title: Increasing subtropical North Pacific Ocean nitrogen fixation since the Little Ice Age.
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.
authors: 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.
authors: Owen A
authors: OA
authors: Sherwood
authors: 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.
authors: Thomas P
authors: TP
authors: Guilderson
authors: Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.
authors: Fabian C
authors: FC
authors: Batista
authors: Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.
authors: John T
authors: JT
authors: Schiff
authors: Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.
authors: Matthew D
authors: MD
authors: McCarthy
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D000818:Animals; D037421:Anthozoa / Q000737:chemistry / Q000378:metabolism; D059001:Aquatic Organisms / Q000378:metabolism*; D057231:Climate Change; D017753:Ecosystem; D006254:Hawaii; D049668:History, 15th Century; D049669:History, 16th Century; D049670:History, 17th Century; D049672:History, 19th Century; D049673:History, 20th Century; D049674:History, 21st Century; D049690:History, Ancient; D049691:History, Medieval; D046448:Ice Cover; D009566:Nitrates / Q000032:analysis; D009586:Nitrogen Fixation; D009587:Nitrogen Isotopes / Q000032:analysis; D010140:Pacific Ocean; D010933:Plankton / Q000378:metabolism; D055110:Radiometric Dating; D012623:Seawater / Q000737:chemistry; D013997:Time Factors; D014329:Tropical Climate
chemical_list: D000596:Amino Acids; D009566:Nitrates; D009587:Nitrogen Isotopes