Ocean circulation, carbon cycle and oxygen cycle

Anand Gnanadesikan

FESD Meeting

January 13, 2012

How much carbon does the ocean actually hold?

Ocean actually holds about 34000 GtC

Atmosphere only holds 800 GtC.

What form is this carbon in?

Average concentration ~2000 mmol/m3!

32322 2HCOCOOHCO

Why so much carbon?- Alkalinity

1. Ocean can hold a lot of carbon- because carbon dioxide is buffered by carbonate ion.

2. Increasing carbon dioxide will result in reducing the amount of carbonate ion (ocean acidification).

Orr et al., Nature, 2005

Role of biology

Nutrients are released, oxygen consumed when reaction runs backwards.

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The setup• Vertical exchange

moves water high in nutrient and carbon low in oxygen into surface layer.

• Biology takes up carbon, produces oxygen.

• “Biological pump” exports carbon to deep ocean.

• Remineralization results in oxygen consumption in deep ocean

The problem

• If vertical exchange increases…

• So does export.

• Which one wins?

Diagnostic ocean models• Restore ocean surface to observed values of T,S,

nutrients. • Apply “observed” fluxes of momentum, heat, freshwater• Predict internal structure, flows using dynamics.• Run depletion scenarios setting nutrients to zero

What do we see from a suite of models

• Solid points have less efficient biology (longer restoring time).

• As we move to right we have higher vertical exchange.

• No clear relationship between productivity and and CO2.

• Increasing vertical exchange gives higher CO2.

Marinov et al., GBC, 2008

What fraction of phosphate is associated with carbon?- Look at deep oxygen

Extending this back we get nonzero “preformed” phosphate!

High preformed nutrients= high surface nutrients

Points to dominant role of Southern Ocean, potential rolefor other regions.

When we look at preformed phosphate we see a much better

relationship!

Do we have a theory to explain why preformed phosphate is such a

good predictor?

zedRemineraliPreformedTotal

zedRemineraliPreformed0 dd

0)()(:2 solDICPPRMpCO eqpreftotalPCa

So ocean could hold more carbon…

0.1 mmol/m3 phosphate = 11.7 mmol/m3 C

~200 Gt C ~90 ppmv (!)= 900 ppmv/M

But… remember the buffering equation

So over a long time, much of the response to changing preformed nutrients will be compensated.

Additionally, time scale for this to be realized is limited by supply of preformed nutrients to the surface.

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Extending this idea

0)(:2 oceqocprefPCa MsolDICMPRMpCO

RDIC

pCO

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)(ln 2

22 )/( pCORpCODICDIC eqeq

Revelle Buffer factor ~12 so that a 12% decrease in pCO2 leads to a 1% decrease in solubility-induced carbon burden.

So we can rewrite this equation

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CRC

COCSC

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pCOOCSC

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Theory fits models very well for fast gas exchange

Gas exchange allows for more carbon to be trapped

So what sets the preformed nutrient?- 3 Box Model

In this model the preformed nutrient is just the value from the high latitude box.

If low latitude surface nutrients are essentially zero...

Balance of phosphate in HL box

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MPMPMP

ProdTPPPf

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Key idea that emerged from this model was that increased productivity or decreased vertical exchange during last ice age could explain lower values.

But in our models surface nutrients aren’t changing much!

Preformed nutrients in a four-box model

Now we can change pCO2 by changing the mix of waters coming from the high preformed south vs. the low preformed north.

Implication

• If total exchange drops, but high-nutrient south drops more than low-nutrient north– Total productivity drops– Carbon storage increases– Oxygen in deep ocean goes down.

• This is what happens under global warming in our coupled model

In modern ocean, increasing diffusion or winds…

Increases impact of Southern, high-preformed source

Tabular demonstration of this..

Note- carbon-13 shows similar behavior.

Final point

• What holds for carbon holds for oxygen in reverse.

• To get low oxygen in the global ocean, we need – Deep water from a warm source (low

preformed oxygen for given nutrient).– Deep water from a low-nutrient source (high

remineralized carbon)

Questions for FESD

• With only one continent, what does the balance of deep waters look like?

• With a shallow marginal sea at low latitudes what does the balance of deep waters look like?

• With a narrow Atlantic, what does the balance of deep waters look like?

Questions for FESD

• Theory assumes constant R (i.e. constant alkalinity). What happens as alkalinity changes?

• Is there a possibility of changing P? By how much?