Devils Hole paleotemperatures and implications for oxygen isotope equilibrium fractionation

Subaqueous calcite in Devils Hole, Nevada, was growing continuously from slightly super-saturated groundwater, providing a 570 kyr-long delta O-18 paleoclimate record. Due to its very slow growth it has been assumed to have grown under conditions of isotopic equilibrium. However, its Holocene delta O-18 value is 1.5 parts per thousand higher than predicted by laboratory-precipitation-based oxygen isotope thermometer calibrations. The suggestion that Devils Hole calcite anchors the isotope thermometer to more O-18-enriched values has stirred a debate as to which paleothermometer calibration is relevant for paleoclimate and casts doubt on the validity of delta O-18-based paleotemperatures. We used clumped isotopes to test the assumptions of the Devils Hole alternative O-18-thermometer. Carbonate clumped isotopes are a temperature proxy that measures the abundance of C-13-O-18 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 degrees C between 27 and 180 ka, similarly to the modern groundwater temperature of 32.8-34.3 degrees 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 delta O-18-temperature calibrations should be reconsidered. The constant water temperature over the glacial-interglacial cycles indicates that the long Devils Hole delta O-18 record reflects only variations in the groundwater delta O-18 values and as such, represents a valuable archive of paleoclimate and isotope paleohydrology. (C) 2014 Elsevier B.V. All rights reserved.