Project Title: Investigation of Late Quaternary Ground-Water Discharge Deposits in the Mojave Desert, Eastern California
Mendenhall Fellow: Jeffrey Pigati, (520) 670-5508, jpigati@usgs.gov
Duty Station: Tucson, AZ/Denver, CO
Start Date: January 7, 2007
Education: Ph.D. (Geosciences), University of Arizona, 2004
Research Advisors: David Miller, (650) 329-4923, dmiller@usgs.gov; Marith Reheis, (303) 236-1270, mreheis@usgs.gov; Chris Menges (520) 670-5022, cmmenges@usgs.gov; Darrell Kaufman, Northern Arizona University, (928) 523-7192, Darrell.Kaufman@nau.edu
Project Description: Ground-water discharge deposits, also called “paleowetland” or “spring” deposits, form in arid environments as water tables rise and breach the ground surface during periods of enhanced effective precipitation. In addition to providing an important water source for local fauna, emergent water tables support hydrophilic vegetation, which in turn acts as a natural catchment system for eolian sediments. The interplay between emergent water tables, ecological and biological systems, and wind-blown sediments results in a unique and complex depositional environment that contains information on the timing (age of deposits) and magnitude (faunal and ostracode assemblages, isotopic data) of past climate change. Spring deposits also clearly demarcate the position of ground-water highstands on the landscape, providing a direct window into past hydrologic conditions.
Although much of the Mojave Desert is extremely arid today, large, interconnected lake systems, as well as numerous springs and wetlands, dominated the valley floors of the Mojave during the Late Pleistocene. Approximately 130 localities that contain fossil spring deposits have been identified by USGS personnel during recent mapping efforts. Little is known about the age of these deposits, how long high water table conditions were sustained, or the contribution of local versus regional recharge to the aquifer systems.
The initial phase of this project includes investigations at three localities, Valley Wells, Piute Valley, and Fenner Wash, all of which are located in the eastern Mojave Desert. The primary goals of this phase are to:
- establish a stratigraphic framework for the deposits at each locality
- determine the ages and rates of deposition of the deposits using radiocarbon dating of terrestrial gastropods and fossil organic matter, and optically-stimulated luminescence dating of eolian sediments
- reconstruct local paleoenvironmental conditions using sedimentology, fossil ostracodes and gastropod assemblages, geochemical indices, and stable isotopic analyses
The second phase of this study will include similar investigations of ground-water discharge deposits in the central and western regions of the Mojave Desert to determine if water tables spread out across the desert rose and fell in unison over time. The results of this study will allow us to better understand how hydrologic conditions in the desert biomes of the American Southwest varied with past changes in climate. The results may also be useful to those charged with managing this fragile ecosystem in light of pressures from a growing population and future climate change.
Valley Wells, California. Carbonate-rich spring deposits record the position of past ground-water highstands in arid environments. Such highstands resulted from a combination of cooler temperatures and increased precipitation at high elevations, like the Clark Mountains that sit ominously in the background here.
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