STABLE-ISOTOPE STUDIES ON MONO LAKE (CALIFORNIA) .1. DELTA-O-18 IN LAKE-SEDIMENTS AS PROXY FOR CLIMATIC-CHANGE DURING THE LAST 150 YEARS

Mono Lake is a hypersaline, alkaline lake in the Mono Basin located at the eastern base of the Californian Sierra Nevada. Its lake-level his tory since 1912 has been recorded instrumentally, showing the decline of lake-surface elevations initiated by the 1941 artificial diversion of stream inflow. We have made high-resolution oxygen isotopic measure ments on the total carbonate fraction of lake sediments and shown that the delta(18)O record parallels the lake-level fluctuations rather we ll. The measurements were carried out on sediments that had been leach ed with deionized water to isolate the isotopic signals of the calcium carbonate from those of pore water and water-soluble carbonate salts in the sediment. Extending the delta(18)O record back in time, we foun d that lake level changed markedly during the past 150 yr, reflecting climatic variations and resultant runoff fluctuations. Lake levels wer e high around 1845, 1880, and 1915 and low around 1860, 1900, and 1933 . This study demonstrates that closed-basin lake sediment delta(18)O p rovides an effective means of probing past precipitation variations in arid to semiarid regions such as the Great Basin in the western Unite d States.