HR: 16:45h
AN: B44C-03    [Abstracts]
TI: Deriving Paleotemperatures From Coral Skeleton Using a Rayleigh Fractionation Model for Coral Biomineralization
AU: * Gaetani, G A
EM: ggaetani@whoi.edu
AF: Dept. Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Cohen, A L
EM: acohen@whoi.edu
AF: Dept. Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Wang, Z
AF: Dept. Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AB: Paleotemperature proxy records are typically derived from coral skeleton using empirical relationships between elemental ratios and water temperature calibrated using the skeletons of living organisms grown under known conditions. While this approach has produced significant advances in our understanding of Earth's climate system, its accuracy is limited by the influence of physiological processes ("vital effects") on compositional variability within the skeleton. "Vital effects" are evident as differences in the composition of carbonates precipitated experimentally and accreted by organisms at the same conditions, as well as differences in composition amongst skeletons of the same group of organisms, or even the same species, accreted under identical environmental conditions. Several recent studies have identified the importance of Rayleigh fractionation in producing "vital effects" in coral skeleton [1-3]. On the basis of this advance in our understanding of coral biomineralization, we have developed a new approach to deriving paleotemperature estimates from coral skeletons through knowledge of their combined Mg/Ca, Sr/Ca and Ba/Ca ratios. Using experimentally determined partition coefficients for Mg, Ca, Sr, and Ba between abiogenic aragonite and seawater [1] combined with a Rayleigh fractionation model for the precipitation of aragonite from a calcifying fluid, the temperature at which the aragonite was precipitated can be accurately determined. To test the accuracy and precision of this approach, which relies only on experimentally determined partition coefficients for abiogenic aragonite and the Rayleigh equation (i.e. contrary to conventional paleothermometer calibrations, no prior knowledge of water temperature is required), aragonite skeletons from 2 different coral species were analyzed for Mg/Ca, Sr/Ca and Ba/Ca ratios using the Cameca 3f ion microprobe at WHOI: (1) tropical coral Diploria labyrinthiformis collected from Bermuda and (2) deep water coral Lophelia pertusa collected from Tisler Reef, NE Skagerrak. Using only the elemental ratios and experimentally determined partition coefficients, the model predicts seasonally-resolved ocean temperatures to within 0.5 °C of the recorded temperatures at the sites the corals were collected. The predictive capability of our approach is outstanding, and is completely independent of any empirical calibration of coral skeleton from these environments. From these results it is clear that our new approach to coral skeleton paleothermometry yields ocean temperatures that are both accurate and precise to within a few tenths of a degree, and that the approach is applicable both across species and to corals growing in vastly different environments. References: [1]Gaetani, GA and Cohen, AL (2006) Geochim Cosmochim Acta 70, 4617-4634; [2] Cohen, AL, Gaetani, GA, Lundälv, T, Corliss, BH and George, RY, (2006), Geochem, Geophys, Geosys 7, Q12004, doi:10.1029/2006GC001354.; [3] Gagnon, AC, Adkins, JF, Fernandez, DP, Robinson, LF (2007) Earth Planet Sci Lett 261, 280-295.
DE: 0419 Biomineralization
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 4916 Corals (4220)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting