The Southern Ocean Biogeochemical Divide

As the picture above shows, the Southern Ocean is a region where nutrients important for plankton growth are high at the surface. This is surprising, if you leave a piece of steak on your lawn, it rapidly disappears! The reasons for this are controversial, but probably have something to do with a combination of lack of iron, low light levels, and low temperatures. The fact that there are so many unutilized nutrients in this region has important implications for the global cycle of carbon.

Tiny marine plants called phytoplankton take up nutrients and carbon in the surface ocean. When these organisms die, around 10 billion tons of carbon sinks into the deep ocean as particulate organic matter. As the particulate matter rots, the carbon in the organic matter gets turned into carbon dioxide. Because of this "biological pump" of carbon to the deep ocean, atmospheric carbon dioxide is lower than it otherwise would be. However, the high nutrient waters of the Southern Ocean, represent an "open window" through which carbon dioxide can escape back to the atmosphere, reducing the efficiency of the biological pump. An increase in the efficiency of the biological pump during glacial periods is the leading candidate for explaining why carbon dioxide was lower during glacial periods.

Recent work by Marinov, Gnanadesikan, Toggweiler, and Sarmiento shows that different regions of the Southern Ocean have very different impacts on the biological pump. This is because the upwelling deep waters split, with some portion going north and sinking to intermediate depths, and another portion going to the south and filling the deep ocean. Marinov et al. find a biogeochemical divide (see below) between these regions. The region to the north of the divide controls the magnitude of the biological pump in low latitudes. In other words, changes in the northern part of the circulation have the potential to affect tropical fisheries and ecosystems. By constrast, changes in the southern branch have virtually no direct impact outside the Antarctic, but have a big impact on atmospheric carbon dioxide.

Studies of changes in the biological pump have been bedeviled by the fact that it is difficult to find a mechanism that acts in both the Subantarctic zone (affecting the northern branch) and the Antarctic zone (affecting the Southern branch). One key result of this paper is that these regions can vary independently. Thus if one wants to explain lower glacial carbon dioxide, the key region is the Antarctic zone. If one is instead trying to explain past changes tropical productivity, the key region is the Subantarctic zone.

A second key result is the demonstration that changes in the Subantarctic zone have the potential to drastically impact the tropical zone. A number of investigators have suggested fertilizing the Subantarctic zone to increase krill production and draw down atmospheric carbon dioxide. These results suggest that such a strategy may have serious consequences for tropical ecosystems.

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References

Marinov, I., A. Gnanadesikan, J.R. Toggweiler, J.L. Sarmiento, The Southern Ocean Biogeochemical Divide,Nature, 441,964-967, 2006. Full text version (Nature subscribers)

Dunne,J.P., R.A. Armstrong, A. Gnanadesikan, J.L. Sarmiento and R.D. Slater, Empirical and mechanistic models for particle export ratio, Global Biogeochemical Cycles, 19, GB4026, doi:10.1029/2004GB002390, 2005. PDF

Toggweiler, J.R., R.J. Murnane, S. Carson, A. Gnanadesikan and J.L. Sarmiento, The strength of carbon pumps in box models, GCMs and the real ocean, Part 2: The biological pump, Global Biogeochem. Cyc. 17(1),1027, doi:10.1029/2001GB001841,2003. PDF