Benthic methane fluxes and oxidation over the Western Indian Shelf: No evidence of pelagic methanotrophic denitrification.

Despite its pelagic supersaturation and wide occurrence of gas-charged pockets in the sediments, methane occurs at unusually low concentrations in the shelf waters off Western Indian coast, even during euxinic events, compared to other anoxic coastal systems of the world. To understand the reason and benthic biogeochemistry of CH, we measured benthic CH flux rates through whole-core incubations, and carried out CH-spiked N-labeled incubations of the suboxic/anoxic shelf waters during 2011-2013. We observed very low rates of benthic CH influx or efflux (-0.23 to 0.37 μmol m d) during normoxia, and low to moderate rates of benthic CH efflux (2.45-74.89 μmol m d) during seasonal anoxia. However, high rates of the potential benthic CH efflux (23.82-154.25 μmol m d) implied massive benthic CH oxidation. The diffusing CH was oxidized within the sediments up to 99-100% during normoxia and up to 51-89% during anoxia. The benthic-released CH was further oxidized in the water column aerobically at the rate of 0.84 μmol m d during normoxia, and anaerobically at the rate of 0.16 μmol m d during anoxia. However, we did not find any evidence for NO (NO and/or NO)-dependent anaerobic CH oxidation (N-DAMO) i.e. methanotrophic denitrification in the shelf waters during anoxia, which suggested the potential role of Fe and Mn and/or SO as the alternative oxidants in the anaerobic CH oxidation necessitating further research. The total annual benthic CH- flux rate and oxidation rate from the entire innershelf were estimated to be 9.45 Gg y and 30.71 Gg y, respectively. Our study revealed that benthic- and pelagic anaerobic CH oxidation combined together reduce the water column CH concentration by 78-98%. Therefore the shelf sediments act as an effective CH filter by substantially reducing the benthic CH flux and ultimately the sea-air CH emission.