Evaluating the skeletal chemistry of Mytilus californianus as a temperature proxy: Effects of microenvironment and ontogeny

Molluscan shell chemistry may provide an important archive of mean annual temperature (MAT) and mean annual range in temperature (MART), but such direct temperature interpretations may be confounded by biologic, metabolic, or kinetic factors. To explore this potential archive, we outplanted variously sized specimens of the common mussel Mytilus californianus at relatively low and high intertidal positions in San Diego, California, for 382 days with in situ recording of ambient temperature and periodic sampling of water chemistry. The prismatic calcite layer of eight variously sized specimens from each intertidal position were then serially microsampled and geochemically analyzed. Average intraspecimen delta O-18 values significantly covaried only with temperature, whereas Mg/Ca values showed a strong and significant positive correlation with growth rate. To assess intra-annual variations in shell chemistry as proxy for MART, each specimen's delta O-18 record was ordinated in the time domain and compared to the predicted isotopic equilibrium delta O-18 values from environmental data. Observed specimen values were significantly correlated with predicted equilibrium values, but show O-18 enrichments of 0.2 to 0.5 parts per thousand. In contrast, Mg/Ca values were poorly correlated with temperature due to significant positive relationships with growth rate and intertidal position. Within the extrapallial fluid, pH, carbonate solution chemistry, Rayleigh fractionation and/or an undetermined source of disequilibrium may cause delta O-18 values to deviate from predicted equilibrium precipitation for ambient seawater. Despite this consistent O-18 enrichment, intraskeletal variations in delta O-18 values readily characterize the instrumental MAT and 5-95% MART values, making M. californianus a valuable source of information for paleoceanographic reconstructions.