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

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    Date
    2025
    Author
    Pratihary,A.K.
    Shenoy, D.M.
    Araujo, J.
    Shirodkar, G.
    Sahu, Sunanda
    Manjima, A.P.
    Naik, Richita
    Karapurkar, S.G.
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    Abstract
    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 CH4, we measured benthic CH4 flux rates through whole-core incubations, and carried out CH4-spiked 15N-labeled incubations of the suboxic/anoxic shelf waters during 2011 2013. We observed very low rates of benthic CH4 influx or efflux (?0.23 to 0.37 ?mol m?2 d?1) during normoxia, and low to moderate rates of benthic CH4 efflux (2.45 74.89 ?mol m?2 d?1) during seasonal anoxia. However, high rates of the potential benthic CH4 efflux (23.82 154.25 ?mol m?2 d?1) implied massive benthic CH4 oxidation. The diffusing CH4 was oxidized within the sediments up to 99 100% during normoxia and up to 51 89% during anoxia. The benthic-released CH4 was further oxidized in the water column aerobically at the rate of 0.84 ?mol m?2 d?1 during normoxia, and anaerobically at the rate of 0.16 ?mol m?2 d?1 during anoxia. However, we did not find any evidence for NOx? (NO3? and/or NO2?)-dependent anaerobic CH4 oxidation (N-DAMO) i.e. methanotrophic denitrification in the shelf waters during anoxia, which suggested the potential role of Fe3+ and Mn4+ and/or SO42? as the alternative oxidants in the anaerobic CH4 oxidation necessitating further research. The total annual benthic CH4- flux rate and oxidation rate from the entire innershelf were estimated to be 9.45 Gg y?1 and 30.71 Gg y?1, respectively. Our study revealed that benthic- and pelagic anaerobic CH4 oxidation combined together reduce the water column CH4 concentration by 78 98%. Therefore the shelf sediments act as an effective CH4 filter by substantially reducing the benthic CH4 flux and ultimately the sea-air CH4 emission
    URI
    https://doi.org/10.1016/j.marenvres.2024.106922
    http://drs.nio.res.in/drs/handle/2264/9414
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