HR: 10:50h
AN: PP32A-03    [Abstracts]
TI: Radiocarbon Calibration Between 30,000 and 50,000 Years Before Present Using Fossil Corals
AU: * Chiu, T
EM: tcchiu@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, New Core Lab, Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Fairbanks, R G
EM: fairbanks@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, New Core Lab, Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Mortlock, R A
EM: mortlock@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, New Core Lab, Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AB: Early attempts to calibrate the radiocarbon time scale using lake sediments, deep sea cores, and cave flowstones relied on assignment of calendar ages based on correlations to other time series or modeled ages (Kitagawa and van der Plicht, 2000; Voelker et al., 2000; Hughen et al., 2004) or modeled ground water chemistry (Beck et al., 2001). These calibrations are too disparate to be adopted for routine radiocarbon conversion. Fossil corals are considered an excellent archive for extending the calibration beyond the tree ring records because corals can be directly dated by both U-series and radiocarbon methods. Coral data sets, however, are not consistent between 30,000 and 50,000 years before present (Yokoyama et al., 2000; Cutler, 2000) and show considerable scatter. Our experiments show that much of the scatter in corals can be explained by minor amounts of diagenetic calcite and trace amounts of organics, thereby significantly biasing the radiocarbon dates toward younger ages. A greater than 50% etching of coral samples immediately preceding radiocarbon measurements is the accepted pretreatment for radiocarbon dating (Yokoyama et al., 2000). In fossil corals that were exposed to the vados or phreatic freshwater environments and contain minor amounts of diagenetic calcite, the acid etching pretreatment enriches the fraction of secondary calcite by 200% to 400%. Our experiments and simple model demonstrate the enrichment of secondary calcite shifts the age of the fossil coral hundreds to thousands of years younger. Previous studies set the lower limit of acceptable calcite in corals between 1% and 5% (Yokoyama et al., 2000; Cutler, 2000; Paterne et al., 2004) or report a detection limit of 1% (Bard et al., 1998). However, our model and experiments show that only corals with $<$0.2% calcite fall within the age uncertainty of radiocarbon dating after progressive etching. We demonstrate a detection limit of 0.2% calcite in aragonite can be routinely achieved by X-ray diffraction analysis, and adopt $<$0.2% calcite as an essential criteria for inclusion of coral U-series and radiocarbon data in our calibration data set. Another significant source of radiocarbon error in fossil corals that have been exposed to vados environments during lower sea level or due to uplift is trace organic matter that is not removed by the standard acid etching procedures. Our experiments show progressive whitening of coral samples over a 3-day period by ultrasonic cleaning in 30% peroxide and a corresponding increase in radiocarbon age due to the oxidation of acid resistant organic matter.
DE: 4825 Geochemistry
DE: 4860 Radioactivity and radioisotopes
DE: 4267 Paleoceanography
DE: 1035 Geochronology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting