HR: 1340h
AN: PP13B-1274    [Abstracts]
TI: Groundtruthing the Neodymium Isotope Proxy in Deep-Sea Corals
AU: * van de Flierdt, T
EM: tina@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, 61 Route 9W, Palisades, NY 10964,
AU: Robinson, L F
EM: lrobinson@whoi.edu
AF: Woods Hole Oceanographic Institution, Clark 448, MS25, Woods Hole, MA 02543,
AU: Adkins, J F
EM: jess@gps.caltech.edu
AF: California Institute of Technology, MS 100-23, 1200 E. California Blvd., Pasadena, CA 91125,
AB: The Nd isotopic composition of marine precipitates is increasingly recognized as a powerful tool in paleoceanography. Unlike nutrient proxies such as δ¹³C or Cd/Ca, Nd isotopes are not thought to be altered by biological processes, and thus may serve as a quasi-conservative water mass mixing tracer. However, any archive, which is used to extract authigenic Nd isotopes, needs careful examination, to test the integrity of the inferred seawater signal. Here we present first data on cleaning experiments and modern calibration experiments on different species of deep-sea corals. Seven different coral samples ranging in age from modern to ~220ka were selected for experiments designed to remove ferromanganese crusts and / or organic residues that may contain high concentrations of Nd and Th. The aim was to determine whether the rigorous chemical procedure we use to remove Th associated with these crusts is effective at removing Nd, and whether it causes any fractionation in the Nd isotopic composition of the coral aragonite. Crusts were found to contain Th-232 concentrations of up to ~160ppm, with ²³²Th/²³⁰Th ratios dependent on the oceanic location of the coral. Un-cleaned corals had Th-232 concentrations of up to 8ppb and the cleaning procedure reduced these values to less than 0.2ppb in both modern and fossil specimens. Neodymium isotopic compositions reveal that for modern corals, with no visible coating, a pre-cleaning step is sufficient to yield the isotopic composition of ambient seawater. The ferromanganese coating around fossil corals however may have a very different isotopic composition than the coral aragonite since it may be a time-integrated signal biased towards modern values. This bias is observed for intermediate water depth D. dianthus corals from stage 3 in the northwest Atlantic. Modern D. dianthus skeletons from the northwest Atlantic and the Drake Passage reflect the seawater Nd isotopic composition, and we are extending this modern calibration to include five different species from diverse oceanographic settings.
DE: 1040 Radiogenic isotope geochemistry
DE: 4825 Geochemistry
DE: 4916 Corals (4220)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting