A new method for the determination of the δ¹⁸O composition of bone phosphate: applications to the thermal physiology of vertebrates

Abstract

A new method for the preparation of bone samples for δ¹⁸O-PO₄ analysis has been developed. The phosphate from bone samples is separated and purified using ion exchange chromatography, and then precipitated as silver phosphate using the Firsching method. O₂ is then extracted by reaction with bromine pentafluoride and converted to CO₂ for isotopic analysis. Advantages of the silver phosphate technique over the bismuth phosphate technique are (1) the simplicity of the method, (2) silver phosphate is not hygroscopic so atmospheric water is not an oxygen contaminate, and (3) very small samples (2 mg to 10 mg) can be analyzed with an increase of precision compared to previous techniques. Iron and silica oxides added to Ag₃PO₄ standards produce no offset, which suggests that this new technique is not sensitive to interferences from cements commonly found in fossil bone material. Analyses have been performed on standards ranging in size from 30 mg to 2 mg with a σ₁ of ± 0.05 per mil.

The δ¹⁸O-PO₄ compositional pattern of the bones of modern vertebrates have been analyzed to provide a base for the interpretation of the δ¹⁸O-PO₄ pattern of fossil vertebrates. The average %PO₄ in modern mammals and reptiles is approximately 15% with a σ₁ of 2-3 %. The average %PO₄ of fossil reptiles range from 10 to 25% with σ₁ variations of up to 10%. The %PO₄ variation can be related to cementation of void spaces in the bone material. XRD analysis reveals that calcite, dolomite, ankerite, and silica cements are commonly present. FTIR (Fourier Transform Infrared Spectroscopy) yields low indices calculated according to the formula of Shemesh, 1990 [GCA 54(9):2433]. Thin sections of the fossil bone material show detailed structures. All this evidence suggests that the fossil bone δ¹⁸O-PO₄ composition is pristine and not altered by diagenesis. The total δ¹⁸O-PO₄ σ₁ variation of 10 or more skeletal elements analyzed for each individual specimen varies from 0.1 per mil for a domestic cow (Bos) to 0.8 per mil for a Komodo dragon (Varanus). Variations in between appear to be related to thermal physiology and variations in drinking water δ¹⁸O due to seasonality or migration.