تغییرات محیط زیستی و پوزوژنیک در دشت های مرکزی دشت مرکزی ویسکانسین تاکنون
Abstract: During the middle Wisconsinan, the Gilman Canyon Formation (GCF), consisting of three loess units and three soils, formed on the loess plateaus of the central Great Plains about 40-25 ka. Stable carbon isotope analysis of the lower two GCF loess units (L1 and L2) at the type locality in southwestern Nebraska, Buzzard\'s Roost, revealed a mixed C3/C 4 grassland. Strongest pedogenesis (GCF S3) and C4-dominance correlated strongly with a peak in summer insolation for the northern latitudes. Soil 2 (S2), L3 and a weakly-expressed S1 developed successively as insolation values declined and δ13C values synchronously dropped back, reflecting the emerging boreal environments of the Late Wisconsinan and deposition of the Peoria Loess. As the Pleistocene ended, Peoria Loess deposition waned sufficiently for pedogenesis to prevail across the central Great Plains, resulting in development of the upland Brady Soil and temporal equivalents in other landscape positions. Climate warmed rapidly between early (13-11 ka) and late (10-9 ka) development of the Brady Soil. Early Holocene temperatures, inferred from carbon isotopic signatures of the Brady Soil, rose 8-10°C above those of the Late Pleistocene. The Holocene record in loess sections of the central Great Plains generally lacks resolution for environmental reconstruction, but canyons systems of the upper Republican River suggest massive Altithermal erosion from c 8-5 ka. T-2 alluviation, with periodic soil formation, ensued from c 5 ka to onset of the Medieval Warm Period (c 1 ka), when erosion initiated a fresh gully cycle. Modern soils at Konza in northeastern Kansas suggest that δ 13C in C4 environments such as those of the GCF S3 and the Brady Soil are insensitive to minor variability with landscape position and can therefore be confidently used for paleoenvironmental reconstruction despite the often unknown paleolandscape position of sampled paleosols. Further research is needed, however, to resolve the issue of low surface soil δ 13C values relative to biomass found in modern soils, a phenomenon that could lead to an under-representation of C4 plant contributions.