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: The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean 1. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters 2,3. It is thought that this apparent paradox can be explained by a shift in plankton community structure from mostly eukaryotes to mostly nitrogen-fixing prokaryotes 2-4. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend 5. Here we analyse records of bulk and amino-acid-specific 15N/14N isotopic ratios ([delta]15N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago; these isotopic records serve as a proxy for the source of nitrogen-supported export production through time. We find that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, [delta]15N decreases between 1850 and the present. The total shift in [delta]15N of -2 per mil over this period is comparable to the total change in global mean sedimentary [delta]15N across the Pleistocene-Holocene transition, but it is happening an order of magnitude faster 6. We use a steady-state model and find that the isotopic mass balance between nitrate and nitrogen fixation implies a 17 to 27 per cent increase in nitrogen fixation over this time period. A comparison with independent records 7,8 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

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