HR: 0800h
AN: PP21C-1572    [Abstracts]
TI: Uranium Series Diagenesis in Corals Exposed to Fresh Water: Toward Better Prospecting for Closed System Samples for High Accuracy Dating
AU: * Mey, J
EM: mey@amnh.org
AF: American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 United States
AU: * Mey, J
EM: mey@amnh.org
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: * Mey, J
EM: mey@amnh.org
AF: The Graduate Center, City University of New York, 365 Fifth Avenue, New York, NY 10016 United States
AU: Fairbanks, R
EM: fairbanks@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Fairbanks, R
EM: fairbanks@ldeo.columbia.edu
AF: Department of Earth and Environmental Sciences, Columbia University, 2960 Broadway, New York, NY 10027 United States
AU: Mortlock, R
EM: mortlock@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Bradtmiller, L
EM: louisab@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Bradtmiller, L
EM: louisab@ldeo.columbia.edu
AF: Department of Earth and Environmental Sciences, Columbia University, 2960 Broadway, New York, NY 10027 United States
AB: A better understanding of U-series diagenesis in corals exposed to the vadose and phreatic freshwater environments is required to help prospect for closed-system samples needed for high-accuracy dates. Disequilibrium U-series are common in corals exposed to fresh water and a priori screening is generally insufficient to reject samples prior to dating. As a result, a vast number of U-series dates reported in the literature are open system and numerous authors have resorted to various correction models (Bender et al., 1979; Gallup et al., 1994; Thompson et al., 2003; Villemant and Feuillet, 2003; and Scholz et al., 2004). The majority of studies assumes or models continuous or episodic addition of ²³⁴U and/or ²³⁰Th over time and relies on the low solubility of ²³⁰Th and/or ²³⁴Th. Several recent studies emphasize progressive production of ²³⁴U via α-recoil (Thompson et al., 2003; Villemant and Feuillet, 2003), however the application of these models may lead to over-parameterization and are mainly idiosyncratic to host sample locations (Scholz et al., 2004). Already, some studies make model corrections to open system ages and draw critical conclusions about sea level change (e.g. Scholz et al., 2004; Thompson and Goldstein, 2005). Characteristic of all of these studies is a general correlation between the ²³⁴U/²³⁸U activity ratio and the ²³⁰Th/²³⁸U activity ratio between intra-reef samples. On Barbados, the largest activity ratio offsets appear in the youngest samples exposed to fresh water; just the opposite finding of most diagenesis models. Our measurements are consistent with the observations of Scholz et al. (2004) from the Red Sea. The greatest addition of ²³⁴U, ²³⁰Th, and ²³¹Pa to the Barbados samples occurred during the first exposure to fresh water, in this case Marine Isotope State 3 (MIS3), when ²³⁴U, ²³⁰Th, and ²³¹Pa are added to samples in similar proportions, but varying amounts. The bulk of reef sands and rubble is aragonite, which releases U-series isotopes during transformation to calcite relatively soon after exposure to fresh water. Barbados reef sands are inverted to calcite within 100,000 years and reef corals are generally in a state of partial dissolution. We propose that the correlation between the ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios seen in coral samples from the same reef or within samples is set early during the first fresh water exposure when the ``diagenetic potential'' of reef sand and rubble is high, and that later partial dissolution of corals progressively reduces the ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U anomaly. Because of this early diagenetic setting, the general ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U anomaly trends survive through time and the anomalies decay over time maintaining the same ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity proportions as evidenced by trends on activity plots.
DE: 1120 Isotopic disequilibrium dating
DE: 1165 Sedimentary geochronology
DE: 1641 Sea level change (1222, 1225, 4556)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005