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PALEOCEANOGRAPHY, VOL. 20, PA3003, doi:10.1029/2004PA001114, 2005

Diatom-bound ¹⁵N/¹⁴N: New support for enhanced nutrient consumption in the ice age subantarctic

Rebecca S. Robinson

Department of Geosciences, Princeton University, Princeton, New Jersey, USA

Daniel M. Sigman

Department of Geosciences, Princeton University, Princeton, New Jersey, USA

Peter J. DiFiore

Department of Geosciences, Princeton University, Princeton, New Jersey, USA

Melissa M. Rohde

Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada

Tracy A. Mashiotta

Byrd Polar Research Center, Columbus, Ohio, USA

David W. Lea

Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, California, USA

Abstract

Diatom-bound ¹⁵N/¹⁴N was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show δ¹⁵N of 5 to 6‰ during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2‰. The effect of either diatom assemblage or physiological change on the diatom-bound ¹⁵N/¹⁴N is unknown and cannot yet be ruled out as a possible explanation for the observed change. However, the consistency between Indian and Pacific sector records and with other paleoceanographic data suggests that the glacial-interglacial difference in diatom-bound ¹⁵N/¹⁴N was driven by higher consumption of nitrate in the subantarctic surface during the last ice age. Such a change in nutrient consumption may have resulted from atmospheric iron fertilization and/or decreased glacial mixed layer depths associated with sea ice melting. Enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of CO₂ in the ice age atmosphere. It also would have reduced the preformed nutrient content of the low-latitude thermocline, leading to decreases in low-latitude productivity, suboxia, and denitrification.

Received 9 November 2004; accepted 28 March 2005; published 22 July 2005.

Keywords: subantarctic; diatom-bound ¹⁵N/¹⁴N; glacial-interglacial CO₂.

Index Terms: 4912 Paleoceanography: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling (0470, 1050); 4870 Oceanography: Biological and Chemical: Stable isotopes (0454, 1041); 4806 Oceanography: Biological and Chemical: Carbon cycling (0428).

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Citation: Robinson, R. S., D. M. Sigman, P. J. DiFiore, M. M. Rohde, T. A. Mashiotta, and D. W. Lea (2005), Diatom-bound ¹⁵N/¹⁴N: New support for enhanced nutrient consumption in the ice age subantarctic, Paleoceanography, 20, PA3003, doi:10.1029/2004PA001114.

Copyright 2005 by the American Geophysical Union.