Subaqueous calcite in Devils Hole, Nevada, was growing continuously from slightly super-saturated groundwater, providing a 570 kyr-long δO18 paleoclimate record. Due to its very slow growth it has been assumed to have grown under conditions of isotopic equilibrium. However, its Holocene δO18 value is 1.5‰ higher than predicted by laboratory-precipitation-based oxygen isotope thermometer calibrations. The suggestion that Devils Hole calcite anchors the isotope thermometer to more 18O-enriched values has stirred a debate as to which paleothermometer calibration is relevant for paleoclimate and casts doubt on the validity of δO18-based paleotemperatures. We used clumped isotopes to test the assumptions of the Devils Hole alternative 18O-thermometer. Carbonate clumped isotopes are a temperature proxy that measures the abundance of 13C18O bonds in CaCO3. This proxy is independent of the water composition and therefore gives independent estimates of temperatures when calcite forms at thermodynamic and isotopic equilibrium. We find that Devils Hole water paleotemperatures were constant at 30.6 ± 2.6°C between 27 and 180 ka, similarly to the modern groundwater temperature of 32.8-34.3 °C. The proximity of the Devils Hole clumped isotope data to values expected based on modern groundwater temperatures supports the notion that Devils Hole calcite grew under equilibrium conditions. Therefore, the commonly used laboratory-based δO18-temperature calibrations should be reconsidered. The constant water temperature over the glacial-interglacial cycles indicates that the long Devils Hole δO18 record reflects only variations in the groundwater δO18 values and as such, represents a valuable archive of paleoclimate and isotope paleohydrology.
Bibliographical noteFunding Information:
The research was funded by the German Science Foundation DFG (Forschungsstipendium KL2391/1-1 to T. Kluge) and the National Science Foundation ( NSF-EAR-0842482 to H.P. Affek). Y. Dublyansky and C. Spötl acknowledge permission by the Death Valley National Park Service to study and sample Devils Hole #2 cave, and Hypo Tirol Bank for initial funding. We thank Shikma Zaarur, Casey Saenger, Yige Zhang, and Peter Douglas for fruitful discussions and Glendon Hunsinger of the Earth System Center for Stable Isotope Studies for technical assistance. We are grateful for thoughtful comments and suggestions of four anonymous reviewers and the editor that helped to improve the manuscript.
- Clumped isotopes
- Degassing effects
- Equilibrium fractionation
- Growth rate
- Oxygen isotopes