CARBON, OXYGEN AND STRONTIUM ISOTOPES OF ARAGONITIC FOSSILS FROM THE MIDDLE JURASSIC OF SCOTLAND: EVIDENCE FOR SEASONALITY OF TEMPERATURE, RAINFALL, AND EVAPORATION

The Jurassic Great Estuarine Group (GEG) in Scotland yields a diverse suite of well-preserved aragonitic fauna that preserve detailed paleoenvironmental information in the form of isotopic and trace elemental chemistry. Our study focuses on fossils recovered from shell beds of the Kildonnan Member of the GEG. Analysis of the geochemistry of fossils collected from single shell beds provides an opportunity to reconstruct the isotope paleohydrology of lagoons in what is widely believed to be a marginal marine environment. Detailed paleohydrological data on these lagoons permits assessment of seasonal patterns of temperature, rainfall and evaporation that characterize the Bathonian of western Europe. Micromilled mollusc d¹³C_(CaCO3) and d¹⁸O_(CaCO3) values range from +1.3‰ to 3.6‰ VPDB and 0.1‰ to -3.6‰ VPDB respectively. Shell fragments with values outside this range suggest that supra-seasonal variation may be greater than seasonality. Otoliths from bed 3g have d¹³C_(CaCO3) and d¹⁸O_(CaCO3) values that range from -5.4‰ to +1.5‰ VPDB and -5.3‰ to -1.9‰ VPDB respectively. These data represent the oldest stable isotopic record of paleodiet, paleoecology, and fish migration to date. Otoliths and molluscs record variation in d¹⁸O_(H2O) values from -3.7‰ to -2.0‰ (VSMOW) indicating that rainfall must have an average d¹⁸O_(H2O) value lower than -3.7‰ (VSMOW). Because stable isotope values are dependent upon water chemistry, we use ⁸⁷Sr/⁸⁶Sr values as an additional determinant to separate salinity/evaporative effects from temperature signals. ⁸⁷Sr/⁸⁶Sr values as well as data previously presented by others (molluscan fauna data, lithostratigraphy, paleogeography, and paleocirculation models) agree that mixing of fresh water and seawater occurred. Significantly, it appears that the d¹⁸O_(H2O) values of the Great Estuarine Group water are predominantly determined by a balance of evaporation of fresh and brackish water rather than simple mixing of freshwater and seawater. d¹⁸O values and high Sr/Ca ratios agree with data on Bathonian tree ring research by others in support of a seasonally arid Mediterranean-type environment, with low seasonality of temperature (mean=23 °C) and a short-lived rainy season.