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

Abstract

[1] We examined the potential utility of Mg/Ca SHELL ratios recorded in shells of the estuarine oyster Crassostrea virginica as a temperature proxy because oxygen isotope ratios (d 18 O) in estuaries are complicated by the simultaneous fluctuation in temperature and salinity, whereas Mg/Ca WATER 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 d 18 O, d 13 C, and Mg/Ca ratios. Measured d 18 O SHELL and d 13 C SHELL 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/Ca SHELL ratios and temperature. When all Mg/Ca SHELL data were compared to temperature, a weak but statistically significant correlation was observed (r 2 = 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/Ca SHELL ratios and D Mg (the partition coefficient) emerged (Mg/Ca SHELL , r 2 = 0.30, p < 0.01; D Mg , r 2 = 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. Citation: Surge, D., and K. C Lohmann (2008), Evaluating Mg/Ca ratios as a temperature proxy in the estuarine oyster, Crassostrea virginica,

Oyster Shells As Recorders of Short-Term Oscillations of Salinity and Temperature During Deposition of Coral Bioherms and Reefs In the Miocene Lorca Basin, SE Spain

“…8A) are within the range of values reported in the literature for modern oysters (Surge and Lohmann 2008;Wisshak et al 2009), and the covariant trends observed for Mg/Ca vs. Sr/ Ca for the O. edulis var. lamellosa samples may indicate that there is a temperature effect on the Mg and Sr uptake (Surge and Lohmann 2008).…”

“…8A) are within the range of values reported in the literature for modern oysters (Surge and Lohmann 2008;Wisshak et al 2009), and the covariant trends observed for Mg/Ca vs. Sr/ Ca for the O. edulis var. lamellosa samples may indicate that there is a temperature effect on the Mg and Sr uptake (Surge and Lohmann 2008). We have no straightforward explanation for the higher Mg/Ca ratios of the oyster #1c-2 and #1a-1B and D samples, but we speculate that the increased ratios might reflect periods of increased metabolic activity (Rosenberg and Hughes 1991;Steuber 1999;cf.…”

“…lamellosa could therefore reflect the ecological niche preferred by C. gryphoides. In addition, the Miocene oyster shells have stable-isotope and Mg/Ca and Sr/Ca values that are similar to those of modern oyster shells (Surge and Lohmann 2008;Wisshak et al 2009), and the stable-isotope values moreover seem to reflect paleo-environmental conditions:…”

Past daily light cycle recorded in the strontium/calcium ratios of giant clam shells

“…The Mg/Ca data also appear to show a diurnal cycle, but the amplitude is rather large and highly irregular. A few reports have described a positive correlation between the Mg/Ca ratio and temperature for Mytilus trossulus 24 and Crassostrea virginica 25 , although these earlier data were obtained using a conventional analytical method. There is a slight positive correlation between the Mg/Ca ratio and seawater temperature ( Supplementary Fig.…”

Sclerochronological analysis of archaeological mollusc assemblages: methods, applications and future prospects

“…The partitioning of Mg/Ca between solid and liquid phases is thought to be temperature-dependent (Mucci 1987) and thus potentially tracks changes in ambient temperature acting as a palaeothermometer (Klein et al 1996). However, correlations between temperature and Mg/Ca can be weak with temperature reconstructions requiring precise knowledge of the environment during shell growth (Surge and Lohmann 2008), making Mg/Ca only suitable as an auxiliary proxy. Ratios of Sr/Ca have also been explored as a proxy for palaeotemperature (Freitas et al 2006); however, it was found that correlations were generally weak or inconsistent.…”