Gas hydrate destabilization and methane release events in the Krishna–Godavari Basin, Bay of Bengal

Abstract

Methane release events have been linked to global warming, alteration of the carbon cycle and influence on biota. However, unequivocal evidence of paleomethane release events are limited. We report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core (MD161-8) from the Krishna-Godavari (K-G) Basin, Bay of Bengal. The most negative δ 13 C spikes are recorded during the marine isotope stages MIS-4 and at the transition of MIS-5 to 4. Occurrence of highly 13 C depleted (average δ 13 C = -48±2.4 ‰ VPDB) authigenic high magnesian calcite are also reported within this time window from the core MD161-8. In the present work an unequivocal explanation for the observed 13 C depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical/ electron microscopy or C-O stable isotope ratio analyses due to possible influence of diagenetic alteration. We attribute the observed episodic methane expulsion events, as inferred from the negative δ 13 C excursions and earlier reports on the occurrence chemosynthetic bivalves and Mo concentration anomaly to the destabilization of the base of gas hydrate stability zone (BGHSZ). Sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here. Shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage. Shale and salt tectonics in the passive continental margins being a globally observed phenomenon, its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time. Additional evidence of methane emission from site MD161-15 further supports the link between shale tectonics and methane emission.

Coupled C–S–Fe geochemistry in a rapidly accumulating marine sedimentary system: Diagenetic and depositional implications

“…The δ 13 C TIC reported here is partially influenced by the presence of foraminifera with the calcareous test having δ 13 C value ranging from −1 to +1‰ [ Joshi et al ., ] and pure authigenic carbonates resulting from AOM could have been even more 13 C depleted. The δ 34 S CRS profile shows significant depletion in 34 S within the depth zone of 13–16 mbsf.…”

“…Approximately 6& down core enrichment in carbon isotope ratio of methane observed below SMTZ may be attributed to the continued preferential removal of the isotopically lighter molecules from the carbon pool (CO 2 ) during methanogenesis resulting in a progressive shift in the residual substrate toward enriched stable carbon isotope ratios following Rayleigh fractionation [Whiticar, 1999]. Mazumdar et al [2012aMazumdar et al [ , 2014b reported down core (below SMTZ) increase in residual d 13 C CH4 values ranging from 3 to 19& from K-G and Mahanadi basin cores. Much greater enrichment is expected at greater depths .…”

Tracing the Paleo sulfate‐methane transition zones and H2S seepage events in marine sediments: An application of C‐S‐Mo systematics

“…δ 13 C CH4 values (−60 to −85.5‰) suggest a possible biogenic origin of methane at the studied location [ Mazumdar et al , 2009]. The δ 13 C TIC reported here is partially influenced by the presence of foraminifera with the calcareous test having δ 13 C value ranging from −1 to +1‰ [ Joshi and Mazumdar , 2011; Joshi et al, manuscript in preparation, 2012]. High TIC contents (Figures 2c and 3c) corresponding in depth to the depleted carbon isotope ratios suggest the accumulation of authigenic carbonate in the sediment.…”

Short communication: Massive erosion in monsoonal central India linked to late Holocene land cover degradation

“…2) is representative of the Godavari continental slope (Mazumdar et al, 2009;Ramprasad et al, 2011;Joshi et al, 2014) and is driven by changes in the siliciclastic sedimentation rate as dilution by biogenic carbonates is less than 5 % ). Despite a lower sea level at the time, the flux was relatively small in the early Holocene (∼ 25 g cm −2 kyr −1 ) but began to increase after 6000 years ago (∼ 40 g cm −2 kyr −1 ), as soon as the monsoon started to decline but well before the adoption of Neolithic agriculture and settlement of the savannah zone of the central peninsula (∼ 4500 years ago; Fuller, 2011).…”