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hello everybody um welcome

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my name is adrian bros um i'm glad that

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you're all here today

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um today i'm going to be talking to you

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about um

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the potential preservation of

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biosignatures in

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some of earth's earliest soils also

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known as paleosols

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and so on the menu for today we're

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broadly discussing two research

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questions so for one

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are there signatures of life preserved

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in earth's earliest soils

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and second are these soils rich in

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sulfur like uh

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rocks on mars and so we're going to take

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a series

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of two different field studies we're

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going to first look at archaean

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three billion year old acid sulfate

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paleosol from australia and the feral

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quartzite

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and then we'll move to an archaean acid

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sulfate uh paleocell from

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greenland that appears to be about 3.7

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billion years old from the isola

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supercrustal belt so

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more broadly and to take a step back um

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why are we doing this work so

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um for one we want to understand uh

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should putative paleocells on mars be

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targeted for future exploration whether

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that's for

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biosignature investigation or for mars

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sample return

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and specifically we want to understand

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well do these archaean age paleocells

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from earth preserve detectable organic

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carbon

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and second is the preservation of

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organic carbon related to the sulfur

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content of paleozoics so to begin

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also to take a step back what is a paleo

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salt

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well it is a buried fossilized

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surface environment or soil which is now

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lithified into a sedimentary rock

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and most commonly paleocells form from

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rapid burial

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whether that be from volcanoes

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landslides

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dust storms sedimentation from flooding

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ash and tough deposits which cap and

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bury the soil

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and here we can see here sort of an

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anthropocene on the left version of a

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paleocell being created in real time by

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a uh by a lava flow on the big island of

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hawaii

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and then sort of on the right a um a

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paleo version of this

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burial event which is burying the soil

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this is on the right of myocene age

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intra basaltic paleo saw this red layer

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here

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and that has been buried by flood

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basalts from the columbia river

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basalt so uh

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how do we recognize paleocells in the

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field well there's a couple different

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ways morphologically

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most often they have a what's called a

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sharp top which is the burial layer or

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the cap

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and this most often in the case of

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sedimentation from flooding as the

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burial mechanism

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leads to cross bedding which is then

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capping

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you can see the underlying bed here that

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this destruction of bedding

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which is characteristic of bioturbation

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and pedoturbation during

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soil development it just kind of

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destroys that original

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and so the fossil record of soils is uh

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from on earth from the archaean to the

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holocene and everywhere in between

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on the left here we can see a pleisocene

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age buried

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molosol here this black layer which is

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buried by approximately

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two or three meters of less um and on

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the right we can see

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a potential paleosol from greenland

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we'll be looking a little bit more on

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this one in this

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talk that has a burial cap up here and

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then a series of

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diffuse horizons which are a common

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characteristic

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of soils and our rationale for studying

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pillar cells on earth that are really

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old

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is that our key in age ones from about

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2.6 to 2.7 billion years

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preserve biogenic organic carbon

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probably derived from cyanobacterial

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mats

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second there's mounting evidence of

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widespread surface weathering leading to

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pedogenic weathering

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on mars billions of years ago and

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third paleo cells have recently been

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named a high priority site for

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biosignature detection and

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mars sample return and so why paleo

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is on mars well based on all the

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mineralogical and remote sensing

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evidence

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it's possible that at gale crater

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curiosity could encounter

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a paleosol similarly at the nowakian age

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jezreel crater

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on this pelia lake with an extensive

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delta system where perseverance

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landed last february there's on the

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western delta and sort of the northern

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fan there's these ubiquitous and

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widespread dioctahedral aluminum clay

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and silica deposits that are associated

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with these point bar

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settings which suggest surface

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weathering or the presence of pele saws

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we know so much about the surface of

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mars because of our orbital remote

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sensing

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primarily in the visible and near

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infrared range

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which across nowakian age terrains on

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mars has

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really detected in thousands of

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locations diohedral aluminum and iron

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magnesium clays that are associated with

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formation

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in pedogenic or soil settings and when

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you overlay stratigraphy on top

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of these mineralogical maps by

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overlaying the mineralogy with a digital

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elevation model

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we can see that these there's this

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pedogenic pedogenic-like stratigraphy

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where aluminum clays overly iron and

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magnesium clays

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and they're all uh nowakin in age 3.7

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billion years

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and older so we know so much about the

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surface of mars in situ because of the

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sample analysis of mars sam instrument

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which the goal is to assess the passive

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ability and isotopic

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composition of the surface in the

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atmosphere and this is one of the

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instruments we use to characterize the

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archaean paleocells so we'll get into

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talking about it in our methods in a

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minute

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but it operates largely in two modes sam

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ega mode which detects both gases and

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same gcms mode which detects

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through molecular separation uh

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individual organic molecules

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but here we're only going to run it in

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same ega mode to look at our archaean

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paleosols for the first time

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and so with this instrument sam onboard

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curiosity

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organic carbon has been detected in

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association with uh with sulfur so

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either through sulfurization reactions

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which cross link and stabilize

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um different forms of organic carbon

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which in this case look to be carrigen

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like

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up to 10 parts per million or

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it's locked up in the crystal structure

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in the crystal lattice of sulfate

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minerals like gypsum basin

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bassinite and pyrite and so

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uh it's thought that sulfur through

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either sulfurization or

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or reactions with mineral surfaces or

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locking in

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sulfate minerals it's sulfur has aided

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in the preservation

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of organic matter on mars for billions

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of years

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and so on earth we see evidence both in

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geochemistry and morphology

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of ancient rocks and sediments that uh

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we might have had some undergoing

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acid sulfate weathering on the surface

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of early earth like on mars

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and so for this work we will first

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compile the previously published

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observations of archaean paleosols

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including total organic carbon

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and both geochemistry we then performed

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a sam ega analysis

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on a greenland paleosol to look at the

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influence of

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mineralogy on total organic carbon and

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then we

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related in the australian palette i saw

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total organocarbon with a number of

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pyrite framboids

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so that's sort of a metric of of of the

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sulfate content or the ancient sulfur

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content of these soils

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so the sam ej instrument works like this

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it heats our sample up in an oven

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here and here to about

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900 degrees centigrade and the sample

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decomposes into volatile gases

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those gases get sucked into a mass

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spectrometer and identified based on

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their mass charge ratio

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this constrains mineralogy and carbon

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content to the samples

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and without the need for any harsh acid

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pre-treatment to remove any inorganic

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carbonates

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so here's the greenland soil it's a

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potential

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3.7 billion year old acid sulfate paleo

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salt based on its geochemistry

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and its morphology and so we took this

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sample and ground it down and subjected

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it to

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total organocarbon and isotopic dell 13c

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analysis

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so stable carbon isotopes and we can see

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here with depth there's this marked

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increase both in total organic carbon

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which was remarkably high up to 1.6

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weight percent

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and dell 13c ranging between 24 to 27

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per ml

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with slight rayleigh distillation at the

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surface which is a common feature in

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soils

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decomposition of organic matter in the

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surface enriched horizons

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and so we can see some really

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interesting trends here with our

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evolved gas analysis so on the y-axis is

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co2 ion current and

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so2 ion current here in the yellow and

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red respectively on the x-axis is

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temperature

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and on this second y-axis we can see we

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have heat flow the dotted line here so

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this is uh telling us whether we have an

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endothermic reaction

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or an exothermic reaction and so at

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around 400 c we see this big

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burst of co2 this big release of uh

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probably organic carbon which is

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accompanied by a slight sulfur so2 peak

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here

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but most notably we also have a

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potentially what looks to be a carriage

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in peak so much higher temperature

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much more thermally stable peak

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accompanied by a

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large exothermic heat release here

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and this is characteristic of organic

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carbon decomposition

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so this are either from mineral

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associated carbon kerogen compounds or

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as we can see here tailing off with a

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co2 release at really high temperature

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around 900 degrees centigrade organo

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sulfur compounds

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and so um yeah we have yeah trying to

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still figure out what this all

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00:09:55,030 --> 00:09:52,720
means but it looks to be like there is

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potentially some preserved organo sulfur

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compounds in these ancient soils

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00:10:01,430 --> 00:09:59,839
and so we're not sure what the source of

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those is at this point so we can't rule

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them

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as being biogenic or abiogenic but

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moving on now to the second soil

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shifting gears a little bit towards

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australia we're looking now at a 3.0

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00:10:15,350 --> 00:10:13,839
potential acid sulfate halisal from a

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feral quartzite

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00:10:22,630 --> 00:10:19,200
in the pilbara craton in australia

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and morphologically these profiles look

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really similar to

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uh soil profiles in modern times

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especially acid sulfate soils and so we

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measured here on the right

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um total organic carbon as a function of

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depth

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and we uh previous work has shown the

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presence

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through thin sections on the there are

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these carbonaceous microfossils

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which have been embassaged to be things

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like a terrestrial community of bacteria

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bacteria and

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methanogenic archaea

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also in these samples there is

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framboidal pyrite

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00:11:04,230 --> 00:11:01,760
in abundance so this is the iron sulfate

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mineral it's framboidal which is sort of

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00:11:13,030 --> 00:11:04,800
a

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00:11:15,910 --> 00:11:13,040
number of

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00:11:16,310 --> 00:11:15,920
opaque pyrite framboids in each sample

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uh

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with the total organic total organic

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00:11:23,350 --> 00:11:21,120
carbon content of each sample

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and we found a uh a significant

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00:11:28,069 --> 00:11:24,640
relationship across

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00:11:30,949 --> 00:11:28,079
uh seven or uh sorry uh 10 or 11 samples

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00:11:31,269 --> 00:11:30,959
um that the more pyrite framboids we

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00:11:34,389 --> 00:11:31,279
have

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generally the more organic carbon we did

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00:11:37,509 --> 00:11:36,160
have this this outlier which i did

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00:11:40,150 --> 00:11:37,519
choose to include

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00:11:41,910 --> 00:11:40,160
because it was a sample with both the

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00:11:42,310 --> 00:11:41,920
highest organic carbon content and the

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00:11:47,190 --> 00:11:42,320
most

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00:11:50,629 --> 00:11:47,200
framboids so this suggests that sulfur

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00:11:52,870 --> 00:11:50,639
is in some way or shape or form

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00:11:55,509 --> 00:11:52,880
related to the preservation of organic

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00:11:58,629 --> 00:11:55,519
carbon in these soils

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so um that's what we're at right now uh

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this is for our preliminary and ongoing

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research so we can't draw any

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conclusions

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as to whether um these uh these ancient

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00:12:09,350 --> 00:12:07,120
soils contain

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biogenic organic carbon because there

348
00:12:14,150 --> 00:12:11,519
are many ways that abiotic

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00:12:14,949 --> 00:12:14,160
organic carbon can be formed we also

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00:12:18,389 --> 00:12:14,959
need to know

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00:12:20,710 --> 00:12:18,399
um you know like if there's any modern

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00:12:22,310 --> 00:12:20,720
contamination which we haven't done yet

353
00:12:24,069 --> 00:12:22,320
and so radiocarbon dating of these

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00:12:25,910 --> 00:12:24,079
samples would be a way to rule out any

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00:12:26,710 --> 00:12:25,920
recent or modern contamination from

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00:12:29,350 --> 00:12:26,720
stuff

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less than about fifty thousand years old

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00:12:32,470 --> 00:12:31,040
so that's all i got um thank you guys