Devils Hole, Nevada: Revisiting a unique paleoclimate archive

The subsurface of Nevada harbors a very special site for the science of past climate evolution: Devils Hole. Calcite is very slowly precipitating from a regional groundwater system giving rise to thick subaqueous crusts coating the walls of fractures. Already two decades ago these deposits have been shown to provide an amazing record of climate change over the past 500,000 years. Our project proposes to take a new, fresh look at this site and to push the record back to at 800,000 years and possibly beyond one million years before present. This will allow for the first time to examine at least eight glacial-interglacial cycles in a terrestrial archive outside Antarctica. We shall also employ a new paleothermometer to quantify temperature changes in this groundwater across hundreds of thousands of years. Finally, we shall use these samples to reconstruct a history of fluctuations of the groundwater table on these long timescales, thus providing a record of humid episodes in this arid region.

Hidden in the Amargosa Desert of south-central Nevada (USA) is an underwater cave that provides a wealth of information about this regions climate history. As groundwater passes through the cave, calcite slowly precipitates on the submerged walls. Over time, the slow yet continuous deposition of calcite has formed thick crusts that store the geochemical signature of groundwater reflecting the regional climate over the last approximately 1 million years. Three decades ago, researchers analyzed these deposits to reconstruct a 500,000-year record of hydroclimate change in the southwest USA. The timing of these changes, however, triggered widespread debate in the paleoclimate community. Central to this debate was whether changes in incoming solar radiation due to variations in Earths orbital configuration acted as the primary driver of glacial cycles during the Pleistocene, or whether these shifts in climate states arose from internal forcings and feedbacks. The aim of this project was to return to the now famous cave, known as Devils Hole, in order to re-examine the precise timing of hydroclimate changes in the southwest USA. By determining the age of the end of penultimate glacial cycle (about 130,000 years ago), our results revealed a systematic offset in the apparent age and vertical sampling depth. The observed sampling-depth bias provided an explanation for the temporal offsets in the original Devils Hole record, thereby solving a long-standing debate surrounding the timing of climate changes in the southwest and showing that the Devils Hole record is consistent with the Milankovic theory. We also reconstructed the last 350,000 years of water-table fluctuations recorded in Devils Hole caves. We discovered that the timing of past water-table rises and falls (>9 m in amplitude) closely coincide with the expansion and reduction of North American ice sheets, which in turn influences the position and intensity of winter storms that transport Pacific- sourced moisture to the southwest USA. Superimposed on this long-term trend are millennial-scale high-stands recorded during the last three glacial cycles that coincide with cool periods in the North Atlantic region. In the final phase of this project, we examined subsurface processes driving changes in groundwater 234U over time, which in turn provides insight into paleo-moisture conditions at recharge zones. Well dated paleo-hydroclimate records are extremely rare in desert regions. Our new Devils Hole record is currently the longest paleo-moisture record from North America. The resulting data represents a significant advancement in the understanding of this unique regions climate history and the forcings that influence moisture availability in the southwest USA.