U.S. Geological Survey Open-File Report 01-257
By Kinga M. Revesz, Jurate M. Landwehr, and Jerry Keybl
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. To obtain reproducible and accurate results,
optimal conditions for the H3PO4
- CaCO3 reaction had to be determined. At
the acid-carbonate reaction temperature suggested by the equipment manufacturer,
the oxygen isotope ratio results were unsatisfactory (standard deviation (
)
greater than 1.5 per mill), probably because of a secondary reaction. When the
acid-carbonate reaction temperature was lowered to 26ºC and the reaction time
was increased to 24 hours, the precision of the carbon and oxygen isotope ratios
for duplicate analyses improved to 0.1 and 0.2 per mill, respectively.
The method was tested by analyzing calcite from Devils Hole, Nevada, which was formed by precipitation from ground water onto the walls of a sub-aqueous cavern during the last 500,000 years. Isotope-ratio values previously had been obtained by the classical method for Devils Hole core DH-11. The DH-11 core had been recently re-sampled, and isotope-ratio values were obtained using this new method. The results were comparable to those obtained by the classical method. The consistency of the isotopic results is such that an alignment offset could be identified in the re-sampled core material, a cutting error that was then independently confirmed. The reproducibility of the isotopic values is demonstrated by a correlation of approximately 0.96 for both isotopes, after correcting for an alignment offset. This result indicates that the new method is a viable alternative to the classical method. In particular, the new method requires less sample material permitting finer resolution and allows automation of some processes resulting in considerable timesavings.
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