CHEMISTRY OF LIVING AND FOSSIL PEARL OYSTER SHELLS ( PINCTADA RADIATA ) FROM THE PERSIAN/ARABIAN GULF AS A PALEOENVIRONMENTAL PROXY

Abstract

Oyster shell chemistry as a paleoenvironmental proxy has shown great potential; however, the chemical composition of bimineralic oysters from the genus Pinctada remains unexplored. Pearl oysters (Pinctada radiata) from shallow subtidal environments along the Qatari coast, Persian/Arabian Gulf, provide an excellent opportunity to explore this relationship. The bimineralic nature of P. radiata also permits the investigation of vital effects on the chemistry of the calcite middle prismatic and aragonitic inner nacreous layers. The δ18O of calcite and aragonite ranges between −1.2‰ and +2.2‰ and −1.1‰ and +1.3‰, respectively. Calculated seasonal δ18O temperatures from calcite are between 19°C and 39°C, which are similar to the range of seawater temperatures recorded by buoys deployed near oyster buildups. There is no correlation between the δ18O values from aragonite and temperature, suggesting vital effects control the δ18O composition of the nacreous layer. The δ13C for calcite and aragonite ranges between −1.6‰ and +0.9‰ and −0.61‰ and +1.75‰, respectively, with no relationship to buoy data, which again implies vital effects. Trace element concentrations and their ratios (Na, Mg, B, Al, P, Cl, K, Fe, Sr; Mg/Ca, Na/Ca, Sr/Ca) exhibit strong correlations with salinity levels (40 to 55‰) and pH values (7.8 to 8.7) across both minerals. This was determined using a novel combination of principal component analysis and multivariate regression techniques (R2 = 0.77 and 0.81, respectively, with p-values of 0.05). Using these relationships, fossil shells yield calculated Holocene temperatures, salinities, and pH values between 23°C and 39°C, 45‰ and 54‰, and 7.81 and 8.2, respectively. Pleistocene values are between 22°C and 43°C, 39‰ and 47‰, and 7.7 and 8.2. These results are like modern seawater values and indicate that P. radiata shells reliably preserve a record of ambient seawater conditions.