Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Wang, Xingchen Tony, Sigman, Daniel M., Prokopenko, Maria G., Adkins, Jess F., Robinson, Laura F., Hines, Sophia K., Chai, Junyi, Studer, Anja S., Martínez-García, Alfredo, Chen, Tianyu, Haug, Gerald H.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: National Academy of Sciences 2017
• Subjects: Southern Ocean | nutrient consumption | atmospheric CO2 | ice ages; Antarctic; Southern Ocean; The Antarctic; Antarc*
• Online Access: https://doi.org/10.1073/pnas.1615718114; https://www.ncbi.nlm.nih.gov/pmc/PMC5380069

The Southern Ocean regulates the ocean's biological sequestration of CO_2 and is widely suspected to underpin much of the ice age decline in atmospheric CO_2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the ^(15)N/^(14)N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The central robust observation is of higher ^(15)N/^(14)N across the Southern Ocean during the Last Glacial Maximum (LGM), 18–25 thousand years ago. These data suggest a reduced summer surface nitrate concentration in both the Antarctic and Subantarctic Zones during the LGM, with little surface nitrate transport between them. After the ice age, the increase in Antarctic surface nitrate occurred through the deglaciation and continued in the Holocene. The rise in Subantarctic surface nitrate appears to have had both early deglacial and late deglacial/Holocene components, preliminarily attributed to the end of Subantarctic iron fertilization and increasing nitrate input from the surface Antarctic Zone, respectively. © 2017 National Academy of Sciences. Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved February 15, 2017 (received for review September 20, 2016). We thank two anonymous reviewers for their constructive comments. This work was supported by National Science Foundation Grants OCE-1234664 (to M.G.P. and D.M.S.), PLR-1401489 (to D.M.S.), and OCE-1503129 (to J.F.A.), the Charlotte Elizabeth Procter Fellowship of the Graduate School at Princeton University (to X.T.W.), the Grand Challenges Program of Princeton University (D.M.S.), European Research Council Grant 278705 (to L.F.R.), and Natural ...