Paleohydrology of southwest Nevada (USA) based on groundwater U-234/U-238 over the past 475 k.y.

摘要

Subaqueous calcite deposited on the walls of Devils Hole 2 cave (Nevada, USA) represents a unique archive for geochemical variations within the regional aquifer. Here, we present a 475,000-year record of initial U-234/U-238 activity ratios in delta notation (delta U-234(0)). Results show a range in values from 1851-1616 parts per thousand. Variations in delta U-234(0) coincide with interglacial-glacial cycles over the past 475,000 years. Maximum delta U-234(0) values correspond to the last five glacial intervals, during which southwest Nevada experienced cool, pluvial conditions. Minimum delta U-234(0) values correspond to interglacial intervals, during which this region experienced warm, arid conditions. We propose that an elevated water table during glacial periods inundated previously dry bedrock and basin sediments, thereby leaching excess U-234 accumulated in these materials. We interpret Devils Hole 2 cave delta U-234(0) as a proxy for water-rock interactions in this regional aquifer, which is ultimately tied to the surface moisture conditions at recharge zones. The mechanism proposed here serves as a testable hypothesis and possible analogue for future subaqueous speleothem studies in similar hydrogeologic settings. Due to its unprecedented duration, the Devils Hole 2 cave delta U-234(0) record provides the first paleo-moisture record in southwest Nevada for marine isotope stages 10-12. In addition, high-precision delta U-234 measurements of modern groundwaters sampled from Devils Hole 2 cave are presented.