Significant flux of atmospheric nitrous oxide from the northwest Indian Ocean

Abstract

INTEREST in nitrous oxide (N₂O) has increased considerably in the light of its deleterious effect on the ozone layer¹, and contribution to the greenhouse effect². There are many sources of atmospheric N₂O, both anthropogenic (for example, combustion) and natural, but the global budget is still inadequately defined. Despite the fact that most of the world's oceans are close to equilibrium with the atmosphere³, water bodies depleted in oxygen have been identified as areas of N₂O production^4,5, and so the oceans represent a potential source of atmospheric N₂O. Here we report elevated N₂O concentrations in the northwest Indian Ocean, an area that exhibits upwelling and high oxygen consumption in the water column. We found that N₂O was supersaturated in both oxygen-saturated surface waters (up to 246% N₂O saturation) and oxygen-depleted sub-surface waters (1,264% N₂O saturation). The calculated flux to the atmosphere indicated that upwelling in the northwest Indian Ocean (15–25^° N) represents one of the most significant marine sources of N₂O, contributing between 5 and 18% of the total marine flux from a surface area of only 0.43% of the world ocean. These data suggest that the oceanic flux of N₂O to the atmosphere shows strong spatial heterogeneity which should be considered in global budgets and ocean–atmosphere models.