PALEOCLIMATE AND PALEOENVIRONMENTAL RECONSTRUCTION ACROSS THE PLEISTOCENE BLANCAN-IRVINGTONIAN TRANSITION IN SOUTHWEST KANSAS

The Meade Basin of southwest Kansas contains a rich archive of vertebrate fossil assemblages, with a particularly well-resolved rodent biostratigraphy. Recent paleontological investigations in the early Pleistocene Crooked Creek Formation have revealed an abrupt turnover in rodent communities occurred between 2.11-1.95 Ma, consisting of the loss of warm-adapted lineages common in the Pliocene and earliest Pleistocene and an influx of cool-adapted later Pleistocene-Recent communities. This turnover has been proposed to represent the Blancan-Irvingtonian North American Land Mammal Age transition. In this study, we test the hypothesis that rodent community change is driven by climate by means of paleopedologic and geochemical analysis at two localities that bracket the faunal transition. The latest Blancan Borchers fossil site is located directly on the Huckleberry Ridge Ash (HRA, 2.11 Ma). A paleosol situated immediately beneath the HRA is a floodplain Inceptisol with a somewhat poorly developed Bw horizon, limited ped development, little clay accumulation, and few vertic features. The Short Haul site, the earliest Irvingtonian assemblage in the Borchers Badlands, is preserved on top of alternating, weakly developed vertic paleosols (Bss horizons) and thinly bedded floodplain muds approximately 1 m above the HRA. Elemental proxies for climate from paleosol B horizons indicate that mean annual precipitation values beneath Short Haul were not substantially different from values in the paleosol beneath the HRA. Likewise, there was no difference in predicted mean annual temperature between the two sampled levels, though temperatures were slightly cooler than modern values. We supplemented weathering indices with micromorphological observations and conclude that differences in elemental chemistry are likely the result of minor variations in landscape position. These results suggest that either paleosol-based proxies are insensitive to the hypothesized climate change, the climate did not change significantly across the Blancan-Irvingtonian boundary, or significant rodent turnover was driven by acute environmental changes associated with the Huckleberry Ridge ash-fall.