Late Quaternary loess in northeastern Colorado: Part II - Pb isotopic evidence for the variability of loess sources

A new application of the Pb isotopic tracer technique has been used to dete rmine the relative importance of different silt sources for late Wisconsin loess in the central Great Plains of eastern Colorado. Samples of the Peori a Loess collected throughout the study area contain K-feldspar derived from two isotopically and genetically distinct sources: (1) glaciogenic materia l from Early and,middle Proterozoic crystalline rocks of the Colorado provi nce, and (2) volcaniclastic material from the Tertiary White River Group ex posed on the northern Great Plains. Pb isotopic compositions of K-feldspar in loess from two dated vertical sections (at Beecher Island and I,ast Chan ce, Colorado) vary systematically, implying climatic control of source avai lability. We propose a model whereby relatively cold conditions promoted th e advance of Front Range valley glaciers discharging relatively little glac iogenic silt, but strong,winds caused eolian erosion of White River Group s ilt due to a decrease in vegetation cover. During warmer periods, valley gl aciers receded and discharged abundant glaciogenic silt, while surfaces und erlain by the White River Group were stabilized by vegetation. Isotopic dat a from eastern Colorado loess sections record two warm-cold-warm cycles dur ing late Wisconsin time between about 21 000 and 11000 radiocarbon Jr BP., similar to results from other studies in the United States and Greenland.