PALEOPEDOLOGIC AND GEOMORPHIC EVIDENCE FOR HOLOCENE CLIMATE VARIATION, SHORTGRASS STEPPE, COLORADO, USA

Radiocarbon dates of paleosols in northeastern Colorado indicate disti nct periods of stability and soil formation with intervening periods o f instability resulting in soil truncation or burial, A combination of pedologic and geomorphic indicators were used to resolve the duration of, and prevailing climate during, these periods, Five sites, each ha ving a paleosol, were examined using both traditional soil analyses an d grain-size statistics, the latter to decipher the mode of parent mat erial deposition. Twenty local stream, dune and bedrock deposits were analyzed using grain-size statistics to establish benchmarks for compa rison with soils, Field investigation supported by grain-size frequenc y statistics indicate early Holocene, middle Holocene and contemporary soils all formed in alluvium. Organic C and phytolith data suggest th e early and middle Holocene climatic conditions were more favorable fo r plant productivity than the present climate. Soil development in ear ly and middle Holocene paleosols suggests wetter soil moisture regimes than present. Low parent material carbonate contents suggest an eolia n source for the carbonate in the Bk and Btk horizons. The presence of paleosol Btk horizons suggests a decrease in precipitation at the end of soil-forming intervals followed by drought and subsequent soil bur ial.