By Marith C. Reheis
 Figure 1. View of Silver Lake (see Fig. 2) in February 2005, an El Niño year, after large winter floods on the Mojave River filled this normally dry playa lake to a depth of 1 meter. |
Our approaches to studying the late Quaternary paleoenvironment of the Mojave Desert focus on the depositional records of drainages and pluvial lakes, as well as related settings and materials such as groundwater-discharge deposits, pollen, and packrat middens. In one approach, we employ analysis of physical, chemical, isotopic, and magnetic properties of sediment cores and stratigraphic sections to construct detailed paleohydrologic records. In a second approach, we use geologic mapping and stratigraphic analysis of sediments, soils, and geomorphic features exposed in outcrop along the drainage pathways. Results from these studies will be integrated and compared with those of nearby marine records from the Gulf of California and the southern California Pacific margin to determine how on-land paleoenvironmental conditions responded to changes in marine circulation and positions of continental glaciers. These reconstructions will enable better forecasting of future effects of global climate change on a marginal desert region that is under increasing population pressure. As part of this effort, we are co-sponsoring Mendenhall postdoctoral fellow Jeff Pigati, who conducts research on "Paleohydrology and Paleoclimate of Deserts from Study of Past Wetland Deposits."
The southwestern United States is under rapidly increasing pressure from population increases and demands for water. Most global climate-change models predict that this region, especially the Mojave Desert, will experience hotter temperatures and probably a drier effective climate in the coming century, but changes in patterns of atmospheric moisture transport are more difficult to predict. Most of the Mojave Desert lies outside the influence of tropical, monsoon-driven summer moisture, but abrupt fluctuations in the dominant air masses that bring moisture to this area are apparent in the historical record. The best known of these shifts is the ENSO (El Niño-Southern Ocillation) cycle in which El Niño conditions every 3-7 years bring substantial winter precipitation to the area (Fig. 1) via westerly Pacific frontal storms. Paleoclimate studies suggest that both monsoon-driven moisture and frontal storms reached this area during the late Pleistocene and early Holocene, but with timing that substantially differs from lake records farther north and east in the Great Basin. More precise and accurate records of paleoenvironmental conditions in the Desert Southwest during pluvial periods, especially during the times of transition from relatively wet to dry climates, and correlation of these records with those from nearby marine cores, will help us extract the fundamental record of climate and resulting air-mass shifts and improve our ability to forecast expected conditions under a changing global climate.
The Mojave Desert (Fig. 2) occupies a transition zone between western California, which dominantly receives westerly Pacific moisture, and areas to the east that dominantly receive southerly summer monsoon moisture. Climate records from the Mojave may potentially contain a sensitive history of the changing influence of these two moisture sources. Comparison of western North American paleolake records shows both local and regional differences in the timing of paleolimnologic indicators such as lake level, total dissolved solids (salts), stable isotope values (18O/16O and 13C/12C ratios), and paleontologic content. Differences between records are commonly related to local issues such as dating, physiography, and hydrology (including surface area to volume ratio), sources of water, and factors such as tectonics, that affect the basin shape and depth. Regional differences are mainly related to climate and, in particular, to resident air masses whose characteristics are determined by continental ice sheets, sea surface temperature, seasonal and annual position of the polar front, solar variability on short (output) and long (orbital) time scales, and other factors. However, few detailed paleolake studies have been published for the Mojave Desert region, and none for periods prior to the latest Pleistocene.
The present focus of this project is to obtain detailed paleoclimate records from pluvial Lake Manix, east of Barstow (Fig. 2). Presently, the Mojave River flows through a valley incised into lake sediments representing the former Lake Manix and through the deep gorge of Afton Canyon (Fig. 3). The canyon was cut during late Pleistocene time, when Lake Manix drained east to form pluvial Lake Mojave in the Soda Lake and Silver Lake basins near Baker. Previous studies (Wells et al., 2003) suggest that, much like Lake Estancia in New Mexico, Lake Mojave reached high levels before and after more northerly lakes. Our detailed studies of Lake Manix will allow an understanding of its response to both local factors, such as changing inputs from the Mojave River due to basin integration events, and regional factors such as climate change. Our preliminary study of ostracodes from a Lake Mojave sediment core shows strong time series shifts from a summer-rain-sourced lake to a winter-precip-sourced lake that are likely related to changes in marine conditions, thus linking oceanographic and land-based climate studies.
| Figure 3. Photograph of high fluvial deposits and landscape above rim of Afton Canyon; view is to northwest. High fluvial deposits and strath terraces lie above cemented Tertiary fan gravel comprising vertical canyon wall. Photo taken from south canyon rim. |
In addition to external forcing by climate, a better understanding of the history of drainage integration along the Mojave River is needed to interpret the sediment record properly. Mojave lakes are supported by long-distance river systems, each of which was integrated through time. As a result, there is potential to compare environmental histories in different parts of the desert with environmental records from Death Valley, for example, if the integration events for those rivers are understood. Previous studies of exposed sediments of Lake Manix, the terminus of the Mojave River before its integration downstream to form Lake Mojave, show that a long, middle to late Pleistocene paleolake sediment record is preserved. These sediments potentially contain a detailed record of the previous glacial-interglacial transition from marine oxygen-isotope (OIS) 6 to 5. Segmentation of the Mojave River drainage can be used to separate local from regional climate effects at various times once the overflow history is known. Similarly, understanding the integration history of the Amargosa River system can yield climate response data. New interpretations of the integration history of the Mojave and Amargosa Rivers also have the potential to revise the biogeographic history of aquatic species in the Desert Southwest; that is, when certain fish or aquatic snails were able to migrate between dry basins where they now reside in isolated springs, and their rates of evolution. (see "Geologic and Biotic Perspectives on Late Cenozoic Drainage History of the Southwestern Great Basin and Lower Colorado River Region: Conference Abstracts").
Reheis, M.C., Miller, D.M., and Redwine, J.L., 2007, Quaternary stratigraphy, drainage-basin development, and geomorphology of the Lake Manix basin, Mojave Desert--Guidebook for fall field trip, Friends of the Pleistocene, Pacific Cell, October 4-7, 2007: U.S. Geological Survey Open-File Report 2007-1281, 31 p.
Wells, S.G., Brown, W.J., Enzel, Y., Anderson, R.Y., and McFadden, L.D., 2003, Late Quaternary geology and paleohydrology of pluvial Lake Mojave, southern California, in Enzel, Y., Wells, S.G., and Lancaster, N., eds., Paleoenvironments and paleohydrology of the Mojave and southern Great Basin Deserts: Boulder, Colo., Geological Society of America Special Paper 368, p. 79-114.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/mojave/
Page Contact Information: ESP Web Team
Page Last Modified: Wed 27-Feb-2008 15:13:07 MST
