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[Music]

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[Applause]

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thank you very much I can't speak for my

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co-author

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but I'm from Earth that is to mean my

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background is looking at the earth and

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looking at the early Earth and paleo and

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I build model tools for looking at some

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paleo questions primarily it means that

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when looking at exoplanets the tools are

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well-established but they bring with

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them a lot of baggage of earth-like

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assumptions and overall this this work

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is it's somewhat a work in progress as I

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try and apply tools really designed for

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Earth to look at non earth-like

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situations

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I sort of wish this wasn't a was a joke

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but it's actually sort of my perspective

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of at least how I build a model tools

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there's a rotating rock it's got a

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bathtub of water that sloshes around and

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that's sort of the important bit for

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this talk and for a lot of sort of

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questions in in in past evolution of

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life in the ocean and logical cycles and

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things unfortunately there are no rubber

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ducks these are actually smaller than

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the grid of the models so they're not

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resolved well the duck really represents

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there is at least on on earth we we

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don't have all water and there is like a

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very convenient amount of land that come

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whether it can supply nutrients it can

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supply cations also things to the ocean

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that can fuel biology and can lead to

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things being deposited and lost to the

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system that are important and I sort of

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concentrate on and this is sort of like

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the thinking for the talk is is what

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goes on within the ocean so we have some

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at least on earth on non fully water

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world plants we have significant supply

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of nutrients to the ocean importantly

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these are very heavily recycled today

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and and through much of Earth's history

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there is productivity at the at the

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ocean surface and you op taking

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nutrients fixation of carbon material

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sinks with Mary with very may be varying

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efficiency through

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Earth's history typically recycled very

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rapidly near the surface and then

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material a small fraction maybe 5% is is

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reaching a number of kilometers depths

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and in today's ocean maybe because the

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compounds are federate Kallstrom because

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the material is is part of our gets sink

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very quickly or maybe there is some

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protection of compounds absorbed within

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within minerals in today's ocean ocean

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circulation on earth and other planets

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will be very important because sustained

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over time in a very even a very small

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fraction of nutrients and material

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reaching that the bottom of the ocean or

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on a planet will tend to then accumulate

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and you need the nutrients back at the

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surface if your if your to have a very

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vigorous and cycle of biochemical

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cycling why does that matter if you can

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bring in a lot of nutrients you could

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lose nutrients and if you can lose

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nutrients you can lose carbon with the

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nutrients and if you're losing carbon

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effectively you're releasing oxygen so

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the like honor the earlier earth perhaps

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mostly the the burial of organic Matt

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Carpenter over the rise of oxygen and

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the great transition of the oxidation of

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the Earth's surface the formation of

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ozone and then sort of potentially

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biosignatures from that the other the

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other thing that a vigorous logical

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cycle in in the in the ocean and the

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earth or any ocean can do for you it can

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create significant heterogeneity in for

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instance redox you have the release of

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oxygen at the ocean surface is gonna be

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released into the atmosphere if you have

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a very vigorous biogeochemical cycle

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that consumes all the oxygen further

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down in the ocean interior you might

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produce hydrogen sulfide dissolved in

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the water you might produce methane once

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your if you have like a low sulfate

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ocean there is the potential of these

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waters that are high in hydrogen sulfide

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or methane also to be exposed to the

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atmosphere you could have the release

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both both of oxygen and hydrogen sulfide

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or methane so you can have species that

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really shouldn't be together being

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really stems here at the same time

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and and this could also provide ways of

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detecting life anyway so on a water only

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world you don't have the luxury of

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overland and weathering and supply of

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fresh nutrients there will be some sort

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of supply of dust micrometeoroids being

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obliged in in the atmosphere but this is

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pretty small on the earth today compared

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with with weathering you may have

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release of nutrients and substances from

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the ocean floor from hydrothermal

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systems but that's at the bottom of the

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ocean you've got to get this back up to

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the surface

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so so really just saying on on an ocean

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world even more than than today it's

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critical that the ocean mixes pretty

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well either you want the materials

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released at the ocean floor to be

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efficiently moved to the surface so

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they're available to photosynthetic life

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I would just say I'm here I'm assuming

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for the purpose of thinking about how

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productive a water world might be in

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terms of life I'm thinking of

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photosynthetic life and if you have a

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very small flux of nutrients to the

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surface from space you don't want this

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loss nor the bottom the ocean again you

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want you want an ocean circulation but

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that those things around quite quite

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nicely and efficiently so here's a

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question well how deep might the ocean

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on the water will be I mean we we sort

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of think the earth sort of frames I

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think a lot of work in terms of ocean

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depth and the model tools the previous

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talk was really interesting this is a

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very common thing to have a cellphone

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model with a few kilometers of ocean

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depth I'm trying to think here a little

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bit more like well how deep might the

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ocean be and what that what are the

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consequences are much deeper than than a

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modern ocean sort of really thinking my

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motivation for the entire talk was just

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thinking is is the is the is a depth of

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an ocean on the water world such that

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you wouldn't actually return material

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from the depth so if you had a very

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small nutrient supply to the ocean

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surface and you lose a very small

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fraction that's a great depth then you

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never return that then you've got a

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really very low production biosphere you

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have very little capable

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of burying organic matter were using

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that oxygen making any sort of great

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spatial heterogeneity and things in the

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ocean and and sort of your ability to

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detect sort of biosignatures may be very

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very limited so I can sort of consider

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just really sort of ocean depth and then

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I can consider some other factors and

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that might help control the circulation

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principally energy at the surface and

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energy at the at the ocean for the model

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is it's it's an earth model and there

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are caveats to that and there are the

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issues with with with spin up and the

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generality of the physics this model is

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run on a very low resolution just to

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start to explore the parameter space the

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grid is only eighteen by by eighteen at

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the surface and as I'll show the

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variable number of layers there are all

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sorts of modern earth assumptions

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entrained in this it's just gonna be a

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normal ocean you know normal salty ocean

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it's gonna be modern nutrients you know

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I'm really just considering a modern

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earth-like ocean but covering the whole

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planetary surface but now but just

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really just thinking about the variable

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depth the ocean and this is my favorite

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plot because there's no both imagery in

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this model if there is on a order water

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world what is the better retreat you

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have spreading ridges on a water world

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you have like volcanic surrounds is it's

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smooth at the bottom you know the answer

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could be anything you want so this is

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gonna be a smooth ocean bottom for that

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purpose of this talk this model also

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differs from the one in the previous

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talk CSM in that this is very simply

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forced at the surface I do not have a

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dynamical atmosphere which which saves

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me a lot of computational time so it's

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having a zonal in stress profile applied

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to drive ocean circulation I'm almost

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always also assuming a very simple as in

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the average planetary albedo and

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in the absence of the dynamical

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atmosphere in clouds I'm running the

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model for 10,000 years I'm spinning up

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from cold which means that the ocean is

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is starting at 0 degrees and we'll just

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see how it pans out after that so a

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typical modern cell the constant modern

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co2 concentration atmosphere which would

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have sea ice that there is some sea ice

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at each Paul that spreads out a little

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bit but but a little bit like the zones

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today so how deep should it be well the

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mean depth of today's oceans about 3.5

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kilometers if you're going to take away

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all the crayons and make it a true water

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world the average eques don't you about

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2.5 kilometres typically in the the

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modern model I use the the maximum depth

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is 5 kilometers with 16 levels I'm going

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to keep the layer structure so that I'm

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not changing the thickness of the

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surface layer where life lives I'm

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basically just going to add more and

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more layers deeper down and I'm going to

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test these difference not all have

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successfully run but all these these

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difference assumptions about ocean depth

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so the 2.5 is just the water on today's

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ocean spread over a smooth billiard ball

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there is a 3.5 which is the the mean

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depth of today's ocean 5 which is which

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is a typical reasonably deep bit of the

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ocean today I'm going to just keep sort

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of adding levels 10 15 21 I've not yet

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successfully run 13 kilometer deep

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oceans but it's it's a it's a technical

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spin up problem the max in the phase

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diagram at least in this off Wikipedia

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the phase diagram of water in in

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temperature and pressure space you could

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you could get to a hundred kilometers or

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so before you start worrying about the

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water becoming ice so and you just

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pursue just show you very briefly the

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initial results of some little ensemble

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experiments where I'm just testing the

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different configurations the model 2.5

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3.5 etc kilometer ocean depth of a pure

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water world with otherwise modern like

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orbits and solar constant and density of

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the water and

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and salinity etc and in this grid of

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different su-24 things so this is

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relative to pre-industrial so x 1.is

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would be a climate like today I'm going

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to do x 4 x 16 I'd also make it a little

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bit colder so a little bit below the

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last glacial in terms of at least in

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terms of greenhouse gas forcing a

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hundred milli watts is the modern sort

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of mean geothermal heat input so I've

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got a similar like factor of four each

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time so 25 is is getting close to

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nothing four times and 16 times the

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modern geothermal heat input so really

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I'm trying to different things I know

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trying heat or call it from the surface

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I'm going to try different sort of

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heating's from them from the ocean floor

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and like I say so far in these very

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initial experiments because it's a it's

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a modern earth earth system model and

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when trying a 32 kilometer

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spin up there are some teething problems

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so there are going to be some plots

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where the computer is cheating at

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tic-tac-toe and not all the results of

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them so this is the simplest case of

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just the water on the modern earth we

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disputed over a billiard ball this is

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just a plot of sea ice extent so there's

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there's nothing you wouldn't guess here

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at all low C you to there's more sea ice

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highest you to that's less yes so I

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don't think there's a Nobel Prize in

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this there is some impact of this change

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of the geothermal Andy yep I'm almost

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done

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there is no it's like I say there is

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some effect of the the geothermal heat

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input so there's less sea ice as you go

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to 16,000 milli watts or 1600 milli

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there's sort of the sort of the most

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useful results so far this is the the

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export production of these these

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different water worlds you had the

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highest export production in a inner

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planet or in a world where you have a

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fairly cold forcing at the surface but

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you have high geothermal heating but so

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it's really maybe not very surprising

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but but here we can start to quantify

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what sort of how much more biological or

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productivity would you have in a world

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where you dis stabilized you

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deliberately destabilize the ocean by

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having higher higher heat input at the

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surface and lower co2 and cooling at the

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at the surface high you know he didn't

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go to depth there cooling at the surface

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and the rest really they're not all spun

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up with a case that the game really is

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just a run through the different

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ensembles different configurations look

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at how the patterns change from one

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configuration to going to a deeper and

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deeper ocean to another so they didn't

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all pan out and the the very deepest

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ones there are still some issues

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spinning up such a system

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so really the other thing I've taken

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away you initially is is just the the

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thought may be that younger planets

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water worlds might have a much higher

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productivity a much more potential for

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biosignatures in in that the the Sun

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will tend to be have less output so you

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might tend to have a like less less

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heating at the surface but you have

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mobile radio to decay still in the ocean

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in the planetary interior to heat the

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the bottom of the ocean so you have a

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situation where you have much more

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vigorous circulation and much more

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recycling of what nutrient supply there

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is and thereafter your planets of Ages

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calls inside the Sun warms up you tend

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to go progressively to a to a less

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vigorous circulation and and a lower

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productivity and potential for

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biosignatures

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thank you

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[Applause]

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okay we have time for one very quick

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question yeah in the case where you have

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a lot of pressure therefore you have a

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the rock and then you have the ice on

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top of it but don't you have enough heat

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coming out of that rock that it would

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make either cracks or plumes through

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that ice and so therefore wouldn't be a

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very effective barrier to the nutrients

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that you're trying to get up so the the

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ice with sea ice at the surface I I

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don't have any ice at depth so the the

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the depth of the ocean is not sufficient

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to have ice between the water and the

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rock and so far the equation of state is

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very very simple so I'd not aggressed

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too far beyond taking a model developed

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really for the last few tens of millions

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of years to more generalized conditions

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on on other worlds okay let's thank the