EARLY HOLOCENE ISOTOPIC SHIFT IN THE BRADY PALEOSOL, CENTRAL GREAT PLAINS (USA)

Shift in the carbon isotope signal derived from SOC in the Brady paleosol of the loess-mantled terrain of the central Great Plains documents dramatic change in plant communities associated with climate change at the end of the Pleistocene (MIS 1-2 transition). The Brady paleosol developed during a time of landscape stability (diminished loess deposition) and environmental change occurring about 11,000-9,000 yrs BP (13,000-10,000 cal yrs BP). Sample series were collected from cores and exposures of the Brady paleosol at locations distributed from eastern Kansas to eastern Colorado and from central Nebraska to southwestern Kansas. Due to the complexities of vertical change in the isotopic record within the solum, data series from modern grassland soils were used to interpret the subtle aspects of the paleosol records. Results document the rapid shift from C3 plant-dominated communities of the Late Pleistocene to C4 grass-dominated communities of the Holocene, and, for some sites, C contributed to the soil by C4 grasses increased by more than 80 percent. Variability in the isotopic record since the Early Holocene has been relatively small in comparison. To better appreciate the nature of shift in the plant community, changes in temperature and precipitation estimates were computed using modern analog data. Spatial patterns of isotopic signatures from the Late Pleistocene and from the Early Holocene display differing east-west and north-south gradients. Isotopic analyses obtained on samples from coeval paleosols within alluvium and eolian sand suggest similar, but more muted community shifts during this time of environmental change. Biogenic opal (phytolith) assemblages from selected sites serve to corroborate the isotopic database.