COLLABORATIVE RESEARCH: Dating Quaternary Deposits and Soils with TIMS U-Series on Pedogenic Carbonate and Silica

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9810209 Ludwig Funds are requested to further develop and apply TIMS (Thermal Ionization Mass Spectrometry) U-series dating to pedogenic carbonate and silica. The research will seek to understand the potential and limitations of U-series dates on chemically/mineralogically-characterized, micro-sampled clast rinds by detailed work in two climatically and lithologically and distinct areas. Two localities will be studied, using techniques that build on the results of recent, independent studies by the PI's. Kyle Canyon (Nevada) provides a sequence of carbonate-clast alluvial deposits and morphologic surfaces ranging from Holocene to 780 ka, whereas the Wind River Basin (Wyoming) provides a sequence of granitic-clast dominated alluvial deposits from Holocene to early Pleistocene age. In both localities, the Quaternary geology and pedology have been well studied by one or more of the investigators of this proposal. This is a collaborative proposal among 3 institutions: Ken Ludwig and Warren Sharp at the Berkeley Geochronology Center will be responsible for sampling and uranium-series analysis, using the new TIMS facility and clean lab at BGC; Ron Amundson at U.C. Berkeley and Oliver Chadwick at U.C. Santa Barbara will provide field and laboratory expertise in Quaternary geology and pedology, as well as familiarity with the study localities. All of the named investigators will participate in the chemical and microscopic characterization of the pedogenic carbonate and silica, and Amundson will be responsible for the stable-isotopic work and interpretation. The single-sample/single-date approach developed by Ludwig and J.B. Paces for a recent study at Crater Flat, Nevada, in which sub-millimeter laminae of individual carbonate/silica rinds are micro-sampled to avoid detrital contamination, will be applied to carefully-characterized clast rinds of the deposits in both localities. The limitations of the approach will be explored, including the reliability of the detrital 230Th corrections , the textrual and geochemical indicators of open systems, and the practicality of extending the method into the Early Pleistocene using 234U/238U. Analyses of Holocene clast-rinds. Knowledge of this time-lag (and its variability in different climatic and lithologic contexts) will be essential in using U-series dates of innermost clast-rinds to date the Quaternary deposits themselves. Some development of micro-sampling techniques will be required to explore the limits of sample size and spatial resolution, but this can be done with relatively modest equipment. The laboratory challenges involve mainly those of small-sample 230Th analyses, in which one of the PI's has extensive experience. The results of the dating studies will be applied to understanding the history and timing of erosion and sedimentation in Kyle Canyon, and of glacial events in the Wind River Basin. A limited number of stable-isotope analyses will be used to evaluate the usefulness of the rinds as a paleoclimate record as well as to check for open-system histories of the mid-and early Pleistocene rinds.