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carbon and oxygen

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are two elements which are critical to

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supporting life

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and these two elements are intimately

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linked by the process of photosynthesis

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this relationship is also evident in the

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geologic record

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in these two figures i'm showing the

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delta 13c of marine carbonates and

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atmospheric oxygen levels for the last

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three and a half billion years

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delta 13c is essentially the ratio of

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carbon 13 to carbon

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12 with more carbon 13 resulting in the

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in higher delta 13c when we look at the

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oxygen curve through time we can see

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that atmospheric oxygen rises in two

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major steps

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first in the paleo during what is called

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the great oxidation event

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and a second time during the

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neoproterozoic during what is called

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the neoprotozoic oxygenation event

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this figure shows that these major

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oxygenation events

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coincide with positive carbon isotope

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excursions in marine carbonates

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when we look at the delta 13c curve we

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can see that for most of earth history

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delta 13c has a baseline of around zero

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per ml

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so whenever delta 13c deviates from zero

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we call it carbon isotope excursion

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there is a reason to explain why

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increases in atmospheric oxygen

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are accompanied by positive carbon

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isotope excursions

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co2 is removed in two forms inorganic

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carbonate which has a relatively high

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delta 13c

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of around 0 per ml and organic carbon

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which has a low delta 13c

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around -25 per mil the ratio

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of total carbon that is buried as

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organic carbon is called

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f org and when f org increases

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more carbon is buried as organic carbon

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which raises oxygen

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due to the burial of photosynthetic

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products

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increases in f org also increase the

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delta 13c of marine carbonates

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because organic carbon is preferentially

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organic carbon preferentially takes up

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carbon 12

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in the carbon pool from which marine

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carbonates form

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so increased f org is one possible neck

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mechanism to drive the coupled increase

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of atmospheric oxygen

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and the positive carbon isotope

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excursions observed in the proterozoic

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and while the mechanism of increased f

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org

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driving positive carbon isotope

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excursions

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and oxygenation events is scientifically

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sound

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some research has brought it into

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question for one

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some studies have shown that oxygen may

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have begun to rise prior to the onset of

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the carbonite stop excursion

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making it difficult to explain both

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events with increased effort

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additionally these positive carbon

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isotope excursions are extremely

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long-lived

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lasting for hundreds of millions of

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years and it has been difficult to

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provide any biological mechanisms

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to sustain increased f-org for such long

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periods of times

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time scales of hundreds of millions of

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years align better with processes

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occurring in the mantel's deep carbon

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cycle and are on the same magnitude

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mantle overturn times so for the rest of

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the presentation

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i'll be talking about a model which uses

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carbon cycle processing

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occurring in the mantle to explain the

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co-evolution of carbon and oxygen

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at earth's surface so here i'll talk

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about how we can increase

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oxygen with no carbon isotope excursion

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so the main process that controls the

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flux of carbonate production is the

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weathering of continental rocks

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which deliver cations such as calcium

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and magnesium to the oceans

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which stimulates the precipitation of

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carbonates

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this weathering flux is sensitive to

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atmospheric co2

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and the weather ability of rocks which

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is controlled by

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composition land area and precipitation

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among other things so if the weathering

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flux increases

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the carbonate production flush also

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increases

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the production flux the production of

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organic carbon

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is also controlled by nutrients such as

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phosphorus which are derived from

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continental weathering

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can increase production of both

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carbonate and organic carbon

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and if they increase proportionally then

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there's no change in f

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org to accompany the increased burial of

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organic carbon

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and rise in atmospheric oxygen

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subsequently

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organic carbon and carbonate are buried

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on the seafloor and subducted deep into

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the mantle

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which is where we propose the carbon

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isotope excursion originates

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so after being subjected to the elevated

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temperatures in the mantle

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organic carbon will metamorphose to

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graphite so here i show

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how graphite and carbonate behave

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differently in the mantle

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this figure on the right shows how much

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of the originally subducted organic

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carbon

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how much of the originally subducted

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carbon will remain in this abducting

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slab

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as a function of pressure which can be

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thought of as how deep the slab has

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subducted into the mantle this figure is

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generated from

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high pressure experiments conducted on

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graphite and carbonate under mantle

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conditions

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the black curve shows graphite retention

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while the red and blue curves show

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carbonate retention

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this figure shows that carbonate is more

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efficiently released from subducting

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slabs compared to graphite

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and what this means is that carbonates

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subducted into the mantle

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are more easily recycled back to the

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surface compared to graphite

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which opens up the possibility of

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driving carbon isotope excursions by

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fractioning graphite

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and carbonate in the mantle so here's a

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schematic diagram

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to outline the mechanisms for driving

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carbon isotope excursions by deep carbon

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cycle processes

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this shows a cross section of the earth

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with three different volcanic centers

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mid-ocean ridges arc volcanoes which

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receive

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significant contributions from materials

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removed from the subducting slab

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and ocean island volcanoes which are

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thought to be fed by upwelling mantle

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plumes

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which carry material from the deep

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mantle

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and often contain subducted slabs

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mid-ocean ridges are are mostly sourced

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from depleted upper mantle

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and this is thought to contain mostly

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primordial carbon which has a delta 13c

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of around -5 per ml and most previous

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research has used this value of around

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minus 5 per ml

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for all volcanically emitted co2 and

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assume that this value never changes

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however this may not be the case as

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shown in the previous slide

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at sub-bar conditions carbonate is more

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easily removed from the subducting slab

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compared to graphite

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therefore arc volcanoes are likely to be

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enriched in carbonate derived

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co2 which has a delta 13c

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of around zero per ml therefore arc co2

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is likely to have delta 13c

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greater than minus 5 per ml so if the

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arc flux increases

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then the delta 13c of global co2

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emissions

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will increase and initiate a positive

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carbon isotope excursion

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now after subducting past subarc depths

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slab will have lost most of its

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carbonate

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and now will be relatively enriched in

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gravitized organic

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carbon these deeply subducted slabs may

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then become entrained in upwelling

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mantle plumes

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and contributes to ocean island

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volcanism releasing the subducted

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organic carbon

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this will result in a spike of co2

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emissions with delta 13c

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less than minus 5 per ml and this will

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decrease the delta 13c of global co2

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emissions

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and will terminate the positive carbon

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isotope excursion

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and possibly even drive a negative

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so now that i have outlined the

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mechanism for driving carbon

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carbon isotope excursions via deep

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carbon cycling i want to share some

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results from a box model we developed

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to account for these behaviors of carbon

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in the mantle i won't go into too much

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detail on all the flux and

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fluxes and reservoirs in the man in the

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model but i will outline some of the

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more

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some of the important assumptions of the

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model first

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the carbonate burial flux is controlled

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by the equation

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k times co2 to the nth power where k is

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a variable which accounts for how

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weatherable the continents are

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and co2 is the amount of co2 in the

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atmosphere

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f org is always kept at 0.2 throughout

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the model

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and atmospheric oxygen is proportional

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to the amount of organic carbon in all

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reservoirs

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co2 released at mid-ocean ridges is

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always minus five per ml

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and co2 release at rx

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is heavily influenced by the delta 13c

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of subducted carbonates

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while the delta 13cm co2 released at

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oceanine volcanoes

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is heavily influenced by subductor

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organic carbon

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next carbonates are recycled on the

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order of tens of millions of years

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while organic carbon is released on the

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order of hundreds of millions of years

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so now i'm going to walk you through the

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model results with some red shadings to

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help

265
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emphasize where the processes are

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occurring on the schematic and how it

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translates to the model

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so the top panel shows the delta 13c of

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marine carbonates with natural data in

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blue

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and model results in orange the second

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panel shows atmospheric oxygen levels

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through time

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with estimates with estimates based on

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proxy measurements in blue

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and model results in orange the third

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panel shows carbon fluxes from

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weathering or

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or carbon burial and different volcanic

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settings while the final panel shows k

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which is a variable variable that

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controls how sensitive

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continental weathering is to atmospheric

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co2

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now k is the only variable we are

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prescribing changes to in the model to

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generate the model

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run shown so at 2.4 billion years

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ago we increased k which increases the

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weathering flux

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00:10:06,310 --> 00:10:04,720
resulting in increased carbon burial and

292
00:10:08,790 --> 00:10:06,320
subduction

293
00:10:09,910 --> 00:10:08,800
this increased carbon barrel results in

294
00:10:12,550 --> 00:10:09,920
an increase in

295
00:10:14,710 --> 00:10:12,560
oxygen and shortly after there is

296
00:10:17,350 --> 00:10:14,720
increase

297
00:10:19,430 --> 00:10:17,360
in the arc co2 flux due to the release

298
00:10:22,069 --> 00:10:19,440
of subducted carbonates

299
00:10:22,710 --> 00:10:22,079
after the initial increase in k we

300
00:10:27,030 --> 00:10:22,720
prescribe

301
00:10:29,670 --> 00:10:27,040
k to decay to a lower value therefore

302
00:10:32,230 --> 00:10:29,680
the weathering flux also decays now the

303
00:10:35,750 --> 00:10:32,240
arc flux rapidly responds to the surface

304
00:10:37,750 --> 00:10:35,760
changes so the arc flux decays as well

305
00:10:39,430 --> 00:10:37,760
after about 300 million years the

306
00:10:41,750 --> 00:10:39,440
increased flux of organic carbon is

307
00:10:43,590 --> 00:10:41,760
released at ocean island volcanoes

308
00:10:45,350 --> 00:10:43,600
and since the arc flux has already

309
00:10:47,910 --> 00:10:45,360
decayed the ocean island flux

310
00:10:48,710 --> 00:10:47,920
dominates global co2 emissions and this

311
00:10:51,829 --> 00:10:48,720
results

312
00:10:53,829 --> 00:10:51,839
in a negative carbon isotope extrusion

313
00:10:55,030 --> 00:10:53,839
directly following the positive carbon

314
00:10:57,670 --> 00:10:55,040
isotope excursion

315
00:10:58,790 --> 00:10:57,680
which has been observed in the natural

316
00:11:01,910 --> 00:10:58,800
data

317
00:11:03,670 --> 00:11:01,920
now we repeat this k increase at 1.4 and

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00:11:06,069 --> 00:11:03,680
0.8 billion years ago

319
00:11:06,790 --> 00:11:06,079
and the model does a good job of

320
00:11:09,350 --> 00:11:06,800
reproducing

321
00:11:15,350 --> 00:11:09,360
both the delta 13c and oxygen curve for

322
00:11:17,110 --> 00:11:15,360
nearly the entirety of earth history

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00:11:19,190 --> 00:11:17,120
so i'd like to wrap up by talking a

324
00:11:21,350 --> 00:11:19,200
little bit about how this process is

325
00:11:24,550 --> 00:11:21,360
related to tectonic cycling

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00:11:26,949 --> 00:11:24,560
on the right i had a panel at the bottom

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00:11:29,509 --> 00:11:26,959
that shows how zircon count

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00:11:30,310 --> 00:11:29,519
that shows zircon count through time and

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00:11:31,750 --> 00:11:30,320
zircon

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00:11:34,310 --> 00:11:31,760
is a mineral that is predominantly

331
00:11:36,870 --> 00:11:34,320
produced at convergent margins

332
00:11:38,389 --> 00:11:36,880
therefore times of high zircon

333
00:11:40,310 --> 00:11:38,399
production are thought to signal

334
00:11:42,310 --> 00:11:40,320
supercontinent formation

335
00:11:45,269 --> 00:11:42,320
while times of low zircon production are

336
00:11:47,590 --> 00:11:45,279
thought to signal supercontinent breakup

337
00:11:49,190 --> 00:11:47,600
i put vertical br gray bands which

338
00:11:49,910 --> 00:11:49,200
encompass all periods of enhanced

339
00:11:51,910 --> 00:11:49,920
weathering

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00:11:53,110 --> 00:11:51,920
in the model and we can see that all

341
00:11:55,590 --> 00:11:53,120
high weathering periods

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00:11:56,310 --> 00:11:55,600
initiate during troughs in the zircon

343
00:11:58,550 --> 00:11:56,320
record

344
00:11:59,430 --> 00:11:58,560
which suggests that supercontinent

345
00:12:01,350 --> 00:11:59,440
breakups

346
00:12:02,949 --> 00:12:01,360
may be driving time periods of enhanced

347
00:12:04,550 --> 00:12:02,959
continental weathering

348
00:12:06,629 --> 00:12:04,560
and there are several reasons that

349
00:12:07,750 --> 00:12:06,639
supercontinent breakup may be tied to

350
00:12:10,310 --> 00:12:07,760
enhanced weathering

351
00:12:11,990 --> 00:12:10,320
for one continental breakups are

352
00:12:13,269 --> 00:12:12,000
accompanied by the eruption of large

353
00:12:15,509 --> 00:12:13,279
volumes of basalts

354
00:12:17,590 --> 00:12:15,519
which are highly weatherable and emit

355
00:12:19,430 --> 00:12:17,600
large amounts of co2 which can also

356
00:12:22,230 --> 00:12:19,440
enhance weathering

357
00:12:24,069 --> 00:12:22,240
second continental breakup increases the

358
00:12:26,710 --> 00:12:24,079
area of continental margins

359
00:12:28,310 --> 00:12:26,720
which are the primary locations for the

360
00:12:29,829 --> 00:12:28,320
deposition of organic carbon and

361
00:12:31,910 --> 00:12:29,839
carbonates

362
00:12:34,150 --> 00:12:31,920
so in conclusion we propose that

363
00:12:34,949 --> 00:12:34,160
supercontinent breakups drive enhanced

364
00:12:37,750 --> 00:12:34,959
weathering

365
00:12:37,990 --> 00:12:37,760
which leads to enhanced carbon burial

366
00:12:39,910 --> 00:12:38,000
and

367
00:12:42,470 --> 00:12:39,920
when this is coupled with a deep carbon

368
00:12:44,790 --> 00:12:42,480
cycling can explain oxygenation

369
00:12:45,829 --> 00:12:44,800
oxygenation events and carbon isotope