Search

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.

A growing number of materials and environmental settings are studied using the carbonate clumped isotope ( Δ 47 ) thermometer. The method has been applied in both biogenic and non-biogenic carbonate systems, in marine and terrestrial settings, over a wide range of geological timescales. The current Δ 47 temperature calibration gives good temperature estimates for most biogenic materials, however, systematic biases are commonly observed at low temperatures. In this study we report additional calibration data, that covers a wider temperature range, at more evenly distributed temperatures, and are measured at higher analytical precision than the original calibration. Combining these data with the original calibration yields a Δ 47 –T relationship that is similar to the original calibration, though slightly less steep: Δ 47 = ( 0.0526 ± 0.0025 ) × 10 6 / T 2 + ( 0.0520 ± 0.0284 ) . This revised calibration is in better agreement with biogenic carbonates, including those grown at low temperatures. The difference between the original and revised calibrations is significant for carbonates forming below 16 °C or above 49 °C ( Δ 47 values of 0.68‰ and 0.56‰). Additionally, we include a comprehensive analysis of the sources of error associated with Δ 47 measurements and estimated temperatures and recommend measurement strategies for obtaining the desired precision. As an illustration, we apply the revised calibration and uncertainty analysis to 3 previously published studies. At low temperatures, the revised calibration results in significant differences from the original calibration and hence affects the interpretation of the environmental signal recorded. In light of our Δ 47 errors analysis, in cases where the temperature signals are small, we find that replicate analyses are critical to identify a temperature signal.