Nitrogen isotopic studies in the suboxic Arabian Sea |
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| Authors: | S W A Naqvi T Yoshinari J A Brandes A H Devol D A Jayakumar P V Narvekar M A Altabet L A Codispoti |
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| Institution: | (1) National Institute of Oceanography, 403004 Dona Paula, Goa, India;(2) Wadsworth Center, New York State Department of Health, and School of Public Health, State University of New York at Albany, P.O. Box 509, 12201 Albany, NY, USA;(3) School of Oceanography, University of Washington, WA, 98195 Seattle, USA;(4) Present address: Geophysical Laboratory, Carnegie Institution of Washington, 20015 Washington, DC, USA;(5) Department of Chemistry and Biochemistry and Center for Marine Science and Technology, University of Massachusetts, Dartmouth, 02747 North Dartmouth, MA, USA;(6) Center for Coastal Physical Oceanography, Old Dominion University, 23529 Norfolk, VA, USA |
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| Abstract: | Measurements of15N/14N in dissolved molecular nitrogen (N2), nitrate (NO
3
−
) and nitrous oxide (N2O) and18O/16O in N2O expressed as δ15N and δ18O, relative to atmospheric N2 and oxygen (O2), respectively] have been made in water column at several locations in the Arabian Sea, a region with one of the thickest
and most intense O2 minima observed in the open ocean. Microbially-mediated reduction of NO
3
−
to N2 (denitrification) in the oxygen minimum zone (OMZ) appears to greatly affect the natural isotopic abundances. The δ15N of NO
3
−
increases from 6‰ in deep waters (2500 m) to 15‰ within the core of the denitrifying layer (250–350 m); the δ15N of N2 concurrently decreases from 0.6‰ to 0.20‰ Values of the isotopic fractionation factor (ε) during denitrification estimated
using simple advection-reaction and diffusion-reaction models are 22‰ and 25‰, respectively. A strong decrease in δ15N of NO
3
−
is observed from ∼ 200m (> 11‰) to 80m (∼ 6‰); this is attributed to the input of isotopically light nitrogen through nitrogen
fixation. Isotopic analysis of N2O reveals extremely large enrichments of both15N and18O within the OMZ, presumably due to the preferential reduction of lighter N2O to N2. However, isotopically light N2O is observed to accumulate in high concentrations above the OMZ indicating that the N2O emitted to the atmosphere from this region cannot be very heavy. The isotope data from the intense upwelling zone off the
southwest coast of India, where some of the highest concentrations of N2O ever found at the sea surface are observed, show moderate depletion of15N, but slight enrichment of18O relative to air. These results suggest that the ocean-atmosphere exchange cannot counter inputs of heavier isotopes (particularly18O) associated with the stratospheric back flux, as proposed by previous workers. This calls for additional sources and/or
sinks of N2O in the atmosphere. Also, the N2O isotope data cannot be explained by production through either nitrification or denitrification, suggesting a possible coupling
between the two processes as an important mechanism of N2O production. |
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| Keywords: | Stable isotopes nitrogen oxygen denitrification Arabian Sea |
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