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Show Robert M. Negrini Department of Physics and Geology, California State University, Bakersfield, CA 93311 Northwestern Great Basin Lacustrine History The Great Basin hydrological province has long been studied as a source of non- marine paleoclimate data ( Antevs, 1948; Morrison, 1991; Benson and Thompson, 1991; Oviatt et al, 1992; Grayson, 1993; Benson et al, 1990; Morrison, 1991). This notoriety is, in part, based on the fact that the sizes of its internally- drained lakes are especially sensitive to changes in precipitation and temperature. Furthermore, these size changes are recorded by a wide range of observable features in the lake system including tufa deposits, strandlines, and, also, the geochemistry, fauna, flora, physical lithology, and magnetism of sediments deposited in these lakes. Because most of the basins have been actively subsiding along marginal faults for much of the late Cenozoic Era, their deposits potentially carry paleoclimate records throughout the Quaternary and earlier into the latest Tertiary Period. The northwestern part of the Great Basin is an especially interesting region of study due to its location downwind from the southern half of the Cascade volcanoes. Consequently, volcanic ashes are common constituents of the pluvial lake deposits in this region. For example, the 17 m of section exposed in the Ana River Canyon near Summer Lake Oregon contains 56 tephra layers within the sediments of Pluvial Lake Chewaucan. The geochemical signatures of these ashes allow excellent intrabasinal correlations between outcrop and core as well as extrabasinal correlations with other well- dated sequences of lake sediments or volcanic flows. The ashes can also be dated directly using either thermoluminescence ( TL) or radiometric methods thus extending the age control of these sediments beyond the range of radiocarbon dating ( Berger, 1991; Sarna- Wojcicki and Davis, 1991). Our knowledge of the pluvial lacustrine history of the northwestern arm of the Great Basin is bimodal in nature. On the one hand, no other pluvial lake in the Great Basin except for Lake Bonneville has been studied as extensively or as well as Lake Lahontan. On the other hand, though significant advances have been made regarding the prehistory of the other pluvial lakes in the northwestern Great Basin during the last 50 years, we still know little about these other lakes relative to what is known about Lake Lahontan. The emphasis on Lahontan is understandable. At highstand, this pluvial system covered more than 8,500 square miles, approximately 50% more than all of the other pluvial lakes in the northwestern Great Basin combined ( Grayson, 1993). Its shear size leads to the probability of finding key exposures of critical geomorphic features and lake sediments. Furthermore, the presence of large remnant lakes, particularly Pyramid Lake and Walker Lake, provide for modern analogs and, at times, for sites of continuously- deposited sediments enabling the potential retrieval of long, nearly continuous climate records. The pluvial history of Lake Lahontan prior to the last 30 ka is the subject of another contribution in this symposium ( Reheis, 1997) and will, therefore, not be discussed here. The pluvial history of Lake Lahontan during the 30 ka has been reviewed in great detail several times ( e. g., Grayson, 1993; Benson et al., 1990; Thompson et al., 1990). This history will be briefly summarized in this contribution based primarily on these previous reviews, supplemented in places with key results from more recent contributions. Despite the complexity of the Lahontan system ( 6 major streams feeding 6 major sub basins separated at different elevations by 7 sills) the overall lake level history has converged toward a coherent story, especially in light of very recent advances ( Benson et al., 1995; 1996; 1997). In brief, moderate to low level lakes were present from about 30,000 to 23,000 14C yr B. P. presumably in all of the major sub basins. Over the next 1,000 years or so, the lake rose to a level high enough to connect all of the major sub basins except for Walker Lake. The lake remained stable near this elevation until - 16,000 14C yr B. P. when it underwent a series of high amplitude oscillations ending with the |