HR: 10:50h
AN: V11H-03 INVITED     [PDF]
TI: 230Th/U Dating of Pedogenic Carbonate
AU: * Sharp, W D
EM: wsharp@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: Ludwig, K R
EM: kludwig@bgc.org
AF: Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709 United States
AU: Amundson, R
EM: earthy@nature.berkeley.edu
AF: Div. of Ecosystem Sciences, University of California, Berkeley, CA 94720 United States
AU: Chadwick, O A
EM: oac@mailhost.geog.ucsb.edu
AF: Dept. of Geography, University of California, Santa Barbara, CA 93106 United States
AB: Quantitative determination of the timing of pedogenesis of calcic soils is of fundamental interest for many aspects of soil science and Quaternary geology. For example, if the onset of soil development is rapid, pedogenic materials can provide useful constraints on the ages of Quaternary deposits themselves, thereby facilitating studies of geomorphic change, neotectonics and paleoclimate. Furthermore, accurate dating can provide a quantitative time-axis for paleoenvironmental information commonly preserved in the oxygen and carbon isotopic compositions of pedogenic carbonates. Newly formed pedogenic carbonate generally contains ppm-levels of U and is depleted in Th relative to secular equilibrium because U is immensely more soluble than Th in oxygenated, infiltrating soil waters. Moreover, the initial concentration of 234U in pedogenic carbonate generally exceeds secular equilibrium because it is enriched in soil waters by recoil and enhanced solubility relative to 238U. Therefore, middle Pleistocene and younger pedogenic carbonate is potentially suitable for dating via the 230Th-234U-238U system. Potential pitfalls for U-series dating of pedogenic carbonate include correction for initial U-Th isotopes introduced by detrital contamination and failure of the U-Th system to remain closed. Moreover, if age-estimates for clastic deposits are sought, the interval between surface stabilization and formation of datable carbonate must be estimated. Several studies now show that reliable and precise ages for dense pedogenic carbonate coatings on the undersides of alluvial gravel-clasts (termed rinds) can be obtained using carefully selected, milligram-size samples drilled from polished slabs and analyzed by TIMS. Rinds from well-sorted alluvium, such as river terraces, sampled using this approach commonly have high 230Th/232Th activity ratios (generally $>$10, and commonly $>$100); thus, corrections for initial U and Th isotopes are minor and are made using a model-silicate detrital composition with 232Th serving as an index isotope. This "single sample, single date" approach precludes meaningless 230Th/U ages that can sometimes result from chemical fractionation of Th from U by leach-residue techniques. Furthermore, small samples allow the long (up to $>$100 kyr), and sometimes visibly discontinuous, depositional histories of individual rinds to be resolved. Using large data sets, reliability of rind-ages may be assessed from the ages themselves; that is, valid ages should be reproducible along microstratigraphic horizons and preserve microstratigraphic order within individual rinds. Datable carbonate can form within a few thousand years of surface stabilization, as shown by comparison with independent ages, allowing ages of Quaternary deposits and surfaces to be closely estimated. For example, a glacio-fluvial terrace of the Wind River basin, Wyoming (USA) that formed penecontemporaneously with the last Rocky Mountain glacial maximum yields rind-ages that overlap with cosmogenic surface exposure ages for last-glacial pavements, moraines and terraces. Moreover, rind dating provides the best available age constraints for terraces formed during the penultimate Rocky Mountain glacial cycle. These ages show that, contrary to previous suggestions, final maximum advances of the penultimate Rocky Mountain glaciation were broadly synchronous with the global ice-volume maximum of marine isotope stage 6.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
DE: 1886 Weathering (1625)
DE: 3344 Paleoclimatology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting