HR: 0800h
AN: PP31B-1532    [Abstracts]
TI: Calibration and Natural Variability of Skeletal Stable Isotopes and Sr/Ca in two species of Pacific sclerosponges (Acanthocheatetes wellsi and Astroclera welleyana)
AU: * Grottoli, A G
EM: grottoli.1@osu.edu
AF: Ohio State University, Department of Geological Sciences, Columbus, OH 43210 United States
AU: Adkins, J F
EM: jess@gps.caltech.edu
AF: California Institute of Technology, MS100-23, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Moots, K
EM: k.moots@ru.ac.za
AF: South African Institute for Aquatic Biodiversity, Private Bag 1015, Grahamstown, 6140 South Africa
AB: Sclerosponges are reef organisms that deposit a calcium carbonate exoskeleton in layers and can live for centuries. Increasingly, the isotopic and elemental compositions of their skeletons are being used to reconstruct tropical paloeceanographic records. Here preliminary data on the in situ calibration of the skeletal stable carbon (δ¹³C), stable oxygen (δ¹⁸O), and strontium to calcium ratios (Sr/Ca) in the Pacific sclerosponges Acanthocheatetes wellsi and Astroclera welleyana are presented. Further, the natural variability underlying these calibrations was evaluated by analyzing the skeletal δ¹³C, δ¹⁸O, Sr/Ca, and maximum linear growth in multiple specimens of A. wellsi (high-Mg calcite) and A. welleyana (aragonite) sclerosponges grown between 8.2 and 11 m depth at The Grotto in Saipan (15^oN, 145^oE) and Short Drop Off Reef in Palau (7^oN, 134^oE). Each sclerosponge was stained in situ with Alizarin Red in July 2001, cemented onto the reef, and collected in July 2003. Near-weekly seawater samples were collected during the two year interval and analyzed for DI-δ¹³C and δ¹⁸O of seawater. Hourly seawater temperature was recorded in situ throughout. Growth was measured from the stain line to the growing edge. Bulk skeletal samples were extracted from the same interval. In addition, high-resolution skeletal samples were extracted at 0.1 mm increment (~monthly). The stable isotopic and Sr/Ca values were measured in all skeletal samples. Based on the bulk skeletal analyses, intraspecific variation in δ¹³C and δ¹⁸O was below analytical detection limits at a given site. This implies that a proxy record from one specimen per site is sufficient. δ¹⁸O and growth did not differ between species. A difference in δ¹³C between species was due primarily to calcite-aragonite fractionation effects. However, growth did vary enough among individuals that age estimates for a hypothetical 5 cm tall A. wellsi or A. welleyana specimen could differ by up to 34 and 6 years, respectively. Thus, combining modern bomb-signal Δ¹⁴C dating techniques with sub-annual δ¹⁸O variability is needed to establish the chronology. Between sites, significant differences in δ¹³C, δ¹⁸O, and growth appear to be due to differences in the oceanographic conditions at each site. Results for the natural variability in Sr/Ca are forthcoming. High-resolution analyses revealed that δ¹⁸O in Palau records do not track changes in sweater temperature and appear to be driven primarily by changes in seawater salinity. In Saipan where seasonal variation in seawater temperature is pronounced, δ¹⁸O appears to be a faithful recorder of temperature. Sclerosponge δ¹³C did not correlate directly with seawater DI-δ¹³C. It is suspected that as slow-growing organisms with high filtering capacity, sclerosponges may be able to meet their carbon demand for calcification by utilizing only the CO₂ component of the DIC. Additional modeling and analytical results exploring this possibility will be presented.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4220 Coral reef systems (4916)
DE: 4273 Physical and biogeochemical interactions
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005