HR: 1340h
AN: PP13A-0590    [Abstracts]
TI: Radiocarbon and Cd/Ca in Deep-sea Coral Desmophyllum cristagalli: A Modern Calibration of Cd/Ca and a Record From the Younger Dryas
AU: * Eltgroth, S F
EM: selene@caltech.edu
AU: Adkins, J F
EM: jess@gps.caltech.edu
AB: Here we report our latest radiocarbon and Cd/Ca results from a deep-sea coral that captures a very short event in the deep ocean during the Younger Dryas. Our Younger Dryas D. cristagalli sample was dredged from 1069-1235m near the Azores and previously U/Th dated using the method of Cheng et al. (2000) to 11,980+/-120 yr BP. Adkins et al. (2000) demonstrated that deep-sea coral radiocarbon reflects the radiocarbon of the seawater in which they grow. AMS radiocarbon ages in the Younger Dryas coral range from 10,900 to 10,400 14C yr. With a total span of 5cm and assuming an aragonite accumulation rate of 1mm/yr, this coral individual lived approximately 50yr. Thus, the range in 14C age must result from conditions other than coral growth. Along the direction of coral growth the radiocarbon age is oldest at the base, reaches its youngest age in the middle, and returns to an older age at the top. This age reversal reveals the competing influence of radiocarbon enriched and depleted water. Radiocarbon depleted water could be produced by slowing the rate of deep-water formation and/or by increasing the relative proportion of southern source water at the site. In order to resolve the deep-water formation rate from mixing, we measure Cd/Ca as a nutrient proxy by isotope dilution ICP-MS. Previous studies have shown that Cd/Ca in foraminifera and surface corals reflects the Cd/Ca of the water in which the organism grew. A straightforward Cd/Ca calibration has proven elusive in the case of D. cristagalli. Modern D. cristagalli Cd/Ca ratios may yet trend with water Cd/Ca, but several of our modern corals have unexplained high Cd/Ca and high variability. Our results show that Cd/Ca ratios do not vary with the coral banding structure but do vary systematically with coral growth, reaching a maximum value at or near the top of the coral. Cd/Ca in the Younger Dryas sample increases from about 0.25 to 0.65micromol/mol, significantly larger than the range of the modern samples measured to date. If this is indicative of the water in which it grew, it reflects the increasing influence of high Cd southern source water toward over the lifetime of this coral and is qualitatively consistent with the radiocarbon record. The changing signal could be caused by a either moving a front of 14C depleted water a short distance, implying that there were 2 water masses with vastly different radiocarbon signature coexisting in close proximity to each other, or by moving a front of 14C depleted water a significant distance, implying that there was a large shift in the relative proportions of NADW and AAIW bathing this site during the Younger Dryas.
DE: 4825 Geochemistry
DE: 4267 Paleoceanography
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting