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

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 [sup.15]N/[sup.14]N isotopic ratios ([δ.sup.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.sup.15]N decreases between 1850 and the present. The total shift in [δ.sup.15]N of -2 per mil over this period is comparable to the total change in global mean sedimentary [δ.sup.15]N 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. Gly 0.99 0.31 1.00 0.13 Glx 0.92 0.43 0.88 0.23 -0.01 0.03 Gly 0.36 0.005# 0.78 Glx 0.20 0.049# 0.46 0.001# 0.00 0.880
Recent satellite observations have shown that globally, the permanently oligotrophic subtropical gyres are expanding at a rate of 1% to 4% per year, generally commensurate with global decreases in net [...]