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Strontium and uranium isotopes from lacustrine carbonates as a paleohydrologic tracer in arid wetlands: An example from the Pahranagat Valley, Nevada
  • Kevin Theissen,
  • James Paces,
  • John Duggan
Kevin Theissen
University of St Thomas

Corresponding Author:kmtheissen@stthomas.edu

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James Paces
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John Duggan
University of St Thomas
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The combination of strontium (87Sr/86Sr) and uranium (234U/238U) isotopes is an especially useful tool to track and quantify mixtures of water sources in arid wetlands, where chemistry and lighter isotopic tracers are strongly influenced by evaporation and transpiration. Those isotopes were used to understand modern water supply to the Pahranagat National Wildlife Refuge (PNWR) in southern Nevada (Paces & Wurster, 2014, J. Hydrol. 517). We investigate the possibility that this isotopic combination might also track paleohydrological changes in such settings. Here, we present Sr and U isotope data for authigenic carbonates in a sediment core spanning 5800 14C cal years from Lower Pahranagat Lake (LPAH), a shallow, alkaline lake within the PNWR. Modern surface waters in the PNWR are comprised of mixtures of discharge from two high volume springs from the regional carbonate aquifer in the northern valley, and smaller amounts of local discharge from the shallow volcanic aquifer in the southern valley. Modern surface water from LPAH has U isotopic values similar to the most recently formed LPAH carbonates; however, Sr isotopic values in LPAH surface water are somewhat lower than values in those same carbonates. Combined Sr and U isotope values in Holocene LPAH carbonates fall within the range defined by the three primary spring sources and reflect varying mixtures of those sources supplying LPAH from mid-Holocene to modern time. Values in the oldest samples (~5800 14C cal yr BP) have distinct 87Sr/86Sr and 234U/238U values that nearly match the local spring end member, suggesting that LPAH water at that time was dominated by proximal volcanic aquifer sources. By ~5300–5200 14C cal yr BP, LPAH water was comprised of a nearly equal mixture of the three spring sources, marking a dramatic shift in hydrologic conditions that allowed contributions of surface flow from distal carbonate springs to the north. Values in samples from ~1,000–3,000 14C cal yr BP indicate decreased contributions from distal carbonate springs during two drought intervals; however, by 730 14C cal yr BP, surface flow from carbonate springs had resumed. Our results indicate that combined Sr and U isotopes preserve evidence of past changes in water sources in arid wetlands useful for interpreting evolving hydrologic conditions.