HR: 0800h
AN: U31B-0300    [Abstracts]
TI: Evidence for Recent Melting at the Base of the GISP2 Ice Core From Uranium-Thorium Disequilibrium Measurements
AU: * Goldstein, S J
EM: sgoldstein@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663, MS J514, Los Alamos, NM 87544,
AU: Murrell, M T
EM: mmurrell@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663, MS J514, Los Alamos, NM 87544,
AU: Nunn, A J
EM: nunn@lanl.gov
AF: Los Alamos National Laboratory, PO Box 1663, MS J514, Los Alamos, NM 87544,
AU: Nishiizumi, K
EM: kuni@ssl.berkeley.edu
AF: University of California - Berkeley, Space Science Laboratory, Berkeley, CA 94720,
AB: We measured ²³⁸U-²³⁴U-²³⁰Th disequilibria by mass spectrometric methods for a set of dusty ice samples from the base of the GISP2 Greenland ice core, at a depth of 3040-3052 m. The goal of this work was to further test the Fireman (1986) recoil-based model for producing uranium-series disequilibria in dusty ice on samples thought to be > 150 ka in age based on layer counting. However, the base of the GISP2 core is greatly disturbed in chemistry and dustiness relative to upper portions of the core. Samples consisted of 11 cm sections of ice core with sample weights of 340-430 g. We separated the samples into several fractions by filtration and analyzed the < 0.05 um fraction. This fraction had exceedingly high U and Th concentrations (2.5- 5 ppb U; 1.4-2.7 ppb Th). These U and Th concentrations are a factor of 1000 higher than measured for ice at Allan Hills, Antarctica. Low Th/U ratios of 0.51-0.65 indicate that a large portion of the uranium present in the samples is dissolved and not associated with particles, which are expected to have Th/U ratios around 3. However, ²³⁴U/²³⁸U activity ratios range from 0.972-0.992 (+/- 0.001), indicating a depletion of ²³⁴U relative to secular equilibrium of 1-3%. In addition, ²³⁰Th/²³⁴U activity ratios are quite low (0.18-0.24), suggesting either recent Th loss and/or U addition to the samples. This recent Th/U fractionation is not consistent with an age > 150 ka. Since liquid water would be characterized by ²³⁰Th/²³⁴U activity ratios ≪1, the low ²³⁰Th/²³⁴U activity ratios likely indicate that recent melting/freezing event(s) have occurred at the base of the GISP2 core. We can model these results with a two component mass balance calculation, with dissolved and particulate pools for each radionuclide. Although several assumptions are required to calculate ages, preliminary results of these calculations suggest that the melting events may be as young as <10 ka.
DE: 0724 Ice cores (4932)
DE: 1120 Isotopic disequilibrium dating
SC: Union [U]
MN: 2007 Fall Meeting