METHANE INDEX: TOWARDS A QUANTITATIVE PROXY FOR PAST MARINE ANAEROBIC METHANE OXIDATION

Anaerobic methane oxidation (AOM) carried out by consortia of sulfate-reducing bacteria and methanotrophic archaea (ANME groups), represents a major sink (~90%) for methane in the ocean. Examining the variability of AOM in Earth's history is therefore fundamental to understanding the global carbon and sulfur cycle. Diagnostic lipid biomarkers of microorganisms associated with AOM have been widely used to reconstruct the past methane cycling, yet are limited to confirming the presence of AOM. Based on the rationale that a major group of methanotrophic archaea (ANME-1) preferentially synthesizes certain tetraether lipids, Methane Index (MI) has been used as a qualitative indicator of ancient methane impact on marine sediments. Since MI can be calculated simultaneously when archaeal lipids are used for TEX₈₆ – a popular ocean temperature proxy, MI has been widely applied on different timescales from all major parts of the world’s ocean. Here, we report new data and compile existing data to show that MI is quantitatively linked to sedimentary methane flux in the modern ocean. High MI values are strongly associated with high methane and sulfate diffusive fluxes, but insensitive to small amounts of free methane in the pore water. When accounting for little variability in environmental factors (e.g., seawater sulfate concentration, geothermal gradient) and sedimentary porosity, the MI – methane flux relationship can then be applied to reconstruct methane history and diagenetic zonation using marine sediments. To test the quantitative applications of MI, we reconstructed the history of methane seepage in two sites in the Cascadia Hydrate Ridge (Sites 1248 and U1328 from the International Ocean Discovery Program). Overall, this study highlights the potential of MI to serve as a powerful, quantitative proxy to evaluate past methane cycling, sedimentary biogeochemistry, and the global cycling of carbon and sulfur.