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[Music]

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[Applause]

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I'm gonna be talking about

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micrometeorites as a new proxy for co2

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in the Archaean so we're all pretty

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familiar with co2 right just heard about

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it a little bit and on the modern

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Earth's right it's a it's a great

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greenhouse gas and even though it's just

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a small fraction of the total

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atmospheric ah mass it plays a very

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important role in controlling the

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surface temperature on the modern earth

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and of course this has been true and

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throughout history and we should a

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little bit about the Archaean right and

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that co2 levels were likely much higher

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than because the earth wasn't frozen at

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that time but in order for it to have

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this liquid surface but need more co2 to

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have that higher surface temperature but

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exactly how high yeah

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exactly how high the surface temperature

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was is something that's still debated

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and we have those sort of two schools of

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thought that we saw in the previous talk

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well we have this low temperature regime

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which for our purposes is under 40

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degrees C and of his Ibis isotopic

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evidence and phosphates to support this

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in addition you see evidence for

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glaciations potentially in the Archean

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which would support at least a

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transiently cool

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earth at that time on the other hand if

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you look at oxygen isotopes in the

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shirts you see a much warmer temperature

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between maybe 55 and 85 degrees C and

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you also see that thermal reconstruction

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a reconstruction of thermostable

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proteins and the RTM supports this

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high-temperature early Earth and there

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may be evidence for a low viscosity

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ocean at that time which would further

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bolster this high temperature claim and

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a big part of the uncertainty here is

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how much co2 was available at this time

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on the early Earth to one the surface

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and of course when we care about this

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because this is when the first evidence

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for life is showing up so we'd really

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like to know what these surface

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conditions were like and how much co2

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was there it can help answer that

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question of course constraints on co2

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aren't great for the RTN and if we look

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at the proxy data we see that we have on

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the low end about 10 to the minus 3 bar

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and on the upper end close to 1 bar is

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our estimates as we go back in time so

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we have almost three orders of magnitude

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between our upper and lower bounds on

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co2

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and models should see this nice model

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here from Chris Hansen ton has made his

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way into all the talks so far you see

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this nice model prediction for them with

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their model mean and the black curve

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here and so co2 is increasing with time

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and then the uncertainty of their model

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is shown by the gray shaded region so

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with both models and the proxy data you

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see here there's a great deal of

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uncertainty now fortunately this is

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where micrometeorites might be able to

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help us and so that's what you're

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looking at here these are for

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micrometeorites that were extracted from

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a 2.7 billion year old limestone from

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australia and if you're not familiar

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with micrometeorites they're essentially

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just sand sized space dust that falls to

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earth quite often on the modern earth

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and probably has throughout Earth's

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history and in particular when I say

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micrometeorites in this talk I mean iron

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rich micro meteorites so a special type

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and that's what you're looking at here

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and the reason we care that the

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therapies iron rich particles is if you

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take metallic iron and you put it into

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the atmosphere very rapidly it will melt

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and when you have molten iron with an

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atmosphere of like co2 rich or oxygen

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rich oxidized and that's what we're

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interested in and in addition as you can

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see from the beautiful preservation of

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all four of these there are 2.7 billion

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years old clearly these iron rich micro

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meteorites are able to stand the test of

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time and show up quite nicely even on

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the earth today so if you were to look

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at these 2.7 billion year old micro

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meteorites you would see that they were

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highly oxidized so they contain

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magnetite and moose tight and they

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actually looked remarkably similar to

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modern micro meteorites in terms of the

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amount of oxidized iron in them and so

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this has led to speculation that the

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Archaean earth had an oxygen-rich upper

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atmosphere to explain the presence of

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most it--and magnetite

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these oxidized iron species but if we

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look at oxygen through time on the earth

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as we see here at present earth and we

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go back in time these micro meteorites

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preserved right around here where the

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total oxygen and the Earth's atmosphere

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was quite low and surface proxies

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constrain the atmospheric oxygen to

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below about one part per million so

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basically a trace gas that's

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non-existent in the atmosphere

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and even proxies for the atmosphere at

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altitude constrain it to a fairly minor

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component at best and this is an upper

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bound in the upper atmosphere of you

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know a fraction of a percent and if we

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take that with our understanding of

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atmospheric mixing it seems perhaps

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unlikely that we could have had modern

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levels of oxygen so an oxygen level up

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here and the upper atmosphere and

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nothing at the surface in the Archaean

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to oxidize these micrometeorites so

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that's sort of why we started with this

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work is if oxygen seems perhaps unlikely

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then co2 is maybe a better choice to

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oxidize these micrometeorites on the

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early Earth and so that's what we wanted

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to look at can we build a model where if

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we take a micrometer and these are

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examples of modern micro meteorites can

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we take one of these and can we measure

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the fraction of metallic iron which you

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see here in the white region and the

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oxidized region here is shown in gray

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can we measure that and then back out

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what the oxidizing power of the

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atmosphere was that these particles

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entered in I just want you to make a

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mental note of what these look like cuz

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you're gonna see a model simulation a

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moment we're gonna reproduce micro

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meteorites that look just like this in

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cross-section where you see an

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unoxidized metallic bead in the center

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and then an oxidized rim around it so of

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course we started with the modern

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atmosphere so we have no an atmospheric

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composition and we can collect modern

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micro meteorites like you just saw and

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then we can try and back out the

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atmospheric composition and so our model

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starts out with the assumption that we

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have a pure iron micro meter at the top

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of the atmosphere and then we pick

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radius for that particle we pick a

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velocity and an entry angle and then we

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drop it into the atmosphere essentially

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and see what happens and so we start

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with this very fun animation at 180

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kilometers above the Earth's surface and

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there's our beautiful particle and

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before I run the simulation I want to

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point out some of the parameters that

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were in a track that happens pretty fast

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so we're gonna be looking at the

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particle temperature the velocity and

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it's starting at about 12 kilometers a

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second so it's going really fast this is

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about a 50 micron and radius particle

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and its iron fraction is that a hundred

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percent by assumption right we're

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starting out with a pure iron particle

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and then we're also tracking the

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altitude and so the the kernel right

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here this is the cross-sectional area

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and so you'll notice that it's all

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dark gray which is matching the metallic

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iron so this is showing you the

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composition is currently 100% metallic

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iron and as we run this forward you're

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gonna see this shrink down to that

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central bead similar to what you saw

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for those modern micro meteorites and

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you'll see an oxidized exterior form ice

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will run it forward and so you'll watch

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that the temperature is is pretty

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minimal and it's gonna climb really fast

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and you'll see this particle get

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white-hot and melt as it exceeds 2,000

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degrees Kelvin and warms up and then if

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you watch you can see the cross-section

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shrinks and we end up at the end of our

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simulation with a particle that looks

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like this

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so you're seeing this essentially

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running in real time here where we have

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15 or so seconds from start to finish

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and the particle is only molten during

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these orange portions of the curve here

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so just a few seconds or you go from

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your iron particle and you very rapidly

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melt and you produce something that

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looks like this as you oxidize the

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exterior so we did this for the modern

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atmosphere and we essentially recorded

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this cross-section right here for each

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of our particles and then we took that

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and we bend it horizontal axis here

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based on that you know the size of that

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iron bead in the center right the

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fraction of the area of the

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cross-section particle that was this

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metallic iron and when you do that from

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the modern earth we generated a

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histogram in blue right here so 500

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randomly generated particles and we see

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that if we took the average cross

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sectional area we get around 2.3 which

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was that iron bead in terms of area and

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then we can compare that to modern

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micrometeorite data which was shown in

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orange collected from Antarctica and

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you'll see that we do a pretty good job

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of reproducing the distribution and the

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mean of our model and the data mean vary

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quite nicely so the next step in was

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right to apply this to the Archean and

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so that's what you see here and this is

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the main result from our model or once

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again you're looking at that fractional

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area of the micrometeorite and this

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black curve here is showing you right

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that fe fractional area means so you

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know up here low co2 which is the

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increasing on the horizontal axis so as

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you increase co2 concentrations you see

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this monotonic decrease in that fe

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fractional area that you would expect to

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find and collected micrometeorites and

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this is pretty exciting right because

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you could go out and find a couple of

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micro meteorites you section them and

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you measure that Fe

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area in your section micrometeorites you

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find the mean and then you basically

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just come over to the line you can drop

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down and back out the co2 concentration

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of the atmosphere at that time and

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that's exactly what we did with the 2.7

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billion year old micrometeorites which

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is shown by this orange dot here you

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come down right we just went over and so

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you find you know it's about 64% co2 the

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arrow bars are quite bad on this

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measurement and I'll get to that in a

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moment

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but there's one of the things that I

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want to point out here which is this -

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blue curve along the bottom here that

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starts to come off zero at around 70%

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and that line is showing you the

276
00:09:38,210 --> 00:09:36,690
fraction of my communities that were

277
00:09:40,100 --> 00:09:38,220
fully oxidized so if you think back to

278
00:09:42,110 --> 00:09:40,110
the simulation where we had that iron

279
00:09:44,510 --> 00:09:42,120
bead in the centre shrinking well if you

280
00:09:45,740 --> 00:09:44,520
encounter enough oxidant then you can

281
00:09:47,389 --> 00:09:45,750
actually oxidize the micrometer right

282
00:09:49,340 --> 00:09:47,399
all the way through and in our model

283
00:09:51,410 --> 00:09:49,350
that happens the red around 70 percent

284
00:09:55,160 --> 00:09:51,420
higher in terms of co2 you start seeing

285
00:09:56,840 --> 00:09:55,170
these fully oxidized particles so what

286
00:10:00,110 --> 00:09:56,850
does that tell us about co2 in the

287
00:10:01,850 --> 00:10:00,120
Archaean well the reason that the error

288
00:10:04,010 --> 00:10:01,860
bars are so bad on that that orange dot

289
00:10:06,380 --> 00:10:04,020
that you see here is that we only have

290
00:10:08,120 --> 00:10:06,390
two measurements from the Archaean data

291
00:10:09,470 --> 00:10:08,130
to constrain that point so we have

292
00:10:10,760 --> 00:10:09,480
really big uncertainties on that but

293
00:10:12,040 --> 00:10:10,770
this is something that could be easily

294
00:10:15,079 --> 00:10:12,050
addressed by collecting additional

295
00:10:17,329 --> 00:10:15,089
micrometeorites unfortunately there were

296
00:10:20,600 --> 00:10:17,339
fully oxidized micrometeorites in the

297
00:10:22,460 --> 00:10:20,610
2.7 billion year old samples and so we

298
00:10:24,440 --> 00:10:22,470
can get a tentative lower bound on co2

299
00:10:27,560 --> 00:10:24,450
in the Archaean of about 70 percent of

300
00:10:28,940 --> 00:10:27,570
the atmosphere by volume based on the

301
00:10:35,600 --> 00:10:28,950
presence of those fully oxidized

302
00:10:36,890 --> 00:10:35,610
micrometeorites and I put some some nice

303
00:10:38,840 --> 00:10:36,900
and OGIS here to say that one is not

304
00:10:43,070 --> 00:10:38,850
great I'm gonna get the green smiley

305
00:10:45,440 --> 00:10:43,080
that one is pretty cool right so these

306
00:10:47,090 --> 00:10:45,450
improved co2 constraints are really

307
00:10:48,740 --> 00:10:47,100
important for the Archaean earth we can

308
00:10:51,110 --> 00:10:48,750
not only potentially address the

309
00:10:53,690 --> 00:10:51,120
temperature question for the Archaean so

310
00:10:56,810 --> 00:10:53,700
if we took our sort of 70% atmospheric

311
00:10:58,819 --> 00:10:56,820
co2 estimate and if we assume a paleo

312
00:11:00,980 --> 00:10:58,829
pressure of about 0.5 bar and the

313
00:11:02,840 --> 00:11:00,990
Archaean then with a very simple climate

314
00:11:05,000 --> 00:11:02,850
model we can estimate about 30 degrees

315
00:11:08,360 --> 00:11:05,010
Celsius for the surface temperature as a

316
00:11:09,740 --> 00:11:08,370
potential upper limit there but these

317
00:11:11,949 --> 00:11:09,750
improved constraints can also help

318
00:11:14,240 --> 00:11:11,959
inform our understanding of exoplanets

319
00:11:16,940 --> 00:11:14,250
if you think of the earth as an analogue

320
00:11:18,830 --> 00:11:16,950
sort of a different habitable world and

321
00:11:20,750 --> 00:11:18,840
in addition I think this is when life

322
00:11:23,000 --> 00:11:20,760
the evidence for life first appears on

323
00:11:24,470 --> 00:11:23,010
earth is during this time and the co2

324
00:11:25,790 --> 00:11:24,480
concentration of the atmosphere plays a

325
00:11:28,070 --> 00:11:25,800
really important role in controlling

326
00:11:30,740 --> 00:11:28,080
surface temperature and the pH of the

327
00:11:32,950 --> 00:11:30,750
ocean which we really like to know for

328
00:11:36,620 --> 00:11:32,960
its period of Earth's history

329
00:11:37,820 --> 00:11:36,630
so from this model right we see that we

330
00:11:40,040 --> 00:11:37,830
could potentially get much better

331
00:11:41,990 --> 00:11:40,050
constraints on our ki and co2 if we go

332
00:11:43,880 --> 00:11:42,000
and collect more micrometeorites and in

333
00:11:45,860 --> 00:11:43,890
addition this same technique could

334
00:11:48,200 --> 00:11:45,870
actually be applied potentially for the

335
00:11:50,270 --> 00:11:48,210
Proterozoic where co2 may remain the

336
00:11:51,980 --> 00:11:50,280
main oxidant at altitude and so these

337
00:11:54,050 --> 00:11:51,990
micrometeorite oxidations could still be

338
00:11:55,820 --> 00:11:54,060
sensitive to total co2 concentration in

339
00:11:58,370 --> 00:11:55,830
the atmosphere and be able to provide

340
00:12:00,290 --> 00:11:58,380
insight there but for all this work we

341
00:12:01,940 --> 00:12:00,300
need more micrometeorites so we need

342
00:12:08,710 --> 00:12:01,950
people to go collect them and measure

343
00:12:15,920 --> 00:12:13,730
Thank You Owen we'll take questions yeah

344
00:12:18,290 --> 00:12:15,930
I think this is the currying at about

345
00:12:21,050 --> 00:12:18,300
100 kilometers yes so these are melting

346
00:12:22,250 --> 00:12:21,060
between about 70 and 90 in the modern

347
00:12:24,800 --> 00:12:22,260
Earth's atmosphere now I've been told

348
00:12:26,270 --> 00:12:24,810
that under the troposphere you have

349
00:12:28,160 --> 00:12:26,280
mixing of the gases but as you get

350
00:12:30,800 --> 00:12:28,170
further up if you get some segregation

351
00:12:31,820 --> 00:12:30,810
density and co2 is rather heavy so I

352
00:12:33,320 --> 00:12:31,830
would have thought that there was some

353
00:12:34,970 --> 00:12:33,330
type of gradient in the co2

354
00:12:37,910 --> 00:12:34,980
concentrations as a function of height

355
00:12:40,940 --> 00:12:37,920
when you get to those altitudes the

356
00:12:43,460 --> 00:12:40,950
other problem I have is you said half a

357
00:12:45,290 --> 00:12:43,470
bar presumably there that's quite a

358
00:12:47,210 --> 00:12:45,300
there's an uncertainty on that it could

359
00:12:48,650 --> 00:12:47,220
be I don't know two bar of ten bar I'm

360
00:12:50,450 --> 00:12:48,660
not sure how large it is I'll have to

361
00:12:53,270 --> 00:12:50,460
ask somebody else but that also

362
00:12:55,700 --> 00:12:53,280
increases the air bars I suppose on your

363
00:12:57,620 --> 00:12:55,710
death yes so both those great points so

364
00:12:59,329 --> 00:12:57,630
to address the question of mixing first

365
00:13:01,010 --> 00:12:59,339
and so on the modern earth right even

366
00:13:02,900 --> 00:13:01,020
though the purpose was the troposphere

367
00:13:05,180 --> 00:13:02,910
is where you have turbulent mixing oh

368
00:13:07,130 --> 00:13:05,190
sorry you have connected mixing so it's

369
00:13:08,690 --> 00:13:07,140
very rapidly mixed you still have mixing

370
00:13:09,920 --> 00:13:08,700
up to about a hundred kilometers which

371
00:13:11,600 --> 00:13:09,930
is the homo Paz region and the

372
00:13:13,340 --> 00:13:11,610
atmosphere remains well mixed below that

373
00:13:15,440 --> 00:13:13,350
due to turbulent mixing of breaking

374
00:13:17,510 --> 00:13:15,450
gravity waves so you get that from

375
00:13:19,190 --> 00:13:17,520
airflow versus a topography or from jets

376
00:13:21,290 --> 00:13:19,200
in the atmosphere and that would occur

377
00:13:22,430 --> 00:13:21,300
throughout Earth's history so even in

378
00:13:24,199 --> 00:13:22,440
the our team we would expect the same

379
00:13:27,400 --> 00:13:24,209
thing so these are melting at a pressure

380
00:13:28,749 --> 00:13:27,410
below that well mixed height so it

381
00:13:30,100 --> 00:13:28,759
the atmosphere should be well mixed up

382
00:13:31,990 --> 00:13:30,110
to the altitude where these are melting

383
00:13:34,720 --> 00:13:32,000
regardless of how much co2 is in the

384
00:13:36,790 --> 00:13:34,730
atmosphere and then to address the

385
00:13:38,379 --> 00:13:36,800
second point of the total pressure of

386
00:13:40,360 --> 00:13:38,389
the atmosphere that is a great point to

387
00:13:41,980 --> 00:13:40,370
bring up and so this technique is not

388
00:13:43,150 --> 00:13:41,990
sensitive to the total pressure of the

389
00:13:44,679 --> 00:13:43,160
atmosphere because it's it's only

390
00:13:47,499 --> 00:13:44,689
sampling the upper atmosphere

391
00:13:50,079 --> 00:13:47,509
essentially and so if you had you know

392
00:13:51,490 --> 00:13:50,089
1/2 bar atmosphere then the altitude the

393
00:13:53,699 --> 00:13:51,500
altitude at which the micrometeoroids

394
00:13:56,319 --> 00:13:53,709
melt would shift closer to the surface

395
00:13:57,850 --> 00:13:56,329
but the overall reaction that's

396
00:13:59,019 --> 00:13:57,860
occurring would be identical and the

397
00:14:00,369 --> 00:13:59,029
same is true if you're putting on a 10

398
00:14:01,900 --> 00:14:00,379
bar atmosphere then they would be

399
00:14:04,509 --> 00:14:01,910
melting at a higher altitude with the

400
00:14:06,100 --> 00:14:04,519
same pressures and so you know we can't

401
00:14:07,449 --> 00:14:06,110
constrain the total surface pressure so

402
00:14:09,249 --> 00:14:07,459
you need an alternative method to

403
00:14:10,869 --> 00:14:09,259
constrain total pressure and then this

404
00:14:12,490 --> 00:14:10,879
can give you the fraction of the

405
00:14:15,579 --> 00:14:12,500
atmosphere that's co2 I would have

406
00:14:19,480 --> 00:14:15,589
thought that when you change the ball

407
00:14:23,889 --> 00:14:19,490
pressure you it's true that you have you

408
00:14:25,990 --> 00:14:23,899
guess oxidized you could oxidize a

409
00:14:27,160 --> 00:14:26,000
larger when you're higher it could be a

410
00:14:28,240 --> 00:14:27,170
longer time for example you should have

411
00:14:30,220 --> 00:14:28,250
certain periods that it's only molten

412
00:14:32,650 --> 00:14:30,230
for about 2 or 3 seconds doesn't that

413
00:14:34,240 --> 00:14:32,660
number depend on how high you are which

414
00:14:36,040 --> 00:14:34,250
would then depend on the pressure and so

415
00:14:37,329 --> 00:14:36,050
just described yes so we actually we did

416
00:14:38,860 --> 00:14:37,339
test in the mall and it's a very minor

417
00:14:41,170 --> 00:14:38,870
effect really the only difference is for

418
00:14:42,309 --> 00:14:41,180
the gravity term with scaling so you

419
00:14:44,139 --> 00:14:42,319
know off the top of my head I don't know

420
00:14:46,030 --> 00:14:44,149
it's actually where the corresponding

421
00:14:47,800 --> 00:14:46,040
level 10 bar atmosphere would be but

422
00:14:50,319 --> 00:14:47,810
presumably it's not a huge difference in

423
00:14:51,639 --> 00:14:50,329
total altitude and so we didn't look at

424
00:14:53,710 --> 00:14:51,649
a 10 bar we looked at actually just

425
00:14:54,670 --> 00:14:53,720
decreasing pressure on that effect but

426
00:15:01,929 --> 00:14:54,680
it's definitely something that could be

427
00:15:03,879 --> 00:15:01,939
considered so rather naive question in

428
00:15:06,160 --> 00:15:03,889
your physical samples the iron core

429
00:15:08,379 --> 00:15:06,170
that's left over seems to be shifted in

430
00:15:09,879 --> 00:15:08,389
one direction do you think that there's

431
00:15:13,329 --> 00:15:09,889
a possibility that some of these could

432
00:15:15,129 --> 00:15:13,339
just fully segregate the remaining solid

433
00:15:16,629 --> 00:15:15,139
core from the molten portion and that's

434
00:15:19,210 --> 00:15:16,639
actually what you're seeing in some of

435
00:15:20,740 --> 00:15:19,220
these fully oxidized samples yes so that

436
00:15:22,329 --> 00:15:20,750
is definitely a possibility that the

437
00:15:24,939 --> 00:15:22,339
sort of more dense iron bead could

438
00:15:27,579 --> 00:15:24,949
separate from the mic from meteorite and

439
00:15:29,199 --> 00:15:27,589
so again Jackie has a number of papers

440
00:15:31,150 --> 00:15:29,209
on that where he speculates that that

441
00:15:32,769 --> 00:15:31,160
isn't likely so they don't think that

442
00:15:33,970 --> 00:15:32,779
that's occurred because you don't see

443
00:15:35,049 --> 00:15:33,980
that often when you're looking at Mike

444
00:15:37,150 --> 00:15:35,059
from meteorite so you don't see ones

445
00:15:38,199 --> 00:15:37,160
that are sort of partially separated and

446
00:15:40,090 --> 00:15:38,209
you would expect me when it separates

447
00:15:40,840 --> 00:15:40,100
that the central bead would maybe split

448
00:15:43,809 --> 00:15:40,850
and so you don't know

449
00:15:45,129 --> 00:15:43,819
with other other features I'm not super

450
00:15:47,439 --> 00:15:45,139
confident on that but I do believe that

451
00:15:49,449 --> 00:15:47,449
that has been somewhat addressed but it

452
00:15:51,189 --> 00:15:49,459
is definitely a concern that you could

453
00:15:53,019 --> 00:15:51,199
have you out of the beat separate so

454
00:15:54,280 --> 00:15:53,029
actually that's why in our model we

455
00:15:55,689 --> 00:15:54,290
don't consider fully oxidized

456
00:15:57,519 --> 00:15:55,699
micrometers we're only looking at ones

457
00:15:58,480 --> 00:15:57,529
that still have the metallic bead so

458
00:16:00,340 --> 00:15:58,490
that way we can be certain that we're

459
00:16:02,170 --> 00:16:00,350
looking at well as certain as we can be

460
00:16:03,879 --> 00:16:02,180
right that we're looking at partially