Seasonal Isotopic Ca, B, O, C and Sr Fractionation in Late Oligocene Oysters (C. Gigantissima)

Stable isotope compositions preserved in shell calcite may record various aspects of the environment at the time of calcification. Data from natural samples of C. gigantissima from the Belgrade Formation, North Carolina 27+-1 Ma), suggest that Ca and B isotope fractionation is temperature dependent and is also a function of water composition. Also, Ca and B isotope fractionation seems to be highly species dependent and for paleotemperature reconstruction it is important to study each species in order to elucidate their response to both temperature and paleo environment. These relationships make seasonal variations in shells extremely useful for interpreting the paleoecology of fossil organisms, as isotope fractionation is different in seawater and brackish water (as seen with oxygen isotopes). In this study, Ca, B, C and O isotopic values were measured along a profile perpendicular to skeletal growth increments, known as stable isotope sclerochronology. This method enables to resolve intra-annual differences in stable isotopes. High resolution O and C isotopic profiles show that C. Gigantissima formed skeletal growth increments annually at seasonally varying growth rates, with convex and concave bands forming during summer and winter, respectively (Kirby, 2000). Assuming shell growth ceased at 10C, the estimated paleotemperature derived from oxygen isotopes gives a seasonal range of temperature of ca. 15C, with summer temperatures reaching ca. 25C. Samples from winter and summer bands show a small range of total variation in ⁴⁴Ca/⁴⁰Ca of 0.5 permil. The Ca isotope fractionation is positively correlated to temperature with more positive δ ⁴⁴Ca of ca. -1.43+-0.1 permil in the summer and more negative Ca isotopic compositions of ca. 1.93+- 0.1 permil in the winter. The data imply a temperature dependence that is ca. 0.03 permil per C. This is significantly less than the 0.24 permil per C reported by Naegler et al (2000) for cultured Globigerina sacculifer. However, aragonite precipitated in the laboratory at temperatures of 10 and 40C appears to show an even smaller temperature dependence of approximately 0.01 permil per C (Deyhle et al., 2002) to 0.015 permil per C (Gussone et al., 2003). Additional data gained on O, C and B isotopes further confirm that skeletal growth increments in C. Gigantissima show an annual variation, with average seasonal changes of 3.2+- 0.4 permil, 1.1+- 0.4 permil and 4.3+- 0.5 permil, respectively. Interestingly, B isotopes which were previously used as paleo-pH proxy, clearly show an intra-annual temperature dependent fractionation of almost 0.3 permil per C, suggesting that B may be a useful paleo-temperature proxy. Likewise δ ¹³C, B isotopes may also be influenced by the amount of respired CO₂ reaching the site of calcification. ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.70809 +- 0.000018 are the same within our analytical uncertainty and thus suggest fully marine conditions with no change in any freshwater input summer to winter. In summary, stable isotope analysis show significant intra-annual changes which are influenced by temperature and possibly other factors. Further studies need to be conducted on Ca and B isotopes to better understand whether temperature is the main fractionation mechanism on a seasonal time scale or if parameters like shell growth rate and other vital effects play also a critical role in isotopic variations.