Resolving and correcting for kinetic biases on methane seep paleotemperature using carbonate Δ47/Δ48 analysis.

Methane-derived authigenic carbonate often constitutes the sole remaining record of relic methane seeps. The clumped (Δ47) and oxygen isotopic composition of seep carbonates often yield inaccurate temperatures, attributed to kinetic isotope effects and modification of seawater isotope composition by hydrate water. Here, we analyzed the dual-clumped isotope (Δ47/Δ48) composition of authigenic carbonate from a modern methane seep. We demonstrate that aragonite forms closest to isotopic equilibrium such that its Δ47 can directly yield the correct formational temperature, whereas calcite is unambiguously biased by kinetic isotope effects. Numerical models show that the observed bias in the isotopic composition arises from rate-limiting dehydration/dehydroxylation of HCO3-alongside diffusive fractionation, which can be corrected for with analysis of carbonate Δ47/Δ48 values. We demonstrate the utility of dual-clumped isotope analysis for studying seep carbonates, as it reveals the origin and magnitude of kinetic biases and can be used to reconstruct paleotemperature and seawater δ18O. [ABSTRACT FROM AUTHOR]