LONG-TERM STABLE ISOTOPIC RECORD OF CENOZOIC PALEOENVIRONMENTS IN THE GREAT PLAINS FROM PEDOGENIC AND SEDIMENTARY CARBONATES

The Great Plains is the largest continuous grassland in North America, and grasses using the C₄ photosynthetic pathway dominate modern, native biomass south of ca. 43° N. Although the evolution of the Great Plains ecosystem is certainly a Cenozoic phenomenon, diverse data (paleosols, plant macrofossils, phytoliths, mammalian faunal structure and paleodiets) do not yet suggest a consistent or precise chronology for the origin of open, grass-dominated habitats or the modern C₄-dominated grasslands in the region. To help constrain the environmental context of grassland origins in the Great Plains, I present an initial compilation of >500 stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope compositions of pedogenic and sedimentary carbonates from 40 measured sections in South Dakota, Nebraska, and Kansas that range in age from late Eocene (ca. 35-34) to early Pleistocene (ca. 2.5-1 Ma). The δ¹³C values of these carbonates are sensitive to atmospheric pCO₂, the δ¹³C of atmospheric CO₂, the relative abundance of C₄ biomass, soil temperature, and aridity and light stress; the δ¹⁸O values are sensitive to soil temperature and soil water δ¹⁸O, which reflects local meteoric water and evaporation of soil water. The region was tectonically inactive through much of this interval, although presumably sensitive to Cordilleran tectonics to the north and west and global climatic and atmospheric changes. Mean δ¹³C values increase from -7.0±0.55‰ (VPDB) in the late Eocene, perhaps reflecting aridity and low soil productivity, to -8.5±0.58‰ in the late Oligocene, and then increase consistently to the early Pleistocene (-2.6±1.18‰). The first unambiguous evidence for C₄ grasses is in the early Miocene, and modern abundance of C₄ biomass is first reached by ca. 1.3 Ma in Kansas. Carbonate δ¹⁸O values exhibit substantial temporal variability; mean values increase consistently from 20.6±1.62‰ (VSMOW) in the late Eocene to 26.3±0.59‰ in the late Oligocene, decrease sharply to 19.3±1.85‰ in the latest Oligocene, then vary without clear pattern from 21.8 to 23.3‰ over the Neogene. Comparing this isotopic record to those from fossil mammals in the region and benthic foraminifera globally could provide a means to distinguish local and regional environmental factors in the evolution of the Great Plains from those that are global in scope.