Naqvi¹, S.W.A., D.A. Jayakumar², T. Yoshinari³, P.V. Narvekar¹, H. Naik¹, W. D’Souza¹, R. Alagarsamy¹ and L.A. Codispoti⁴

¹National Institute of Oceanography, Dona Paula, Goa 403 004, India, Tel: 91-832-2456700; Fax: 91-832-2456702, E-mail naqvi@nio.org, ²Guyot Hall, Geosciences, Princeton University, Princeton, NJ 80544, USA, ³Wadsworth Center, New York State Department of Health, and School of Public Health, State University of New York at Albany, P.O. Box 509, Albany, NY 12201, USA and ⁴Horn Point Laboratory, P.O. Box 775, 2020 Horns Point Road, Cambridge, MD 21613, USA

Nitrous oxide cycling in the Arabian Sea

Housing one of the ocean’s largest and best developed O₂-deficient zones, the Arabian Sea contributes very substantially to the global cycling of this important greenhouse and ozone-depleting gas. A large amount of data has been collected in the last 15 years under JGOFS and related programmes on both the concentration and isotopic composition of dissolved N₂O in the Arabian Sea. The results show extreme variability in N₂O distribution both in space and time. Large accumulation of N₂O seems to occur at the peripheries of the perennial suboxic zone of the open Arabian Sea, whereas the bulk of the denitrifying layer is characterized by N₂O depletion presumably due to its reductive loss to N₂. Upwelling and vertical mixing bring up N₂O-rich water to the sea surface sustaining high rate of emission to the atmosphere. The highest surface concentration (up to 436 nM) occurs within the narrow upwelling zone along the Indian west coast. However, this region, experiencing seasonal sub-surface O₂-deficiency during late summer and autumn, is distinguished by frequent N₂O accumulation in unprecedented levels (maximum ~800 nM) through denitrification. Moreover, even when denitrification results in a net loss of N₂O here, the isotopic fractionation is much smaller than in the suboxic zone of the open ocean where denitrification results in huge enrichment of heavier isotopes in residual N₂O (d¹⁵N and d¹⁸O values, relative to air, up to 37.5 and 83.6 per mil, respectively). Isotopic data collected during the summer and winter monsoon seasons show that the upwelled-water N₂O does not have an isotopic composition markedly different from that of the troposphere except when upwelling extends to the offshore denitrification zone. By contrast, the ventilation of heavy N₂O from the suboxic zone to the atmosphere is much more important during the winter monsoon. The observed spatial and temporal changes in the isotopic composition of N₂O emitting from the Arabian Sea imply a variable isotopic flux from the ocean.