BIOTIC RESPONSE TO A LATE CRETACEOUS ASH FALL: COMPARATIVE FAUNAL ANALYSES FROM A METHANE SEEP AND NON-SEEP ECOSYSTEM WITHIN THE WESTERN INTERIOR SEAWAY

The Western Interior Seaway (WIS) of North America was subject to multiple ash falls during the Late Cretaceous Period. The Pierre Shale, which represents offshore sedimentation, contains multiple bentonite beds that have been radiometrically dated. These bentonites have been utilized for stratigraphic correlation across the Western Interior; however, the effect of ash deposition on faunal communities has received little to no attention. Additionally, the WIS was peppered with numerous cold methane seeps during this time interval. In this study, we document a relatively thick (~30-40 cm), laterally continuous bentonite at a single site in the Pierre Shale from the Baculites compressus Zone (upper Campanian) in southwestern South Dakota. At this locality, the bentonite cuts through a methane seep deposit. Seep carbonates appear below, within, and above the bentonite, indicating that the seep co-occurred with ash deposition. We examined fauna below, within, and above the bentonite both at the seep and ~90 meters away, in order to reconstruct both pre-and post-event community dynamics at a seep and non-seep ecosystem. A grid framework was constructed in order to systematically collect in-situ fauna from 12.5 cm intervals below and above the bentonite. Preliminary results suggest that seep fauna are consistently richer and more abundant than fauna at the non-seep site at all stratigraphic intervals. Fauna are preserved in the bentonite at both sections. However, above the bentonite at the non-seep site, we document a noticeable decline in both fossil richness and abundance: the sediments are depauperate up to 50cm above the bentonite. At the seep site, faunal assemblages immediately overlying the bentonite are slightly less abundant than underlying layers; however, the seep assemblage returns to “normal” (i.e., resembles beds below the bentonite) within ~12.5cm. It appears that the seep community recovered more quickly following a large ash fall than neighboring non-seep communities. This suggests that seeps may have acted as a refuge for organisms during otherwise detrimental environmental perturbations. If seep emissions remained active during ash fall, a continued supply of nutrients may have sustained seep fauna, while non-seep sites were most likely covered in a thick layer of nutrient-restricting ash.