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I'm Parker casselberry this is the

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possible photochemical origins of banded

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iron formations so just some

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introduction and background first our

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story starts in space you have an

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exoplanet you want to find life on it

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okay that's great let's look for

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something in the atmosphere say oxygen

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easy to detect spectroscopically and

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associated with life but the thing is

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we're not entirely sure exactly what

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controls its build up in the atmosphere

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so to do that we look at the earth

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specifically like the great oxidation

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event so about two and a half billion

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years ago earth's oxygen rose

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dramatically and the atmos the

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environment went from neutral to

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oxidizing so and the details of this

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though are kind of puzzling and the more

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we find the more the mystery deepens for

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example evidence of global atmospheric

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oxygen is about 2.5 2.4 billion years

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old but we have evidence of just

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oxygenation on a local scale at 3

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billion years ago so Lori investigate

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more this like this and all these

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investigations though rely on

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geochemical proxies some of which are

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very subtle effects out of effects of

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oxygen well it's often ignored is

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photochemistry the potential for

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ultraviolet light to cause this

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oxidation and there was no ozone layer

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back then too so this might completely

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be confounding our detective work so one

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of these signs of oxygenation and really

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big ones are banded iron formations

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there are layered sedimentary deposits

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of iron oxide the oldest are actually

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well before the great oxidation event

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but most are leading up to it so they're

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classically thought to be formed by

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oxygen turning aqueous iron to into iron

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3 but there's more to the story so

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there's something called photo Farrah

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trophy which is a alternate idea so it's

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an toxigenic photosynthesis that

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directly oxidizes the iron and there's

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been experiments that have shown that

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this could make ifs but then there's

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also photooxidation you have an

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ultraviolet photon knocking electron off

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of aqueous iron to turning into iron 3

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iron species like feo h plus have very

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broad UV absorbance so that gives some

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interesting results so making a bit with

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white you have dissolved iron in

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seawater from a hydrothermal vent and

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then feo h plus to absorb a photon

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electron gets knocked off you form iron

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hydroxides that electron creates

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hydrogen gas by combining with h plus in

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solution and then things precipitate

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then over time you get the minerals of

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AB if so the implication here is that

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you don't need oxygen or even biological

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activity so then are these banded iron

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formations really signs of oxygen life

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or are they completely abiotic so how to

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test this experimentally previous work

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has been done late 70s it was first

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proposed and then in 83 there were

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simple experiments that found that it

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could more than account for Biff's in 92

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more complete simulations concluded the

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same in 2007 under completely different

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conditions but more complete they

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concluded that it would be minor

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compared to photo Farrah trophy but all

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these experiments are very different

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they used different

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compositions of their sea water simple

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versus complex mixtures iron

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concentrations were an order of

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magnitude different things different

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light sources one of which having a

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hundred nanometer bandgap right in the

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middle of the UV spectrum and they

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concluded different things but then

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there is actually a direct conflict the

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83 paper they tested different

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wavelengths of UV light specifically and

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they found that UVA causes precipitation

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that's where the fao h+ absorbances but

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the later paper they tested UVA

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specifically in a simple solution as the

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previous experiment and found absolutely

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no effect that is the red line on the

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graph the black line is their control

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yeah major conflict so then this is

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where I come in so my experimental

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objectives are to test this photo

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chemical mechanism get quantitative

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rates by measuring iron loss and

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solution and compare that to estimated

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rates of actual Biff's and hopefully

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help resolve the current dispute in the

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literature so what i'm doing differently

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is having realistic best estimates of

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seawater composition haven't quite

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figured that out yet want to make sure I

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can actually get things working first

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but that could affect speciation other

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non iron species can absorb and also

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there's great potential for dark

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reactions between oxidized iron and

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other components so the other thing I'm

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having more rigorous control of my

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atmosphere and temperature previous

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experiments some of them did not control

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their temperature at all and I'm using

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something called a solar simulator so

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solar simulator is a powerful light

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source i vizi on our clamp that

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recreates the solar spectrum with

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special filters and I'm using one for

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the extra-terrestrial spectrum to

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simulate an earth with no

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zone as I said before there we go um so

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this is my setup there i have my solar

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simulator some optics there's a beam

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turner which is a really fancy name for

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a 45-degree mirror there's my glass

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chamber and i have this big machine to

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teflon lid that bolts on with 10 bolts

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at pain water-cooling jacket control

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temperature sensors ph and oxygen

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sensors with the USB sensor interface so

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i can just walk away and have the

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computer continuously log data for me

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all this tubing for a gas system so then

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my initial experiments i tested simple

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solutions of iron salt bicarbonate

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buffers basically repeats of the

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previous experiments sealed it with a

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positive pressure for psi so hopefully

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if anything gets in it'll get pushed out

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ran for over 18 hours this is what I saw

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and it beautiful you can see the iron

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precipitation there and all over in the

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water I was so excited here's lost so

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this is Iron remaining in solution over

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time so it looked like yeah um the only

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reason why that point is out there is

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this was on a weekend I wanted to sleep

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in so I didn't come in as a really to

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sample it uh but so basically the dark

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control showed exactly as much

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precipitation as the experiment with the

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light on and so essentially what

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happened is my oxygen sensor also rose

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to about ten percent of atmospheric

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oxygen so

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I was leaking oxygen significantly which

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was quite worrisome but at least I

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confirmed that oxygen makes iron

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precipitate I confirmed that I convinced

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myself of that so my colleagues went to

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our house University in Denmark to do

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experiments with a group who does they

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have this really fancy extremely

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sensitive oxygen detector and in doing

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that they realize that their setups were

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not nearly as oxygen tight as they

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thought so they've designed extremely

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oxygen tight experiments so I'm learning

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from them essentially what they said is

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that plastic which I used a lot of

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because I wanted to avoid using like

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metal because I was working with iron

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turns out that plastic is extremely

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permeable to oxygen so yeah that was

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kind of bad but the good news is is that

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glass is actually oxygen does not

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permeate well through it so you can

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build a nice reactor out of glass so

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right now I'm working with the glass

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shop to build something like this this

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is just a shiny 3d model so the

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interesting things eh oxygen sensor

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ports there's quartz window to let the

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light in stopcock and septum for

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headspace sampling remember how I said

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earlier that this reaction produces

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hydrogen I want to try to sample the

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headspace to look for that hydrogen so

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that I confirmed that I have photo

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chemistry going on um this entire thing

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can be placed in a water bath hooked to

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a chiller for temperature control

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because it will heat up it is getting

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hit with the power

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the sun so and then this is an

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interesting this injection in sample

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port so mix things up the seawater

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separately deoxygenated it and then

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inject it in to the chamber ah through

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this long skinny tube and you don't even

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have to seal it it's so long and thin

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that it will take oxygen longer than the

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course of the experiment to diffuse down

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it but when you want to sample it you

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just stick a needle right down in there

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it's absolutely genius thank you guys

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endeavor for that so I expect some

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amount of iron precipitation due to

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photooxidation to occur as has been seen

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in the simple experiments once i get all

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oxygen out more complex solution depends

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on speciation and dark reactions

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especially like if i add in some organic

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carbon that could interact with the iron

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so hopefully i'll answer if this can be

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made by UV light which would take the

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first step in investigating

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photooxidation how it might affect our

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geochemical oxygen proxies in general so

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okay we have time for a few questions

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yes great talk you may already know what

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questions i'm going to ask because we

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briefly discussed at the banquet i'm

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interested in actually kind of two

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things first off what kind of ph is

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you're going to be trying to keep your

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solution ette because it definitely

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depends you know with microbes

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especially are much faster and oxidizing

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iron a very low ph s and even oxygen is

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and also how you're going to also

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sterilize make sure you're not getting

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microbes in there um sort of the UV

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light sufficient yeah I was for

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sterilization I haven't exactly thought

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about that but I do wash this with fifty

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percent hydrochloric acid in between so

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the glassware is acid washed to remove

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the iron in between experiments so

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that'll get a lot of things out but and

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then as for pH the thinking is that

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things were relatively neutral back then

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um so that's what I'm shooting for um I

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actually broke the pH sensor for those

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experiments because it's this really

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long skinny glass thing um so I don't

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have pH data for that but ok question oh

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hi so in the archaean ocean when you

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precipitate iron you don't get a pure

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iron oxide you get an iron silica co

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precipitate or gel because the ocean is

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super saturated in silica so I saw that

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in the complex solutions of Khan hauser

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they have silica in that complex

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solution but and then the other ones

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don't have the silica so I'm wondering

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if you've considered the role of silica

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and discrepancy between the two and if

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you're going to try to model are keen

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seawater more with extent that is one

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thing that I most definitely want to do

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including silica um so I never

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we thought about that effect on the

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precipitation that you mentioned but

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yeah I mean the other experiments didn't

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have it so that very well could be a

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cause for discrepancy or the other

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numerous things that they have different

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okay real quick yeah I just saw this in

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the question I might have just missed it

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but what was the half reaction that you

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were making the hydrogen gas uh it was

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an electron combining with um h+ in the