Quantifying sources of gas hydrate through diagenetic modeling

Abstract Determining the sources of methane for hydrate formation is critical for hydrate exploration and for understanding its implications on the global carbon cycle, climate change, ocean chemistry, and the biosphere throughout Earth's history. However, quantifying the gas sources for ancient hydrate records presents a considerable challenge due to the absence of direct evidence. Authigenic carbonates (AC), formed through the anaerobic oxidation of methane (AOM), have partially retained the δ13C signature of ancient hydrates, providing a unique opportunity to identify their gas sources. In this study, we simulate the variations in δ13C values of AC precipitation associated with AOM under different AOM rates and specific methane source scenarios. Our findings suggest that the observed changes in δ13CAC with heavy δ18OAC (mostly ~2.0‰ to 7.7‰), across global compilations associated with AOM, are best explained by varying ratios of biogenic versus thermogenic methane and differences in AOM rates. Furthermore, our analysis indicates that thermogenic methane contributions at half of the investigated gas hydrate seeps range from 17.3% to 81.5% (average 47.1%; n=3) by volume. The compiled ages, coinciding with δ13C values of AC, suggest that amplified methane release, associated with an increased contribution from thermogenic methane and high AOM rates, preferentially occurred during interglacial periods over the past 150,000 years. The previously underestimated role of thermogenic methane, which likely played a positive feedback role in global warming, necessitates a reevaluation of the impact of methane seeps on global carbon cycles and climates dynamics throughout Earth's history.