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Show Detailed reconstructions of vegetation changes over the past 30,000 years in the eastern Great Basin have been made based on plant remains preserved in packrat middens coupled with fossil pollen data ( e. g. Beiswenger, 1991; Rhode and Madsen, 1996; Thompson, 1990, 1992; Wells, 1983). During the late Pleistocene, subalpine plants descended as much as 1000 m below their modern elevational limits, and steppe plants dominated the valleys. Many of the modern woodland taxa were absent and lived in the modern Southwestern deserts. Preliminary reconstructions of late Pleistocene climates in the Great Basin region suggest that temperatures were 6° C ( or more) lower than today and precipitation was 2.5 to 3.0 times the modern values ( during at least some periods). In contrast with the lacustrine record, the late Pleistocene paleovegetation data do not indicate millennial- scale high- amplitude changes. Discussion Through the Pliocene and early Pleistocene the climate of the eastern Great Basin region was characterized by sustained levels of effective moisture slightly above those of today. Aridity may have occurred during the late Pliocene glacial period and a regional trend toward drier conditions occurred between the Olduvai Subchron and the Bruhnes/ Matuyama boundary. Throughout this period, it appears that times of global warmth were correlated with enhanced wetness in the western United States and that continental glaciations were associated with periods of aridity. A different pattern of climatic variations is evident for the Brunhes, with long periods of aridity being punctuated by short- lived intervals of enhanced moisture, when fresh- water lakes of larger size than those of the late Pliocene and early Pleistocene were created. In contrast to the earlier period, continental glaciations appear to be associated with relatively wet climates in the Great Basin during the Brunhes. This pattern holds for the last 30,000 years, and is probably due to the deflection of the westerlies to the south of their current trajectory by the Laurentide and earlier Ice Sheets ( COHMAP, 1998; Thompson et al., 1993). In the western Great Basin ( Reheis et al., 1993; Reheis, 1996; Reheis, this volume) the largest pluvial lakes occurred early in the Brunhes Chron, whereas in the eastern Great Basin the late Pleistocene lakes were larger than their middle Pleistocene antecedents. This could be due to changes in the configurations of the ice sheets during the different glacial periods creating different pathways for the westerlies. Alternatively, increasing height of the Sierra Nevada and Transverse Ranges through the Brunhes could have influenced the pathways by which moisture entered the Great Basin. Changes in drainage patterns through time may also have influenced the relative sizes of the pluvial lakes. References Cited Beiswenger, J. M., 1991, Late Quaternary vegetational history of Grays Lake, Idaho. Ecological Monographs, v. 6, p. 165- 182. Benson, L. V., Burdett, J. W., Kashgarian, M., Lund, S. P., Phillips, F. M., and Rye, O. R., 1996, Climatic and hydrologic oscillations in the Owens Lake Basin and adjacent Sierra Nevada, California. Science, v. 274, p. 746- 749. Benson, L. V., Currey, D. R., Dorn, R. I., Lajoie, K. R., Oviatt, C. G., Robinson, S. W., Smith, G. I., and Stine, S., 1990, Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 78, p. 241- 286. Cande, S. C. and Kent, D. V., 1995, Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research, v. 100, No. B, p. 6093- 6095. COHMAP members, 1988, Climatic changes of the last 18,000 years: observations and model simulations. Science, v. 241, p. 1043- 1052. Davis, O. K. and Moutoux, T. E., 1996, Quaternary of the eastern Great Basin: palynology of the Great Salt Lake. Program and Abstracts of the 14th Biennial Meeting, May 20- 22, 1996, Flagstaff, Arizona. P. 159. Forester, R. M., 1991, Pliocene- climate history of the western United States derived from lacustrine ostracodes. Quaternary Science Reviews, v. 10, p. 133- 146. Eardley, AJ., Shuey, R. T., Gvosdetsky, V., Nash, W. P., Picard, M. D., Grey, D. C., and Kukla, G. J., 1973, Lake Cycles in the Bonneville Basin, Utah. Geological Society of America Bulletin, v. 84, p. 211- 216. |