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alright thanks for coming out guys so

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we're going to travel back to the mezzo

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archaean today but before we do I wanted

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to thank my funding sources which is the

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current na I can cycle the Lewis and

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Clark grant and the GSA graduate

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research grant I also like to thank my

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collaborators so I'm going to start

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today talking about the motivation

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behind my research but I'm going to go

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into background into the sumotori iron

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reduction I'm then I'm going to go into

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the approach behind my research and get

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some background to my field area and

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then I'll present some are an isotope

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data put it into context than the model

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and tie everything together with some

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implications and conclusions so the

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question driving my research is was

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there a chemical footprint of the

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sumotori iron reduction three billion

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years ago and I ask this question

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because the sumotori iron reduction is

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then has great astrobiological potential

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its electron acceptor iron is widespread

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in the solar system and this metabolism

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of the sumotori arm reduction is deeply

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rooted in the tree of life so let's move

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on to some background this equation

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right here is showing the matura RN

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reduction with the electron acceptor and

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blue and the electron donor in red so in

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this process you oxidize the organic

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matter at the same time that you reduce

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the ferric iron and this produces

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ferrous iron the purple boxes here are

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indicating the phyla that are capable to

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similar Tory I reduction so you can see

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that those phyla are deeply rooted and

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their present in both the bacterial and

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archaeal branches of life so how do we

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find dir in the rock record what

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fingerprints are there it basically

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boils down to isotopes and minerals so

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if you guys aren't familiar with iron

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isotopes we've got the notation here so

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iron isotopes are measured on relative

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to a standard which is the average

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igneous rocks iron 56 is the heavy

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isotope of iron and iron 54 is the light

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isotope so if you get a positive Delta

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50

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6-iron value that means you are enriched

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in the heavy isotope relative to igneous

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rocks and if you get a negative value

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that means you're depleted in the heavy

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isotope now this process it also enables

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the formation of certain minerals such

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as magnetite satellite and Vivian night

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so combining isotopes and minerals we

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can find dir in the rock record so for

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my study I did iron isotopes on whole

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rock samples microgel pyrite and

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magnetite separates these samples come

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from the mezzo archaean Witwatersrand

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supergroup and they represent a wide

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range of lithologies and depositional

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facies so where is the witwatersrand

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govardhan supergroup is part of the cat

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fall Cretan of South Africa and it's

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composed of the older West ran group and

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the younger more famous Central Rand

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group and Central Rand group is very

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famous because it's a it has lots of

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gold and but my samples are from the

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west rand group and we are lucky to get

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those samples because when they're

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trying to sample the central ran groups

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sometimes the drill cores went down too

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far and they accidentally sample the

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western group so I'm very thankful for

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that so there's a couple really

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fantastic aspects of the Western group

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the first is that it's experienced

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low-grade lo green show species

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metamorphism which sounds pretty scary

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to anyone that works with younger rocks

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or modern sediments but for the mezzo

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archaean it's pretty much the best we

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can hope for I these rocks represent a

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complete depositional Basin from just

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offshore to the starve shelf and these

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rocks have been used as evidence for

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atmospheric oxygen and that has come in

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the form of chromium isotopes selenium

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concentrations in pyrite and molybdenum

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isotopes so I say that we have this

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whole complete depositional Basin where

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so you can see here that this is a sort

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of model ocean basin and so we have

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samples from the braids approval depo

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phases which is just off shore and then

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slightly ensure the pro delta depth of

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faces and they should be considered

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proximal the transitional shelf would be

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intermediate and the outer shelf and

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starve shelf would be the distal

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depositional facies so let's look at

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some data this is showing iron isotopes

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plotted against tol iron concentrations

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which is normalized to Luminum and we

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normalized to aluminum because we want

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to look at iron enrichment beyond the

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trityl flux so the cooler blue colors

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are more proximal depositional facies

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and the more red warmer colors are the

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more distal depositional phases the

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diamonds are micro joule pyrite the

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circles are whole rocks and the squares

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are magnetite separates so we can see

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that as we move from the proximal part

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of the basin to the more distal part of

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basin we're getting enrichment in the

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iron contents so this line here is

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showing the iron concentration of the

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average our crust and to the left of

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this line we have mostly proximal

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samples which means they are depleted an

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iron relative to the average crust and

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on the right side we see the more distal

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samples and they are enriched in iron

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relative to the crust you can also see

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that the same time we have iron

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enrichment we're getting increasingly

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negative Delta 56 iron values and this

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line here is representing the average

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Delta 56 iron value of our key and crust

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so above the line we have mostly

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proximal samples which means that they

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are enriched in the heavy isotope of

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iron and then below this line we have

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the more negative Delta 56 samples the

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delt the distal depo faces and that

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means that they are depleted in the

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heavy isotope relative to the crust so

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this inverse correlation between the

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iron concentration and the iron isotope

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values has been seen before in modern

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environments and it's indicative of

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something called a benthic iron shuttle

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or microbial iron shuttle

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so this work was pioneered by silca

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sever minh and in her 2008 study on the

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black sea they found that I sitaki light

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iron was being produced on the shelf by

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microbes and this light iron was

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preferentially removed and deposited

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entrapped in the more distal deep basin

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this meant that a residual heavy pool of

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iron was left on the shelf now another

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important aspect of the study is that it

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required a redox boundary in order to

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trap the benthic iron flux through the

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microbial iron flux and in fact every

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study that I've come across of microbial

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iron shuttle requires a redox boundary

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in order to get that inverse correlation

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between iron concentration and iron

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isotopes so this is modern environment

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study how does this apply to the measure

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can and this is where it gets fun so

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we've got a model and the first part of

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the model is we need food for the

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dissymmetry iron reducing bacteria and

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so because other papers have come out

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suggesting that oxygen was present in

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this mess rocky atmosphere I'm

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suggesting that there is a redox

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stratified water column with oxic

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surface waters on top and anoxic

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ferruginous waters below so iron would

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precipitate and go to the shelf and

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these ferric iron would be reduced by

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the dir bacteria and a produce I stop

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relight iron now this isotope cool iron

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would be preferentially removed and

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trapped below the redox climb and this

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would leave the heavy iron isotopes on

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the shelf this is why we have the more

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heavy iron isotopes on the shelf and

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they get increasingly light as we

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increase the distance from the shelf so

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one of the remaining question is what

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minerals are the reservoirs for the

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benthic iron flux I address this

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question by doing normative calculations

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based on bulk element data and iron

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speciation data and so you can see that

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the green represents a silicate as the

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iron bear

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in face the red it would indicate

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magnetite the blue pyrite and then the

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purple hematite and as we move from the

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proximal to the more distal part of the

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basin we're having seen an increasing

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proportion of magnetite and this leads

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us to hypothesize that it's magnetite is

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the main reservoir for the benefit iron

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flux you can also see that the size of

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the pie chart is increasing as we're

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moving to the distal basin and that

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represents iron enrichment but it's not

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just iron that's being enriched this a

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trend also holds for manganese manganese

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is increasing as we go from the proximal

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to the distal part of the basin and

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another previous study on the banded

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iron formations of the starved shelf of

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these rocks found that manganese was

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anomalously enriched compared to all

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other banded iron formations of the

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archaean so if we plot iron isotopes

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against manganese concentrations you can

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see that we have an inverse correlation

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here as well where we have increasing

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manganese content and that corresponds

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with more and more light iron isotope

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values so this leads me to hypothesize

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that manganese is hitching a ride on the

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microbial iron shuttle so manganese is

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being shuttled as well so to tie

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everything together we see an inverse

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correlation between the iron content and

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the iron isotopes which is indicative of

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microbial iron shuttle and the inverse

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correlation between iron isotopes a

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manganese content is suggesting that

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manganese is being shuttled as well the

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iron enrichment is corresponding to an

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increasing proportion of magnetite in

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the samples and this study is

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particularly exciting because it is the

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oldest record of microbial iron shuttle

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that we have and it shows that the

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sumotori iron reduction was a left of

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basin-wide footprint three billion years

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ago so this implies that there is an

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active iron cycling microbial community

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and the redox stratified ocean and

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lastly i think this emphasizes the

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importance of using basin-wide samples

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to address basin

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research questions because if I had only

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sampled the proximal part of the basin

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argc heavy iron eyes took values and

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think maybe some iron was oxidized if I

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only sampled the distal part of the

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basin or just seeing negative iron eyes

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took values and think there's dir there

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and that would be the end of the story

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it was only with the complete basin that

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I could be able to piece together a

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model of a mezzo archaean iron shuttle

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so with that are there any questions so

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you mentioned how you're moving the

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ferrous iron from the surface ocean into

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the deep ocean so if the surface ocean

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is oxidized in the deep ocean is reduced

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or an toxic I guess or dis oxic

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depending on you want to freeze it how

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are you getting the ferrous iron through

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the oxygen and then immobilizing it in

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the deep ocean that's an excellent

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question so there's multiple ways that

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or there's multiple different transport

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mechanisms that have been invoked in the

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literature for microbial iron shuttles

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the first would be a ferrous ion

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traveling along the redox klein and then

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going down below another one of via the

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iron is transported by ligands and that

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would be able to travel through the

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octopus waters that way and the last is

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that the bed the garrn flux is being

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transported as a solid a nanoparticulate

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solid and that is on what I favor but it

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requires that all of the benthic iron

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flux is oxidized if you're going to

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preserve that heavy the light iron