PLEISTOCENE OVERFLOW, BASIN INTEGRATION, AND AN EXPLANATION FOR THE DISTRIBUTION OF OIS 6 AND OIS 2 HIGHSTANDS IN THE GREAT BASIN

Across the Great Basin (GB) of the western US, there is a complicated pattern of pluvial lake highstands dating from oxygen isotope stages (OIS) 6 and 2. Here we focus on the spatio-temporal pattern of highstand ages that reflect the integrative effects of climate, basin overflow, and basin integration. Basins known or suspected to have had larger OIS 6 lakes than OIS 2 lakes include Diamond, Newark, Long, Ruby, and Railroad valleys in the central GB and Mono, Columbus, and Death Valley (DV) in the southwest GB. None of these basins changed their drainage basin size between OIS 6 and 2 except for DV, which received spill from the Owens River system in OIS 6 but not OIS 2. The observations that OIS 6 lakes were larger in these basins suggest that climate was effectively wetter in OIS 6 than in OIS 2.

Lakes known to have had larger OIS 2 lakes than OIS 6 lakes include Lahontan and Bonneville, both of which increased their drainage basin size between OIS 6 and 2. The Bear River was diverted into Bonneville at some point prior to OIS 2, causing the lake to rise to its all time highstand during OIS 2. During OIS 6, the Walker Lake subbasin was apparently separated from the rest of the Lahontan system as evidenced by OIS 6 shorelines at Walker Lake about 20 m higher than the OIS 2 highstand. In the rest of the Lahontan system, the putative OIS 6 highstand is about 5 to 7 m lower than the OIS 2 highstand. At some point during or after OIS 6, Walker Lake was integrated with the rest of Lahontan via Adrian Valley so that the OIS 2 highstand was continuous between the formerly separated water bodies. In addition, the Lake Diamond drainage was also added to the Lahontan system via downcutting of a sill between 22 and 13 ka. The relatively small increase in drainage basin size in Lahontan between OIS 6 and 2 was apparently enough to overcome the effects of a slightly less “wet” OIS 2 climate. In the Bonneville basin, the OIS 2 lake was greatly magnified because the newly added Bear River is the largest river in the GB.

The implications of these interpretations are that many other valleys in the GB that have not modified their drainage basins between OIS 6 and 2 may have slightly higher but previously unrecognized OIS 6 highstands. Alternatively, the spatial-temporal complexity of GB highstand ages is more complicated than previously thought.