Here, we report for the first time, the genus Lamellibrachia tubeworm and associated chemosynthetic ecosystem from a cold-seep site in the Indian Ocean. The discovery of cold-seep was made off the Cauvery–Mannar Basin onboard ORV Sindhu Sadhana (SSD-070; 13th to 22nd February 2020). The chemosymbiont bearing polychaete worm is also associated with squat lobsters ( Munidposis sp.) and Gastropoda belonging to the family Buccinidae. Relict shells of chemosynthetic Calyptogena clams are ubiquitous at the seep sites. The Lamellibrachia tubes were found to be firmly anchored into the authigenic carbonate crusts. The authigenic carbonate crusts (chemoherm) are packed with large Calyptogena shells (whole shell and fragments). Very high concentrations (3800–12900 $\mu$M) of hydrogen sulfide (H$_{2}$ S) in the interstitial waters (40 cmbsf) is responsible for the sustenance of chemosymbiont bearing tubeworms. The posterior end of the tube penetrates downwards into the H$_{2}$ S-rich zone. The high concentration of H$_{2}$ S at ${\sim}$ 40 cmbsf is attributed to sulfate reduction via anaerobic oxidation of methane (AOM) pathway. Methane hydrate was observed within the faults/fractures in the sediments. The presence of ethane and propane along with methane in the headspace gases and $\delta^{13}$C$_{ CH4}$ values (–28.4 to –79.5per thousand VPDB) suggest a contribution of deep-seated thermogenic methane.

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-\grave{a}-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 (${\sim}$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.