Paleo-spring discharge activity in the southern Great Basin responded to changes in recharge, hence climate changes, in high mountain areas during the late Quaternary. In our study, we examined four stratigraphic sections in southern Nevada in order to reconstruct paleohydrologic change spanning the last two major discharge cycles. The largest discharge event in those sections is expressed as extensive wetland deposits (Unit B) that fall beyond the range of radiocarbon dating (>41 ka). We tentatively correlate this event with marine isotope stage 6, which is so conspicuously represented in cores from Death Valley and Owens Lake. Major wetlands were also present during last glacial maximum (Unit D) deposited between 16.4 and <26.3 14C ka. The absence of any dates between 16.4 and ca. 14.5 14C ka may indicate a period of relative aridity. Wetlands are also strongly expressed between ca. 13.9 and 13.5 14C ka in several sections, followed by contraction beginning between 12.9 and 12.8 14C ka. The region witnessed a modest resurgence of spring activity, expressed as black mats and spring-fed channels, starting at 11.6 14C ka, and peaking between 11 and 9.5 14C ka, followed by desiccation of most springs between 9.5 and 7 14C ka. Detailed analysis of ostracode taxa from three stratigraphic sections shows that a complex depositional mosaic composed of wet meadows, seeps, flowing springs, streams, and wetlands covered the valley bottom during the last two glacial periods. Differences in ostracode species assemblages suggest that climate associated with the earlier discharge cycle (Unit B) was colder and perhaps wetter than that of the younger cycle (Units D and E). δ18O values from >400 ostracode shells vary by ∼5‰, and there is no consistent, section-wide, difference in isotopic values between standing water and spring taxa. This pattern strongly suggests short residence times for water in local basins, due to loss of water from basins by outflow as groundwater or overflow, rather than by evaporation. We used the δ18O value of fossil ostracodes to place constraints on paleotemperature in the valley bottoms during glacial periods. This analysis entails at least three key assumptions: no vital or evaporation effects during valve formation of the ostracode Cypridopsis vidua, short transit imes in the aquifer, and the basic relationship between modern air and spring water temperature holds for the past. If these and other assumptions are satisfied, we estimate that mean annual air temperature during the penultimate wet period (Unit B2) in the valley bottom was at least 10.8 °C colder than today, and at least 5.6 °C colder during the last glacial maximum (Unit D). If a vital effect of 0.8-1‰ is assumed using δ18O values from groundwater candonids, then the above estimates of maximum valley-bottom temperatures during Unit D time increase by ∼2-3 °C.
|Original language||English (US)|
|Number of pages||24|
|Journal||Special Paper of the Geological Society of America|
|State||Published - 2003|
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