U.S. Geological Survey Open-File Report 01-257
By Kinga M. Revesz, Jurate M. Landwehr, and Jerry Keybl
Calcite from Devils Hole, Nevada was used to test this new method. Devils Hole is a tectonic cave formed in the discharge zone of a regional aquifer in south-central Nevada. Dense vein calcite has precipitated from the ground water onto the walls of this subaqueous cavern during the last 500,000 years (Winograd and others, 1992). Devils Hole Core DH-11 is a 36-cm long core taken from the wall of the cave at about 30 meters below the water table; it contains an approximately 500,000-year-old continuous record of the paleoclimate (Landwehr and others, 1997). The core was originally sampled along its length in approximately 1.27 mm intervals by milling. A new slab was cut from DH-11 and it was re-sampled in 1998 for other purposes. Material from the new slab was used for this study rather than the original material cut in 1997. This study analyzed material that had been identified as being re-sampled at distances of 165.7 mm to 266.0 mm from the free (outer) face of the core, which would correspond to precipitate from approximately 320 ka to 450 ka (thousands of years before present), respectively. The ratio of carbon isotopes (C13/C12) and of oxygen isotopes (O18/O16) are of interest for the study of global paleoclimatic conditions.
Calcite re-sampled from core DH-11 was analyzed by the new method. The analytical results, compiled into a data table organized by the reported re-sampling depth of each sample, are given in Appendix C. The stable isotope data plotted with the re-sampled depth is shown in Figure 4. An inverse relation results between the oxygen and carbon isotope ratio data, consistent with the pattern reported by Coplen and others (1994).
For a check of method consistency, three samples of the re-sampled material (at approximately 232, 233, and 234 mm) also were analyzed using the classical method. These data are shown in Figure 4 as stars and listed at the bottom of Appendix C. All results differ by less than 0.1 per mill and this corroborates the consistency of the two methods.
When the re-sampled series (filled symbols in Fig. 4 and 5) were compared with the original data (open symbols), two offsets that increased with depth were noted (Fig. 4). This observation was confirmed to accord with periodic re-positioning of the specimen during milling when it was observed to be slipping in the vise (A.C.Riggs, private communication, 2001). A mathematical correction to the recorded cutting depths was applied to accommodate these re-sampling conditions (Fig. 5).
When the results of the new method, with corrected sampling depths, are compared to the classical method (Fig. 5), it can be seen that the new method reproduced the classical method. The reproducibility of the isotopic values is demonstrated by a correlation of approximately 0.96 for both isotopes, after correcting for an alignment offset. Hence the new method can be used as an alternative to the classical method. However, care must be taken to check the precision of the standards, since reruns might be necessary. These additional analyses require time, but overall the automation of the GasBench II method results in considerable timesavings. The capability to make use of smaller sample sizes makes the GasBench II method superior to the classical method when sample amount is limited and/or finer sampling resolution is desirable.
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