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Nature. 2014 Jan 2;505(7481):78-81. doi: 10.1038/nature12784. Epub 2013 Dec 15.

Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age.

Author information

1
1] Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA [2] Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309, USA.
2
1] Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA [2] Lawrence Livermore National Laboratory, Livermore, California 94550, USA [3] Institute for Marine Sciences, University of California, Santa Cruz, California 95064, USA.
3
Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.

Abstract

The North Pacific subtropical gyre (NPSG) plays a major part in the export of carbon and other nutrients to the deep ocean. Primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. 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. It remains uncertain, however, whether the plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. Here we analyse records of bulk and amino-acid-specific (15)N/(14)N isotopic ratios (δ(15)N) 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, δ(15)N decreases between 1850 and the present. The total shift in δ(15)N of -2 per mil over this period is comparable to the total change in global mean sedimentary δ(15)N across the Pleistocene-Holocene transition, but it is happening an order of magnitude faster. 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 suggests that the increase in nitrogen fixation might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.

PMID:
24336216
DOI:
10.1038/nature12784
[Indexed for MEDLINE]

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