LIGHT ISOTOPE ANALYSIS AND BIOGEOGRAPHY OF LATE CRETACEOUS METHANE COLD-SEEPS OF THE PIERRE SHALE, SOUTH DAKOTA

Methane cold-seeps from the late Cretaceous in the Pierre Shale of South Dakota tend to have dense faunal communities including baculites and Inoceramids. These communities depend on intricate relationships between the geochemically driven formation of the seep substrate, pressure gradients allowing for methane migration, anaerobic oxidation of methane combined with sulphate reduction, and interactions between fauna and microorganisms. These functions are necessary to sustain life within the cold-seep community. American Museum of Natural History locality 3520A, from South Dakota, was gridded into 24 quadrants each approximately 4’ by 4’, mapped out, and specimens were collected for light isotope analysis, 18O and 13C, and biogeographic analysis. Nacre from Inoceramids and baculites were extracted from larger samples, coated with Au and Pd, and imaged using a Hitachi S4700 FE- scanning electron microscope to determine the preservation index, quality of preservation. Samples with a preservation index of 3 or greater were sent out for δ¹³C and δ¹⁸O isotope analysis. Faunal distance between central pipes at the seep and location in the mound were examined for any distribution relationships. There was no correlation found between the preservation index of the samples and the distance of the main conduit of the cold-seep. Baculites were found in a greater concentration to the central conduit of the methane-seep and become scarcer moving away from the main conduit. Inoceramids are well distributed throughout the seep. Seep 3520A does not appear to have a concentric distribution of fauna around the central conduit as frequently mentioned in the literature. Light isotope readings were consistent with baculite species, both proximal and distal to the main conduit. Inoceramid species isotope readings were variable and reflect no spatial patterns.