Clumped and oxygen isotopes reveal differential disequilibrium in the formation of carbonates from marine methane seeps

Recent studies using the CO2 carbonate clumped isotope (C-13-O-18-O-16, i.e., Delta(47)) paleothermometer and bulk isotopes (delta C-13, delta O-18) have brought new insights into the prevailing conditions during carbonate formation around marine methane seeps. These studies mostly revealed delta O-18 or paired delta O-18 and Delta(47) disequilibria between precipitating minerals, water and DIC species. Here, we have sampled bivalves and cements from two modern, slow-release methane seeps located in the St. Lawrence Estuary. The bivalve shells have marine isotopic signals, whereas the cements show wide Delta(47) (up to +0.05 parts per thousand) and minor delta O-18 (+0.8 parts per thousand) positive offsets relative to marine equilibrium, with a narrow delta C-13 range (-33.5 to -31.1 parts per thousand). The observed isotopic trends show that, unlike in previous studies, the aragonite shells precipitated at full isotopic equilibrium, whilst the cements suggest differential disequilibrium precipitation. We propose that the suite of results were generated by distinct Delta(47) and delta O-18 pathways, as supported by DIC-water isotopic exchange numerical experiments. The observed trends typify slow rates of methane oxidation and carbonate precipitation from residual gas with high-delta O-18 and low-delta(47) values after methane-derived CO2 diffusion. Overall, we suggest that the shells represent potential archives of seep conditions, and that the seep cements define a new Delta(47) and delta O-18 domain widening the known spectrum of marine conditions producing natural carbonates. Our results, compiled with literature data, help refine the basis for interpreting carbonate precipitation mechanisms in ancient seeps or other geological settings. Crown Copyright (C) 2021 Published by Elsevier Ltd.