HR: 0800h
AN: PP31C-0540    [Abstracts]
TI: Variation of initial Th-230/Th-232 and limits of high precision U-Th dating of shallow-water corals
AU: * Shen, C
EM: river@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Li, K
EM: wawa0114@yahoo.com.tw
AF: Department of Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Sieh, K
EM: sieh@adelphia.net
AF: Tectonics Observatory, California Institute of Technology, Pasadena, CA 91125, United States
AU: Natawidjaja, D
EM: danny@geotek.lipi.go.id
AF: Tectonics Observatory, California Institute of Technology, Pasadena, CA 91125, United States
AU: Cheng, H
EM: cheng021@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AU: Wang, X
EM: wang0452@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AU: Edwards, R
EM: edwar001@umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455, United States
AU: Lam, D
EM: ddinhlam@yahoo.com
AF: Institute of Geology, National Centre for Natural Sciences and Technology, Hanoi, 084, Viet Nam
AU: Meltzner, A
EM: meltzner@gps.caltech.edu
AF: Tectonics Observatory, California Institute of Technology, Pasadena, CA 91125, United States
AU: Taylor, F
EM: fred@utig.ig.utexas.edu
AF: Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78759, United States
AU: Quinn, T
EM: quinn@ig.utexas.edu
AF: Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78759, United States
AU: Chiang, H
EM: d91224004@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, Taipei, 106, Taiwan
AU: Kilbourne, K
EM: kilbourn@cbl.umces.edu
AF: Physical Sciences Division R/PSD1, NOAA, Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305, United States
AB: One hundred seventy eight U-Th data of 25 pristine modern and Holocene corals and 33 seawater samples were analyzed to understand the variability of coral initial ²³⁰Th/²³²Th (²³⁰Th/²³²Th₀). Samples were collected from 2 continental shelf sites, Nanwan of southern Taiwan in the western Pacific and Son Tra of central Vietnam in the South China Sea; 1 open ocean site, Santo of Vanuatu in the western tropical Pacific; and 6 intertidal sites around the islands of Simeulue, Lago, North Pagai and South Pagai of Sumatra in the eastern Indian Ocean. Using site-specific ²³⁰Th/²³²Th₀ values or isochron techniques, our study demonstrates that a coral ²³⁰Th dating method with a precision as good as 1 year is achievable for corals with an age less than 100 years. The results show that thorium is mainly attributed to the dissolved phase of seawater and the variation of ²³⁰Th/²³²Th₀ depends on local hydrology. ²³⁰Th/²³²Th₀ atomic ratios of 4.9 x 10^-6 and 3.2 x 10^-6 with a 10% variation are respectively observed in Nanwan and Son Tra with intense terrestrial material input. At the Santo site, we find a value of 5.6 x 10^-6 at 4 horizons and one high value of 24 x 10^-6 in 1974.6 ± 0.5 AD, attributed to the upwelling of cold water during a La Nina event from 1973-1976 AD. The natural dynamics of ²³⁰Th/²³²Th₀ at the intertidal sites in the Sumatran islands are complicated and vary significantly from 3.0-9.4 x 10^-6. Discordant ages, ranging from -23 to +4 years, are observed in 2.2% of the 139 total subsamples, accounting for detrital material with anomalous high and low ²³⁰Th/²³²Th values. Duplicate measurement of coeval subsamples is therefore recommended to verify the age accuracy. A high standard quality control program, with careful sampling and subsampling processes, contamination-free storage methods, and low chemistry ²³⁰Th blank levels, is required.
DE: 1000 GEOCHEMISTRY
DE: 1040 Radiogenic isotope geochemistry
DE: 1105 Quaternary geochronology
DE: 1120 Isotopic disequilibrium dating
DE: 4860 Radioactivity and radioisotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting