LATE HOLOCENE PLANKTON DOMAIN SHIFTS IN THE NORTH PACIFIC SUBTROPICAL GYRE REVEALED BY AMINO ACID SPECIFIC δ13C AND δ15N RECORDS FROM PROTEINACEOUS DEEP-SEA CORALS

Abstract:

Recent observations from station ALOHA have framed a new paradigm about the dynamic nature of subtropical ocean gyres. These vast regions are now known to vary physically and biologically, over a range of timescales, with important implications for the export of carbon to the deep ocean. In the largest of these gyres, the North Pacific subtropical gyre (NPSG), primary production 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 recent plankton community domain shift can be linked to cyclical climate variability or a long-term global warming trend. To establish historical trends, we analyzed nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopic records preserved in the skeletons of extraordinarily long-lived, proteinaceous deep-sea corals, which feed on, and therefore serve as a proxy for, exported productivity. Specimens of Hawaiian gold coral (Kulamanamana haumeaae) were collected from the Hawaiian archipelago and sampled across the skeletal growth rings to generate high-resolution (5 yr), millennial-length records of “bulk” δ¹⁵N and δ¹³C. After a millennium of relatively minor fluctuation, δ¹⁵N decreased by up to 2 per mil between 1850 and the present. Analysis of amino-acid-specific δ¹⁵N on a subset of the samples, combined with isotopic mass balance between nitrate and nitrogen fixation, implied a 17 to 27 % increase in nitrogen fixation as the underlying cause for the observed trends. This interpretation is supported by analysis of the δ¹³C of essential amino acids, which serve as isotopic fingerprints of primary producer origin. Together, these independent lines of evidence describe a domain shift from a dominantly eukaryotic to dinitrogen-fixing prokaryotic plankton community. This shift has been ongoing since the end of the Little Ice Age, concurrent with increasing temperatures in the northern hemisphere, and most likely the result of increasing stratification and nutrient limitation in the NPSG. These results highlight the value of novel paleo-archives combined with amino-acid-specific isotope methods in addressing unresolved problems in marine biogeochemistry and climate change.