Carbon and oxygen isotopic variability in Neogene paleosol carbonates: Constraints on the evolution of the C₄-grasslands of the Great Plains, USA

Understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas. We focus on the central and southern portions of the Great Plains, which is the largest continuous grassland in North America and has a well-documented mammalian fossil record. Modern biomass in this region is dominated by C₄ grasses, but paleontological evidence indicates the region was forested prior to the middle Miocene, implying the region was formerly dominated by C₃ plants. We examine long- and short-term variations in the abundance of C ₄ grasses and climatic conditions in the central and southern Great Plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region. The carbon isotope composition (δ¹³C) of paleosol carbonate reflects the proportion of C₃ and C₄ plants that grew in an ancient soil. The long-term pattern of δ¹³C values indicates that the percentage of C₄ grasses in the Great Plains was moderate (ca. 20%) throughout most of the Miocene, increased from 6.4 to 4.0 Ma, and reached modern levels by 2.5 Ma. Based on the range in δ¹³C values within sections and the mean δ¹³C values for different carbonate morphologies that reflect different intervals of carbonate accumulation, the abundance of C₄ biomass did not vary substantially on short time scales. The variability in C₄ biomass on short time scales does not correlate with age of section, latitude, longitude, or the mean percentage of C₄ biomass. The oxygen isotope composition (δ¹⁸O) of soil carbonate is controlled by soil temperature and the δ¹⁸O value of soil water. Great Plains paleosol δ¹⁸O values vary considerably in relation to time and latitude. Removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of Neogene climate change. Quantitative interpretation of the δ¹⁸O record is difficult without independent control on temperature or the oxygen isotope composition of soil water, both of which have probably varied with long-term climate change. Comparisons of measured δ¹⁸O values for each section with predicted values based on modern temperature data and estimated modern meteoric water δ¹⁸O values suggest that (1) most Miocene sections have δ¹⁸O values consistent with warmer temperatures and more positively shifted soil water δ¹⁸O values than today, (2) three Miocene sections in western Nebraska probably had soil water compositions that were more negative than similar-aged localities in the region, and (3) the Plio-Pleistocene sections have δ¹⁸O values consistent with cooler temperatures and more negative soil water δ¹⁸O values than today. The paleosol δ¹³C values do not correlate strongly with the pattern of climate change recorded by the δ¹⁸O values, suggesting that long-term change in mean temperature is not the main control on the abundance of δ¹³C biomass in the Great Plains. We propose two testable hypotheses to account for our data and other published data that bear on the evolution of the Great Plains ecosystem.