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[Music]

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[Applause]

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thanks for invitin I'm somewhat an

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outsider to this field I work mostly in

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modern lake sediments but I have got

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involved into in the early Earth working

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with Sean Crowe on Lake Matano and a key

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an analog and I had an enthusiastic PhD

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student who wanted to keep modeling this

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stuff so I'll talk about how we

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interprets the sulfa set of records in

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Keene sediments and also what we mean

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maybe missin current currently from

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those models so as was just presented

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there is pretty good evidence for

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coastal oxygenation just before the

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great oxidation event before the

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atmosphere got oxygenated so we're

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interested in how this may have affected

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the sulfate concentrations possibly

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methane fluxes as well and how we can

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separate the signals that propagate from

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the atmosphere so the oceanic water

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column and get preserved in the

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sediments so when we look at this

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sulfides it obviously typical trends

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that we see or that mass independence

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also isotope fractionation disappeared

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goe

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and the predominant paradigm for some

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mass independent relation Asians is that

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ultraviolet radiation separate positive

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Delta negative Delta into the elemental

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sulfur pool and sulfate pool and these

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are incompletely mixed so the positive

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excursions get preserved the elemental

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sulfur can just proportionate also

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generate some sulfate but basically we

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are looking at the sulfides preserved as

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pyrite in marine sediments there is a

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relationship between mass independent

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fractionation Delta Center City and

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Delta 34 so for a decrease in Delta

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sata3

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that is probably a decreased role of

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elemental sulfur we also see lighter

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isotopes and sulfur which probably

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when we try to simulate preservation of

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these signals we use diagenetic models

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which importantly are analogous to the

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models that we use in modern sediments

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that is we consider these sediments as

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analogous to modern sediments with

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sulfate diffuses from the water column

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into the sediments the sulfate reduction

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happens and that's how we get our pyrite

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we also use fractionation microfracture

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nations that will learn in modern

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systems with more than microbial

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capabilities for the lack of better

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information so this this is a picture

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from Lake Madonna which has high iron

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concentrations this is in the water

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column you may as well consider

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stratified sediments sulfate diffuses in

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produces sulfide as sulfate diffuses in

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and gets reduced

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we have Rayleigh effects fractionation

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so sulfate gets isotopically havior

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sulfide while must track must be getting

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as rapidly heavy as well the reason why

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we don't get exactly zero fractionation

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is that the system is open so because

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lighter isotopes are preferentially used

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up in the sediment emergency it's deep

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water column the gradients for the

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lighter isotopes are steeper so there

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are more lighter isotopes diffusing in

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we get essentially the lighter sulphide

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if we assume that at low sulfate

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concentrations fractionation x' decrease

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because essentially all of the sulfate

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is being used up we get load isotopic

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differences Delta 34s between sulfate

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and sulfide and we could match this with

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a key and pyrite for fairly low sulfate

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very long actual sulfate can situations

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below 5 micro molar and this work we can

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develop the model further in more recent

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paper we included the oxidative cycling

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of sulfur where sulfide can get oxidized

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so in coastal environments also put in

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disproportionation of elemental sulfur

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that wasn't really considered there and

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what we found out is that oxygen well

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primarily limits sulfate reduction

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because organic matter mineralization

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happens to some degree through a orbital

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this increases Delta 34 the oxidative

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recycling but paradoxically it also

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increases preservation of mass

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independent fractionation because you

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get less sulfate reduction so more

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signal gets carried through from

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elemental sulfur if we put this in the

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context of sulfides atop records what

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this what this seems to explain is the

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increase in Delta status 3 leading up to

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the goe

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coincident with the increase in Delta 34

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and if we try to put constraints on the

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sulfate concentrations that's consistent

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with this it looks like the sulfate can

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situation increased from about 50

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micromolar we relaxed the previous

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constraints on sulphate concentration to

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about 50 micromolar up to about 200

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micro molar and oxygen concentrations

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that could reach cup well 25 so micro

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molar in the zeldo environments to

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reproduce this chance this also seems to

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suggest perhaps increase in organic

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productivity is the deposition of

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organic carbon if we carry simulations

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further in time into the Proterozoic we

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could look at the sulfur budget in the

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Proterozoic ocean so in further simple

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model that considers coastal ocean

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surface deep ocean and deep deep ocean

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for realistic fluxes input and output

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fluxes what we seem to have is that

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sulfate concentrations were in the

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hundreds of micro moles they were not

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as high as previously believed the

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important results from the sensitivity

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analysis is that the sulphate

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concentrations in the ocean seem to be

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controlled by the presence of this

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anoxic a huge anoxic compartment if you

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oxygenate the water column this D

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increases the sulphate concentrations

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tremendously by removing this deep sink

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and this is how you can get millimolar

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or higher levels of sulphate but until

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the ocean deep ocean becomes excision

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ated sulfate concentrations may have

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remained for a little low so this is

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this figure basically addresses this

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there are some exceptions to this result

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so Lagoon G seems to have high sulfate

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concentrations which may suggest that

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the water column have been oxygenated

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this have been suggested in the

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literature their genetic models show

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that basically the these sulfate

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concentrations are consistent with

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sulphur isotope records if we consider

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tens of microns Aegean in the in the

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surface ocean at these low sulfate con

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situations methane fluxes are sufficient

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to maintain concentrations above 25 ppm

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v which have been suggested as

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sufficient to maintain the Earth's

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glaciation free so I have in the

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Romanian a few minutes I'd like to talk

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about the factors that we may not have

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put into this model like I said

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previously we can say that sulfate

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diffusion in from the water column that

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was the primary source of sulfur to the

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sediments well if we look into the

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modern lake sediments low sulfate lakes

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sulfate or salt and sulfide seem to be

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generated within the sediment from

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organic matter well these are the plots

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from Lake Superior which is oxygen like

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Malawi which is borderline ferruginous

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of

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sulphate concentrations in the top

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layers of sediment are actually above

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the layers the levels of sulphate in the

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overlying water so sulphate actually

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fluxes out of the sediment well in lake

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montana serious fraction which is

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basically pyrite plus elemental sulfur

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increases but the total amount of sulfur

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actually decreases so we have a

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conversion from non serious fraction to

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share a fraction and this decrease is

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consistent with the organic sulfur being

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mineralized and so the ratio of sulfate

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sulfur total sulfur to total organic

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carbon remains remarkably constant

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within the sediment which can suggest

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the role for organic sulfur now this has

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a profound effect instead of considering

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rayleigh distillation in the sediment

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column of water column we have sulfate

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and sulfide being generated within the

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sediment possibly with the isotopic

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effect of afghan excel for hydrolysis

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and so this changes the paradigm of how

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we generate the isotopic signatures and

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sediments if we put if we construct a

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model that simulates how much pyrite it

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you can generate turns out you can

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generate at low sulphate concentrations

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you can generate all pyrite in the

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sediment from organic sulfur and so if

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you consider organic sulfur in organic

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debris prokaryotic cells actually leg

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Lac plus six like sulfates but they do

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have sulfonates which is plus four and

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of course they have plus minus two

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oxidation state so you generate both

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sulfide and sulfite from mineralization

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and sulfite reduction is actually

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energetically more favorable than

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sulfate reduction so if you form pyrite

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out of this source the resultant

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signatures are also consistent with the

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ork record it's also consistent with

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some other lines of evidence that show

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while sulfate reduction has a

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volved laters and sulfate reduction in

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fact as late as 2.7 billion years ago

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and this also seems to suggest that you

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don't need as much sulfate in the water

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column so if you look at the sulfur to

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carbon ratio in living cells it's pretty

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similar to phosphorous to carbon ratio

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about 1% so these levels sulfate could

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have been a limiting nutrient for life

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the well depending on what you believe

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about a key in productivity

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mineralization of organic sulfur could

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generate enough sulfur to account for

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global pyrite fluxes and it's comparable

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to the flux of sulfur in from the

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hydrothermal system or actually exceeds

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that works so organic sulfur probably

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needs to be considered if you try to

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forecast the effects on the isotopic

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signatures this is Delta set of 3

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Delta's 34 depending on what you believe

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about the simulation of organic sulfur

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into the organisms it's probably false

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on the continuum between elemental

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sulfur and sulfate sulfide the microbial

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reactions would make would shift the

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result and solve pyrite to the left here

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interestingly around this area we find

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sulfides in microbial mats which again

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organic reach system with abundant

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organic sulfur I have just a few more

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minutes I want to briefly comment on

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other factors that other uncertainties

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in our interpretations we don't really

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know what happens at low concentrations

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and low concentrations and low rates are

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not the same this is a figure from a

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recent review that showed that these are

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the bars

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are the specific cell specific sulfate

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reduction rates in more than marine

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sediments these are the data points

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found in more than experiments we will

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lack data on low sulfate reduction rate

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specifics sulfate reduction rate low

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reduction rates however cell specific

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production rates are not necessarily

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associated with low concentrations below

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about 500 micro molar of sulfate sulfate

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reduction rate starts being limited by

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sulfate but organic matter is still

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important so when you look at the

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software production rates in lakes where

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concentrations of sulfate adjust tens of

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micrometer they're not that different

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from marine sediments were

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considerations of sulfate are a thousand

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times greater because they are

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controlled mostly by organic matter so

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we probably don't need to look that deep

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into marine sediments to those low cell

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specific rate and finally we have to be

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conscious of the sampling bias the

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sulfate reduction modern sediments

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happens predominantly in coastal

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sediments and coastal sediment extremely

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heterogeneous environments if we we try

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to say something about modern

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environments by looking at the more than

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Ocean at just a few locations that try

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to extrapolate to the entire planet we

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probably would have been laughed at but

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that's Excel essentially what we do for

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the early Earth for the lack of better

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alternative I think we have to be

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conscious of that I'll end here

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[Applause]

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going from you know the sulfate

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reduction pathway or disproportionation

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how is the sulfur complex to organic

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matter once it gets out of the cell but

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does that matter in terms of the

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isotopic signature fractionation or the

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way it's process in other words it can

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be bound to different types of organic

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functional groups right I think it's

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very important especially which step

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during our mineralization is a rate

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limiting we don't have that much

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information on how sulfur gets

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fractionated there are there is some

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information primarily from lab studies

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but I think that's what we need to