Abstract
The Pahranagat National Wildlife Refuge (PNWR) is a U.S. Fish and Wildlife Service (USFWS) managed, spring-fed system of arid wetlands in southern Nevada that serves as a sanctuary for migratory birds. Spring water feeding the PNWR wetlands is alkaline and the sediments are carbonate-rich. Since 2011, USFWS staff have facilitated our research and instructional activities in the PNWR. We collected and investigated lake sediment cores, fossil microbialites, and modern microbial mats to explore two lines of geoscience research: 1) The use of this existing spring-fed system as an analog for Miocene age carbonate rocks in the Lake Mead National Recreation Area (LMNRA) and 2) Reconstruction of the record of Holocene paleohydroclimate for the southern Great Basin using sedimentology and applying stable isotopic proxies. Through textural analysis, carbonate sedimentology, and petrology we found the sediments, and the modern and fossil microbial features within and on the margins of the Lower Pahranagat Lake (LPAH), are useful modern analogs for the Miocene microbialite limestones of the LMNRA. We applied O (delta (super 18) O), C (delta (super 13) C), Sr ( (super 87) Sr/ (super 86) Sr), and U ( (super 234) U/ (super 238) U) stable isotopes to carbonates from an approximately 5.8 ka LPAH core record to understand trends in Holocene paleohydroclimate. The isotopic records collectively show century to millennial scale wet and dry periods, including a notable period between 3.1-1.8 ka characterized by isotopic values that indicate drought with a significant reduction in water supply from the high volume carbonate aquifer that supplies approximately 90% of the water to the PNWR today. Clumped isotopic analysis of PNWR carbonates shows promise to get surface water paleotemperature estimates and produce hydrological models that could be useful to water resource management efforts. Preliminary clumped isotopic results from the LPAH core record indicate trends of increasing lake water and air temperatures and evaporation over the past 1000 years and highly variable temperatures over the Holocene. The PNWR is an ideal setting for geoscience and environmental science field methods instruction. To date, we have trained nearly 80 undergraduate students in a January field methods course and >10 students went on to do PNWR-focused collaborative research projects, contributing significantly to the work described above.