Here we report the discovery of cold-seep ecosystem and shallow methane hydrates (2-3 mbsf) associated with methane gas flares in the water column from the Indian EEZ for the first time. The seep-sites are located in the Krishna-Godavari (K-G) basin at water depths of 900-1800 m and are characterized by gas flares in the water-column images. The occurrence of methane gas hydrates at very shallow depths (2-3 mbsf) at some of the seep-sites is attributed to high methane flux and conducive P-T conditions, necessary for the stability of methane hydrate. Chemosymbiont bearing Bivalves (Vesicomidae, Mytilidae, Thyasiridae and Solemyidae families); Polychaetes (Siboglinidae family) and Gastropods (Provannidae family) are also identified from seep-sites.
The Krishna-Godavari, the Mahanadi, and the Cauvery basins comprise the major peninsular sedimentary basins along the western margin of the Bay of Bengal. The genesis of these pericratonic basins has been attributed to the Late Cretaceous break-up of the Gondwana land and initiation of the rift basins (Fuloria et al., 1992; Sastri et al., 1981). The upper Cretaceous to present sedimentary stratigraphies of these basins are described based on oil well drillings and other studies (
Pore fluid chemistry of sediment cores from two coring sites collected from Mahanadi basin (Bay of Bengal) during IODP-353 was studied to understand the intra-basinal variation in microbially mediated sediment biogeochemistry as well as methane hydrate occurrences. The ionic concentrations of SO 42shows quasi-linear profiles reflecting the steadystate situation in a dominantly diffusion controlled system. Pore-water (DTA + DCa 2+ + DMg 2+ )/DSO 4 2ratios and d 13 C DIC depth profiles suggest the influence of dual sulfate reduction pathways (organo-clastic and anaerobic methane oxidation) on the sediment fluid chemistry. d 13 C CH4 values indicate microbial origin of methane. The lowest d 13 C CH4 values in the present study are reported from the sulfate methane transition zone (SMTZ) and attributed to secondary methane generation via microbial recycling of 13 C depleted DIC produced through AOM. Based on the drop in Clconcentrations relative to the ambient sea-water, we predict the occurrence of disperse/pore-filling methane hydrates in the site U1445 proximal to the basin in which the SMTZ is slightly deeper from the surface compared to the distal (northern) U1446 site.
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