DELTA C-13 AND DELTA O-18 ISOTOPIC RATIOS OF CACO3 MEASURED BY CONTINUOUS FLOW ISOTOPE RATIO MASS SPECTROMETER: STATISTICAL EVALUATION AND VERIFICATION BY APPLICATION TO DEVILS HOLE CORE DH-11 CALCITE

A new method was developed to analyze the stable carbon and oxygen isotope ratios of small samples (400±20 µg) of calcium carbonate. This new method streamlines the classical phosphoric acid - calcium carbonate (H₃PO₄ - CaCO₃) reaction method by making use of a Thermoquest-Finnigan GasBench II* preparation device and a Delta Plus XL continuous flow isotope ratio mass spectrometer. Conditions for which the H₃PO₄ - CaCO₃ reaction produced reproducible results and minimal error had to be determined. Results from analyses of three different reference materials were evaluated statistically; the method was seen to produce unbiased and precise estimates of delta C-13 when the acid reaction time was ³ 24 hours, and of delta O-18 when the reaction time was > 34 hours, with precision £ 0.1 and £ 0.2 per mill, respectively. Screening rule for aliquot analysis was shown to further improve the quality of the results. The utility of the method was verified by analyzing calcite from Devils Hole, Nevada, for which isotope-ratio values previously had been obtained by the classical method (Landwehr and others, 1997). Devils Hole core DH-11 had been recently re-cut and re-sampled, and isotope-ratio values were obtained using this new method. The consistency of the isotopic results is such that an alignment offset was identified in the re-sampled core material. This confirmed that a cutting error had occurred during re-sampling. After correcting for this alignment offset, the results were comparable to those obtained by the classical method with correlation=+0.96 for both isotopes. This result indicates that the new method is a viable alternative to the classical method. In the case of DH-11 sample, both methods allow one to stratigrafically align cores by matching stable isotope patterns, a useful capability in paleoclimate studies, however, since the new method requires less sample material it permits finer resolution when it is necessary. Also, the new method allows automation of some processes resulting in considerable timesaving. °°°°°°°. Reference: Landwehr, J.M, Coplen, T.B., Ludwig, K.R., Winograd, I.J., and Riggs, A.C., 1997, Data for Devils Hole Core DH-11: U.S. Geological Survey Open-File Report 97-792, 8pp.