First record of cold-seep induced enhanced water column methane concentrations from the EEZ of India

Abstract

A scientific expedition was carried out onboard RV Sindhu Sadhana (12th to 23rd February, 2020) for gas hydrate/cold-seep exploration in Mannar basin, northern Indian Ocean. We observed for the first time gas flares in the water column images of the multi-beam echo-sounder off Mannar basin. Water sampling was carried out by using CTD through the gas flares to understand the nature and distribution of the gas in the water column. The CTD operation was monitored using a sub-bottom profiler (SBP) to keep track of CTD location vis-�-vis the gas flare. High dissolved methane concentrations up to 48.7 nmol/L were detected at the depth of 1075 mbsl, within the gas cloud observed in SBP data, whereas very low dissolved methane concentrations (~0.6 nmol/L) were observed beyond the gas cloud. Enhanced methane concentrations in the water column can be correlated with the presence of cold-seep ecosystem and shallow gas hydrate in the Mannar basin. The stability of the methane bubbles within the gas flares may be attributed to the formation of a thin hydrate layer at the gas-water interface of the gas bubbles.