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Originally published in Science Express on 18 December 2008
Science 9 January 2009:
Vol. 323. no. 5911, pp. 244 - 248
DOI: 10.1126/science.1165787
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Reports

Foraminiferal Isotope Evidence of Reduced Nitrogen Fixation in the Ice Age Atlantic Ocean

H. Ren,1* D. M. Sigman,1 A. N. Meckler,2 B. Plessen,3 R. S. Robinson,4 Y. Rosenthal,5 G. H. Haug6,7

Fixed nitrogen (N) is a limiting nutrient for algae in the low-latitude ocean, and its oceanic inventory may have been higher during ice ages, thus helping to lower atmospheric CO2 during those intervals. In organic matter within planktonic foraminifera shells in Caribbean Sea sediments, we found that the 15N/14N ratio from the last ice age is higher than that from the current interglacial, indicating a higher nitrate 15N/14N ratio in the Caribbean thermocline. This change and other species-specific differences are best explained by less N fixation in the Atlantic during the last ice age. The fixation decrease was most likely a response to a known ice age reduction in ocean N loss, and it would have worked to balance the ocean N budget and to curb ice age–interglacial change in the N inventory.

1 Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544, USA.
2 Geological and Planetary Sciences Division, California Institute of Technology, Pasadena, CA 91125, USA.
3 Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum (GFZ), Potsdam 14473, Germany.
4 Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA.
5 Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University, New Brunswick, NJ 08901, USA.
6 Geological Institute, Department of Earth Sciences, ETH Zürich, Zürich 8092, Switzerland.
7 DFG Leibniz Center for Earth Surface Process and Climate Studies, Institute for Geosciences, Potsdam University, Potsdam D-14476, Germany.

* To whom correspondence should be addressed. E-mail: hren{at}princeton.edu

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