COSMOGENIC NUCLIDE DATING OF PLUVIAL SHORELINES IN THE WESTERN GREAT BASIN

Although evidence of the most recent lake highstand in the western Great Basin has been intensively studied for over 100 years, comparatively little is known about the longer-term lacustrine history of the basin. A better understanding of the timing of these older lake-level changes could answer questions regarding the response of the western Great Basin to the forcing of Pleistocene climate changes. Based on geomorphic and soil properties, ages of several shorelines above the latest Pleistocene level appear to increase as a function of elevation. Shoreline deposits at three sites in western Nevada were sampled for cosmogenic nuclide (36Cl and 10Be) dating to test this apparent increase. Preliminary results of 36Cl analysis on several Thorne Bar shorelines near Walker Lake, Nevada indicate that the lake highstand thought to be related to the Last Glacial Maximum (LGM) may instead correlate with oxygen isotope stage 4 in age, and that the shorelines above it are older.

It is possible that the lowest shoreline sampled is not the LGM shoreline but a recessional feature from the previous highstand. Under this scenario, the LGM shoreline may be superimposed on and less prominent than the older features. If the shorelines we see preserved are progressively younger with decreasing elevation, then Lake Lahontan and other lakes in the western Great Basin filled in response to several cooling periods in the middle to late Pleistocene, and each episode was smaller in magnitude than the previous one. Future work will focus on further investigation of the relationship between the basin-filling cycles and periods of global cooling.