Changes in North Atlantic nitrogen fixation controlled by ocean circulation


In the ocean, the chemical forms of nitrogen that are readily available for biological use (known collectively as ‘fixed’ nitrogen) fuel the global phytoplankton productivity that exports carbon to the deep ocean1,2,3. Accordingly, variation in the oceanic fixed nitrogen reservoir has been proposed as a cause of glacial–interglacial changes in atmospheric carbon dioxide concentration2,3. Marine nitrogen fixation, which produces most of the ocean’s fixed nitrogen, is thought to be affected by multiple factors, including ocean temperature4 and the availability of iron2,3,5 and phosphorus6. Here we reconstruct changes in North Atlantic nitrogen fixation over the past 160,000 years from the shell-bound nitrogen isotope ratio (15N/14N) of planktonic foraminifera in Caribbean Sea sediments. The observed changes cannot be explained by reconstructed changes in temperature, the supply of (iron-bearing) dust or water column denitrification. We identify a strong, roughly 23,000-year cycle in nitrogen fixation and suggest that it is a response to orbitally driven changes in equatorial Atlantic upwelling7, which imports ‘excess’ phosphorus (phosphorus in stoichiometric excess of fixed nitrogen) into the tropical North Atlantic surface5,6. In addition, we find that nitrogen fixation was reduced during glacial stages 6 and 4, when North Atlantic Deep Water had shoaled to become glacial North Atlantic intermediate water8, which isolated the Atlantic thermocline from excess phosphorus-rich mid-depth waters that today enter from the Southern Ocean. Although modern studies have yielded diverse views of the controls on nitrogen fixation1,2,4,5, our palaeobiogeochemical data suggest that excess phosphorus is the master variable in the North Atlantic Ocean and indicate that the variations in its supply over the most recent glacial cycle were dominated by the response of regional ocean circulation to the orbital cycles.

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Figure 1: Core locations, surface winds, excess P at 20-m depth and main surface currents.
Figure 2: FB-δ15N in ODP Site 999 and its relationship to SST, dust flux and water column denitrification.
Figure 3: Comparison of FB-δ15N with changes in precession-paced equatorial Atlantic upwelling and glacial–interglacial Atlantic intermediate water source.
Figure 4: Effect of changes in glacial–interglacial circulation on N fixation in the Atlantic.


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We thank M. A. Weigand, S. Oleynik and S. Bishop for technical assistance and U. Röhl and V. Lukies for X-ray fluorescence scanning support. Funding was from SNF grant 200021-131886/1, US NSF grant OCE-1060947 and the Grand Challenges Program of Princeton University. This research used samples provided by the ODP, which is sponsored by the NSF and participating countries under the management of the Joint Oceanographic Institutions. X-ray fluorescence scanning was supported by the DFG-Leibniz Center for Surface Process and Climate Studies at the University of Potsdam.

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M.S., D.M.S. and G.H.H. designed the study. M.S. performed the FB-δ15N analysis and wrote the first version of the manuscript with D.M.S.; A.N.M. generated the Zr/Al record at ODP Site 658. A.M.-G. performed the statistical analysis of the data with M.S.; H.R. was involved in the laboratory and data analysis. All authors contributed to the interpretation of the data and provided significant input to the final manuscript.

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Correspondence to Marietta Straub.

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Straub, M., Sigman, D., Ren, H. et al. Changes in North Atlantic nitrogen fixation controlled by ocean circulation. Nature 501, 200–203 (2013).

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