Geochemical characteristics of gas hydrate-bearing sediments: insights from a multi-proxy approach

Gas hydrate-bearing sediments are highly sensitive to redox fluctuations, primarily driven by methane fluxes associated with gas hydrate decomposition or cold seep activity. These sediments serve as valuable archives, preserving records of past redox states and methane emission events. Trace elements and rare earth elements (REE) have been widely applied as proxies to reconstruct such environmental conditions. Anoxic, methane-rich depositional environments characteristic of gas hydrate systems promote the enrichment of redox-sensitive major and trace elements (e.g., Fe, Mo, U, Ce) and REE in seep carbonates and surrounding sediments, suggesting that these settings act as important sinks for these elements. Methane also strongly shapes the geochemical signatures, with dynamic seepage events identified through indicators within the sulfate-methane transition zone (SMTZ), such as the barite front, molybdenum enrichment, and authigenic carbonate precipitation. However, relying solely on bulk sediment analysis often leads to ambiguous interpretations. Here we highlight that geochemical signals are fractionated differentially across these phases: carbonates capture fluid signatures, while pyrite hosts sulfur and chalcophile elements (e.g., Mo, As). This review synthesizes and evaluates previous findings, highlighting current knowledge, identifying uncertainties, and outlining directions for future research.