Stable carbon isotope record of holocene environmental change in the central great plains

Ratios of stable carbon isotopes preserved in soil organic matter record vegetation and climate change and are applicable to the dry environment of the central Great Plains. We sampled two loess sections and one sand sheet swale in southwest Nebraska to study a continuous record of late Pleistocene and Holocene climate change. Carbon isotope (δ13C) data are combined with previously reported and new optically stimulated luminescence (OSL) age control to establish the timing and magnitude of vegetation change. Stratigraphic units studied include the late-Pleistocene Peoria Loess and Brady Soil, Holocene Bignell Loess, and four buried soils within Bignell Loess. Isotope ratios and OSL ages indicate a shift from a C₃ to C₄ vegetation community during formation of the Brady Soil, reflecting climatic warming spanning the Pleistocene-Holocene transition. By the early Holocene, between 10,100 and 9100 yrs. ago, drought became a persistently recurring feature in the region as indicated by the deposition of Bignell Loess that originated from reworking of devegetated dunes and sand sheet deposits. Periods of rapid loess deposition are marked by an increase in C₃ vegetation, which we interpret to indicate vegetation adapted to high rates of landscape disturbance. Conversely, C₄ vegetation types dominated during periods of landscape stability and soil formation, when precipitation was sufficient for vegetation to grow on dune fields. We conclude that total annual precipitation and landscape stability are more important factors than seasonality of precipitation and atmospheric CO₂ concentration in determining the chronology of C₃-C₄ vegetation change during the Holocene.