USING VOLCANIC ASH TO CONSTRAIN THE HISTORY OF GLACIAL-INTERGLACIAL LAKE LEVEL CHANGES AT SUMMER LAKE, PLUVIAL LAKE CHEWAUCAN, OREGON

During the ice ages, many large lakes existed in the western United States in an area known as the Great Basin. This area includes parts of Nevada, Utah, Oregon, and eastern California; nearly all of this land is desert today. As large ice sheets elsewhere advanced across the continent and then melted away, these lakes expanded and dried up over time. These lakes contain a long term record of multiple ice age cycles and related changes in water availability and regional ecology, often covering much of Pleistocene time and in some cases extending back through the Pliocene. Most available data on these lake fluctuations only capture relative changes in water level. At Summer Lake, Oregon, located in a sub-basin of pluvial Lake Chewaucan, we have found that exposures of ancient lake sediments at different elevations around the basin make it possible to constrain water levels at different times to produce information on absolute lake levels. Preserved lake sediments at a given site indicate that the area was under water at that time, and evidence of erosion indicates that the lake level fell below the site elevation. In some places, shoreline deposits are also preserved. Using field-based research in Oregon to document lake sediment deposits and unconformities and analysis of volcanic ash (tephra) layers for age information using Concord University’s electron microprobe, we are able to correlate between different locations and determine a timeline for the changing water levels. Based on the volcanic ash chronology, we have constructed preliminary correlations to specific glacial and interglacial periods. This data set will make it possible to do more quantitative modeling of the causes of changes in water availability.