17 Geochemistry and Petrology (Posters)

Thursday, 4 April 2013
1:30 PM - 5:30 PM
Classroom 107

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

Paper number: 17-6
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

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.