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

The production by microorganisms of nitrous oxide (N 2 O), 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 N 2 O. 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 N 2 O accumulation. This study found two‐ to ten‐fold greater accumulation of N 2 O during the autumn intermonsoon (November) than for other seasons described in earlier reports. The maximum N 2 O concentration (136 nmol kg −1 at 16°N, 88°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 N 2 O production, but earlier studies using nitrate or nitrite analysis did not confirm denitrification in the BoB. Large seasonal variation of N 2 O implicates the BoB as an important N 2 O source, similar to the Arabian Sea and eastern tropical Pacific, if the accumulated N 2 O is emitted to the atmosphere during the subsequent monsoon season. During the autumn intermonsoon, greenhouse gas/ozone depleting gas N 2 O accumulates around the oxygen minimum zone in the Bay of Bengal (BoB) Bacterial denitrification and archeal nitrification play important roles in the N 2 O production Large seasonal variation of N 2 O implicates the BoB as an important N 2 O source to the atmosphere