HR: 0800h
AN: PP31B-1522    [Abstracts]
TI: Initial and Diagenetic Behaviour of U Isotopes in Corals: Implications for U-series Dating
AU: Robinson, L F
EM: laurar@gps.caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: * Adkins, J F
EM: jess@gps.caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Fernandez, D
EM: diego@gps.caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Burnett, D
EM: burnett@gps.caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Gagnon, A
EM: gagnon@caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Krakauer, N
EM: niryk@caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Wang, S
EM: jslw@gps.caltech.edu
AF: Caltech, GPS, MS 100-23 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: U-series isotopes in corals are one of the most powerful tools for chronometry in Quaternary Paleoceanography. Modern mass spectrometric techniques routinely reach 1 permil precision for the isotopic ratios of uranium and thorium. However, the corals themselves are often found to be open systems for U-series dating. The most widely used metric for diagenetic alteration of the age is the deviation of the δ²³⁴U ratio from the seawater value of 146‰. In this study we use deep-sea corals that live and are preserved in a constant seawater environment to investigate the causes of this diagenetic alteration. Coral thin-sections display complex visible banding patterns based on the crystal morphology. Fission track maps and MC-ICP-MS measurements performed on micro-milled sub-samples reveal a primary [U]-variability that has a spatial distribution closely related to the visible banding. Sub-samples from fossil corals, ranging in age from 11 ka to 218 ka, have variable δ²³⁴U_initial with the highest δ²³⁴U_initial values in areas of low [U]. A model shows that most of the variability can be explained by two simple processes, direct transfer of alpha recoil ²³⁰Th and ²³⁴Th and, more importantly, preferential movement of alpha-decay mobilised ²³⁴U. Coupling this preferential movement with a high [U] coating such as an organic film provides a source of ²³⁴U to the coral lattice that can account for large δ²³⁴U_initial elevations, with little change to the final age. As surface corals also have large initial [U] gradients, our results demonstrate that many elevated δ²³⁴U_initial values are a natural consequence of the coral's initial [U] gradient. These gradients are biologically induced at the time of calcification and are an example of how better understanding the effects of biomineralization can improve our interpretation of paleotracers.
DE: 1120 Isotopic disequilibrium dating
DE: 4825 Geochemistry
DE: 4916 Corals (4220)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005