CLUMPED ISOTOPE PALEOTHERMOMETRY OF AUTHIGENIC CARBONATES FROM LATE CRETACEOUS (CAMPANIAN) METHANE SEEPS IN THE WESTERN INTERIOR SEAWAY, SOUTH DAKOTA, USA

Cold methane seep deposits are common occurrences in the Pierre Shale of the Upper Cretaceous Western Interior of North America. They occur in Colorado, South Dakota, Wyoming, and Montana and span the Didymoceras nebrascense (~75.2 Ma) to the Baculites baculus (~70 Ma) Zone. The authigenic seep carbonates were precipitated from dissolved inorganic carbon that was imprinted with ¹²C-enriched carbon derived from the anaerobic oxidation of methane (AOM), and exhibit δ¹³C values ranging as low as ~-50‰ (VPDB). In order to better characterize the depositional environment of Western Interior Seaway (WIS) seeps, carbonate clumped isotope paleothermometry was used to investigate the temperatures at which the carbonate formed and the oxygen isotope composition of the associated fluids. Clumped isotope analyses of 14 samples of micritc limestones, and 6 nodules and veins embedded within them, from the Baculites compressus (~73.5 Ma) and Didymoceras cheyennense (~74.7 Ma) Zones, show apparent clumped isotope equilibrium between carbonates and WIS seawater. The average temperature calculated based on the micritic carbonate samples is 22 ± 4°C. Significantly, this value represents bottom water temperatures for the WIS because the seep carbonates formed within the sediments, likely relatively close to the sediment-water interface. These values are also indistinguishable from water paleotemperatures previously reported by conventional δ¹⁸O measurements of well-preserved shells of ammonites (B. compressus) that lived in close association with the seeps. From the seep carbonate samples, the calculated δ¹⁸O value of the pore fluid is 1.1 ± 1.8‰. This value is somewhat greater than, but broadly consistent with, previous estimates of the global seawater δ¹⁸O during the Late Cretaceous (~-1‰), as well as estimates of the δ¹⁸O of water in the WIS during the Late Cretaceous (~-1‰ relative to VSMOW).