Iron availability limits the ocean nitrogen inventory stabilizing feedbacks between marine denitrification and nitrogen fixation
Figure S2: The N* tracer (= ([NO3−] + [NH4+]) − 16 * [PO4−]) averaged over sub-euphotic zone depths (103–215 m) plotted over time for each of the case experiments at global and basin scales. (776.5Kb)
Figure S3: The minimum O2 concentration within the water column at each location plotted from the World Ocean Atlas 2001 data, from our control (year 2050) and case experiment simulations (year 200). (1.187Mb)
Figure S4: Spatial patterns for the N* tracer (= ([NO3−] + [NH4+]) − 16 * [PO4−]) averaged over euphotic zone depths (<103 m) for the WOA2001 and each of our case experiments. (1.165Mb)
Table S2: Global scale fluxes and inventories from our control simulation (year 2050) and the difference from our six case experiments. (2.122Kb)
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Recent upward revisions in key sink/source terms for fixed nitrogen (N) in the oceans imply a short residence time and strong negative feedbacks involving denitrification and N fixation to prevent large swings in the ocean N inventory over timescales of a few centuries. We tested the strength of these feedbacks in a global biogeochemical elemental cycling (BEC) ocean model that includes water column denitrification and an explicit N fixing phytoplankton group. In the northern Indian Ocean and over longer timescales in the tropical Atlantic, we find strong stabilizing feedbacks that minimize changes in marine N inventory over timescales of ∼30–200 years. In these regions high atmospheric dust/iron inputs lead to phosphorus limitation of diazotrophs, and thus a tight link between N fixation and surface water N/P ratios. Maintenance of the oxygen minimum zones in these basins depends on N fixation driven export. The stabilizing feedbacks in other regions are significant but weaker owing to iron limitation of the diazotrophs. Thus Fe limitation appears to restrict the ability of N fixation to compensate for changes in denitrification in the current climate, perhaps leading the oceans to lose fixed N. We suggest that iron is the ultimate limiting nutrient leading to nitrogen being the proximate limiting nutrient over wide regions today. Iron stress was at least partially alleviated during more dusty, glacial times, leading to a higher marine N inventory, increased export production, and perhaps widespread phosphorus limitation of the phytoplankton community. The increased efficiency of the biological pump would have contributed to the glacial drawdown in atmospheric CO2.
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 21 (2007): GB2001, doi:10.1029/2006GB002762.
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