you look at and it's lee suggested that 1694 00:55:23,030 --> 00:55:20,800 there could have been a more widespread 1695 00:55:24,309 --> 00:55:23,040 effect than uh than you might otherwise 1696 00:55:25,430 --> 00:55:24,319 think 1697 00:55:27,750 --> 00:55:25,440 so to take us back to our initial 1698 00:55:30,309 --> 00:55:27,760 motivation here's our cartoon of oxygen 1699 00:55:32,390 --> 00:55:30,319 in the environment through time um 1700 00:55:34,790 --> 00:55:32,400 uh the sort of canonical story that most 1701 00:55:36,470 --> 00:55:34,800 people will uh uh 1702 00:55:38,150 --> 00:55:36,480 recite you who are who are knowledgeable 1703 00:55:39,829 --> 00:55:38,160 about the field and what we think we 1704 00:55:42,470 --> 00:55:39,839 find here is that so here's the the 1705 00:55:45,109 --> 00:55:42,480 great oxidation event um starting around 1706 00:55:46,470 --> 00:55:45,119 2.4 and really taking off around 2.3 um 1707 00:55:49,750 --> 00:55:46,480 what we think is that we have evidence 1708 00:55:51,910 --> 00:55:49,760 now of a whiff of oxygen at 2.5 billion 1709 00:55:53,349 --> 00:55:51,920 years ago here at this point in time and 1710 00:55:56,069 --> 00:55:53,359 you put this together with the biomarker 1711 00:55:59,190 --> 00:55:56,079 evidence in the fifths 1712 00:56:01,270 --> 00:55:59,200 and you start to start to have increased 1713 00:56:02,230 --> 00:56:01,280 confidence in the notion that oxygen may 1714 00:56:04,549 --> 00:56:02,240 well have been present in the 1715 00:56:06,470 --> 00:56:04,559 environment uh well before the great 1716 00:56:08,630 --> 00:56:06,480 oxidation event and we think we've also 1717 00:56:09,829 --> 00:56:08,640 here developed a novel strategy for 1718 00:56:11,670 --> 00:56:09,839 going about looking for evidence of that 1719 00:56:13,190 --> 00:56:11,680 oxygen we very much like to look at 1720 00:56:14,789 --> 00:56:13,200 other sediments of greater antiquity and 1721 00:56:17,349 --> 00:56:14,799 see how far back this particular 1722 00:56:18,710 --> 00:56:17,359 signature might go 1723 00:56:20,150 --> 00:56:18,720 and the final thought i want to leave 1724 00:56:22,470 --> 00:56:20,160 you with is that as we think more and 1725 00:56:24,150 --> 00:56:22,480 more and get get better data and data as 1726 00:56:25,910 --> 00:56:24,160 we have now at high temporal resolution 1727 00:56:28,950 --> 00:56:25,920 looking at high resolution in the drill 1728 00:56:31,030 --> 00:56:28,960 core through time in the archaean i 1729 00:56:34,150 --> 00:56:31,040 think we probably need to start to shift 1730 00:56:36,470 --> 00:56:34,160 our vision of um of 1731 00:56:38,390 --> 00:56:36,480 of what the debate is all about when we 1732 00:56:40,470 --> 00:56:38,400 talk about oxygen in the ancient 1733 00:56:42,230 --> 00:56:40,480 environment the archaean environment the 1734 00:56:44,470 --> 00:56:42,240 debate is often depicted in a cartoon 1735 00:56:46,630 --> 00:56:44,480 fashion between those who argue for an 1736 00:56:48,870 --> 00:56:46,640 anoxic world and those who argue for an 1737 00:56:50,390 --> 00:56:48,880 oxygenated world 1738 00:56:52,309 --> 00:56:50,400 if you read the papers carefully even 1739 00:56:54,230 --> 00:56:52,319 between partisans in that debate you see 1740 00:56:56,390 --> 00:56:54,240 that it's actually not so simple that 1741 00:56:58,069 --> 00:56:56,400 that anoxic never really means no oxygen 1742 00:56:59,589 --> 00:56:58,079 or rarely means no oxygen it can mean 1743 00:57:01,270 --> 00:56:59,599 but not necessarily 1744 00:57:02,549 --> 00:57:01,280 and those who argue for an oxygenated 1745 00:57:03,750 --> 00:57:02,559 environment often are talking about 1746 00:57:05,190 --> 00:57:03,760 something that is a percent of the 1747 00:57:07,990 --> 00:57:05,200 modern or maybe even a bit less has 1748 00:57:09,910 --> 00:57:08,000 oxygenated but but but that's in detail 1749 00:57:11,510 --> 00:57:09,920 if you step back the argument often is 1750 00:57:13,190 --> 00:57:11,520 is between those who say lots of oxygen 1751 00:57:14,390 --> 00:57:13,200 and those who say none and i think in 1752 00:57:16,390 --> 00:57:14,400 the archaean we need to have a somewhat 1753 00:57:17,589 --> 00:57:16,400 more sophisticated view of things and 1754 00:57:19,190 --> 00:57:17,599 and i'm trying to embody that here in 1755 00:57:20,230 --> 00:57:19,200 this figure here so this is a picture 1756 00:57:23,670 --> 00:57:20,240 not of the r key and this is of the 1757 00:57:26,150 --> 00:57:23,680 modern or the recent um from zero to 1758 00:57:29,190 --> 00:57:26,160 650 000 years ago you're looking here at 1759 00:57:31,750 --> 00:57:29,200 data of carbon dioxide and methane in 1760 00:57:33,829 --> 00:57:31,760 antarctic ice cores right so we know 1761 00:57:35,670 --> 00:57:33,839 very well from the modern that these 1762 00:57:36,789 --> 00:57:35,680 gases are trace gases in the modern 1763 00:57:38,870 --> 00:57:36,799 environment 1764 00:57:40,710 --> 00:57:38,880 um present parts per million and parts 1765 00:57:41,829 --> 00:57:40,720 per billion concentrations in the in the 1766 00:57:43,589 --> 00:57:41,839 atmosphere 1767 00:57:45,270 --> 00:57:43,599 and they're variable 1768 00:57:47,990 --> 00:57:45,280 right now you wouldn't say that the 1769 00:57:49,990 --> 00:57:48,000 modern atmosphere is amethanic 1770 00:57:51,510 --> 00:57:50,000 um but the amount of methane is indeed 1771 00:57:53,030 --> 00:57:51,520 very low but it's there's enough there 1772 00:57:55,349 --> 00:57:53,040 that we actually worry about it quite a 1773 00:57:58,069 --> 00:57:55,359 bit and you wouldn't say that the modern 1774 00:57:59,430 --> 00:57:58,079 atmosphere is devoid of carbon dioxide 1775 00:58:01,109 --> 00:57:59,440 even though it's present as a fairly 1776 00:58:02,549 --> 00:58:01,119 trace constituent 1777 00:58:04,150 --> 00:58:02,559 it's even though it's trace it's still 1778 00:58:06,470 --> 00:58:04,160 significant in its impact on the 1779 00:58:08,150 --> 00:58:06,480 environment and i think that oxygen in 1780 00:58:10,549 --> 00:58:08,160 the archaean is probably somewhat of the 1781 00:58:12,390 --> 00:58:10,559 same thing i i think we we need to open 1782 00:58:13,990 --> 00:58:12,400 our minds to the possibility anyway that 1783 00:58:16,309 --> 00:58:14,000 oxygenic photosynthesis might be quite 1784 00:58:18,470 --> 00:58:16,319 ancient um that oxygen that other 1785 00:58:21,109 --> 00:58:18,480 factors beyond the evolution of oxygen 1786 00:58:23,510 --> 00:58:21,119 oxygen photosynthesis kept oxygen low 1787 00:58:25,430 --> 00:58:23,520 for a billion years or more 1788 00:58:26,870 --> 00:58:25,440 and that during that time oxygen behave 1789 00:58:29,190 --> 00:58:26,880 in the atmosphere very much as methane 1790 00:58:31,589 --> 00:58:29,200 and co2 do now that was present in low 1791 00:58:33,109 --> 00:58:31,599 amounts in variable amounts amounts that 1792 00:58:34,390 --> 00:58:33,119 may have been very highly variable 1793 00:58:36,069 --> 00:58:34,400 geographically 1794 00:58:37,829 --> 00:58:36,079 but it was there and some of the debates 1795 00:58:39,349 --> 00:58:37,839 we have between various proxies may not 1796 00:58:41,349 --> 00:58:39,359 be nearly as black and white as we like 1797 00:58:43,829 --> 00:58:41,359 to think it may well be that the amount 1798 00:58:45,589 --> 00:58:43,839 of oxygen bounces around um 1799 00:58:46,870 --> 00:58:45,599 too low to trip one proxy but high 1800 00:58:49,349 --> 00:58:46,880 enough to trip another and then back and 1801 00:58:51,270 --> 00:58:49,359 forth and back and forth and i suspect 1802 00:58:52,870 --> 00:58:51,280 that as we dig deeper and deeper into 1803 00:58:55,430 --> 00:58:52,880 the archaean record we'll emerge with a 1804 00:58:58,309 --> 00:58:55,440 picture somewhat like this of oxygen is 1805 00:59:01,270 --> 00:58:58,319 a biogenic trace gas bouncing around um 1806 00:59:04,710 --> 00:59:01,280 uh various values always low but but 1807 00:59:06,789 --> 00:59:04,720 often but uh often present 1808 00:59:08,950 --> 00:59:06,799 and uh i'll leave you with just a set of 1809 00:59:10,309 --> 00:59:08,960 conclusions i won't walk through them um 1810 00:59:11,589 --> 00:59:10,319 other than to say that we have in 1811 00:59:12,870 --> 00:59:11,599 progress a number of other studies that 1812 00:59:14,789 --> 00:59:12,880 haven't gone into here because there's 1813 00:59:16,390 --> 00:59:14,799 simply no time we're looking at iron 1814 00:59:17,750 --> 00:59:16,400 isotope data we're looking at libnam and 1815 00:59:18,710 --> 00:59:17,760 other isotopes i want to stress in 1816 00:59:20,390 --> 00:59:18,720 particular i should have mentioned this 1817 00:59:21,349 --> 00:59:20,400 the outset that almost all the metal 1818 00:59:22,710 --> 00:59:21,359 data you've seen here have been 1819 00:59:25,670 --> 00:59:22,720 generated by 1820 00:59:28,390 --> 00:59:25,680 my phd student yun dwan and uh 1821 00:59:30,069 --> 00:59:28,400 co-workers here at asu gail arnold no 1822 00:59:32,710 --> 00:59:30,079 longer asu but recent until recently at 1823 00:59:34,549 --> 00:59:32,720 asu and gwyneth gordon and jun is also 1824 00:59:36,390 --> 00:59:34,559 working as part of her thesis on iron 1825 00:59:38,069 --> 00:59:36,400 isotopes and libido isotopes which show 1826 00:59:39,750 --> 00:59:38,079 variations that i'm not comfortable 1827 00:59:41,829 --> 00:59:39,760 sharing in a very public forum like this 1828 00:59:43,829 --> 00:59:41,839 yet but show variations that are quite 1829 00:59:45,589 --> 00:59:43,839 uh sympathetic with the story that i've 1830 00:59:47,349 --> 00:59:45,599 been uh outlining for you up to this 1831 00:59:48,230 --> 00:59:47,359 point so we're looking at other proxies 1832 00:59:49,829 --> 00:59:48,240 we're starting to look at other 1833 00:59:51,349 --> 00:59:49,839 locations in more detail look at that 1834 00:59:53,910 --> 00:59:51,359 south african base in more detail from 1835 00:59:55,510 --> 00:59:53,920 metal content and uh the story so the 1836 00:59:58,150 --> 00:59:55,520 story that you've heard today i think is 1837 00:59:59,589 --> 00:59:58,160 just the first of a richer story to come 1838 01:00:02,549 --> 00:59:59,599 so thanks i'll be glad to take any 1839 01:00:02,559 --> 01:00:06,870 okay ariel thank you very much 1840 01:00:12,470 --> 01:00:10,150 for a great talk and a very clear one uh 1841 01:00:14,309 --> 01:00:12,480 would anybody around the net who has 1842 01:00:15,990 --> 01:00:14,319 questions please raise your hand on 1843 01:00:17,829 --> 01:00:16,000 webex and while you're doing that 1844 01:00:19,750 --> 01:00:17,839 there's a question here at nai central 1845 01:00:21,030 --> 01:00:19,760 from david morrison 1846 01:00:23,109 --> 01:00:21,040 hi there ariel 1847 01:00:24,230 --> 01:00:23,119 thanks very much um 1848 01:00:25,430 --> 01:00:24,240 now i can tell who that is in that 1849 01:00:26,309 --> 01:00:25,440 little picture there 1850 01:00:28,870 --> 01:00:26,319 right 1851 01:00:33,349 --> 01:00:28,880 uh i'm curious about this the period of 1852 01:00:35,109 --> 01:00:33,359 time between 2.5 and 2.3 or so you 1853 01:00:37,750 --> 01:00:35,119 talked about why you couldn't go lower 1854 01:00:39,990 --> 01:00:37,760 in this drill car but is there data from 1855 01:00:42,390 --> 01:00:40,000 the drill car to indicate whether this 1856 01:00:44,630 --> 01:00:42,400 whiff stays around goes up and down or 1857 01:00:45,829 --> 01:00:44,640 whatever in the subsequent uh 200 1858 01:00:47,829 --> 01:00:45,839 million years 1859 01:00:50,789 --> 01:00:47,839 sure sure well this drill core we can't 1860 01:00:51,829 --> 01:00:50,799 really go much much younger 1861 01:00:53,670 --> 01:00:51,839 we're basically looking at the upper 1862 01:00:54,870 --> 01:00:53,680 part that we can that we can work on it 1863 01:00:56,470 --> 01:00:54,880 was it 1864 01:00:58,390 --> 01:00:56,480 that's just the nature of this core 1865 01:01:00,150 --> 01:00:58,400 these there is a south african drill 1866 01:01:01,990 --> 01:01:00,160 core project um 1867 01:01:03,829 --> 01:01:02,000 uh funded by a private foundation the 1868 01:01:05,510 --> 01:01:03,839 agron foundation which actually more or 1869 01:01:08,069 --> 01:01:05,520 less starts where this core leaves off 1870 01:01:10,390 --> 01:01:08,079 and works its way up in time and that's 1871 01:01:11,990 --> 01:01:10,400 what we're eager to dig into in part um 1872 01:01:14,710 --> 01:01:12,000 the story gets a bit more complicated 1873 01:01:17,670 --> 01:01:14,720 because that south african core is 1874 01:01:18,870 --> 01:01:17,680 is from uh a different faces this the 1875 01:01:20,630 --> 01:01:18,880 sediments there were laid down in 1876 01:01:22,390 --> 01:01:20,640 shallower water and so whether those are 1877 01:01:24,470 --> 01:01:22,400 the right whether they provide the right 1878 01:01:26,309 --> 01:01:24,480 kind of environment for sequestering 1879 01:01:27,670 --> 01:01:26,319 molybdenum in the same way so whether we 1880 01:01:29,109 --> 01:01:27,680 could be comparing apples and apples 1881 01:01:30,630 --> 01:01:29,119 apples and oranges is not clear but 1882 01:01:32,549 --> 01:01:30,640 that's that's one of the directions 1883 01:01:34,069 --> 01:01:32,559 we're going in um within this core 1884 01:01:35,829 --> 01:01:34,079 though i mean it is 1885 01:01:37,109 --> 01:01:35,839 you know we look let me find the right 1886 01:01:40,230 --> 01:01:37,119 figure here 1887 01:01:42,150 --> 01:01:40,240 that's this one i want so as i i tried 1888 01:01:43,990 --> 01:01:42,160 to stress but it's worth um 1889 01:01:45,990 --> 01:01:44,000 stressing again so so you look at these 1890 01:01:47,109 --> 01:01:46,000 data and you see this this this 1891 01:01:49,109 --> 01:01:47,119 excursion 1892 01:01:50,950 --> 01:01:49,119 and um it's very tempting especially 1893 01:01:52,549 --> 01:01:50,960 when you use a word like with to imagine 1894 01:01:54,150 --> 01:01:52,559 that this is a transient phenomenon and 1895 01:01:56,390 --> 01:01:54,160 i even i even said that at the outset 1896 01:01:58,150 --> 01:01:56,400 that it might be transient and it might 1897 01:01:59,670 --> 01:01:58,160 be somewhat transient but if you look at 1898 01:02:00,789 --> 01:01:59,680 this in detail 1899 01:02:03,109 --> 01:02:00,799 i tried to stress before i'm going to 1900 01:02:04,470 --> 01:02:03,119 stress it again the malignant enrichment 1901 01:02:06,390 --> 01:02:04,480 factors 1902 01:02:07,589 --> 01:02:06,400 here are very very low 1903 01:02:11,430 --> 01:02:07,599 up here 1904 01:02:13,190 --> 01:02:11,440 they're not as high as they are here but 1905 01:02:14,870 --> 01:02:13,200 they're not so low it looks like there 1906 01:02:16,710 --> 01:02:14,880 was molybdenum in the environment in the 1907 01:02:19,589 --> 01:02:16,720 aqueous environment here more so than 1908 01:02:21,510 --> 01:02:19,599 there was down here so and if you look 1909 01:02:22,950 --> 01:02:21,520 at the molybdenum um to organic carbon 1910 01:02:24,069 --> 01:02:22,960 ratio 1911 01:02:25,190 --> 01:02:24,079 here 1912 01:02:26,390 --> 01:02:25,200 as i mentioned i don't want to make too 1913 01:02:27,270 --> 01:02:26,400 much of it but 1914 01:02:29,510 --> 01:02:27,280 um 1915 01:02:31,990 --> 01:02:29,520 you go from this unit at the very bottom 1916 01:02:33,589 --> 01:02:32,000 to this here this is the lower part of 1917 01:02:34,390 --> 01:02:33,599 the upper unit where the big excursion 1918 01:02:36,710 --> 01:02:34,400 happens 1919 01:02:38,150 --> 01:02:36,720 and then as you go up you you settle 1920 01:02:39,670 --> 01:02:38,160 down into this slope here which is a 1921 01:02:41,430 --> 01:02:39,680 little shallower but it's still much 1922 01:02:42,630 --> 01:02:41,440 steeper than what's down here this is a 1923 01:02:44,870 --> 01:02:42,640 more sophisticated way of saying the 1924 01:02:46,870 --> 01:02:44,880 same thing that what comes after that 1925 01:02:48,870 --> 01:02:46,880 excursion is still different than what 1926 01:02:50,710 --> 01:02:48,880 came before and and what we say in the 1927 01:02:52,150 --> 01:02:50,720 paper is that this may well have been we 1928 01:02:54,950 --> 01:02:52,160 may all be seeing an irreversible 1929 01:02:56,230 --> 01:02:54,960 transition of some sort here um 1930 01:02:58,630 --> 01:02:56,240 uh so 1931 01:03:00,230 --> 01:02:58,640 you know bets are off but but it's quite 1932 01:03:01,670 --> 01:03:00,240 possible that that's what what we're 1933 01:03:04,789 --> 01:03:01,680 seeing here is 1934 01:03:09,190 --> 01:03:06,549 yeah that's great but it still tells me 1935 01:03:11,270 --> 01:03:09,200 you need more data in those uh couple of 1936 01:03:14,549 --> 01:03:11,280 hundred million years before this 1937 01:03:14,559 --> 01:03:19,029 we need more drilling 1938 01:03:19,039 --> 01:03:22,230 penn state has a question 1939 01:03:26,710 --> 01:03:25,270 yeah hi hi ariel this is jim casting i i 1940 01:03:31,510 --> 01:03:26,720 like your story 1941 01:03:34,069 --> 01:03:31,520 wondering about the implications when 1942 01:03:37,190 --> 01:03:34,079 you talk about a whiff of oxygen there 1943 01:03:39,990 --> 01:03:37,200 you compared oxygen in the uh reduced 1944 01:03:42,710 --> 01:03:40,000 archaean atmosphere to methane in the ox 1945 01:03:45,109 --> 01:03:42,720 in the present oxygenic atmosphere 1946 01:03:46,870 --> 01:03:45,119 it's it's a good analogy in some way i 1947 01:03:48,549 --> 01:03:46,880 think but on the other hand there's a 1948 01:03:50,549 --> 01:03:48,559 significant difference and that's that 1949 01:03:52,150 --> 01:03:50,559 today you've got oxygen and ozone so a 1950 01:03:54,630 --> 01:03:52,160 good uv screen 1951 01:03:57,190 --> 01:03:54,640 back then if you didn't then 1952 01:04:00,230 --> 01:03:57,200 the photolysis rates are much faster and 1953 01:04:02,230 --> 01:04:00,240 so the lifetime of oxygen in a methane 1954 01:04:04,309 --> 01:04:02,240 rich atmosphere is much shorter than the 1955 01:04:06,549 --> 01:04:04,319 lifetime of methane in an oxygen-rich 1956 01:04:09,109 --> 01:04:06,559 atmosphere so i guess the difference in 1957 01:04:12,309 --> 01:04:09,119 my mind is that it's hard to have a very 1958 01:04:14,549 --> 01:04:12,319 low level level of oxygen like one one 1959 01:04:16,470 --> 01:04:14,559 ppm or something like that that's that's 1960 01:04:18,309 --> 01:04:16,480 a global signal it'll just be a plume 1961 01:04:20,470 --> 01:04:18,319 and i i think your whiff may actually be 1962 01:04:23,109 --> 01:04:20,480 bigger than you think 1963 01:04:25,190 --> 01:04:23,119 uh it could be or i mean i mean much of 1964 01:04:26,309 --> 01:04:25,200 the unless i'm wrong much of the logic 1965 01:04:28,549 --> 01:04:26,319 you just presented is in a sort of 1966 01:04:30,069 --> 01:04:28,559 steady state sense and you mentioned 1967 01:04:32,309 --> 01:04:30,079 plumes at the end so you could imagine 1968 01:04:33,990 --> 01:04:32,319 that you have a world where you have 1969 01:04:35,910 --> 01:04:34,000 plumes and wax of oxygen that aren't 1970 01:04:37,829 --> 01:04:35,920 very long-lived but they do get around 1971 01:04:39,349 --> 01:04:37,839 not globally perhaps but in the regional 1972 01:04:40,710 --> 01:04:39,359 sense and maybe that's what we're seeing 1973 01:04:42,870 --> 01:04:40,720 uh i don't know i mean like like all 1974 01:04:45,109 --> 01:04:42,880 analogies um you know it's poetry and 1975 01:04:46,470 --> 01:04:45,119 it's imperfect and i leave it to 1976 01:04:48,069 --> 01:04:46,480 those who think more deeply about the 1977 01:04:48,870 --> 01:04:48,079 atmospheric physics and chemistry to 1978 01:04:51,029 --> 01:04:48,880 really 1979 01:04:53,990 --> 01:04:51,039 dig into it but 1980 01:04:55,990 --> 01:04:54,000 but but the major point 1981 01:04:57,910 --> 01:04:56,000 we talked about this in our seminar here 1982 01:04:59,670 --> 01:04:57,920 last week and and uh you know the 1983 01:05:02,710 --> 01:04:59,680 question is what do you need to weather 1984 01:05:03,829 --> 01:05:02,720 molybdenum molybdenum which i can't say 1985 01:05:06,309 --> 01:05:03,839 properly 1986 01:05:08,470 --> 01:05:06,319 right but maybe maybe you do need a 1987 01:05:10,870 --> 01:05:08,480 global signal in which case maybe it's a 1988 01:05:13,270 --> 01:05:10,880 pretty big puff of oxygen 1989 01:05:14,470 --> 01:05:13,280 maybe maybe what we really need is is uh 1990 01:05:16,710 --> 01:05:14,480 well we need more drilling as david 1991 01:05:18,549 --> 01:05:16,720 alluded to but we also uh we need more 1992 01:05:19,910 --> 01:05:18,559 experimental work you know the amount of 1993 01:05:22,789 --> 01:05:19,920 experimental work that goes on that 1994 01:05:24,870 --> 01:05:22,799 actually tries to tie to archaean and 1995 01:05:26,950 --> 01:05:24,880 protozoa geochemistry is very small and 1996 01:05:28,150 --> 01:05:26,960 that's you know it would be nice to 1997 01:05:29,829 --> 01:05:28,160 to revisit the kind of thing that 1998 01:05:32,870 --> 01:05:29,839 williamson and rimstep did with with 1999 01:05:33,589 --> 01:05:32,880 pyrite um but with uh with more sulfides 2000 01:05:42,630 --> 01:05:33,599 and 2001 01:05:46,390 --> 01:05:44,309 ariel i've got a question that's a 2002 01:05:47,910 --> 01:05:46,400 little far afield and i'd encourage 2003 01:05:48,950 --> 01:05:47,920 other people to raise their hands on 2004 01:05:51,349 --> 01:05:48,960 webex 2005 01:05:54,470 --> 01:05:51,359 to ask questions as well 2006 01:05:57,029 --> 01:05:54,480 and that is that there has been work 2007 01:05:58,870 --> 01:05:57,039 particularly by dave dimaray's group 2008 01:06:00,470 --> 01:05:58,880 including dave and tori holler and brad 2009 01:06:02,710 --> 01:06:00,480 beabout about 2010 01:06:04,750 --> 01:06:02,720 the role that the production of reduced 2011 01:06:07,589 --> 01:06:04,760 gases may have played somewhat 2012 01:06:10,150 --> 01:06:07,599 counter-intuitively in oxidizing earth's 2013 01:06:12,069 --> 01:06:10,160 atmosphere in that these microbial 2014 01:06:14,470 --> 01:06:12,079 communities that exist today and 2015 01:06:16,549 --> 01:06:14,480 presumably existed uh in this time 2016 01:06:18,069 --> 01:06:16,559 period two and a half billion years ago 2017 01:06:19,430 --> 01:06:18,079 they were putting out oxygen during the 2018 01:06:21,990 --> 01:06:19,440 daytime but they were putting out 2019 01:06:23,829 --> 01:06:22,000 hydrogen and methane at night and that 2020 01:06:25,589 --> 01:06:23,839 hydrogen and methane goes up to the top 2021 01:06:28,230 --> 01:06:25,599 of the atmosphere the hydrogen escapes 2022 01:06:30,950 --> 01:06:28,240 and you have a net oxidation so 2023 01:06:33,990 --> 01:06:30,960 what do you think might be 2024 01:06:36,230 --> 01:06:34,000 the contribution or the balance between 2025 01:06:38,789 --> 01:06:36,240 the oxidation of the atmosphere by the 2026 01:06:41,750 --> 01:06:38,799 loss of hydrogen to space 2027 01:06:44,630 --> 01:06:41,760 versus the photosynthetic production of 2028 01:06:48,630 --> 01:06:44,640 molecular oxygen 2029 01:06:49,990 --> 01:06:48,640 um so you want a quantitative answer 2030 01:06:52,390 --> 01:06:50,000 any kind of answer you want to try 2031 01:06:53,829 --> 01:06:52,400 giving i i don't even have a qualitative 2032 01:06:55,430 --> 01:06:53,839 feel i mean i mean the loss of hydrogen 2033 01:06:57,990 --> 01:06:55,440 to space and you know there's a lot of 2034 01:07:00,069 --> 01:06:58,000 ideas you know lee comp is here online a 2035 01:07:01,910 --> 01:07:00,079 lot of good ideas leave an author of of 2036 01:07:05,430 --> 01:07:01,920 one of the better ones um 2037 01:07:07,349 --> 01:07:05,440 uh uh of what actually was changing to 2038 01:07:08,950 --> 01:07:07,359 cause a change of oxygen um the one 2039 01:07:11,829 --> 01:07:08,960 thing that we do know goes on is loss of 2040 01:07:14,470 --> 01:07:11,839 hydrogen to space right and so um that's 2041 01:07:16,150 --> 01:07:14,480 one that we can't avoid but 2042 01:07:17,589 --> 01:07:16,160 to actually try to quantify the relative 2043 01:07:19,430 --> 01:07:17,599 importance 2044 01:07:22,069 --> 01:07:19,440 i you know of the various policies 2045 01:07:24,150 --> 01:07:22,079 involved you know 2046 01:07:32,230 --> 01:07:24,160 tough problem maybe lee would like to 2047 01:07:35,109 --> 01:07:33,670 i think jim's the one who might want to 2048 01:07:37,190 --> 01:07:35,119 comment on that 2049 01:07:39,750 --> 01:07:37,200 i have a potential postdoc who wants to 2050 01:07:41,990 --> 01:07:39,760 work on that problem yeah 2051 01:07:43,430 --> 01:07:42,000 colin goldberg 2052 01:07:47,190 --> 01:07:43,440 so crawl for a little bit of money you 2053 01:07:47,200 --> 01:07:53,910 okay i hear you 2054 01:07:59,029 --> 01:07:56,870 okay we're going once going twice for 2055 01:08:02,630 --> 01:07:59,039 asking ariel questions or continuing 2056 01:08:06,549 --> 01:08:04,870 okay ariel i guess we will thank you 2057 01:08:12,069 --> 01:08:06,559 again for a great talk 2058 01:08:18,070 --> 01:08:14,789 okay remember the next director seminar 2059 01:08:20,309 --> 01:08:18,080 is i think four weeks from today 2060 01:08:22,070 --> 01:08:20,319 december 3rd and it will be by jeff 2061 01:08:25,669 --> 01:08:22,080 marcy and we're going to be hearing 2062 01:08:29,510 --> 01:08:25,679 about low mass extra solar planets so