AVOIDING ARTIFACTS IN THE MEASUREMENT OF δ18O ON PHOSPHATE DERIVED FROM BIOGENIC APATITE

Phosphate oxygen isotope ratios provide a powerful tool for paleoclimate reconstruction. Analyses of the δ¹⁸O of phosphate in biogenic minerals (e.g., Longinelli and Nuti, 1973; Kolodny, 1983) and in solution in hydrothermal experiments (e.g., O’Neil et al., 2003) have revealed a temperature dependent equilibrium offset between the oxygen isotope composition of phosphate and that of ambient water. This temperature dependency has been used extensively for paleothermometry. Bioapatite in fossil tooth enamel and conodont elements is commonly used for the determination of paleotemperatures and the oxygen isotope composition of ancient meteoric and ocean water. The development of techniques for analyzing small samples has been critical for enabling paleoclimate reconstructions using small fossils and for resolving seasonal variation through serial sampling of larger teeth.

We report on artifacts in the measured oxygen isotope composition of silver phosphate that result from certain methods commonly used in the microscale (< 1mg starting material) processing of bioapatite phosphate. Silver phosphate is the purified phase used for phosphate oxygen isotope analysis. We compare buffered (slow) and unbuffered (crash) precipitations of silver phosphate over a range of conditions. Measured δ¹⁸O values of the silver phosphate produced by crash precipitation are sensitive to solution composition. Following routine protocols, we observe systematic shifts in crash precipitated phosphate δ¹⁸O of 1-1.5 per mil, which if undetected would lead to errors in temperature reconstructions of roughly 4-6 °C. A series of careful experiments suggests that Ag₂O coatings on fine-grained Ag₃PO₄ crash precipitates shift the measured isotopic compositions away from the true phosphate oxygen values. Buffered, slow Ag₃PO₄ microprecipitations yield oxygen isotope compositions indistinguishable from classical macroscale (ca. 10-20 mg bioapatite starting material) precipitations.