CONFOUNDING EFFECTS ON DETERMINING TEMPERATURE FROM THE INTRASKELETAL CHEMICAL VARIATIONS OF MYTILUS CALIFORNIANUS

The carbonate within the shells of marine metazoans is often a rich archive of past climate change. However, the precision and accuracy with which oceanographic conditions are recorded may be taxon- or environment-specific, necessitating proxy studies for any fossil taxon under investigation. Here, we examine the intraskeletal oxygen isotope (δ¹⁸O) and minor element ratios (Mg/Ca) from the intertidal mussel Mytilus californianus in order to assess its use as a paleotemperature recorder. A total of 294 specimens from 13 monthly collection dates from a range of ontogeny and two extreme intertidal heights were collected from Santa Cruz, CA.

General linear models were used to assess the explanatory power of various environmental variables (temperature, δ¹⁸O_seawater, mussel size, intertidal position, chlorophyll a) for the observed intraskeletal δ¹⁸O and Mg/Ca variations. The predicted δ¹⁸O and intertidal position together explained 37% of the variation of the measured intraskeletal δ¹⁸O. Temperature and intertidal position explained 24% of the variation of the measured intraskeletal Mg/Ca with mussel size explaining an additional 5%. Tidal elevation differences include a δ¹⁸O enrichment relative to equilibrium at the higher intertidal height during warmer months due to (1) a possible growth slow/shut down at higher temperatures or (2) possible enrichment in δ¹⁸O above a certain temperature threshold. In addition, consistently higher Mg/Ca values at the higher tidal elevation can be explained by differences in temperature or potential feeding time between the two intertidal positions. Thus, M. californianus would not be a reliable tool to recreate past oceanographic conditions based on (1) the large amount of unaccounted variation from correlations with predicted variables (63% for δ¹⁸O and 72% for Mg/Ca), (2) confounding effects of intertidal position and specimen size, (3) the δ¹⁸O enrichment relative to equilibrium during summer at the higher intertidal elevation, and (4) the large number of replicate samples needed for an accurate temperature determination. Using additional data from previous studies, we demonstrate the precision of Mytilus as a paleotemperature recorder increases with percent submersion and, therefore, with lower tidal elevation.