HR: 09:15h
AN: PP51G-06    [Abstracts]
TI: Diagenesis and Paleotemperature Reconstruction: A New Look at Old Coral
AU: * Cohen, A L
EM: acohen@whoi.edu
AF: Woods Hole oceanographic Institution, Department of Geology and Geophysics , Woods Hole, MA 02543 United States
AU: Hart, S R
EM: shart@whoi.edu
AF: Woods Hole oceanographic Institution, Department of Geology and Geophysics , Woods Hole, MA 02543 United States
AB: The strontium to calcium ratio (Sr/Ca) of the aragonite skeleton of massive reef corals decreases with increasing seawater temperature and is widely used as a paleothermometer. However, Sr/Ca measurements of fossil coral from both submerged and uplifted reefs have yielded paleoSSTs several degrees colder than those based on other marine proxies and their accuracy has been questioned. Here we show that abiogenic (secondary) aragonite crystals begin to grow within vacant skeletal pore spaces soon after tissue uplift (evacuation) has occurred. While the presence of secondary aragonite in living and fossil corals cannot be detected by X-Ray Diffraction, these crystals are distinguished from those of the primary skeleton by their large size. (>10 microns width) and irregular orientation. Discrete SIMS ion microprobe analyses of secondary crystals in two chunks of fossil Porites coral retrieved from 79-82m depth in a Tahiti barrier reef drill core (149W, 17S), yielded Sr/Ca ratios greater than 1mmol/mol higher than that of the primary coral skeleton, equivalent to 3 degrees cooler SST. Our results suggest that bulk sample analyses of these fossil corals would yield SSTs several degrees cooler than were actually experienced. While radiocarbon analyses indicate that the secondary crystals in these samples are up to 200 years younger than the primary coral skeleton, their impact on the Tahiti radiocarbon chronology is small relative to the impact on paleoSST reconstructions. Nevertheless, using SIMS ion microprobe, we have extracted Sr/Ca-derived SSTs from these fossil corals by targeting the centers of pristine skeletal septa with a 10-æm diameter ion beam spot, avoiding adjacent pore spaces occupied by the secondary needles. We obtained western Pacific SSTs ~0.5-1.5 degrees C cooler during the Bolling-Allerod relative to the present day, in good agreement with SSTs derived from Mg/Ca ratios of calcitic foraminifera. Our results indicate that employment of microanalytical techniques capable of selective analysis of pristine skeleton is warranted for accurate paleoSST reconstructions from ancient corals.
DE: 3670 Minor and trace element composition
DE: 4267 Paleoceanography
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting