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[Music]

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[Applause]

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great thank you so quiet where this is

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due this is talk of the work by Chad

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Austria who's a graduate student working

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with me he couldn't be here for family

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reasons so he asked me to cover his talk

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and it's work that he began and has

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continued with SUNY Nielsen at Woods

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Hole Oceanographic Institution and

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Jeremy Owens when he was there as well

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as with my group so the stallion work

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I'm going to show you here tell him

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isotopes is pleadin generated at hui and

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this work follows on it's a it's a

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sequel of sequels to the whiff of oxygen

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story that began with his paper in 2007

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on a companion paper led by jake hoffman

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that argued based on molybdenum

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enrichments in 2.5 billion year old

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black shales the Mount Moriah shale that

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there was that this middle of an

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enrichment SAR evidence of trace oxygen

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in the in the environment at that time

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and subsequent to this paper has been a

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number of other studies a number other

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proxies it all kind of point in the same

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direction where the most parsimonious

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explanation is a whiff of oxygen small

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amounts of oxygen in the pre great

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oxidation event the pre goe environment

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and other studies have extended this

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kind of thinking back substantially

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earlier than you know and extra half

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billion years or more so so all of that

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work has has kind of solidified this

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notion that the pre goe environment was

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not devoid of oxygen that there was some

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small amount of oxygen in the in the

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oceans and oxygen Oasis or other such

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environments perhaps emanating into the

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atmosphere and being rapidly consumed by

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by oxidative weathering but the

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outstanding question you know yeah we

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have this kind of vision that that that

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it's an old vision and has become

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solidified but the extent of this

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oxygenation is poorly constrained and so

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this is the question that that has

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really interested Chad and he started

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thinking about whether or not thallium

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isotopes could be useful as a constraint

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which is what I'm gonna gonna present

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here or get to there have been a few

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other studies that really tried we've

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tried to get at this extent of

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oxygenation question in the oceans this

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work here in 2006 by John Eagan broad

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and Kate Freeman I made arguments based

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on carbon isotopes for extensive

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oxygenation on kondal margins a

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widespread oxygenation before the goe

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and

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rhenium enrichments not isotopes which

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is concentration enrichments in South

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African drill cores from about 2.6

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billion years ago to make the argument

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that there was oxygenation down to the

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sediment water interface at least in

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that location

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which promotes rhenium burial and the

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reef was the explanation of the Reem

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enrichment that Brian identified so

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there have been arguments made that

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oxygenation was extensive enough to

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actually give you fully oxygenated water

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columns on continental margins at least

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in some locations so that leads to

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prediction you you might predict then

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that if you really did have at least in

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some places and times fully oxygenated

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water columns that you would have had

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the ability to bury manganese oxides in

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in marginal sedimentary environments the

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basic idea being simply that in order to

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actually bury manganese and sequestered

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in sediments you need to have oxygen

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penetration in order to to allow for the

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preservation of mameys oxides during the

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burial process if you don't have oxygen

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penetration below the seventh water

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interface then you get the manganese

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cycling back out and so so chad

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speculated that well okay if these kind

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of environments were widespread we don't

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actually have those sediments so we have

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to look for them indirectly but if these

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kind environments are widespread with

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manganese oxides being buried perhaps

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there would be fingerprints of that that

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we could find in isotope systems now

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we've done some work previously to this

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with the notion that molybdenum isotopes

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might be useful for this and the basic

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concept for molybdenum isotopes

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schematically is that we know that

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melibea MYSA topes adsorb on to or

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molybdenum absorbs onto manganese oxides

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and there's an isotope fractionation

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when that happens such that light

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molybdenum I saw the light molybdenum is

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preferentially buried with manganese

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oxides leaving seawater heavy as a

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result and in fact modern seawater ship

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is substantially heavy word compared to

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the middle of my step composition River

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inputs because of this process going on

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today because we have copious manganese

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oxide deposition on the seafloor today

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so if you have environments for oxygen

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penetrates a sediment water in your face

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and and you bury manganese oxides you

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should preferentially sequester light

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molybdenum leaving the oceans heavy and

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you can sample that you can probe for in

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theory in principle you can probe for

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the seawater will have my Stoke

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composition by looking at live my stoop

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lived in my isotope composition in

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sediments deposited under uke ciinic

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conditions at least in those places

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where the water masses have enough time

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for the quantitative removal of the

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molybdenum in those environments and so

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we and others in pursue a little MYSA

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topes with that in mind this is worked

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by a former graduate student you and

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Juan in 2010 in the McCray Shale so

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here's the same thing you saw before the

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molybdenum enrichments around 2.5

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billion years ago in the mountain crazy

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Shale down here you have very little

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molybdenum then you have this

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this increase which is the the original

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signature of this width of oxygen

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concept and here are molybdenum isotopes

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and you can see there's a shift to

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heavier values here somewhat correlated

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with this these Malinin Richman's and

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the argument was made that this shift to

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heavier my stove values is indicative of

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a shift to heavier moving isotope values

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in the oceans being sampled at this time

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and that that is due to removable

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preference removal of light molybdenum

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oxidative environments perhaps including

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manganese oxides at the segment water

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interface in some places so that was one

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of the ideas put forward in this paper

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but there are number of ambiguities with

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little isotopes there also absorb up to

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iron oxides it's not a unique tracer for

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manganese in this fractionation there -

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there's complexation oblivion with

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organic matter that can also impart its

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- fractionation and in even in these

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your clinic settings if you don't have

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complete removal of molybdenum there's a

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nice dip fractionation with molybdenum

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forms thio molybdate saw Roxy time lib

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dates so if you don't complete removal

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you can get fractionation even in these

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reduced environments that are supposed

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to be where you're well you're getting a

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good record of the sea water value so

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there number of ambiguities with with

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moving my Stokes that always left a

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question mark about what what that what

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that shift to heavy values really means

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other than in general that has to do

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with oxidation probably so a family my

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stuff's come into play in this way this

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is a slightly complicated system but

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I'll try to give you the this simple the

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simple message out of it so first of all

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we took we talked in terms of the 205 to

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203 ratio that it's a very small

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variation so it's an epsilon units 10 to

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the 4 rather than Delta units 10 to the

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3 Salim comes in from a number of

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sources while Kanak gases rivers

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hydrothermal vents aerosols into

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seawater gets removed primarily into

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three sinks it gets removed in

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association with manganese oxides that's

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kind of key it also gets removed during

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alteration of ocean crust that's a net

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sink for a thallium from the sip from

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the oceans and it gets remove very

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efficiently into an toxic sedimentary

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environments where you have some sulfide

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in the water column the key to the story

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here is that when Sally me is removed

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into these manganese oxide environments

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there is a very strong fractionation 13

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epsilon units here verses -2 in the

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input and -6 and sea waters there's a

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very strong shift strong preferential

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removal of heavy thallium into these

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sediments leaving seawater light this is

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the big fractionation in the system and

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in these reducing environments thallium

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seems to be very nicely a very nice

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recorder of the seawater isotope

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composition there does not seem to be

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fractionation during removal into these

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environments these environments

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sediments deposit and these environments

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can be pretty good probe of seawater and

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the seawater ISTEP composition we think

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should vary as a function of how

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importance this manganese oxide burial

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story is and this is this has been

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reviewed in and in a this nice paper in

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reviews numerology geochemistry there's

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been a number of studies building this

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up by suni Nielsen's group and and

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Jeremy and others critically the

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ambiguities the plague molybdenum

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isotopes don't plague thallium isotopes

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it appears as far as anybody can tell so

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far the family mice notes don't fraction

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ate during adsorption on to iron oxides

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for example so that ambiguity that is

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there with molybdenum is gone and

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thallium doesn't seem to form thio

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complexes and doesn't seem to be subject

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to this incomplete removal problem in

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reducing environments and so it's still

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early days

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all new proxies go through their wave of

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optimism but maybe we're still there

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with thallium but at least at the moment

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it seems to not have the same

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ambiguities in molybdenum and so the

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notion that Chad had was let's go and

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look and here's what he hypothesized we

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ought to see if you have environments

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where you have oxygen penetrating the

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sediment water interface and burial of

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Maggie's oxides you will preferentially

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sequester light isotopes haven't lived

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in them as we said before and you should

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preferentially sequester heavy isotopes

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of thallium that's what the other data

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shows so seawater in these kind of

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situations seawater should be driven

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towards heavy molybdenum ISTEP values

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and light thallium ISTEP values and we

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should be able to probe both of those in

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highly reducing

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sedimentary environments and what you

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should see then is a complimentary ship

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you should see these guys going sort of

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in opposite directions they should

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complement each other so you might

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expect to see if you go from go from a

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anoxic world a more oxidizing one you

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might expect to see that in in black

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shales which are this reduced complement

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to the oxides where we can actually

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track seawater we think you might expect

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to see then thallium going light

274
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molybdenum going heavy and microphones

275
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getting knocked around all sorts of

276
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stuff like that so that's what you might

277
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expect to see so what is what do we

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actually see then so this is work that

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was just published earlier this year in

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Nature Geoscience is so here is again

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this molybdenum enrichment and black

282
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shales here are slides here here's the

283
00:09:53,060 --> 00:09:51,839
molybdenum isotope values so Chad went

284
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and reimagined them live my steps at

285
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much higher resolutions there's a lot

286
00:09:58,490 --> 00:09:57,000
more live my scope data now so you can

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see this shift to heavier valleys of

288
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live and isotopes then shift back down

289
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and an interesting lee another shift up

290
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here trying to figure out what that

291
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means

292
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and here the thallium isotopes and you

293
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can see they mirror you know pretty

294
00:10:11,150 --> 00:10:10,110
nicely for this kind of work anyway to

295
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enough fill in your old sediments and

296
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although things have happened since then

297
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potentially there does seem to be this

298
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complementarity very much the way Chad

299
00:10:22,040 --> 00:10:18,870
predicted and you know it's it's

300
00:10:24,110 --> 00:10:22,050
geochemistry so trends but this is a

301
00:10:27,439 --> 00:10:24,120
significant trend right there's a

302
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correlation there so so the upshot then

303
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is it looks like we would argue that

304
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this is at least a strong argument to be

305
00:10:35,930 --> 00:10:33,390
made that what you're seeing here is

306
00:10:37,040 --> 00:10:35,940
evidence of not just small mats box in

307
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the environment but actually burial

308
00:10:41,180 --> 00:10:39,779
manganese oxides varalu manganese oxides

309
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- in extent it's significant enough that

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it's affecting the ocean budgets of both

311
00:10:46,819 --> 00:10:45,180
thallium and molybdenum so you can do

312
00:10:48,949 --> 00:10:46,829
some simple isotope mass balance

313
00:10:50,780 --> 00:10:48,959
modeling you can come up with for

314
00:10:51,949 --> 00:10:50,790
molybdenum isotopes at the fraction of

315
00:10:53,840 --> 00:10:51,959
molybdenum in the system that's being

316
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removed but mang-nese oxides at that

317
00:10:58,100 --> 00:10:55,670
time was something like 20 to 30 percent

318
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for thallium isotopes it's somewhere

319
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between six and twenty percent big

320
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uncertainties because there's a lot of

321
00:11:05,600 --> 00:11:01,800
uncertainties of course in the budgets

322
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these elements so but these are these

323
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are pretty significant they're smaller

324
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fractions than in the modern oceans but

325
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not dramatically smaller they're smaller

326
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by 10 or 20 percent or so so these are

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these are pretty sizable

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six inks and so it's hard to turn that

329
00:11:20,930 --> 00:11:19,740
into a quantitative measure that's that

330
00:11:23,030 --> 00:11:20,940
requires a lot better understanding of

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00:11:25,520 --> 00:11:23,040
the site of the budgets which which are

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00:11:26,990 --> 00:11:25,530
working on but what emerges more

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strongly than before is the notion that

334
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we might want to be thinking about

335
00:11:34,430 --> 00:11:31,860
Archaean environments in which you the

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the shelf environment was sufficiently

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oxygenated that you actually had

338
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manganese oxides being buried in

339
00:11:42,200 --> 00:11:40,890
sediments you had oxygen fully

340
00:11:44,480 --> 00:11:42,210
oxygenated water columns down to the

341
00:11:46,040 --> 00:11:44,490
water interface persistently for long

342
00:11:47,660 --> 00:11:46,050
periods of time in large parts of the

343
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ocean so when you think about oxygen

344
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Oasis you might really want to think

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about you know substantially oxygenated

346
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shelf environments for long periods of

347
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time coming and going and wafting and

348
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win and waning and so I will end there

349
00:12:12,530 --> 00:12:10,380
and take questions Thanks the whiffs of

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00:12:15,470 --> 00:12:12,540
oxygen shorelines is that where you

351
00:12:17,330 --> 00:12:15,480
expect the most I guess cyanobacteria to

352
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be producing the oxygen it's a

353
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reasonable hypothesis is there any other

354
00:12:24,650 --> 00:12:23,610
hypothesis I mean if you interpret all

355
00:12:27,260 --> 00:12:24,660
this evidence as evidence of o2

356
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production by photosynthesis then

357
00:12:30,290 --> 00:12:28,830
presumably you're talking about santa

358
00:12:31,730 --> 00:12:30,300
bacteria growing those near shore arms

359
00:12:34,160 --> 00:12:31,740
is a very classic idea it's not a new

360
00:12:37,700 --> 00:12:34,170
idea right it's a no land anything oh

361
00:12:38,510 --> 00:12:37,710
the land question Don sitting right

362
00:12:40,010 --> 00:12:38,520
behind you might have a better idea

363
00:12:47,420 --> 00:12:40,020
what's on what might have been on land

364
00:12:53,090 --> 00:12:47,430
and I then then I do but yeah yeah do we

365
00:12:54,650 --> 00:12:53,100
want to believe it but but but again the

366
00:12:56,750 --> 00:12:54,660
fit but just give it back to the actual

367
00:12:58,670 --> 00:12:56,760
story here right the thallium isotopes

368
00:13:00,950 --> 00:12:58,680
are telling you about burial in the

369
00:13:02,420 --> 00:13:00,960
ocean environment right so yeah you

370
00:13:04,460 --> 00:13:02,430
could have had oxygen production on land

371
00:13:07,820 --> 00:13:04,470
you could have biological soil across I

372
00:13:10,280 --> 00:13:07,830
mean I by two but that wouldn't give you

373
00:13:12,110 --> 00:13:10,290
manganese oxide deposition very easily

374
00:13:13,670 --> 00:13:12,120
in the marginal environments I think you

375
00:13:15,560 --> 00:13:13,680
need to have oxygen actually in the

376
00:13:17,720 --> 00:13:15,570
water allowed as an exact size be

377
00:13:22,390 --> 00:13:17,730
preserved and buried that's what the

378
00:13:29,069 --> 00:13:26,400
I'm wondering if you actually need

379
00:13:31,889 --> 00:13:29,079
in the water or if you can do this

380
00:13:36,420 --> 00:13:31,899
process with oxygen production and

381
00:13:38,999 --> 00:13:36,430
benthic mats in these areas so one of

382
00:13:41,699 --> 00:13:39,009
the great things about benthic mats is

383
00:13:44,220 --> 00:13:41,709
that you concentrate the oxygen and can

384
00:13:47,429 --> 00:13:44,230
get very high values over extensive

385
00:13:52,670 --> 00:13:47,439
areas without actually oxidizing the

386
00:13:55,170 --> 00:13:52,680
water but and that oxygen does penetrate

387
00:13:56,100 --> 00:13:55,180
right no I think it's a good question I

388
00:13:58,199 --> 00:13:56,110
mean obviously that's gonna be

389
00:13:59,460 --> 00:13:58,209
restricted to a smaller part of the I

390
00:14:00,780 --> 00:13:59,470
mean the area's gonna be smaller because

391
00:14:02,730 --> 00:14:00,790
it has to be shallower because light has

392
00:14:04,079 --> 00:14:02,740
to penetrate but we don't have a good

393
00:14:06,660 --> 00:14:04,089
enough quantity but we don't have good

394
00:14:09,720 --> 00:14:06,670
enough quantity of you know idea to

395
00:14:10,559 --> 00:14:09,730
really to really get traction on that

396
00:14:12,300 --> 00:14:10,569
but I think that would be interesting

397
00:14:23,220 --> 00:14:12,310
question asked how much environment

398
00:14:25,889 --> 00:14:23,230
could you remember that way Amelia

399
00:14:28,639 --> 00:14:25,899
Hernandez UC Davis you've prevented

400
00:14:30,420 --> 00:14:28,649
you've presented a case of a stratified

401
00:14:32,040 --> 00:14:30,430
ocean at least in the nearshore

402
00:14:35,160 --> 00:14:32,050
environments

403
00:14:37,829 --> 00:14:35,170
what sort of physical processes are

404
00:14:40,139 --> 00:14:37,839
required to maintain the stratification

405
00:14:45,329 --> 00:14:40,149
like such as salinity as presented in

406
00:14:47,280 --> 00:14:45,339
the previous talk so I don't know how

407
00:14:48,449 --> 00:14:47,290
stable 'is stratified all this would

408
00:14:49,470 --> 00:14:48,459
have been these are kind of scenarios

409
00:14:50,970 --> 00:14:49,480
that I think would have been coming and

410
00:14:52,620 --> 00:14:50,980
going so I don't think you should be

411
00:14:53,929 --> 00:14:52,630
thinking of an ocean that is you know

412
00:14:56,999 --> 00:14:53,939
for a billion years

413
00:14:58,949 --> 00:14:57,009
oxidize the surface you clinic wedges

414
00:15:00,090 --> 00:14:58,959
and you know it's things are coming and

415
00:15:01,470 --> 00:15:00,100
going and moving and I think that's what

416
00:15:03,389 --> 00:15:01,480
that's what we see in the geologic

417
00:15:05,970 --> 00:15:03,399
record right we see this little

418
00:15:07,769 --> 00:15:05,980
miniature moons come and go so so what

419
00:15:10,170 --> 00:15:07,779
do you need you need to have a dynamic

420
00:15:14,309 --> 00:15:10,180
enough system where it doesn't get you

421
00:15:15,480 --> 00:15:14,319
know stuck but in terms of I mean you're

422
00:15:16,410 --> 00:15:15,490
asking specific like what salinity you

423
00:15:18,150 --> 00:15:16,420
need to have an oceans and how would

424
00:15:19,379 --> 00:15:18,160
that change things I mean the the

425
00:15:21,030 --> 00:15:19,389
dynamics would change if you change the

426
00:15:26,939 --> 00:15:21,040
salinity the rates of mixing would

427
00:15:30,509 --> 00:15:26,949
change you know so that's a good

428
00:15:31,499 --> 00:15:30,519
question hmm where do you go with I've

429
00:15:32,999 --> 00:15:31,509
looking at Stephanie cuz I'm trying to

430
00:15:36,550 --> 00:15:33,009
I'm thinking well we might move on to

431
00:15:39,580 --> 00:15:36,560
another talk sorry