Pore-water chemistry of sediment cores off Mahanadi Basin, Bay of Bengal: Possible link to deep seated methane hydrate deposit

• Linear type sulfate profiles suggest dominance of AOM induced sulfate reduction.
• δ13C values and C1/(C2 + C3) ratios suggest biogenic origin of methane.
• Enhanced Holocene sedimentation rate possibly affected the stability of methane hydrate.

Organoclastic degradation and anaerobic oxidation of methane (AOM) are known to influence sulfate, methane and bicarbonate concentrations of sediment pore-waters. In this work we have analyzed the pore-fluid composition of three piston cores (36–39 m long) collected from water depths ranging from 1429 to 1691 m. We attribute the observed linear sulfate concentration profiles and (ΔCa + ΔAlk)/ΔSO42− ratios dominantly to anaerobic oxidation of methane (AOM) induced sulfate consumption. The gas rich layers just below the base of hydrate stability zone (BGHSZ) is the possible source of the enhanced diffusive flux of biogenic methane (δ13CCH4δ13CCH4 ranging from −99.7 to −106.3‰ VPDB). Occurrences of disseminated gas hydrate in the Mahanadi Basin within the hydrate stability zone (210–220 m thick) supports the link between linear sulfate profile, high rate of AOM and hydrate occurrence.

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