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GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 19, GB4022, doi:10.1029/2005GB002458, 2005

Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin

Daniel M. Sigman

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


Julie Granger

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


Peter J. DiFiore

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


Moritz M. Lehmann

Geochemistry and Geodynamics Research Center (GEOTOP-UQAM-McGill), University of Quebec at Montreal, Montreal, Quebec, Canada


Ruby Ho

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


Greg Cane

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


Alexander van Geen

Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA


Abstract

Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3 −) 15N/14N and 18O/16O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ18O is up to 3‰ greater than in δ15N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO3 − δ18O and δ15N during NO3 − consumption. Moreover, the maximum in NO3 − δ18O is somewhat shallower than the maximum in NO3 − δ15N. These two observations can be summarized as an “anomaly” from the 1:1 δ18O-to-δ15N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: (1) the addition of low-δ15N NO3 − to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO3 − and NO2 − (coupled NO3 − reduction and NO2 − oxidation) in the suboxic zone.

Received 12 January 2005; accepted 31 August 2005; published 23 December 2005.

Keywords: nitrate; nitrogen cycle; stable isotopes.

Index Terms: 4802 Oceanography: Biological and Chemical: Anoxic environments (0404, 1803, 4834, 4902); 4805 Oceanography: Biological and Chemical: Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4912); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling (0470, 1050); 4870 Oceanography: Biological and Chemical: Stable isotopes (0454, 1041).

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Citation: Sigman, D. M., J. Granger, P. J. DiFiore, M. M. Lehmann, R. Ho, G. Cane, and A. van Geen (2005), Coupled nitrogen and oxygen isotope measurements of nitrate along the eastern North Pacific margin, Global Biogeochem. Cycles, 19, GB4022, doi:10.1029/2005GB002458.