Chemistry of the pearl oyster Pinctada radiata and implications for palaeoceanography

Abstract

Oyster shell chemistry offers major potential for palaeoceanographic studies and temperature reconstructions, particularly bimineralic oysters such as Pinctada radiata. This thesis explores the efficacy of using P. radiata shells from the shallow subtidal environment of the Qatari coast, Persian/Arabian Gulf, as recorders of oceanographic conditions. Collected from the Qatar coast, the chemistry of growth lines from calcitic and aragonitic layers within the shells was analysed and compared with known ocean buoy data (including temperature, salinity, and pH). The oxygen isotopic composition (δ18O) of calcite and aragonite ranges from -1.2 to 2.2‰ and -1.1 to 1.3‰, respectively, with seasonal δ18O temperatures derived from calcite mirroring recorded seawater temperatures. However, δ18O values in aragonite show no temperature correlation to seawater values, suggesting vital effects influence its composition. Carbon isotopic compositions (δ13C) for both minerals also indicate vital effects, with no direct correlation to buoy data. Trace element concentrations and ratios in both minerals correlate with salinity and pH, allowing for the reconstruction of Holocene and Pleistocene seawater conditions using fossilized P. radiata. Additionally, the application of carbonate clumped isotope (Δ47) thermometry to the bimineralic shells reveals discrepancies in temperature estimates. TΔ47 values from calcite layers accurately track seasonal temperature variations, while aragonite layers show consistently higher temperatures due to kinetic fractionation and vital effects. Elevated Ca and Mg concentrations further contribute to temperature overestimation. This thesis also examines the role of environmental stressors on living P. radiata, including temperature fluctuations and tidal exposure. These stressors affect the inner extrapallial fluid (iEPF), pH, and magnesium ion concentrations, facilitating dissolution and Mg-rich carbonate precipitation within the aragonite matrix. This research highlights that diagenesis is an active process throughout the organism's life and can be induced by the mantle itself. It offers new insights into biogenic carbonate preservation and diagenesis prior to its export to the geological record. Collectively, results highlight the potential of P. radiata shells as valuable proxies for understanding past environmental conditions and the complexities of carbonate chemistry in marine settings.