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

• δ13C-δ18O-Δ 47 characterize methane-derived bivalve shells and cements from an Estuary. • Seep shells show full isotopic equilibrium due to slow precipitation. • Cements show unique Δ 47 and δ18O positive offsets from marine signals. • Cements define a new differential disequilibrium domain for natural carbonates. • This isotope study refines the basis for interpreting ancient seep carbonates. Recent studies using the CO 2 carbonate clumped isotope (13C-18O-16O, i.e., Δ 47) paleothermometer and bulk isotopes (δ13C, δ18O) have brought new insights into the prevailing conditions during carbonate formation around marine methane seeps. These studies mostly revealed δ18O or paired δ18O and Δ 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 Δ 47 (up to +0.05‰) and minor δ18O (+0.8‰) positive offsets relative to marine equilibrium, with a narrow δ13C range (−33.5 to −31.1‰). 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 Δ 47 and δ18O 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-δ18O and low-Δ 47 values after methane-derived CO 2 diffusion. Overall, we suggest that the shells represent potential archives of seep conditions, and that the seep cements define a new Δ 47 and δ18O 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. [ABSTRACT FROM AUTHOR]