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okay so I have apparently been

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classified as something resembling a

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geologist but I promise not to talk

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about any rocks or minerals or plate

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tectonics or anything related instead I

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will be talking about the environmental

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consequences of the evolution of oxygen

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acronis synthesis so of course oxygenic

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photosynthesis produces oxygen and

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oxygen is really important because it's

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a strict requirement of complex life

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turns out oxygen is also quite toxic and

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because life differs vastly and its

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ability to tolerate oxygen it's one of

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the fundamental controls on which

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organisms will be living in a particular

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environment oxygen is also important

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indirectly because it profoundly affects

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the cycling of important elements

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through the Earth's system so life is

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impacted by the presence or absence of

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oxygen far beyond its immediate

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environment so for those reasons the

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history of oxygen on earth is a critical

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component of the history of life on

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Earth and possibly elsewhere as well

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this is a somewhat schematic

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representation of the history of

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atmospheric oxygen on earth the details

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of this curve are certainly the topic of

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continued debate but it is widely

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accepted that both the atmosphere and

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the ocean were essentially an toxic for

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the first two billion years of Earth

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history the timing of this dramatic

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increase in atmospheric oxygen is also

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well constrained and it's typically

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blamed on the evolution of cyanobacteria

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that makes for an attractive story

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because oxygen aclaro synthesis is the

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only major source of oxygen on earth but

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the story is apparently more complicated

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than that because biomarkers and isotope

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roxys point to both oxygenic

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photosynthesis and aerobic metabolism up

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to 300 million years before the great

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oxidation event the most popular way to

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attempt to reconcile

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evidence for aerobic metabolism and

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global anoxia is to invoke oxygen

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owaisi's which are locally oxygenated

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environments within an otherwise an

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toxic ocean and beneath an anoxic

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atmosphere but without anything

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resembling a modern analogue it's not

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immediately obvious that this type of

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environment is even possible Jim casting

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was among the first to attempt to

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explore the possibility of oxygen oasis

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and in 1992 he presented this very

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simple box model in which cyanobacteria

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are actively pumping out oxygen in the

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surface ocean and then that oxygen is

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free to enter the atmosphere but upon

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doing so it's consumed immediately by

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reaction with an overwhelming volcanic

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supply of reduced gases such that the

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atmosphere remains anoxic but in this

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scenario the surface ocean is not

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necessarily a toxic because the exchange

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of gases between the surface ocean and

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the atmosphere is quite slow compared to

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biological processes so dramatic

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chemical disequilibrium can be

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established and maintained at steady

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state so for reasonable rates of oxygen

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production casting estimated that

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beneath a strictly anoxic atmosphere

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dissolved oxygen concentrations could

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actually approach 10% modern levels so

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that's encouraging for the oxygen

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owaisi's hypothesis but this simple

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model almost certainly overestimates the

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possible dissolved oxygen concentrations

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and that's because oxygen is not

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chemically inert in the surface ocean

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you know so it's not made and then it

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passively awaits its opportunity to

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enter the atmosphere and even if it were

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chemically inert the circulation of the

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ocean would act to continuously remove

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it from sites of concentrated production

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further reducing the possible

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concentrations so 20 years later I am

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the first to attempt with a 3d ocean

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model that these environments can

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actually exist so my work is motivated

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by three main questions now first are

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these

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vironment impossible if so what did they

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look like how big were they and how much

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oxygen did they contain and i use a

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earth system model of intermediate

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complexity called genie at its core it

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consists of a 3d frictional geostrophic

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ocean model coupled to a 2d energy and

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moisture balance model and in

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combination they simulate the physics of

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the ocean atmosphere system then within

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that framework Genia considers a number

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of biological processes to which I have

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added probable archaean metabolisms that

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are not particularly important today

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including methanogenesis and anoxygenic

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photo trophy Genie is probably most

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famous for its work in predicting Earth

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system evolution and response to

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continue to co2 emission scenarios so

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I'm actually the first person to apply

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it to the ER cute in my model like in

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the original casting model oxygen is

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exclusively supplied by oxygenic

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photosynthesis and surface ocean cells

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and then that oxygen is free to exchange

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with the atmosphere but unlike the

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casting model oxygen is also subject to

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both lateral and vertical transport and

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oxygen participates in a number of redox

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transformations during aerobic

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metabolism another minor difference is

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that my cyanobacteria are nutrient

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limited and nay must compete with an

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toxigenic phototrophs for those

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nutrients so that's another kind of

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indirect control on the oxygen mass

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balance so taking into account all of

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these processes and a number of other

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processes I haven't described this is

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the steady-state dissolved oxygen map

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that my model produces you'll notice

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that I have used a modern continental

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configuration that's because I don't

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know what the art can world look like

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I'm guessing that you don't either so

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dissolved oxygen concentrations range

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from 0 up to about 6 my

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moeller in regions are particularly high

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productivity and for comparison the

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modern ocean has about 250 micromolar

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dissolved oxygen on average so these are

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pretty low dissolved oxygen

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concentrations fish start dying at about

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60 micromolar nonetheless this is a

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pretty exciting result because these is

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alled oxygen concentrations are actually

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three orders of magnitude greater than

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the lowest dissolved oxygen

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concentrations at which aerobic growth

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has been documented for microbial life

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okay so the next slide i'm going to show

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you is a cross-section through my

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hypothetical ocean basin that strongly

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resembles the atlantic ocean on this

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figure you know the surface ocean is at

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the top sea floor at the bottom and then

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the poles are on either end and the

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point here is that the ocean is

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overwhelmingly anoxic zooming into just

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the surface ocean you can see that these

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environments are truly isolated owaisi's

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within another wise in toxic world i

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should note that although i've been

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referring that to them as a way seas and

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it might be tempting to think of them as

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safe havens for aerobic life and you

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know the evolution of multicellular life

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even these environments contain

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poisonous oxygen and ultimately would

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have been lethal for the vast majority

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of life on Earth at this time so a more

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fitting name might be like oxygen death

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traps or something similar don't know

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okay geologists love looking at maps and

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cross sections but in case you don't

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this is the same data in histogram form

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again you know most of the ocean is an

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anoxic and about fifteen percent of the

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surface ocean contains micromolar

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there is of course some uncertainty that

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is inherent to attempting to model the

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ocean two and a half billion years ago

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so to try to understand how that might

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affect my results I have conducted a

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series of experiments in which I

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systematically very key parameters in

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the model and this type of dissolved

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oxygen concentration is reproduced every

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time over a huge range of conditions

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although the exact dissolved oxygen

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concentrations might be sensitive to my

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model configuration the existence or

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possibility of oxygen owaisi's is not so

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in conclusion oxygen owaisi's are not

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only physically possible but I think

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that they are unexpected consequence of

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oxygen Aikido synthesis on an anoxic

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earth dissolved oxygen concentrations on

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the order of 1 to 10 micro molar are

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entirely possible in regions of high

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productivity typically i see

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concentrations around three to five

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micromolar finally this work is further

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support for the argument that aerobic

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ecosystems predate atmospheric oxygen on

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earth and ultimately what this means is

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that in the quest for life elsewhere

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atmospheric oxygen might not be a

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particularly reliable bio signature for

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oxygenic photosynthesis or aerobic life

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so looking at your at your maps you're

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showing concentrations near in an

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equatorial region and then near the two

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poles so in this particular case if

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you've actually got a flow model right

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you've got the Antarctic circumpolar

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current that's naturally going to

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concentrate it there so I mean the

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natural you've also got you know you

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have equatorial bands that are that are

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the the main east-west Jets right so so

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this is all just looks like it's a

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consequence of fluid processes in the

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ocean what happens if you have a Pangaea

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you know that's actually a really great

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question I am currently attempting to

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get Pangaea simulations up and going of

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course the archaean didn't look like

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pangaea either it's probably as

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different from the modern configuration

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so I don't know yet but I don't

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anticipate that the concentrations will

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be profoundly different obviously the

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distribution the map will be somewhat

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different but since oxygens associated

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with like upwelling regions there's kind

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of an inherent limit on the fraction of

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the Earth's surface that can be an

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oxygen oasis yeah that's correct it just

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you know if you get a big continental

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mass interrupting a global flow it'll

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divert things you writing up with

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something completely different I mean

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it'll divert things and modify the big

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changes but ultimately the continents

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don't drive ocean circulation that's a

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function of you know heating a rotating

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sphere so I would anticipate that the

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major features of ocean circulation

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would still be occurring for most types

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hi I was just curious for you into a

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little bit more detail on how you

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modeled the competition between the

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aerobic phototrophs and the anaerobic

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phototrophs okay the an oxygen aquatics

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win the competition for nutrients

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whenever inorganic chemical reductant is

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available yeah oxygen accredit ropes are

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since you're a pseudo geologist right

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now I have a pseudo geochemical question

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for you do you know if the iron banding

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record supports your idea of these oases

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or death traps whatever you want to call

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them um that's actually a really good

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question I don't incorporate a sediment

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burial in my model and so I cannot

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explicitly address that question right

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but outside of the model I thought you

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might have checked the geochemical

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record to see if it supports the

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conclusions of your model that's why I

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was 10 absolutely my work is not

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inconsistent with geochemical proxies

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but it's difficult to predict what the

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signature of these environments would be

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and that's because that's going to be a

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function of both like exposure time in

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the water column and you know and these

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environments are separated from the

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sediments by several kilometers of like

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an toxic water so you need really kind

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of special proxies that can be sensitive

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to that and I don't know what those are

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yet so any other questions for Stephanie