A 800,000 YEAR-LONG RECORD OF PALEOCLIMATE FROM DEVILS HOLE

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. The precipitated calcite captures the geochemical signal of the groundwater, which in turn reflects the regional climate state. Over time, the slow yet continuous deposition of calcite has formed thick crusts containing nearly 1 million years of climate history. By analyzing this unique archive, we can determine the precise timing of past hydroclimate changes in the southwest USA over long (103 to 105 year) timescales. This provides a testbed to examine whether recorded changes coincide with variations in incoming solar radiation a primary driver of glacial cycles during the Pleistocene (Milankovic theory) or whether these shifts in climate states arose from internal forcings and feedbacks. In a precursor project, we extracted samples from the cave, known as Devils Hole, to reconstruct a record of regional hydroclimate changes over the last 200,000 years. We discovered a sampling-depth bias that explained the chronological offset observed in previous paleoclimate records derived from this cave. Our findings solved a long-standing debate surrounding the timing of climate changes in the southwest USA and confirmed that these changes are indeed consistent with the Milankovic theory over the last 200,000 years. In this follow-up project we plan to extend the Devils Hole record to 800,000 years. This will provide the first paleoclimate record of the last eight glacial- interglacial cycles from a terrestrial archive outside Antarctica. In addition, we will determine the height of the paleo-water table in order to examine past moisture changes in this modern-day desert region. The resulting data will represent a significant advancement in the understanding of this unique regions climatic history and the underpinning forcings.

In the south of Nevada, there is a unique underwater cave in the desert that holds a wealth of information about the climatic and environmental history of this part of the USA: Devils Hole. The groundwater that slowly flows through this cave deposited calcite on the walls. Over time, a thick layer formed that has stored the history of the groundwater and its fluctuations, and thus the hydroclimate, for hundreds of thousands of years. In this project, the changes in the groundwater table over more than 700,000 years were reconstructed. During the long glacial periods, it rose to at least 9.5 meters above today's level. Using a numerical groundwater model, it was possible to show that this corresponds to an increase in groundwater recharge of around 240%; a clear indication that more rain fell due to the cooler temperatures and considerably less precipitation evaporated. During interglacial periods, on the other hand, there was always drought and we were able to detect groundwater lowstands of up to 1.6 meters below today's level. These extreme conditions are certainly relevant scenarios for the future climate in this arid region in the southwest of the USA. Furthermore, it could be shown that the groundwater level fluctuations reflect the large climatic fluctuations of the northern hemisphere. The height of the groundwater table and the chemical composition of the groundwater (stored in the calcite layers on the cave walls) is therefore a "calendar" of the natural climate fluctuations and is unique worldwide in this completeness. For example, the Devils Hole deposits show a major change in their isotopic composition at the end of glacial periods when the carbon dioxide content of the atmosphere increased massively. Devils Hole is also sensitive to climate changes on shorter time scales; we were able to detect the effects of the so-called Heinrich events in the calcite layers, times when the ice shelves around the North Atlantic collapsed and the deep water circulation of the ocean came to a standstill. At these times, it was cooler and wetter in the Nevada desert.