STRONTIUM, CARBON AND OXYGEN ISOTOPES IN WELL-PRESERVED AMMONITE SHELLS AND AUTHIGENIC CARBONATES IN COLD METHANE SEEP DEPOSITS FROM THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY OF NORTH AMERICA: CLUES TO SEEP PALEOENVIRONMENT

Cold methane seeps were common in the Late Cretaceous Western Interior Seaway (WIS) of North America. These deposits are characterized by the presence of diverse fossil assemblages, relative to the surrounding shale, and carbonate concretions presumed to be the result of anaerobic oxidation of methane. Sr, C and O isotopes in the well-preserved shells of ammonites found at the seeps and in seep carbonates provide clues to the paleoenvironment of the seeps and the source and transport of fluids associated with them. Several seep deposits exposed in the Pierre Shale in South Dakota (USA) contain well-preserved shell material of ammonite and other molluscan fauna, as well as carbonate concretions. ⁸⁷Sr/⁸⁶Sr ratios are elevated above the coeval seawater value in carbonate concretions (predominantly Mg-calcite) from seep deposits in the Baculites compressus and Didymoceras cheyennense Zones. This implies that seep fluids equilibrated with a radiogenic Sr source at depth and were transported through the sediments toward the sediment-water interface. Such fluids acquired methane en route, and the anaerobic oxidation of methane in the sulfate-methane transition zone of the sediments produced a dissolved inorganic carbon (DIC) reservoir with low δ¹³C values derived from the ¹²C-enriched methane. These low δ¹³C values (as low as ‑50‰ VPDB) are observed in a variety of seep carbonates (tubes, pipes, subspherical concretions). Other seep carbonates (large limestone bodies) have higher δ¹³C values, suggesting mixing of multiple DIC sources in their precipitation. Shells of well-preserved ammonites collected at the seep deposits also show elevated ⁸⁷Sr/⁸⁶Sr and low δ¹³C, indicating that the seep fluids impacted the water column above the seeps. Temperatures of precipitation calculated from δ¹⁸O are those characteristic of the WIS during the Late Cretaceous (20-30°C), but such calculations require assumptions about the δ¹⁸O of the water. Carbonate clumped isotope measurements in both shells and authigenic carbonates will help resolve the temperature of precipitation and δ¹⁸O of the water. They will also help test whether seep authigenic carbonate clumped isotope ratios are in equilibrium with ambient WIS waters.