Resolving and correcting for kinetic biases on methane seep paleotemperature using carbonate ∆ ₄₇ /∆ ₄₈ analysis

Abstract

Methane-derived authigenic carbonate often constitutes the sole remaining record of relic methane seeps. The clumped (∆ 47 ) 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 (∆ 47 /∆ 48 ) composition of authigenic carbonate from a modern methane seep. We demonstrate that aragonite forms closest to isotopic equilibrium such that its ∆ 47 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 HCO 3 − alongside diffusive fractionation, which can be corrected for with analysis of carbonate ∆ 47 /∆ 48 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 δ 18 O.