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

Kathleen A Wendt,Christoph Spötl,Gina E Moseley,Yuri V Dublyansky,Mathieu Pythoud,R Lawrence Edwards

doi:10.1130/b35168.1

Kathleen A Wendt, Christoph Spötl + Show 4 more

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https://doi.org/10.1130/b35168.1

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Abstract

Abstract 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 234U/238U activity ratios in delta notation (δ234U0). Results show a range in values from 1851–1616‰. Variations in δ234U0 coincide with interglacial-glacial cycles over the past 475,000 years. Maximum δ234U0 values correspond to the last five glacial intervals, during which southwest Nevada experienced cool, pluvial conditions. Minimum δ234U0 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 234U accumulated in these materials. We interpret Devils Hole 2 cave δ234U0 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 δ234U0 record provides the first paleo-moisture record in southwest Nevada for marine isotope stages 10–12. In addition, high-precision δ234U measurements of modern groundwaters sampled from Devils Hole 2 cave are presented.

Highlights

The Ash Meadows groundwater flow system (AMGFS) is a large carbonate aquifer located downstream of a potential radioactive waste repository site in southwest (SW) Nevada, USA.Understanding this region’s hydrological variability over longer timescales is required by federal regulations to confidently assess the long-term risk of future radionuclide migration (10 CFR 963)
Results show that Devils Hole 2 (DH2) δ234U0 does not correlate with U concentration (R2 = 0.02) or growth rate (R2 = 0.02) over the past 475 k.y. (Fig. 2)
Five maxima in DH2 δ234U0 are identified at ca. 475 ± 11, 374 ± 6, 278 ± 2, 185.1 ± 0.7, and 43.2 ± 0.2 ka corresponding to glacial marine isotope stages (MIS) 12, 10, 8, 6, and 2, respectively

Summary

Introduction

The Ash Meadows groundwater flow system (AMGFS) is a large carbonate aquifer located downstream of a potential radioactive waste repository site in southwest (SW) Nevada, USA. Understanding this region’s hydrological variability over longer timescales is required by federal regulations to confidently assess the long-term risk of future radionuclide migration (10 CFR 963). The state of disequilibrium for the naturally occurring uranium isotopes 234U and 238U in groundwaters has been widely used to investigate the modern hydrological parameters of the AMGFS and surrounding aquifers (Paces et al, 2002; Neymark et al, 2005; Bushman et al, 2010; Paces et al, 2013; Paces and Wurster, 2014). Studying changes in AMGFS groundwater 234U‐238U disequilibrium over long timescales provides valuable insight into the long-term hydrological variability of this region

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