Abstract
Repeat observations over the Kochi and Mangalore shelves of the southeastern Arabian Sea (SEAS) during April to December 2012 revealed substantial accumulation of methane (CH4) in the nearshore waters (48.6±34.4 nM) compared to the outer shelf (2.9±0.7 nM). Sediment methanogenesis and estuarine discharge appear to be the major sources of CH4 in the nearshore regions during non-upwelling period. But under oxygen deficient conditions that prevail during the upwelling period, extremely low concentrations of CH4 in the nearshore anoxic region of Mangalore (14±2 nM) compared to similar region of hypoxic Kochi shelf (35.5±15.4 nM) have been observed. We propose that this is mainly due to its greater loss through anaerobic oxidation and in part by the reduced sedimentary inputs by weak bioturbation over Mangalore relative to Kochi. On an annual basis, SEAS is found to be a net source of CH4 to the atmosphere with its efflux ranging from 0.03 to 170 µmol m-2 d-1 (21.9±36.7 µmol m-2 d-1). Following a zonal extrapolation approach, the estimated CH4 efflux from the SEAS (7-14oN; 3.2 Gg y-1) accounts for up to ~16 % of the total CH4 emission from the Arabian Sea.
Highlights
Coastal seas, important sites of methane (CH4) production, are estimated to contribute up to ∼20% emission of all greenhouse gases (IPCC, 2013)
The spatio-temporal variations in hydrographical properties, dissolved oxygen (DO) and nutrients observed during this study were presented earlier in detail by Gupta et al (2016) and Sudheesh et al (2016)
Upwelling off Kochi was found to initiate at the shelf break during January-March, progressed steadily to mid shelf by April and to inner shelf by May
Summary
Important sites of methane (CH4) production, are estimated to contribute up to ∼20% emission of all greenhouse gases (IPCC, 2013) Despite their relatively small geographical coverage (16%), the shelf region accounts for 75% of the oceanic CH4 emission (Bange et al, 1994). A good example for this is the eastern Arabian Sea (EAS) coastal (alias west coast of India) upwelling system, one of the most intense oxygen deficient shelf systems of world’s ocean, where the magnitude of CH4 emissions has not been well quantified. Naqvi et al (2005, 2010) have reviewed the importance and cycling of CH4 mostly based on the data generated during the Arabian Sea Process Study (1992–1997) They have reported that the coastal wetlands along the west coast of India potentially contribute CH4 to the nearby coastal regions. The study focused on (i) factors influencing the distribution and variability of CH4 concentrations, (ii) role of deoxygenation and macrofauna on CH4 concentrations, and (ii) its fluxes from the SEAS shelf
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