Abstract
A detailed geochemical investigation of a 5.25 m-long sediment core from the gas-rich shallow seasonally hypoxic shelf off the west coast of India (eastern Arabian Sea) provides the first robust geochemical evidence for deep-seated thermogenic methane (CH4) in this region. The \(\updelta^{13}{\text{C}_{{\text{CH}}_{4}}}\) values (–113.6 to –30.5‰), isotopically enriched higher hydrocarbons (C2–C4), low C1/C2 ratios (<70), together with down-core DIC concentrations and δ13CDIC values, collectively indicate a mixed biogenic–thermogenic CH4 origin, coupled with significant biogeochemical transformation of gas compositions. The down-core DIC concentrations and δ13CDIC values confirm active hydrogenotrophic methanogenesis below the sulfate–methane transition zone (SMTZ). The modified Bernard plot shows clustering of data points across the thermogenic–biogenic CH4 mixing curve, suggesting a mixed gas origin as well as biogeochemical transformation of gas compositions, namely the preferential consumption of CH4, which leads to a significant decrease in δ13C values of residual CH4 and in the C1/C2 ratios. The biogeochemical transformation of gas compositions is attributed to sulfate-dependent anaerobic oxidation of methane (SO42––AOM), with additional possibilities involving Fe–Mn-dependent AOM (Fe–Mn–AOM) below the SMTZ. The widespread occurrence of gas-charged sediments, coupled with the detection of thermogenic hydrocarbons, underscores the hydrocarbon potential of the western continental margin of India.
Research Highlights
First evidence of deep thermogenic CH4 in shelf sediments of west coast of India.
\(\updelta^{13}{\text{C}_{{\text{CH}}_{4}}}\) values −113.6 to −30.5‰ and C1/C2 of 1 to 70 indicate mixed gas sources.
Below sulfate methane transition zone, Fe–Mn oxides may drive intermittent AOM, altering hydrocarbon signals.
Fe–Mn–AOM below SMTZ may further deplete CH concentrations and shift δ13C, causing deviation from Bernard-plot.
