1. Ocean circulation, carbon cycle and oxygen cycle Anand Gnanadesikan FESD Meeting January 13, 2012

2. 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!

3. 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

4. Role of biology Nutrients are released, oxygen consumed when reaction runs backwards.

5. 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

6. The problem If vertical exchange increases… So does export. Which one wins?

7. 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

8. 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

9. What fraction of phosphate is associated with carbon?- Look at deep oxygen Extending this back we get nonzero “preformed” phosphate!

10. High preformed nutrients= high surface nutrients Points to dominant role of Southern Ocean, potential role for other regions.

11. When we look at preformed phosphate we see a much better relationship!

12. Do we have a theory to explain why preformed phosphate is such a good predictor?

13. So ocean could hold more carbon… 0.1 mmol/m 3 phosphate = 11.7 mmol/m 3 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.

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

15. So we can rewrite this equation

16. Theory fits models very well for fast gas exchange

17. Gas exchange allows for more carbon to be trapped

18. 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...

19. Balance of phosphate in HL box 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!

20. 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.

21. 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

22. In modern ocean, increasing diffusion or winds… Increases impact of Southern, high-preformed source

23. Tabular demonstration of this.. Note- carbon-13 shows similar behavior.

24. 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)

25. 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?

26. 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?