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Dale Lake Sand Ramps, Mojave Desert

This study focuses on unraveling late Pleistocene hydrologic/climatic conditions responsible for the Dale Lake Sand Ramp (near Twentynine Palms, California) that contains a 50+-m-high stratigraphic sequence of eolian sands, intercalated with over 10 buried soils and colluvium. Understanding the origins of this record will reveal fundamental process/climate controls on sediment and soil distribution in related eolian, lacustrine, and fluvial settings in the desert SW.

At least 10-11 eolian depositional cycles representing the last 65,000 years make up the Dale Lake Sand Ramp. The last 5-6 episodes of sand deposition took place during the last 37,000 years, indicating that eolian sequences in the southern Mojave are more complex and older than originally believed. We will test the hypothesis that heightened sediment supply to Dale Lake, the source for the Dale Lake sand ramp, may be associated with increased regional precipitation, enhanced monsoonal circulation during the glacial conditions, and later landscape stability with reduced zonal flow. In this hypothesis, periods of sand ramp formation are tentatively associated with glacial periods and/or stadials. Alternatively, this record may reflect variations in sediment supply and vegetation cover associated with particularly warm periods during interstadials and interglacials and thus resulted from interstadial/interglacial aridity variations.

The stratigraphy and age of sand ramps in the northern Mojave are significantly different from those in the southern Mojave. Northern Mojave sand ramps generally range from >50,000 years at their surface to >760,000 years at their base with long intervals of non deposition recorded by soils. In contrast, southern Mojave sand ramps are all generally <35ka and most are believed to be <20ka. Different climatic mechanisms, on different time scales, appear to be responsible for sand movement in latitudinal zones within the Mojave.

Oxygen/carbon isotopic analyses on the soil carbonate, if preserved, may help in defining if carbonate was accreted under "lighter" glacial conditions or "heavier" Holocene-like conditions. The field observations, geochronology and stable isotope analyses will be the basis to develop a process response model for the Dale Lake area. This model will assess under what environmental conditions increases the effective sediment supply for eolian entrainment and subsequent landscape stability. A key focus of this research is to assess the current state of this eolian system and range of response in the late Holocene, with other nearby environmental systems showing substantial drought variability in the past two to four millennia.

For more information, contact John Whitney.