Late holocene fluctuations of Mono Lake, eastern California

Abstract

While long recognized as important sources of information on sub-millenial-to millenial-scale changes in climate, hydrographically closed lakes have seldom been used to illuminate past climatic vicissitudes in the sub-century-to centuries time frame. Mono Lake, an artificially depressed, hydrographically closed water body that abuts the eastern escarpment of California's Sierra Nevada, is well-suited for this type of detailed, high-resolution paleoclimatic reconstruction. Streamcuts through the deltas of the main feeder streams reveal lake-transgressive and -regressive sedimentary sequences with intervening soils, as well as an abundance of datable materials and tephra layers. The expansiveness and clarity of these exposures make it possible to trace individual transgressive and regressive units landward from the lake for as much as 1000 m, enabling the elevations of past high stands and low stands to be defined with a high degree of precision. Twenty-five radiocarbon dates, many of them on the remains of vegetation killed during the lake transgressions, together with 4 tephra units of known age, provide chronometric control for the fluctuation curve. The deltaic sequences, in combination with sedimentary, geomorphic, biotic, tephrostatigraphic, and historic evidence, indicate that during the past 3800 years Mono Lake has fluctuated over a vertical range of 40 m in response to changes in inflow and evaporation. Approximately 3770 cal B.P. thelake reached the Dechambeau Ranch High Stand at ∼1980.8 m, a level that it had not occupied for perhaps 7000 years, and that it has not attained since. By ∼1807 cal B.P. the lake had declined to an extreme low stand (the Marina Low Stand) at 1940.9 m. Over the ensuing six centuries it fluctuated little, remaining within an elevation interval (1945–1952 m) which is low by historic standards. The past 1200 years has been a period of rapid and large-scale fluctuations. During this interval Mono Lake has alternated between high stands (the Post Office, Rush Delta, Danberg Beach, Clover Ranch, and Historic high stands) of up to 1967.7 m, and low stabds (the Lee Vining Delta, Simis Ranch, Navy Beach, Rush Delta, and Pre-Historic low stands) as low as ∼1941 m. The water balance model for the Mono Basin developed by Vorster (1985) was used to assess the changes in hydroclimatic conditions necessary to account for the reconstructed lake behavior. The model indicates that, on the sub-century to centuries time scale, effective inflow to the Mono groundwater basin over the past 3800 years has varied from greater than 134% to less than 68% of the modern (1937–1979) mean value. For the past 2 millennia the lake fluctuations seem to correspond in time to de Vries-type variations in solar activity.