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So in summary, the N2-primed prokaryote pump and the stochastic event-driven diatom pump, are probably the most important with regard to discussions of ocean fertilization by iron.

I will now talk briefly about the Redfield Ratio pump because it is the one we're used to thinking about. The Redfield Ratio concept is a very important one in oceanography and I really don't have time to review the entire history this morning. Dr. Paul Falkowski, who is present here today, has recently written a beautiful summary (Falkowski 2000, J. Phycol. 36, 3-6). This concept assumes that there is a uniform biochemical stoichiometry of life in the ocean. Biology ultimately controls the nutrient elements in the sea, and there's been a cottage industry of studying the Redfield Ratio in the ocean.

Now this is all well and good, and there's plenty of evidence that the Redfield Ratio holds up in many parts of the world's oceans. However, N2 fixation and the process of denitrification (conversion of fixed N back into N2 gas) are two microbiological processes that can, over the long term, disrupt the expected Redfield Ratio.

Furthermore, under phosphorus (P) limitation, which can occur in many portions of the open ocean, the nitrogen-to-phosphorus (N:P) molar ratio in organisms can increase well in excess of the Redfield Ratio of 16N:1P, and under fixed N-limitation, it can decrease to values below the Redfield Ratio.

imageSo these "anomalies," if you will, to the mean climatology of the ocean are very important to our understanding of carbon sequestration in the sea. At the top of this slide, I've presented a schematic of the Redfield Ratio pump, wherein organic matter is produced and exported all in Redfield Ratio, and is subsequently remineralized with a similar C:N:P stoichiometry. The latter regenerated nutrient pool feeds back into the surface ocean to continue the Redfield-balanced production, export and remineralization cycle.

The Redfield stoichiometry pump has no potential for the net sequestration of carbon, because the carbon that flow out of the euphotic zone is equal to the carbon that flows back in, and everything is fully balanced.

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By comparison, I present an example of the role of N2 fixation, as effected via the N2-primed prokaryote pump. In addition to the nutrients upwelled from below, there is an enhancement of the N inventory via surface ocean N2 fixation. Because this new nitrogen enters the system without an equivalent amount of regenerated carbon, this provides the opportunity for the biological pump to export organic matter out of the normal Redfield balance, in favor of excess carbon (C-to-P ratios > 150 compared to a Redfield Ratio of 106C:1P), and N-to-P ratios > 25 compared to a Redfield Ratio of 16N:1P). Under these conditions there can be a net removal of C to the ocean's interior.

One thing about this very unusual N2-primed prokaryote carbon pump is that as export of C-rich and N-rich (relative to P) organic matter continues, there will eventually be a feedback as the exported non-Redfield materials that are remineralized beneath the euphotic zone begin to affect the total dissolved nutrient balance. When these high C:P/N:P materials find their way back into the surface ocean, it will shut down the N2-primed prokaryote carbon pump, because the N2-fixation process is controlled by the availability of fixed N relative to available P. This negative feedback keeps the system from drifting too far out of Redfield balance, and helps to explain why the world ocean is not dominated by N2 fixing microorganisms. At best, this must be considered a transient carbon pump lasting for, at most, periods of 1-2 decades at a time given the known residence time of nutrients in the subeuphotic zone waters.

imageThere are several very important key variables that one needs to understand in order to model the biological carbon pump. First, knowledge of the "e-ratio" or the export ratio; simply the particulate organic matter export scaled to contemporaneous primary production (i.e., e-ratio = C export/primary production). In other words, what percent of the carbon being produced at any one time is escaping from the euphotic zone?

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