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you

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[Music]

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so those nice introduction to a lot of

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the concepts that'll come up in this

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talk and we're kind of getting at the

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same issue but maybe from the opposite

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end so likewise the question that

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motivates this and lot of the stuff I

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like to think about research Rises and

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why are we here on our planet I mean not

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just those humans but us as you carry

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out these complex life forms that

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metabolize aerobic Li and so on and so

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forth am I aren't we just a planet full

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of single cellular you know bacteria and

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archaea like we were for maybe eight

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nine verse history so I'm going to

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approach this question sort of from a

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environmental standpoint its to say what

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are the environmental parameters that

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enable eukaryotes to persist on the

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surface of the earth today and then

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we'll look back and say okay what's the

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distribution of these your space and

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time in Earth's distant past so there's

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a variety of things you could look at as

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these parameters that we need to

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constrain we talked a lot about free

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oxygen for aerobic respiration but just

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like the last talk we were hearing a

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source of fixed nitrogen is really

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important for eukaryotes since they

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can't fix their own and so that's what

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I'll really be getting into in detail

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here just to give a little more

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background for those that weren't in the

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last talk all life as we know it at

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least needs nitrogen and a lot of it

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it's the most abundant sorry Adam

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after CH no so the bulk of your

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carbohydrates and so if it's scarce it

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becomes the rate limiting reactant sort

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of in the reaction that is life on the

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surface of the earth the thing is it's

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not scarce there's tons of nitrogen at

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the surface of the earth and we're

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always bathing in atmospheric n2 gas but

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this is pretty much inaccessible to you

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and I most organisms actually on the

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face of the earth

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since it's very energetically costly to

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break that triple and N bond so that

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means that all eukaryotes rely on a

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source of nitrogen that's fixed by other

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organisms and that's only done by a few

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clades of prokaryotes so just to put

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that simply these prokaryotes apply the

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nitrogen for the whole biosphere and if

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you want to look at this then in terms

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of a cycle what we can see then is if

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you start with nitrogen the atmosphere

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first gets fixed into biomass by

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prokaryotes and only when they die in

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their biomass ckets respired does this

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nitrogen then get released into the

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environment as the ammonium ion this can

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be assimilated into biological tissues

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and so if you were to see this is how

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the nation cycle would operate on a more

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or less anaerobic world where this

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ammonium is the dominant ion in the

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water column but as soon as you

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introduce a little bit of oxygen

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actually then that changes nitrate

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instead is the more stable ion and so

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nitrification will occur sorry again

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where ammonium then gets nitrified into

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nitrate and this is the form of nitrogen

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that eukaryotes really like to

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assimilate today and that they rely on

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for their nitrogen needs so that's a

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similar Tauri metabolism you can also in

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locally subbox akin vironment s-- you

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get dissimilatory reduction of nitrate

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which is denitrification this pathway

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here and that closes the loop on the

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aerobic nitrogen cycle so the question

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we really want to ask them looking back

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through time is when did aerobic

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nitrogen cycling occur through space and

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time in the oceans through first past

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since this is presumed to be critical

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for the distribution of eukaryotes today

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and the way we do that is by using

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nitrogen isotopes or at least a large

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part of the work is done using nitrogen

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isotopes because each of these pathways

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has a characteristic fractionation which

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have been studied quite a bit and just

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to sum up lots of work very quickly the

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processes of nitrogen fixation

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remineralization and nitrification in

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the modern environment at least in part

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relatively small isotopic fractionation

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when compared with this denitrification

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flux which can exert really large

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kinetic isotopic fractionation so it

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removes lighter nitrogen to the

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atmosphere which carries a negative

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Delta 59 value meaning what's left will

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have to be positive by mass balance so

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just again as a guiding principle we

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sort of think that if you were to scoop

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up biomass that's in mud being deposited

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in the bottom of the ocean and it falls

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between maybe minus 2 plus 2 per mil

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relative to 0 which is the atmospheric

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baseline that would be indicative of a

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fixation dominated ecosystem or maybe an

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anaerobic ecosystem whereas

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in order to get these more elevated

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values at least in general it's often

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because of aerobic nitrogen cycle so

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like I said that's been used as a

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guiding principle to answer this

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question and people have done a lot of

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work trying to look at that in ancient

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sedimentary rock so I'm plotting this

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data here everything that's been

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published from bulk marine sedimentary

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rocks through geologic time with this

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grave and then being what we would

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consider our anaerobic nitrogen fixation

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window and so if you look on the right

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side of the screen and the Archaean the

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limited data we do have seem to plot

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close to this atmospheric value and then

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the first significant enrichment you see

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in Delta 59 comes right around what we

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would independently constrained to be

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the great oxidation event in the

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earliest paleo Proterozoic so that makes

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sense

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qualitatively but the first thing we

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wanted to do in this study is fill this

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fairly large temporal gap in this record

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in the immediate aftermath of the goe

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for a few hundred million years

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especially considering this has recently

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been proposed as a time where oxygen may

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have overshot equilibrium values and

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been fairly high on a few hundred

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million year timescale and this could

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have been a critical time for eukaryotic

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evolution so we went ahead and did that

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thanks to our collaborator Andre Becker

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we were able to get samples spanning a

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large geographic extent so for multiple

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continents and filling in this temporal

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gap and just to quickly flash the data

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up there into this old previous plot you

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can see that they are less corroborate

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the story based on the pre-existing data

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which is that from the goe onwards for a

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few hundred million years you have

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values persistently above this fixation

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window or in a qualitative sense you

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have a signal of aerobic nitrogen

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cycling so if we were to keep the just

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sort of the canonical view of this we

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could just stop there and say in a

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qualitative sense that this might be

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indicative of greater nitrate

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availability and that could have played

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a role in the evolution of eukaryotes

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but the thing I want to start to get

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into here and just sort of looking

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forward is asking this question can we

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get more quantitative information from

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this record so maybe starting humbly

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with this we're going to use a very

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simple ice

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to a box model view of this which is

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going to be the same as the nitrogen

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cycle as I outlined at the beginning so

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just walk through it we trace nitrogen

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from the atmosphere that gets fixed in

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the ocean by nitrogen fixers when they

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die a very small percentage of that gets

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buried in marine sediments but most

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actually gets remineralized

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and then if there's any oxygen in the

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ocean nitrified into nitrate and so that

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becomes your surface ocean nitrate pool

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and there's other ways on the modern

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earth to get nitrate into the surface

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ocean you can get it delivered by rivers

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and through atmospheric deposition we

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could modulate these in the Precambrian

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if we think that they were less

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effective but the truth is they're

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really minor fluxes compared to the

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fixation flux so then from your surface

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ocean nitrate pool you have two possible

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outputs either the burial again which is

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a very small percentage of total or

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denitrification which more or less

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balances nitrogen fixation so the thing

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we need to focus on here is that as you

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can see there's two types or two places

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of denitrification that occurs in the

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modern ocean so there's a greatly

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self-explanatory water column

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denitrification which occurs in the

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water column or sedimentary which occurs

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in the poor waters of Suboxone sediments

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the reason we have to distinguish

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between the two is that they actually

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have really different isotopic effects

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so I mentioned before there's a large

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kinetic isotope effect in the water

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column the thing with sediments is that

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they're thought to be diffusion limited

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systems and so you could imagine if you

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bring nitrate into a pore that's

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thermodynamically favorable for nitrate

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reduction you'd exhaust it all reacted

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completely and you get no corresponding

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isotope effect and so you can use either

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a top down isotope mass balance approach

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or a flux estimate approach to actually

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try to pin down how much each of these

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fluxes contributes to the total

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denitrification flux in the modern ocean

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and best estimates are somewhere on the

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order of a one to three ratio of water

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column to sedimentary denitrification so

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if you take that sort of box back to the

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envelope calculation and then just do

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the isotope mass balance it correspond

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to a surface ocean nitrate isotopic

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composition or something like plus five

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or six per mil so this is all

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been calibrated with the modern ocean so

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what we really got thinking then is okay

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the modern ocean is quite a bit

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different from the Precambrian if you

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consider that water combi nitrification

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is 1/4 of the total output despite the

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fact that it has to occur in oxygen

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minimum zone which are a negligible

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fraction of the ocean volume today that

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is really a testimony to the rapid

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kinetics or the faster rate of

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denitrification in the water column as

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opposed to in these sedimentary poor

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waters which are much vaster volume but

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not correspondingly vast reflux and so

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we thought that is well what if you

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cranked the ocean to Precambrian levels

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of maybe approaching 100% anoxia then

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presumably this number will start to

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reach 100% and that has implications yes

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for isotope mass balance but maybe also

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for the rate of nitrogen loss so just to

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flip through the things that model is

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showing us when we did that this is the

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total outputs from that surface ocean

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nitrate box so it's burial sedimentary

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and water column denitrification burial

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again it's really small less than 1% of

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the total output in the modern ocean if

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you just tie it to 25 and 75 balance

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just based on the literature then if you

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crank it towards 100% anoxia you'll see

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that water combi nitrification does

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approach 100 percent of the outputs and

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correspondingly then with this we just

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keep the same x-axis here the total rate

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of nitrogen removal this is our just

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scaled to modern is 1 will also increase

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just due to the greater rate of water

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column denitrification so if we keep

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this as a simple model we're not

285
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changing anything else you increase this

286
00:10:47,920 --> 00:10:46,010
output flux and basically you're not

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bearing as much biomass that's from

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00:10:50,830 --> 00:10:49,760
nitrate assimilating organisms or

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00:10:53,080 --> 00:10:50,840
another way to think of it is there's

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not enough nitrate left around to fuel

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those organisms and so you can see this

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born out then and the outputs for bulk

293
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sedimentary delta 15 n values where if

294
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you tie it to the modern value of about

295
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plus 5 if you were to crank the ocean a

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little more anoxic you get faster

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nitrate reduction in the water column a

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00:11:14,020 --> 00:11:12,290
little more isotopic enrichment but

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there comes a tipping point where you

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00:11:17,400 --> 00:11:15,070
remove

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nitrates so effectively that you don't

302
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leave much behind for nitrate

303
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assimilating organisms or in other words

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if you're in this dominantly anoxic

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portion of the plot then you're

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dominated by nitrogen fixing organisms

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as most of your biomass and you could be

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competitively excluding eukaryotes that

309
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rely on the dissolved nitrate so finally

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what does this mean then for the

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evolution of complex life on earth what

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00:11:43,980 --> 00:11:42,160
was the trajectory we think it took so

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I've taken the data from that first data

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plot and just been them now by

315
00:11:50,850 --> 00:11:48,550
individual formation then put a trend

316
00:11:53,190 --> 00:11:50,860
through it it seems that this transition

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from nitrogen fixation to nitrate

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assimilation dominated ecosystems

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occurred around the goe and then it was

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sustained for a few hundred million

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00:12:04,610 --> 00:12:02,380
years immediately following that so the

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00:12:06,360 --> 00:12:04,620
question is was this a necessary and

323
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sufficient or necessary but not

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sufficient condition for the evolution

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of eukaryotes so the but it plotted here

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is the first compelling body fossil

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evidence for eukaryotic organisms which

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from the later paleo Proterozoic and if

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we were to take that to be the earliest

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occurrence then there would seem to be

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some temporal gap where either there's a

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biological or other environmental hurdle

333
00:12:29,340 --> 00:12:26,770
in the way of their evolution whereas

334
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perhaps with just the poor preservation

335
00:12:33,930 --> 00:12:31,060
of these sorts of fossils in deep time

336
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we're missing an earlier occurrence of

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00:12:36,389 --> 00:12:35,020
these and so distinguishing between

338
00:12:37,740 --> 00:12:36,399
those two I think is important to

339
00:12:39,900 --> 00:12:37,750
understand what are the factors that

340
00:12:42,240 --> 00:12:39,910
ultimately control the emergence of

341
00:12:43,949 --> 00:12:42,250
complex life and other planets and then

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lastly I'll close just by saying that

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the retraction of these aerobic

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environments in the mid Proterozoic

345
00:12:51,630 --> 00:12:49,540
could perhaps help explain why

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eukaryotes may have emerged earlier but

347
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did not take on their dominant

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ecological aspect until the later

349
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Precambrian when these conditions return

350
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and then have stayed for the last

351
00:13:09,190 --> 00:13:02,079
several hundred million years so with

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00:13:18,110 --> 00:13:12,320
thanks very much we have time for a

353
00:13:21,170 --> 00:13:18,120
question using scooch over hey Rick stop

354
00:13:22,670 --> 00:13:21,180
so one of the things that you encounter

355
00:13:24,530 --> 00:13:22,680
when you build a model like this you end

356
00:13:26,480 --> 00:13:24,540
up having to ramp the nitrogen cycle up

357
00:13:30,290 --> 00:13:26,490
to really high nitrogen fixation race

358
00:13:31,550 --> 00:13:30,300
something like ten times modern to

359
00:13:34,250 --> 00:13:31,560
support the very high denitrification

360
00:13:35,570 --> 00:13:34,260
loss as they go on have you looked

361
00:13:38,480 --> 00:13:35,580
you're thought much about that do you

362
00:13:40,070 --> 00:13:38,490
think a notion with 1,400 teragrams of

363
00:13:43,900 --> 00:13:40,080
your nitrogen fixation is totally

364
00:13:46,040 --> 00:13:43,910
reasonable no so we didn't try to

365
00:13:50,140 --> 00:13:46,050
correspondingly increase the amount of

366
00:13:52,190 --> 00:13:50,150
nitrogen fixation to allow a total

367
00:13:53,810 --> 00:13:52,200
nitrogen loss rate that is that much

368
00:13:56,900 --> 00:13:53,820
absolutely higher than today it's just

369
00:13:58,820 --> 00:13:56,910
those so if you basically in this simple

370
00:14:01,100 --> 00:13:58,830
run of it set nitrogen fixation at a

371
00:14:03,530 --> 00:14:01,110
constant we didn't tamper with that and

372
00:14:06,080 --> 00:14:03,540
so it's all relative to that baseline

373
00:14:08,090 --> 00:14:06,090
but if we had been planning on doing is

374
00:14:09,770 --> 00:14:08,100
ultimately running it with that time

375
00:14:11,390 --> 00:14:09,780
component so we just solved it in steady

376
00:14:13,910 --> 00:14:11,400
state or basically we've set a nitrogen

377
00:14:14,990 --> 00:14:13,920
fixation rate and it could be higher or

378
00:14:19,750 --> 00:14:15,000
lower than today

379
00:14:26,390 --> 00:14:24,470
so yes yeah in that scenario what we had

380
00:14:27,620 --> 00:14:26,400
been thinking would be the ultimate way

381
00:14:29,870 --> 00:14:27,630
to constrain it

382
00:14:31,430 --> 00:14:29,880
this is unable to tease out whether

383
00:14:32,750 --> 00:14:31,440
nitrogen versus foster should Alton

384
00:14:34,460 --> 00:14:32,760
Utley be the thing that's limiting

385
00:14:36,500 --> 00:14:34,470
productivity so in a sense we just tie a

386
00:14:39,200 --> 00:14:36,510
total productivity prescribed that based

387
00:14:44,030 --> 00:14:39,210
on phosphorus availability is - the main

388
00:14:45,269 --> 00:14:44,040
goal of it all simple thank you very

389
00:14:45,770 --> 00:14:45,279
much