Stable isotope studies on Mono Lake (California). 1. al80 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 he eastern base of the Californian Sierra Nevada. Its lake-level history 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 measurements on the total carbonate fraction of lake sediments and shown that the S’*O record parallels the lakelevel fluctuations rather well. The measurements were carried out on sediments that had been leached 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 S’“O record back in time, we found that lake level changed markedly during the past 150 yr, reflecting climatic variations and resultant runoff fluctuiitions. Lake levels were high around 1845, 1880, and 19 15 and low around 1860, 1900, and 1933. This study demonstrates that closed-basin lake sediment S180 provides an effective means of probing past precipitation variations in arid to semiarid regions such as the Great Basin in the western United States. Numerous attempts have been made to reconstruct paleolake levels on the basis of geological and stable isotopic evidence from which paleohydrological conditions have been inferred. Studies on closed-basin lakes in semiarid and tropical zones have contributed greatly to paleoclimatic research, particularly for the Late Pleistocene and the Holocene (e.g. Street-Perrott and Harrison 1985; Chivas et al. 1986; Gasse et al. 1991). Water entering closed-basin lakes via rain and runoff leaves by evaporation. Hence, fluctuations in lake level and volume should serve to indicate changes in the moisture budget associated with climatic change. Mono Lake, located in eastern California about 480 km north of Los Angeles (Fig. l), is the only terminal lake in the Mono Basin. Its present ratio of surface area (150 km2) to catchment area (2,072 km2) is -7 : 100, in contrast to the ratio of -33 : 100 for Lake Russell during the last glacial period (Russell 1889; Lajoie 1968; Mono Lake Committee 1981). Since 1912, the levels of Mono Lake have been measured by the U.S. Geological Survey (USGS) and the Los Angeles Department of Water and Power (LADWP) (Blevins et al. 1987). Since its historic high in 1915, the water level within the lake has fallen as a result of climatic changes. The lake level fall has been exacerbated by the diversion of inflow to the lake by LADWP ‘since 1941 and has caused a further increase in salinity and alkalinity. DurAcknowledgments We thank D. E. Hammond and S. P. Lund for insightful discussions. D. R. Cayan kindly made available to us his unpublished data on Nevada City precipitation and streamflow. Comments made by L. V. Benson and an anonymous reviewer greatly helped our presentation. This work was supported by NSF grants ATM 93-03587 (T.-LX.) and OCE 93-14192 (L.D.S.), and by NOAA grant NA16-RC-0084 (R.EA.). ing the diversion period the lake level fell 13 m (from an elevation of 1,956 m to its historic low stand of 1,942 m) between 194 1 and 1982. During the high runoff years between 1982 and 1986 (a wet period caused by strong 19821983 El Nifio), the lake level rose by -2.4 m to 1,944.6 m before falling again. The fluctuations in lake level and volume of a closedbasin lake such as Mono Lake may be recorded in the 6’*0 of authigenic carbonates deposited within the lake sediments. This is feasible because the lake S180 monitors the hydrologic balance between the isotopically distinct inflow and outflow. For example, Benson (1994) monitored the 6’*0 of lake waiter from 1985 to 1992 in Pyramid Lake, Nevada, another closed-basin lake in the Great Basin. His results showed that although the surface-water &*O exhibits seasonal variations, reflecting varying amounts of fresh water discharged to and lost from the lake, the yearly S180 trend over the study period correlates well with that of the lakevolume change (lake-water oxygen isotopic values become lighter as volume increases). This observation suggests that when the lake undergoes a volume change, the hydrological balance is the most important factor influencing the lake 6’*0, although other factors such as stratification, relative humidity, ,air temperature, and the atmospheric @*O may also play a role. If carbonate minerals form in a lake in isotopic equilibrium with lake water, their 6’*0 values can be used toI reconstruct lake volume fluctuations. This approach to lake history investigations forms the basis of several previous studies (Lister et al. 1991; Phillips et al. 1992; Johnson et al. 1991). However, it would be reassuring if a comparison could be made between the reconstructed lakelevel changes with instrumental records. Such a comparison can be conducted for Mono Lake because, as mentioned above, its lake-level record is available for the past 80 yr through the measurements of LADWP and USGS (Blevins