BIODIVERSITY CHANGES FOLLOWING AN ASH FALL IN THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

Offshore marine environments are subject to a variety of physical perturbations which can adversely affect benthic communities. This study examines the biotic response of a methane seep community to widespread volcanic ash deposition in the Late Cretaceous Western Interior Seaway (WIS). The Upper Cretaceous Pierre Shale in South Dakota contains numerous fossiliferous seep deposits and bentonite (ash) beds. Here, we document a single seep from the upper Campanian Baculites compressus Zone where a thick, laterally continuous bentonite cuts through the seep. Seep associated concretions are present below, within, and above the bentonite indicating the seep was active during the ash fall. A grid system was constructed and fossils were collected in 0.125 m intervals from horizons below and above the bentonite. Diversity indices were calculated for all horizons in order to reconstruct paleoecological dynamics prior to and following ash deposition. Additionally, the section was partitioned laterally in 0.3 m intervals to detect faunal variability in relation to seep proximity. This analysis was also conducted on deposits ~90 m away from the seep. These analyses allow us to compare the biotic response of a seep and non-seep community to the same perturbation. Current data shows horizons below the bentonite at both localities are more abundant in fauna. However, at the non-seep site, faunal abundance and richness sharply declines above the bentonite. Faunal abundance at the seep site slightly declines in horizons immediately above the bentonite but is negligible in comparison to the depauperate sediments found at the same horizons at the non-seep site. These data suggest that seep communities within the WIS were more resilient to rapid environmental change than surrounding ecosystems. WIS seeps were productive ecosystems sustained by chemosynthetic microbial communities reliant on methane emissions at the sediment-water interface. Evidence suggests uninterrupted seep activity during the ash fall and thus, an uninterrupted source of nutrients. Elsewhere in the WIS, ash deposition may have smothered benthic communities and restricted nutrient availability. Seeps are self-sustaining ecosystems and our data suggests WIS seeps may have served as a refuge during otherwise detrimental conditions.