INFLUENCES OF MODERN PEDOGENESIS ON PALEOCLIMATE ESTIMATES FROM PENNSYLVANIAN AND PERMIAN PALEOSOLS, SOUTHEAST OHIO, USA

摘要

Paleosols are widely used in paleoclimatic reconstructions. Various proxies based on ratios of major oxides in paleosols have been used to estimate weathering processes, mean annual precipitation (MAP), and mean annual temperature (MAT). The degree of influence that modern weathering and pedogenesis has on the bulk geochemistry of exposed paleosols and the corresponding weathering ratios, however, has not been adequately addressed. In this study, previously analyzed Pennsylvanian and Permian Vertisols and Inceptisols that are actively undergoing pedogenesis were sampled from five roadcuts with known exposure times in southeastern Ohio, USA. Samples were collected at depths of 0, 25, 50, 100, and 150 cm from the outcrop surface. The physical properties of the samples from each depth were described and then analyzed via X-ray fluorescence (XRF) for major-oxide geochemistry. These data were used to calculate molecular weathering ratios, MAP, and MAT at each depth. Results indicate that although physical properties and oxide geochemistry often differ between sampling depths, the differences do not occur in a pattern that supports recent weathering and pedogenesis as a driving factor. Therefore, the decade-level exposure time along these roadcuts effectively limits the extent of pedogenic modification of the paleosols, despite the formation of soils on them. Rather, this study likely captured lateral variations within the paleosols that were the result of small-scale differences in parent material, topography, and biota. Ultimately, for geochemical studies on paleosols in outcrops located on young roadcuts in temperate climates, current sampling techniques appear to be sufficient in mitigating the effects of modern weathering. Recommendations from this study include sampling from 25-50 cm beyond the paleosol surface and taking multiple samples across or into an outcrop to account for lateral variation.