Uranium-trend systematics for dating Quaternary sediments

摘要: A new concept in uranium-series dating called uranium-trend has been tested extensively over the past several years to determine reliability of this technique estimating time deposition surficial deposits ranging age from 5,000 about 800,000 B.P. The open system consists determining a linear trend analyses three ten samples covering various layers or soil horizons formed given depositional unit. In each sample an accurate determination concentrations 238 U, 231*U, 230 Th, and 232 Th is required. Whole-rock are used made on subsamples less than 2 mm-size fraction. Isotopic determined by alpha spectrometry utilizing radioisotope dilution techniques. analytical results plotted where ( U230 Th)/ U vs. 231*U238U)/ 238U ideally yields array which slope line best fit changes systematic way with increasing deposit. rate change half-period uranium flux, F(0), local environment. flux mobile component that migrates either solution colloids through slowly works its void spaces between mineral grains produces time-dependent trail daughter products, 231*U 230Th. An empirical model based distribution products sediment, compensates for F(0) response different lithologies, textures, climatic regimes. Analyses units known ages required calibration model; calibrations at years, 12,000 150,000 600,000 730,000 were provided correlations dated radiocarbon K-Ar. At best, have potential estimated accuracy ± 10 percent older 100,000 years; however, uncertainty strongly dependent quality regarding scatter data points length line. Percentage errors not symmetric throughout range; they greater both young years) deposits. Quaternary variety environments analyzed investigation. These include alluvium, colluvium, eolian sand, till, loess, zeolitized volcanic ash, coastal marine sediments. Most modified pedogenic processes. Depending conditions lithologic compositions, these soils range aridosols spodosols. Some buried such as terrace paleosols. Owing need explain methodology U-trend well influence geomorphic, climatic, factors study areas, report divided into chapters. introductory chapter followed separate chapters co-authors describe (a) Rocky Mountain region, (b) alluvial San Joaquin Sacramento Valleys California, (c) Rio Grande Pecos New Mexico, (d) Nevada Test Site area (e) effects fault disturbance calcareous alluvium Site, (f) southeastern Atlantic Coastal Plain (g) southern California Barbados, West Indies.