Collaborative Research: Coordinated TIMS U-Series Dating and Paleoenvironmental Interpretation of Pedogenic Carbonate

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PROJECT SUMMARY With prior support from NSF, we have shown that TIMS (Thermal Ionization Mass Spectrometry) U-series dating of pedogenic carbonate can provide a precise chronology of pedogenesis and reliable ages for appropriate Quaternary alluvial deposits (Sharp et al., accepted). Though the U/Th ratios of pedogenic carbonate of carbonate-clast dominated alluvium at Kyle Canyon (Nevada) are too low for U-series dating, pedogenic carbonate in granitic-clast dominated gravels of the Wind River Basin (Wyoming) is highly suitable. Precise U-series ages (1-2 percent, 2) may be measured on a few cubic millimeters of pedogenic carbonate and require only very small corrections for detrital 230Th. Suites of ages may be screened for reliability using criteria inherent in the U-series results. The time lag between clast deposition and accumulation of datable carbonate.a critical parameter if U-series dates of innermost clast-rinds are to be used to date host Quaternary deposits.is estimated to be 5+ 5 kyr in the Wind River basin. O- and C-isotope analyses of a trial suite of pedogenic carbonate samples shows that a temporally resolved stable isotope paleoclimatic record can be constructed using pedogenic carbonate dated by U-series. Funds are requested to further develop U-series dating of pedogenic carbonate and its application to the refinement of a Rocky Mountain glacial chronology. Emphasis will be placed on resolving the timing of Bull Lake glacial advances (at their type locality) with respect to other regional and global climate records. Moreover, we propose to characterize the spatial and temporal accumulation of pedogenic carbonate in relict soils of Late-Middle to Late Pleistocene deposits, and measure the O- and C-isotopic compositions of carbonate samples dated by U-series. We will interpret these data to provide a comprehensive history of paleoenvironmental change in the Wind River Basin throughout much of the late Illinoian and Wisconsin glacial cycles. Ultimately, our goal is to correlate the soils-based paleoenvironmental record with the geomorphic expressions of paleoclimatic change.the well-preserved glacial moraines and glacio-fluvial terraces of the Basin.by establishing a common chronological framework based on precise U-series ages. This is a collaborative proposal. Warren Sharp and Ken Ludwig (Berkeley Geochronology Center) will be responsible for U-series analyses and age interpretations; Ron Amundson (U.C. Berkeley) will provide field and laboratory expertise in pedology and stable isotope analysis and interpretation; Oliver Chadwick (U.C. Santa Barbara) will provide expertise in Quaternary geology, geomorphology and soil stratigraphy as well as familiarity with the Wind River area.