PHOSPHATE δ18O DETERMINATION OF MODERN RODENT TEETH BY DIRECT LASER FLUORINATION: AN APPRAISAL OF METHODOLOGY AND POTENTIAL APPLICATION TO PALAEOCLIMATE RECONSTRUCTION

Abstract

A direct laser-fluorination (DLF) method is presented for phosphate δ18O analysis (mass 1 - 2 mg). The automated system heats samples in the presence of excess BrF5 using a 25 W CO2 laser, at 10.66 μm. δ18O ratios of the liberated O2 were measured using a dual inlet Optima mass spectrometer. As DLF measures whole apatite oxygen, non-phosphate bound oxygen must be removed by pre-treatment. Two methods were investigated: 1) heating to 1000°C; and 2) heating to 400°C followed by laser fusing. Method 2 is recommended as samples heated to 1000°C showed evidence of oxygen exchange with atmospheric water. To validate the DLF method, and show the potential of rodent teeth in palaeoclimate reconstruction, modern rodent teeth δ18O results from 2 species are presented (δ18Op). Large inter- and intra-jaw heterogeneity indicates that single teeth cannot be used for palaeothermometry. However, the overall standard deviations were low (Glis glis δ18Op = +10.4 +/- 0.7%% n = 38 and Apodemus sylvaticus δ18Op = +14.4 +/- 1.3%% n = 24). Using equations, derived from lab rodents, an ingested water value of -5.6 +/- 2.2%% was calculated for Apodemus sylvaticus, only -1.3%% lower than measured local water (-4.3%%). This suggests that the phosphate δ18O of rodent teeth can be used as a proxy for palaeoclimate reconstruction.