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We report some main results of multidisciplinary investigations carried out within the framework of the Indian National Gas Hydrate Program in [2002][2003] in the Krishna-Godavari Basin offshore sector, east coast of India, to explore indicators of likely gas hydrate occurrence suggested by preliminary multi-channel seismic reflection data and estimates of gas hydrate stability zone thickness. Swath bathymetry data reveal new evidence of three distinct geomorphic units representing (1) a delta front incised by several narrow valleys and mass flows, (2) a deep fan in the east and (3) a WNW-ESE-trending sedimentary ridge in the south.Deep-tow digital side-scan sonar, multi-frequency chirp sonar, and sub-bottom profiler records indicate several surface and subsurface gas-escape features with a highly resolved stratification within the upper 50 m sedimentary strata. Multi-channel seismic reflection data show the presence of bottom simulating reflections of continuous to discrete character. Textural analyses of 76 gravity cores indicate that the sediments are mostly silty clay. Geochemical analyses reveal decreasing downcore pore water sulphate (SO 4 2− ) concentrations (28.7 to <4 mM), increasing downcore methane (CH 4 ) concentrations (0-20 nM) and relatively high total organic carbon contents (1-2.5%), and microbial analyses a high abundance of microbes in top core sediments and a low abundance of sulphate-reducing bacteria in bottom core sediments.Methane-derived authigenic carbonates were identified in some cores. Combined with evidence of gasescape features in association with bottom simulating reflections, the findings strongly suggest that the physicochemical conditions prevailing in the study area are highly conducive to methane generation and gas hydrate occurrence. Deep drilling from aboard the JOIDES Resolution during 2006 has indeed confirmed the presence of gas hydrate in the KrishnaGodavari Basin offshore.

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In order to investigate whether geochemical, physiographic and lithological differences in two end-member sedimentary settings could evoke varied microbe-sediment interactions, two 25 cm long sediment cores from contrasting regions in the Central Indian Basin have been examined. Site TVBC 26 in the northern siliceous realm (10°S, 75AE5°E) is organic-C rich with 0AE3 ± 0AE09% total organic carbon. Site TVBC 08 in the southern pelagic red clay realm (16°S, 75AE5°E), located on the flank of a seamount in a mid-plate volcanic area with hydrothermal alterations of recent origin, is organic-C poor (0AE1 ± 0AE07%). Significantly higher bacterial viability under anaerobic conditions, generally lower microbial carbon uptake and higher numbers of aerobic sulphur oxidizers at the mottled zones, characterize core TVBC 26. In the carbon-poor environment of core TVBC 08, a doubling of the 14 C uptake, a 250 times increase in the number of autotrophic nitrifiers, a four-fold lowering in the number of aerobic sulphur oxidizers and a higher order of denitrifiers exists when compared with core TVBC 26; this suggests the prevalence of a potentially autotrophic microbial community in core TVBC 08 in response to hydrothermal activity. Microbial activity at the northern TVBC 26 is predominantly heterotrophic with enhanced chemosynthetic activity restricted to tan-green mottled zones. The southern TVBC 08 is autotrophic with increased heterotrophic activity in the deepest layers. Notably, the bacterial activity is generally dependent on the surface productivity in TVBC 26, the carbon-rich core, and mostly independent in TVBC 08, the carbon-poor, hydrothermally influenced core. The northern sediment is more organic sink-controlled and the southern sediment is more hydrothermal source-controlled. Hydrothermal activity and associated rock alteration processes may be more relevant than organic matter delivery in these deep-sea sediments. Thus, this study highlights the relative importance of hydrothermal activity versus organic delivery in evoking different microbial responses in the Central Indian Basin sediments.

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Isotopic (210Pb, 238U‐230Th, 10Be), major and trace elements, and micromorphological and microchemical data, were used to identify recent (∼100 yrs) hydrothermal alteration of a >200 kyr sedimentary record from the flank of a seamount in the Central Indian Basin located at the edge of the 75°30′E fracture zone. Alteration effects are also reflected in 1) the depleted sedimentary organic carbon, 2) dissolution features of radiolarian skeletons, 3) the presence of altered minerals such as smectite and zeolites, and 4) distinctly different magnetic properties in the altered sediments. We interpret a predominant influence of neutral chloride type hydrothermal fluids. This is the first report of recently occurring sediment alteration by shallow circulating sub‐surface fluids along the Indian Ocean intra‐plate seamount environment.