Novel method for determining 234U-238U ages of Devils Hole 2 cave calcite (Nevada)

Uranium-uranium (²³⁴U-²³⁸U) disequilibrium dating can determine the age of secondary carbonates over greater time intervals than the well-established ²³⁰Th-²³⁴U dating method. Yet it is rarely applied due to unknowns in the initial d²³⁴U (d²³⁴U_i) value, which result in significant age uncertainties. In order to understand the d²³⁴U_i in Devils Hole 2 cave, Nevada, we have determined 110 d²³⁴U_i values from phreatic calcite using ²³⁰Th-²³⁴U disequilibrium dating. The sampled calcite was deposited in Devils Hole 2 between 4 and 590 ka, providing a long-term look at d²³⁴U_i variability over time. We then performed multi-linear regression among the d²³⁴U_i values and correlative d¹⁸O and d¹³C values. The regression can be used to estimate the d²³⁴U_i value of Devils Hole calcite based upon its measured d¹⁸O and d¹³C values. Using this approach and the measured present-day d²³⁴U values of Devils Hole 2 calcite, we calculated 110 independent ²³⁴U-²³⁸U ages. In addition, we used newly measured d¹⁸O, d¹³C, and present-day d²³⁴U values to calculate 10 ²³⁴U-²³⁸U ages that range between 676 and 731 ka, thus allowing us to extend the Devils Hole chronology beyond the ²³⁰Th-²³⁴U-dated chronology while maintaining an age precision of ~2 %. Our results indicate that calcite deposition at Devils Hole 2 cave began no later than 736 ± 11 kyr ago. The novel method presented here may be applied to future speleothem studies in similar hydrogeological settings, given appropriate calibration studies.