Extensive Accumulation of Nitrous Oxide in the Oxygen Minimum Zone in the Bay of Bengal

The production by microorganisms of nitrous oxide (N2O), a trace gas contributing to global warming and stratospheric ozone depletion, is enhanced around the oceanic oxygen minimum zones (OMZs). The production constitutes an important source of atmospheric N2O. Although an OMZ is found in the northern part of the eastern Indian Ocean, the Bay of Bengal (BoB), two earlier studies conducted during the later phase of winter monsoon (February) and spring intermonsoon (March-April) found quite different magnitudes of N2O accumulation. This study found two-to ten-fold greater accumulation of N2O during the autumn intermonsoon (November) than for other seasons described in earlier reports. The maximum N2O concentration (136 nmol kg (-1) at 16 degrees N, 88 degrees E) is comparable to those observed around the OMZ in the Arabian Sea or eastern tropical Pacific. Isotopic signatures suggest that bacterial denitrification and archeal nitrification play important roles in N2O production, but earlier studies using nitrate or nitrite analysis did not confirm denitrification in the BoB. Large seasonal variation of N2O implicates the BoB as an important N2O source, similar to the Arabian Sea and eastern tropical Pacific, if the accumulated N2O is emitted to the atmosphere during the subsequent monsoon season.