| Popis: |
Uranium–uranium ( 234 U– 238 U) disequilibrium dating can determine the age of secondary carbonates over greater time intervals than the well-established 230 Th– 234 U dating method. Yet it is rarely applied due to unknowns in the initial δ234 U ( δ234 U i ) value, which result in significant age uncertainties. In order to understand the δ234 U i in Devils Hole 2 cave, Nevada, we have determined 110 δ234 U i values from phreatic calcite using 230 Th– 234 U disequilibrium dating. The sampled calcite was deposited in Devils Hole 2 between 4 and 590 ka, providing a long-term look at δ234 U i variability over time. We then performed multi-linear regression among the δ234 U i values and correlative δ18 O and δ13 C values. The regression can be used to estimate the δ234 U i value of Devils Hole calcite based upon its measured δ18 O and δ13 C values. Using this approach and the measured present-day δ234 U values of Devils Hole 2 calcite, we calculated 110 independent 234 U– 238 U ages. In addition, we used newly measured δ18 O, δ13 C, and present-day δ234 U values to calculate 10 234 U– 238 U ages that range between 676 and 731 ka, thus allowing us to extend the Devils Hole chronology beyond the 230 Th– 234 U-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. |
