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yeah so i'm josh from the University of

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Washington and I'm going to be

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presenting some work I've done with my

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advisor David catling along with Roger

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Buick on the rise of oxygen and to

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motivate this I've put up Earth's

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reflectance spectrum not because this is

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what I do but because I think it

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illustrates quite nicely why thinking

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about the rise of oxygen is important so

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when you look at our spectrum you see

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really obvious other one absorption

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features from abundant biogenic

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atmospheric oxygen and you're

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potentially we could image terrestrial

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exoplanets and and see the same thing

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now on earth over the last few decades

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geologists and geochemists have put

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together this really good descriptive

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picture of the history of atmospheric

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oxygen and this is a different version

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of the figure that that Kevin showed and

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also is kind of a picture version of

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Briana's fame lab talk last night so we

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know that for the first couple of

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billion years of Earth history over here

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there was very little atmospheric oxygen

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then around sort of 2.4 2.5 it shot up

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to maybe one ten percent of modern

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levels were remained until a few hundred

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million years ago right increased again

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to genere modern levels now although

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this descriptive pizza is fairly well

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established the causes of the centuries

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are not particularly well understood and

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so I think if we want to go looking for

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oxygen biosignatures and exoplanets it's

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helpful to first to understand this we

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don't know for instance you know not

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only what caused this but as this is

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this kind of increase a general kind of

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behavior we would expect to see in all

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terrestrial planets with properties x y

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and z what determines the timing of this

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increase we don't know the answers to

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these questions so yeah i'm sure before

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we look for and interpret this i think

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it's helpful to understand this so so

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how might we do that how might we think

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about the cause of the rise of oxygen

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it's useful to kind of

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formalize our ignorance on the matter as

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it were by saying that the the time rate

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of change of the amount of oxygen in the

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atmosphere is equal to you know the

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source fluxes minus the sink fluxes and

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so obviously in this picture there are

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only two possible classes of

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explanations for the rise of oxygen

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either the source fluxes have to have

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increased at some point or the sink

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fluxes had to have decreased at some

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point to cause the imbalance that caused

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it to shoot up now in this talk I'm

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going to be focusing on whether or not

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an increase in the source flux can

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explain the rise of oxygen or more

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specifically where the changes in

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organic burial which I'll explain

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shortly can explain the rise of oxygen

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so and I should add that this fits into

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the isotope session because you get at

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organic burial through looking at carbon

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isotopes so how does this work well you

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will know how oxygen is made right

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plants cyanobacteria they take co2 and

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water produce reduced organic carbon and

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then give off oxygen however generally

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what happens is that when this organic

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carbon is respired or the organism dies

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and decomposes it gets reoxidized back

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to co2 and water and so photosynthesis

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does not increase atmospheric oxygen at

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all unless you bury this organic carbon

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in sediments permanently separating it

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from the surface thereby leaving behind

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one mole of oxygen for every mole of

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organic carbon you bury and so in this

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way the source flux of oxygen into the

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atmosphere which is what we're

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interested in for the rise of oxygen is

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exactly equal to the burial rate of

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organic carbon so to get at the oxygen

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source flux we need to understand the

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carbon cycle this is a really simple

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representation of the geological carbon

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cycle they've got carbon so these are is

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a fluxes of carbon and moles per year or

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whatever

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we've got some flux of carbon into the

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atmosphere ocean system co2 outcasts

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from volcanoes and then we've got carbon

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leaving the surface to the burial of

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organic carbon ie vista the stuff that's

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important for oxygen but also leaving as

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carbonates now life as we've heard

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fraction eats carbon during

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photosynthesis so each of these fluxes

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has a distinct isotopic signature and so

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i'm using this this Delta 13 c notation

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which is thankfully already been defined

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for me but if you don't remember that

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the definition you can just think of it

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as the ratio of rare heavy carbon 13

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atoms to abundant carbon-12 at and

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scaled in a totally counterintuitive way

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so what we can then say given these

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fluxes we can say over long time scales

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what goes in must equal what goes out so

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the input fluxes must balance the output

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fluxes and similarly the isotopic

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composition of what goes in must balance

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the isotopic composition and what goes

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out and I won't go through the math but

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it's it's really simple it's basically

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just two equations two unknowns and you

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get out this expression here which is

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which is really important so this tells

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you that if org which is defined as

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fractional organic barrel the amount of

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carbon that is buried as organic divided

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by the total carbon burial you can show

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using this kind of mass balance model

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that is equal to a function of purely

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the isotopes and so what this tells you

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then is that if you can go and measure

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the isotopic ratios of these various

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sorts and sync fluxes over time then

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that'll give you fractional organic

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burial over time which as I just

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explained gives you the oxygen source

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flux over time so isotopes give your

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organic burial gives your oxygen source

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slugs so people have been trying to do

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this for decades and we think that the

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isotopic composition of what goes in has

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been pretty much fixed so to figure out

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this thing you only have to know the

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isotopic

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position of carbonate and organic carbon

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and so here's the the data this is a

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compilation I put together from various

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other compilations the carbonates are in

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red organics in blue this is over Earth

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history and this is that strange Delta

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13c measurement of the isotopic ratios

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most authors who have looked at this

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this kind of data have basically said

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well it looks like they've both been

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pretty much constant over Earth history

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and so therefore everything on the right

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hand side of this equation is a constant

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as a function of time there for organic

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burial is a constant as a function of

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time therefore the oxygen source flux is

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a constant as a function of time

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therefore you cannot explain the rise of

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oxygen by an increase in this it must

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have been a decrease in some of the sink

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fluxes that caused the rise of oxygen on

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earth that's the most popular

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interpretation but there have been a few

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papers that have tried to draw the lines

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through this data and they've done this

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for one reason or another either

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invoking other kind of carbon cycle

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processes or biases and preservation but

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even without those complications there's

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a fair amount of spread in this data set

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and so it's not obvious the extent to

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which the isotopes really constrain

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organic burial and so what I've been

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trying to do is a fairly rigorous

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statistical analysis of this data set to

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try and determine exactly how well if

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org is as constrained and so I won't go

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through all the details other than to

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say I took a sort of shotgun approach to

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statistics just applying a whole bunch

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of different methodologies hoping to

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derive robust conclusions that don't

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depend too much on the specific

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methodology so here's one one of those

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techniques our regression method similar

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to what you might use in climate science

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say to determine to what extent is there

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a significant increase in global

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temperature over some time spent given

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the tendency of time series data to kind

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of correlate with itself so you fit this

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sort of model to the

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you fit a trend to the carbonates you

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fit a trend to the organics you generate

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uncertainties in a kind of Monte Carlo

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way you plug that in and you get out a

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change in organic burial fraction

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organic burial over Earth history with

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uncertainties and so if you do that you

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get something looks like this this is a

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probability distribution for the change

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in F or fractional organic burial the

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percentage changing it over the rock

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record and so what you see is that you

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know with ninety-five percent confidence

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the change is anywhere from very little

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all the way up to a hundred percent

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increase of doubling this is a very

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broad confidence interval 0 to 100

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percent you can kind of do the same

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thing with with different methodologies

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so rather than impose a regression you

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just take the the organic data in this

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case the crosses you smooth it out with

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some algorithm that gives you

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uncertainties do the same for the

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carbonates smooth it out get

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uncertainties plug goes into your

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equation get out if all over time this

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is fraction organic burial over Earth

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history with uncertainties and you

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basically had a similar kind of answer

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there are subtle differences depending

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on how you do the methodology but you

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get a similar look and distribution so

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what does this mean then for the rise of

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oxygen well if you take the lower end of

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our confidence interval then you can

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show using fairly simple flux arguments

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that whatever you do this isn't enough

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to explain the rise of oxygen at a

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twenty percent increase in organic

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burial will not cause a rise of oxygen

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you need a decrease in some of the sink

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flux flux is but if you take the upper

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end of our confidence interval one

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hundred percent increase in f org over

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Earth history then then yeah if you make

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certain assumptions about the absolute

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fluxes that are plausible then you can

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get out a rise of oxygen so basically

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what this analysis shows is that given

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the uncertainty in the change in organic

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burial over of history we simply cannot

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say whether the rise of oxygen was

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caused by increased source flux so

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that's the basic conclusion I was going

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to talk about

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more complex carbon cycle model but I

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think I'll just skip out of that because

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yeah so that's the basic conclusion the

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isotopes don't constrain the source flux

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of oxygen and so to to make progress

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here we somehow need better better

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isotope data the better represents the

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global average with less spread yes I'll

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just open up for questions and if you

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want to know more also this should

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appear in the American Journal of

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science some time for the end of the

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nas which is a question but I think some

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support to your argument I'm surprised

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you didn't mention the sharp rise in the

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Delta 13 C values around the time of the

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great oxidation event since it's

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believed that that rise in the carbonate

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the carbonate carbon isotope values is

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related to an increase in the burial of

278
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organic carbon and it's coincident with

279
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the great oxidation event yeah so where

280
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are we now so this is what you're

281
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talking about right this ad the

282
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amalgamation of all the data yeah yeah

283
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yeah yeahs 2.3 there's definitely an

284
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increase in the isotopes of carbon which

285
00:12:51,550 --> 00:12:50,060
which could be attributed to well the

286
00:12:52,690 --> 00:12:51,560
increased organic burial could be part

287
00:12:54,160 --> 00:12:52,700
of the explanation for the rise of

288
00:12:56,140 --> 00:12:54,170
oxygen but could also be a consequence

289
00:12:58,329 --> 00:12:56,150
of the Roses of oxygen right answer this

290
00:13:01,480 --> 00:12:58,339
question about the timing and I think it

291
00:13:03,370 --> 00:13:01,490
seems to be coming after the rise of

292
00:13:05,320 --> 00:13:03,380
oxygen as determined by the self enough

293
00:13:07,900 --> 00:13:05,330
signal so i don't know if that is really

294
00:13:11,769 --> 00:13:07,910
supports the idea that enhanced organic

295
00:13:14,350 --> 00:13:11,779
burial cause the rise of oxygen the

296
00:13:17,290 --> 00:13:14,360
great oxidation event is well-timed but

297
00:13:24,530 --> 00:13:17,300
the carbon isotope excursion is not well