News & Events

Nitrogen isotopic analysis of amino acids & record of increasing N2 fixation in subtropical Pacific

Monday, March 17, 2014, 12:00PM - 1:00PM


Owen Sherwood



ARC room 620

Full title

Nitrogen isotopic analysis of amino acids and the record of increasing N2 fixation in the subtropical North Pacific Ocean


Recent satellite data have shown that the ocean’s least productive waters—the vast, permanently oligotrophic, subtropical gyres—are expanding in size at the rate of 1 to 4% per year.  This raises important questions about the efficiency of oceanic uptake of CO2 in a warming world. The North Pacific subtropical gyre (NPSG) is the largest of these subtropical gyres. Paradoxically, primary production in the NPSG has increased in recent decades despite a reduction in nutrient supply to surface waters. This is thought to be the result of a shift in plankton community structure from mostly eukaryotes to mostly dinitrogen-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. Bulk and amino-acid-specific 15N/14N isotopic ratios (d15N) preserved in the skeletons of long-lived deep-sea proteinaceous corals collected from the Hawaiian archipelago serve as a proxy for the source of nitrogen-supported export production through time.  These records show that the recent increase in nitrogen fixation is the continuation of a much larger, centennial-scale trend. After a millennium of relatively minor fluctuation, d15N decreases between 1850 and the present. The total shift in d15N of -2 per mil over this period is comparable to the total change in global mean sedimentary d15N across the Pleistocene–Holocene transition, but it is happening an order of magnitude faster. In a steady-state model, 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 this increase might be linked to Northern Hemisphere climate change since the end of the Little Ice Age.


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