Evaluating Mg/Ca ratios as a temperature proxy in the estuarine oyster, Crassostrea virginica

We examined the potential utility of Mg/CaSHELL ratios recorded in shells of the estuarine oyster Crassostrea virginica as a temperature proxy because oxygen isotope ratios (δ18O) in estuaries are complicated by the simultaneous fluctuation in temperature and salinity, whereas Mg/CaWATER ratios are assumed to be constant above 10 practical salinity units (psu). We tested this assumption and observed a slight mixing effect between 10 psu and normal marine values emphasizing the need to test this assumption in estuarine settings. Microsamples of shell carbonate were analyzed for δ18O, δ13C, and Mg/Ca ratios. Measured δ18OSHELL and δ13CSHELL were compared to predicted values to assign dates to shell samples. Once samples and dates were aligned, corresponding temperature and salinity for a particular sample were known, enabling comparison of Mg/CaSHELL ratios and temperature. When all Mg/CaSHELL data were compared to temperature, a weak but statistically significant correlation was observed (r2 = 0.05, p < 0.01). Because date assignments may become increasingly uncertain further back in time and earlier studies identified ontogenetic effects in juvenile portions of shells from other bivalve species, we examined the relationship from only the last year of growth. When only data from the last year of growth were compared, a slightly stronger positive relationship between Mg/CaSHELL ratios andDMg (the partition coefficient) emerged (Mg/CaSHELL, r2 = 0.30, p < 0.01; DMg, r2 = 0.33, p < 0.01). The improved correlation may result from either ontogenetic effects influencing the incorporation of Mg into the younger portion of the shell and/or inaccurate date assignments. Regardless of the cause, our equations are similar to previously published equations for C. virginica and other taxa. Despite the low correlations, our improved relationship supports further study under controlled experimental conditions.