VARIABILITY IN INITIAL 230TH/ 232TH RATIOS IN CENTRAL TEXAS SPELEOTHEMS IS USED FOR MORE ACCURATE AGE DETERMINATION

Speleothems, calcite deposits in caves, preserve a record of climate in their growth rate, climate sensitive isotope ratios, and trace element concentrations. To produce pre-instrumental records of climate and create accurate climate models for the future, these variables must be tied to precise dates. The ²³⁸U-²³⁴U- ²³⁰Th disequilibrium method can yield very precise dates if there is low uncertainty in the amount of ²³⁰Th. ²³⁰Th concentration is a function of the ²³⁸U concentration and the age of the sample, plus the amount of ²³⁰Th initially in the sample. Although the calcite lattice does not typically accept Th, samples can contain high-Th impurities. It is commonly assumed that either the significant source of sample ²³⁰Th is from the decay of ²³⁸U or that initial ²³⁰Th can be estimated. This estimated initial ²³⁰Th/²³²Th assumes bulk-Earth U/Th values and secular equilibrium in the ²³⁸U decay chain. The uncertainty in this estimate is also assumed. If the amount of initial detrital ²³⁰Th can be better quantified for samples or sites then we can reduce the uncertainties in the dates and thus the time series of climate proxies.

This study determined the initial ²³⁰Th/²³²Th of modern calcite in order to provide more accurate dates for central Texas speleothems. Calcite was grown on glass substrates placed under active drips in central Texas caves. The ²³⁰Th/²³²Th of this modern calcite was determined using thermal ionization mass spectrometry. Results show that: 1) initial ²³⁰Th/²³²Th ratios can be accurately determined in these young samples and 2) that measuring ²³⁰Th/²³²Th ratios reduces the uncertainties in ages determined earlier on samples from under the same drips. Calcite collected on glass substrates from 4 drip sites at Westcave Preserve have initial ²³⁰Th/²³²Th values of 5.96 ± 0.6 ppm, 5.8 ± 0.56 ppm, 15.29 ± 0.67 ppm, and 14.6 ± 0.83 ppm. These values contrast with the value commonly assumed for initial ²³⁰Th/²³²Th, 4.4 ± 4.4 ppm, and are more precise. The two samples with higher ²³⁰Th/²³²Th grew during June and July while those with the lower values of ²³⁰Th/²³²Th grew over two years. This study shows that the commonly assumed initial ²³⁰Th/²³²Th is not always applicable and that initial ²³⁰Th/²³²Th can be determined accurately to determine the ages of young speleothems.