Geochemical evidence (C and Sr isotopes) for methane seeps as ammonite habitats in the Late Cretaceous (Campanian) Western Interior Seaway

Abstract

Methane seeps in the Upper Cretaceous Pierre Shale of the U.S. Western interior contain a rich fauna including ammonites (Baculites, Hoploscaphites, Didymoceras, Placenticeras, Solenoceras), bivalves (Lucina), gastropods, sponges, and crinoids. Occasionally, the shell material in the seeps is very well preserved, retaining the original mineralogy and microstructure. We explored two such seeps from the upper Campanian Didymoceras cheyennense and overlying Baculites compressus Zones (74.7–73.5 Ma) in southwestern South Dakota. Light values of δ13C in the micritic limestones (–11 to –47 ‰) confirm the impact of anaerobic oxidation of methane on the isotopic composition of the dissolved inorganic carbon reservoir. At the seep from the D. cheyennense Zone, δ13C values in well-preserved specimens of Hoploscaphites and Baculites are significantly lighter than those measured in specimens from approximately age-equivalent non-seep deposits. In addition, the 87Sr/86Sr ratio is elevated in the authigenic carbonates and ammonite shells compared with the coeval marine value. This suggests that seep fluids imprinted with a radiogenic Sr signature, perhaps derived from isotopic exchange with granitic deposits at depth associated with the Black Hills uplift, are transported through the surficial sediments into the overlying water. The persistence of these isotopic tracers of seep fluids in the ammonite shells suggests that these mobile animals were likely demersal and were living in close proximity to the seep. A more restricted data set on a single baculite and nautilid from the seep in the B. compressus Zone shows less divergence of δ13C and 87Sr/86Sr relative to non-seep values, suggesting that fluid transport was not as strong at that seep.