Determination of Sr and Ba partition coefficients between apatite from fish (Sparus aurata) and seawater: The influence of temperature
Résumé
The Sr/Ca and Ba/Ca ratios in inorganic apatite are strongly dependent on the temperature of the aqueous medium during precipitation. If valid in biogenic apatite, these thermometers would offer the advantage of being more resistant to diagenesis than those calibrated on biogenic calcite and aragonite. We have reared seabreams (Sparus aurata) in tanks with controlled conditions during experiments lasting for more than 2 years at 13, 17, 23 and 27 degrees C, in order to determine the variations in Sr and Ba partitioning relative to Ca (D(Sr) and D(Ba), respectively) between seawater and fish apatitic hard tissues (i.e. teeth and bones), as a function of temperature. The sensitivity of the Sr and Ba thermometers (i.e. partial derivative D(Sr)/partial derivative T and partial derivative D(Ba)/partial derivative T, respectively), are similar in bone (partial derivative D(b-w)(Sr)/partial derivative T = 0.0036 +/- 0.0003 and partial derivative D(b-w)(Ba)/partial derivative T = 0.0134 +/- 0.0026, respectively) and enamel (partial derivative D(e-w)(Sr)/partial derivative T = 0.0037 +/- 0.0005 and partial derivative D(e-w)(Ba)/partial derivative T = 0.0107 +/- 0.0026, respectively). The positive values of partial derivative D(Sr)/partial derivative T and partial derivative D(Ba)/partial derivative T in bone and enamel indicate that D(Sr) and D(Ba) increase with increasing temperature, a pattern opposite to that observed for inorganic apatite. This distinct thermodependent trace element partitioning between inorganic and organic apatite and water highlights the contradictory effects of the crystal-chemical and biological controls on the partitioning of Ca, Sr and Ba in vertebrate organisms. Taking into account the diet Sr/Ca and Ba/Ca values, it is shown that the bone Ba/Ca signature of fish can be explained by Ca-biopurification and inorganic apatite precipitation, whereas both of these processes fail to predict the bone Sr/Ca values. Therefore, the metabolism of Ca as a function of temperature still needs to be fully understood. However, the biogenic Sr thermometer is used to calculate an average seawater temperature of 30.6 degrees C using the Sr/Ca compositions of fossil shark teeth at the Cretaceous/Tertiary boundary, and a typical seawater Sr/Ca ratio of 0.02. Finally, while the present work should be completed with data obtained in natural contexts, it is clear that Sr/Ca and Ba/Ca ratios in fossil biogenic apatite already constitute attractive thermometers for marine paleoenvironments.