Clumped isotope signatures of methane-derived authigenic carbonate presenting equilibrium values of their formation temperatures

Abstract

The chemical and stable isotopic composition of methane-derived authigenic carbonate (MDAC) can be used to document paleoclimate and biogeochemical information in methane cold seeps, such as temperature records determined through clumped isotope (Δ47) analyses. Previous investigations of MDACs, however, have noted isotopic disequilibrium values, limiting the application of Δ47 signatures as a proxy for the formation temperature of carbonates at present and former gas seep sites. Here we measured 49 MDAC samples collected from 5 areas along the Eastern margin of the Japan Sea and the northwest Pacific region of Japan. All samples present equilibrium Δ47 signatures with respect to seafloor temperatures at the time of precipitation, and the results do not appear to be influenced by the Mg content, mineralogy or mixing effect. The pore water δ18O values estimated by the Δ47 thermometer reveal that most of these MDACs formed during glacial and/or hydrate destabilization periods. While our new data shows equilibrium Δ47 values, the results do not discount the possibility that disequilibrium observed in previous studies could possibly be associated with extremely rapid rates of anaerobic oxidation of methane. Relevant potential mechanisms are discussed. Further investigations are crucial to clarify the utility of Δ47 in MDACs as a novel proxy for constraining the modern/ancient methane seep environment. This proxy also offers insights into formation and dissociation processes of gas hydrate, a potential energy resource in the future.