FOSSIL GASTROPODS AS LATE PLEISTOCENE PALEOENVIRONMENTAL PROXIES, WHITE RIVER BADLANDS, SOUTH DAKOTA, USA

Considering the changing climate, paleoclimate research on Holocene megadroughts within the west-continental U.S. may reveal the relative vulnerability of this region to future droughts — pertinent to the continual stability of agricultural soils. The prehistoric climate of the White River Badlands (WRB), located in the northwestern Great Plains, is poorly understood because of the lack of climate proxies in the area. However, loess preserved on mesas may provide climate information, if the loess contains fossils capable of constraining age and paleoclimate proxies for depositional periods. Here we use gastropod fossils to derive a Late Pleistocene paleoenvironmental proxy for the WRB, with the goal of constraining a temporal scale and climatic threshold for aeolian activity. The gastropod species, Galba obrussa and Gyralus parvus, along with fragments in all but one sample, were collected from the lower 10 m of a 23 m loess section at Red Dog Table (RDT) in the WRB. Samples were processed by wet-sieving collected loess material through 0.5 mm mesh, followed by oven-drying at 100°C for 24 hours and picking out shells and fragments with a fine paint brush. Two samples of Galba obrussa from the mid-lower section were found to have radiocarbon ages of 23.3 (±0.5) and 23.8 (±0.5) ka — near the beginning of the last glacial maximum (LGM) (~26-19 ka). Surprisingly, modern and paleo data for these species indicate they are aquatic-freshwater snails, inhabiting perennial lakes or vernal ponds. Anticipated δ¹⁸O_carb results from the snail fossils may clarify whether xeric conditions in the WRB were driven by waning precipitation and further approximate the (%) precipitation decrease in this region during the LGM. Assuming a terrace existed on the mesa above stream-level, overlap of fossil ages with the start of Peoria Loess deposition (25 ka) would suggest that initial aeolian activity was punctuated by periods of heavy rainfall. In this case, the paucity of snails in the upper 13 m might represent a gradient towards drought-like conditions, coinciding with greater loess deposition.