Millenial-scale Plankton Regime Shifts In The Subtropical North Pacific Assessed By δ13C And δ15N Compound-specific Stable Isotope Analysis Of Deep-sea Corals

Kelton McMahon, Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, CA, United States, Thomas P Guilderson, Lawrence Livermore National Laboratory, Livermore, CA, United States; Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, Livermore, CA, United States, Matthew D. McCarthy, University of California Santa Cruz, Ocean Sciences Department, Santa Cruz, CA, United States, Owen Sherwood, University of Colorado at Boulder, Institute of Arctic and Alpine Research, Boulder, CO, United States, Thomas Larsen, Christian-Albrechts-Universität zu Kiel, Leibniz-Laboratory for Radiometric Dating and Stable Isotope Research, Kiel, Germany and Branwen Williams, Claremont Colleges, W. M. Keck Science Department, Claremont, CA, United States
Abstract:
Future climate change is predicted to alter marine phytoplankton communities and affect productivity, biogeochemistry, and the efficacy of the biological pump. As such, it is imperative to understand recent ecosystem changes in the North Pacific Subtropical Gyre (NPSG), the world’s largest continuous ecosystem, in the context of longer-term trends. We analyzed bulk and compound-specific carbon and nitrogen isotope data in long-lived, deep-sea proteinaceous corals from the Hawaiian archipelago to reconstruct high-resolution records of changing plankton community composition and biogeochemical cycling in the North Pacific Ocean over the past millennium. Essential amino acid–specific δ13C records revealed three major plankton regimes corresponding to northern hemisphere climate periods. Non-N2 fixing cyanobacteria dominated during the Medieval Climate Anomaly (~950-1250C.E.) before giving way to a new regime where eukaryotic microalgae contributed nearly half of export production during the Little Ice Age (~1400-1850C.E.). The third regime, unprecedented in the last millennium, began in the industrial era and is supported by increasing N2-fixing cyanobacterial production. By offering the first direct phylogenetic context for long-term shifts in isotopic records of exported particulate organic matter, our data represent a major new constraint in understanding the evolution of NPSG biogeochemistry. Our δ13C fingerprinting data, showing an increase in N2-fixing cyanobacteria carbon in export production (47%) since the end of the LIA, correspond well with recent evidence of a 17-27% increase in NPSG N2-fixation since ~1850C.E based on amino acid-specific nitrogen isotopes on the same suite of corals. Our new records, coupled with recent evidence that plankton C:P ratios are higher in oligotrophic gyres than upwelling regions, suggest that recent picoplankton community shift may provide a more efficient C pump and thus a negative feedback to rising atmospheric CO2.
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