HR: 0800h
AN: B31D-0613    [Abstracts]
TI: Regional Failure of Intertidal Mussel Mg/Ca as an Independent Temperature Proxy: A Cautionary Tale
AU: * Ford, H L
EM: hford@ucsc.edu
AF: San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, United States
AU: * Ford, H L
EM: hford@ucsc.edu
AF: University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, United States
AU: Schellenberg, S A
AF: San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, United States
AU: Becker, B J
AF: University of Washington, Tacoma, 1900 Commerce St, Tacoma, WA 98402, United States
AU: Deutschman, D
AF: San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, United States
AU: Koch, P L
AF: University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, United States
AB: Paleoceanographic reconstructions depend upon accurate estimations of mean annual temperature (MAT) and mean annual range in temperature (MART). Mollusk shells may offer an archive of seasonal to decadal data on environmental conditions. However, a suite of factors (e.g., biologic, metabolic, kinetic) may confound interpretations of mollusk records. Mytilus californianus shell chemistry was directly compared to in situ environmental data to inspect the influence of intertidal position and growth rate (ontogeny) on δ¹⁸O and Mg/Ca. M. californianus specimens from an ontogenetic spectrum were outplanted from 31 Aug 04 to 17 Sept 05 at low (-0.2 m MLLW) and high (1.1 m MLLW) intertidal positions within Cabrillo National Monument at San Diego, California. Ambient temperature was recorded in situ and water samples were collected at ~2-3 week intervals. After the outplant interval, the prismatic calcite layer of eight specimens spanning the ontogenetic spectrum at each intertidal position were serially microsampled and analyzed for stable-isotope and minor-elemental variations. Average intra-specimen Mg/Ca values for specimens show a strong and significant positive correlation with accretion rate (i.e., younger, faster-growing specimens have higher Mg/Ca). This ontogenetic effect weakens the potential of average intra-specimen Mg/Ca an accurate MAT proxy. In contrast, average intra-specimen. δ¹⁸O provided a relatively accurate temperature proxy regardless of ontogenetic stage. To assess the utility of intraskeletal δ¹⁸O and Mg/Ca variation as a MART proxy, the observed δ¹⁸O record from each specimen was ordinated in the time-domain of the outplant interval and compared to the predicted equilibrium δ¹⁸O inorganic calcite record calculated from ambient seawater temperature and δ¹⁸O. Observed specimen δ&&¹⁸O is well-correlated (r² = 0.50, p<0.001) with predicted δ¹⁸O, but shows an ~+1.05‰ ¹⁸O-enrichment relative to inorganic calcite equilibrium precipitation. In contrast, corresponding Mg/Ca is poorly correlated with temperature (r² = 0.05, p<0.001) due to significant positive relationships with shell accretion rate and intertidal position. Within the extrapallial fluid, pH, carbonate solution chemistry, Rayleigh fractionation and/or an undetermined source of disequilibrium may cause skeletal δ¹⁸O values to deviate from predicted ambient seawater equilibrium precipitation. Despite the consistent ¹⁸O-enrichment, δ¹⁸O _{M. californianus} values capture instrumental MAT and 5-95 percentile MART making M. californianus a valuable proxy for paleoceanographic reconstructions.
DE: 0419 Biomineralization
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1065 Major and trace element geochemistry
DE: 4954 Sea surface temperature
SC: Biogeosciences [B]
MN: 2007 Fall Meeting