Research paperInfluence of total organic carbon deposition on the inventory of gas hydrate in the Indian continental margins

- Long term records of TOC, CaCO3, C/N, and δ13CTOC.
- Proximity to fluvial carbon sources and monsoon climate drive variations.
- TOC is a first order control on methanogenesis.
- High gas hydrate saturations likely linked to fluid advection.

Total organic carbon (TOC) content of marine sediments represents residual carbon, originally derived from terrestrial and marine sources, which has survived seafloor and shallow subseafloor diagenesis. Ultimately, its preservation below the sulfate reduction zone in marine sediments drives methanogenesis. Within the gas hydrate stability zone (GHSZ), methane production along continental margins can supersaturate pore fluids and lead to the formation of gas hydrate. In this paper we examine the inventory and sources of TOC in sediments collected from four regions within the GHSZ along the Indian continental margins. The recovered sediments vary in age from Oligocene to recent. Mean TOC abundance is greatest in the Krishna-Godavari (K-G) Basin and decreases progressively to the Mahanadi basin, Andaman wedge, and Kerala-Konkan (K-K) Basin. This decrease in TOC is matched by a progressive increase in biogenic CaCO3 and increasing distance from terrestrial sources of organic matter and lithogenic materials. Organic carbon sources inferred from C/N and δ13CTOC range from terrestrial (K-G Basin) to mixed marine and terrestrial (Mahanadi Basin), to marine dominant (Andaman wedge and K-K Basin). In the K-G Basin, variation in the bulk δ13CTOC is consistent with changes in C3 and C4 vegetation driven by monsoon variability on glacial-interglacial timescales, whereas in the Mahanadi Basin a shift in the δ13CTOC likely reflects the onset of C4 plant deposition in the Late Miocene. A large shift the δ13CTOC in the K-K basin is consistent with a change from C3 to C4 dominated plants during the middle Miocene. We observe a close relationship between TOC content and gas hydrate saturation, but consider the role of sedimentation rates on the preservation of TOC in the zone of methanogenesis and advective flow of methane from depth. Although TOC contents are sufficient for in situ methanogenesis at all the sites where gas hydrates were observed or inferred from proxy data, seismic, borehole log, pressure core, and gas composition data coupled with relatively high observed gas hydrate saturations suggest that advective gas transport may also play a role in the saturation of methane and the formation of gas hydrates in these regions. Although TOC content may be a first order indicator for gas hydrate potential, the structural and stratigraphic geologic environment along a margin will most likely dictate where the greatest gas hydrate saturations will occur.