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