Pedogenic carbonate stable isotope record of environmental change during the Neogene in the southern Great Plains, southwest Kansas, USA: Oxygen isotopes and paleoclimate during the evolution of C₄-dominated grasslands

Neogene strata in the Meade Basin (southwest Kansas) preserve numerous superposed calcareous paleosols in sections that range in age from the Clarendonian North American Land Mammal Age (NALMA; 12.0-9.0 Ma, early late Miocene) to the early Irvingtonian NALMA (ca. 2.5-ca. 1 Ma, early Pleistocene). The carbon isotope compositions (δ¹³C relative to Vienna Peedee belemnite [VPDB]) of pedogenic carbonates from these sections record the protracted regional increase in the abundance of grasses using the C₄ photosynthetic pathway over this interval, with the first appearance of a modern-like grassland ecosystem having >70% C₄ biomass around 1.3 Ma. We use the stable oxygen isotope composition (δ¹⁸O relative to Vienna standard mean ocean water [VSMOW]) of 194 paleosol carbonates from 19 measured sections to reconstruct the climatic conditions in the Meade Basin during the rise of C₄ grasses to ecological dominance in the region. Pedogenic carbonate δ¹⁸O values are sensitive to soil temperature and the δ¹⁸O of soil water, and hence they are a paleoclimate proxy. Carbonate δ¹³C values do not exhibit consistent trends in relation to δ¹⁸O values, indicating no consistent relationship between short-term climatic conditions in terms of temperature or aridity and the abundance of C₄ biomass. Mean carbonate δ¹⁸O values within biostratigraphic intervals decrease from the Clarendonian (25.3‰ ± 0.72‰) to the early and middle Blancan (21.8‰ ± 0.87‰ and 22.1‰ ± 0.69‰, respectively), and they also exhibit a decreasing trend in the late Blancan-early Irvingtonian, from ~25‰ to ~21‰. The increase in δ¹⁸O values between the end of the middle Blancan and the beginning of the late Blancan-early Irvingtonian sections could reflect the onset of Northern Hemisphere glaciation and a change in the isotope composition of the hydrosphere due to increased ice volume. The long-term trend in δ¹⁸O values suggests that C₄ biomass increased coincident with some combination of decreasing temperature, increasing proportion of winter precipitation recharge of soil water, and/or increasing soil moisture. Thus, the ecological dominance of C₄ grasses in the region today does not seem to be linked to warmer temperatures or increased aridity.