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TitleClumped and oxygen isotopes reveal differential disequilibrium in the formation of carbonates from marine methane seeps
AuthorSavard, M MORCID logo; Jautzy, J JORCID logo; Lavoie, DORCID logo; Dhillon, R S; Defliese, W
SourceGeochimica et Cosmochimica Acta vol. 298, 2021 p. 43-54, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200080
Mediapaper; on-line; digital
File formatpdf
NTS22B/03; 22B/04; 22B/05; 22B/06; 22B/11; 22B/12; 22B/13; 22B/14; 22C; 22F/01; 22F/02; 22F/03; 22F/04; 22F/05; 22F/06; 22F/07; 22F/08; 22G/03; 22G/04; 22G/05; 22G/06
Lat/Long WENS -70.0000 -67.0000 49.5000 48.0000
Subjectsenvironmental geology; geochemistry; marine geology; Nature and Environment; paleontology; Science and Technology; carbon dioxide; diffusion; cement, commodity; marine environments; equilibrium; paleotemperatures; St. Lawrence Estuary
Illustrationslocation maps; tables; plots
ProgramGEM2: Geo-mapping for Energy and Minerals GEM Synthesis
Released2021 02 03
AbstractRecent studies using the CO2 carbonate clumped isotope (13C-18O-16O, i.e., D47) paleothermometer and bulk isotopes (d13C, d18O) have brought new insights into the prevailing conditions during carbonate formation around marine methane seeps. These studies mostly revealed d18O or paired d18O and D47 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 D47 (up to +0.05 per mille) and minor d18O (+0.8 per mille) positive offsets relative to marine equilibrium, with a narrow d13C range (-33.5 to -31.1 per mille). 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 D47 and d18O 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-d18O and low-D47 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 D47 and d18O 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.
Summary(Plain Language Summary, not published)
The Geological Survey of Canada has developed a new indicator, clumped isotopes (13C-18O-16O, i.e., delta-47), for understanding natural conditions during the production of carbonates in various geological contexts. Here we applied this new tool to carbonates (aragonites) associated with two seafloor methane seeps in the St. Lawrence Estuary. The clumped isotopes (13C-18O-16O, i.e., delta-47) in aragonites of bivalve shells reveal regular marine conditions, whereas cements (vug filling material) showing wide delta-47 and minor d18O positive offsets relative to marine equilibrium, coupled with a narrow d13C range reflect precipitation under differential disequilibrium. The cement isotopic tendency is unprecedented in natural carbonates. These findings point towards slow methane delivery, oxidation, and precipitation rates after gas diffusion, and suggest a new pole in the spectrum of cold seep conditions.

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