HR: 0800h
AN: PP21C-1579    [Abstracts]
TI: Seasonal d18O and Sr/Ca Records from Submerged Pleistocene Fossil Corals in the Western Equatorial Pacific
AU: * Gruhn, L M
EM: gruh0005@d.umn.edu
AF: Dept. of Geological Sciences, Univ. of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812 United States
AU: Gallup, C D
EM: cgallup@d.umn.edu
AF: Dept. of Geological Sciences, Univ. of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812 United States
AU: Ravelo, A C
EM: acr@emerald.ucsc.edu
AF: Ocean Sciences Dept., UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Riker-Coleman, K
EM: rike0003@tc.umn.edu
AF: Dept. of Geological Sciences, Univ. of Minnesota Duluth, 1114 Kirby Dr., Duluth, MN 55812 United States
AU: Taylor, F W
EM: fred@utig.ig.utexas.edu
AF: Institute for Geophysics, Univ. of Texas, 4412 Spicewood Springs Rd. #600, Austin, TX 78759 United States
AU: Webster, J M
EM: jody.webster@jcu.edu.au
AF: School of Earth Sciences, James Cook Univ., Townsville, Qld 4811 Australia
AU: Webster, J M
EM: jody.webster@jcu.edu.au
AF: Earth Sciences Dept., UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Silver, E A
EM: esilver@emerald.ucsc.edu
AF: Earth Sciences Dept., UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Potts, D C
EM: potts@biology.ucsc.edu
AF: Dept. of Ecology and Evolutionary Biology, UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AB: We have monthly d18O and Sr/Ca records from three submerged fossil Porites corals from the Huon Gulf, Papua New Guinea and from two modern Porites corals from New Britain and Sanaroa Island, which have been used for calibration. The fossil corals grew on the lower plate of an active convergent margin in the Huon Gulf and formed a series of 100-200 m high platforms between depths of 100 and 2500 m. In a tectonic model (1), the reefs build platforms during glacial/interglacial cycles and drown during periods of rapid sea level rise after glacial events. The platform ages were estimated by combining U-Th coral ages (2,3) with the model (1). Corals were collected by the ROV {IT JASON} from the tops of the platforms, which grew just prior to rapid sea level rise in the model. Three fossil corals, each spanning 3-6 years, from water depths of 1277 m, 1631 m, and 1977 m (model platform age 260-248 ka, 326-302 ka, and U-Th platform age 348 ±10 ka respectively) were slabbed along the growth axis, sampled at 11-30 samples per year, and analyzed for d18O and Sr/Ca. The modern samples S1 and NB19 have mean d18O values of -4.9 and -5.2‰ respectively; fossil corals 135, 166, and 244 have mean d18O of -3.5, -2.8, and -2.1‰ respectively. The mean d18O of the fossil corals is 1.5 to 3‰ heavier than that of the modern samples. This offset suggests fossil samples grew when ice volume was greater than present, supporting Wallace's model of platform drowning. As d18O changes associated with ice volume are expected to be on the order of 1.0‰ or less, the differences in the mean values of fossil and modern corals indicate changes in sea surface temperature (SST) and/or sea surface salinity (SSS) associated with the glacial cycles. The two samples with the heaviest d18O also have Sr/Ca means offset to higher values, corresponding to a change in SST of 7-10°C. Past changes in seawater Sr/Ca (4) or addition of secondary aragonite (5) could increase this offset and the amount of cooling it implies. We have compared the amplitude of the seasonal signal during times of low/intermediate sea levels to modern samples. Based on the mean d18O and Sr/Ca, Sample 244 represents the coldest period of time that we analyzed and has the lowest seasonal d18O and Sr/Ca amplitude. Sample 166, representing the second coldest period, has the highest seasonal d18O and Sr/Ca amplitude, even higher than the modern samples. Thus, there does not appear to be a linear relationship between annual mean d18O and seasonality in either of the proxies that we measured. Assuming that the mean d18O is a first-order measure of the extent of glaciation (cooling and ice volume), we do not see evidence that seasonal variability in the Huon Gulf is predominantly influenced by glacial/interglacial forcing. 1: Wallace, Ph.D. thesis, UC Santa Cruz (2002); 2: Galewsky et al., Geology 24, 819 (1996); 3: Webster et al., Marine Geology 204 (2004); 4: Stoll and Schrag, GCA 62, 1107 (1998); 5: Enmar et al., GCA 64, 3123 (2000).
DE: 4870 Stable isotopes (0454, 1041)
DE: 4875 Trace elements (0489)
DE: 4916 Corals (4220)
DE: 4926 Glacial
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005