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Show Long- Term Climatic Change And Great Basin Aquatic History THOMPSON, R. S., U. S. Geological Survey, Denver, CO 80225 ( rthompson@ usgs. gov); OVIATT, C. G., Kansas State Univ., Manhattan, KS 66506; ROBERTS, A. P., Univ. of Southampton, European Way, Southhampton, SO 14 3ZN UK; and ANDERSON, K. H., INSTAAR, Univ. of Colorado, Boulder, CO The amplitude, pacing, and nature of climatic variations in the Great Basin region has changed over the past three million years. Past periods of wetter- than- modern conditions have occurred both during periods of relatively little global ice cover during the Pliocene, and during the major continental glaciations of the middle and late Quaternary. Although many of the climatic variations in the Great Basin region may have been related to the cycle of continental glaciations, not all changes in past hydrology and regional climates were caused by global climatic changes. Tectonic activity through the late Cenozoic has affected the elevation, topography, and drainage networks of the region. Major mountain masses have risen to the west, and perhaps to the south, creating or modifying regional rain shadows. The past hydrology and environment of the Great Basin region has thus been impacted by complex interplay of climate, topography, tectonics, and other factors. In the following text we summarize current information about past environments and climate in the eastern Great Basin and surrounding region through Pliocene and Quaternary time. Pliocene and early Pleistocene Generally wetter- than- modern conditions occurred during much of the Pliocene and early Pleistocene across much of the western United States ( Thompson, 1991), although interludes of relatively dry climates also occurred. Sediments of the Glenns Ferry Formation were deposited in a large deep lake that occupied the western Snake River Plain of southern Idaho during the period of greater- than- modern global warmth in the middle Pliocene (~ 3.5 to ~ 2.4 Ma). Studies of fossil pollen, fish remains, and ostracodes indicate much wetter- than- modern conditions, with reduced seasonality of temperature, relative to today ( Forester, 1991; Smith and Patterson, 1994; Thompson, 1996). The Pliocene pollen flora, dominated by Pinus, Juniperus, Artemisia, and steppe plants, resembles that of the region today. This suggests that although the climate was relatively wet, it was not greatly different than the modern climate. The Glenns Ferry lake apparently disappeared before the large continental glaciations of the late Pliocene, and the creation of a new outlet through Hells Canyon apparently prevented the creation of deep lakes after that time. Palynological data from the shallow lacustrine deposits of the Bruneau Formation indicates steppe vegetation and associated dry conditions during portions of the early Pleistocene. Recent tephrochronological studies ( Williams, 1994) provide a chronological framework for cored sections traversing the Bonneville Basin ( Eardley et al., 1973), and reinterpretation of sedimentological and paleontological data from these sediments indicate that a fresh- water lake occurred in the central Bonneville Basin during the middle Pliocene. This was apparently of short duration, and for much of the Pliocene and early Pleistocene conditions were not discernibly different than those of today. Periods of increased wetness occurred near Jaramillo time (~ 1 Ma; Cande and Kent, 1995), but no major lake episodes are evident in the sediments from that period. Variations in the relative dominance of steppe taxa in pollen assemblages from cores from Great Salt Lake ( Davis and Moutoux, 1996) suggest that quasi- cyclical changes occurred in Pliocene and Pleistocene climates in this region. Similar results can be seen in the pollen record from Black Rock, Utah, where the Pliocene and early Pleistocene vegetation apparently fluctuated between increasing ( or decreasing) coverage of pine- juniper woodland and sagebrush steppe. These vegetation fluctuations do not appear to have been accompanied by changes in the past shallow lake systems of the southern Bonneville Basin. A trend toward drier conditions occurred through the late Pliocene and |