PLIO-PLEISTOCENE PALEOENVIRONMENTAL RECORD OF THE MEADE BASIN, SOUTHWEST KANSAS

The Meade Basin is a NE-SW trending depositional basin with well-exposed but discontinuous late Miocene to latest Pleistocene strata that outcrop over ca. 50 x 50 km² in SW Kansas and NW Oklahoma. The sequence is predominantly mixed silts, sands, and gravels with minor clay deposited in a variety of continental depositional environments (channels, floodplains, lakes), and accommodation was controlled in part by geomorphic response to salt withdrawal from deeper Paleozoic strata. The Meade Basin is unique in the Great Plains in preserving a sequence of small mammal faunas that document the in situ evolution of the modern local community over the last 4.5 Myr. Here we describe recent field and laboratory work focused on characterizing climatic and environmental changes in the Meade Basin in sections associated with small mammal faunas to constrain the role of abiotic processes in the evolution of community structure. We have employed a suite of geochemical proxies measured on modern soils and on paleosols that include δ¹³C and δ¹⁸O values of carbonates, δ¹³C values of bulk sedimentary organic matter, δD and δ¹³C values of plant-derived n-alkanes and n-alkanoic acids, carbonate clumped isotope paleothermometry, and elemental composition of sediments, in addition to an extensive petrographic survey of carbonates. The δ¹³C values from all substrates are consistent with a long-term increase in the abundance of C₄ grasses from the Miocene to the late Pleistocene, although different carbon records reflect different aspects of vegetation change. In contrast, paleoclimate estimates from carbonate clumped isotope paleothermometry (temperature) and paleosol geochemistry and rock magnetic properties (precipitation) do not indicate any long-term trends. All proxies show high variability in multiple early Pliocene sections that is not evident in younger sections, which may reflect early Pliocene landscape scale heterogeneity in vegetation composition and structure that influenced both soil temperature and hydrology. The lack of trends or cycles in the proxies is surprising and may reflect a bias in deposition or preservation over glacial-interglacial cycles or insufficient stratigraphic resolution to detect climatic changes paced by variation in obliquity over the Pliocene and early Pleistocene prior to ca. 1 Ma.