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Abstract:

Mono Lake is a large, hypersaline, alkaline water body, lying in the rainshadow of the Sierra Nevada. Unlike nearby Walker and Pyramid Lakes, Mono Lake has no major river input of water, but is fed by meltwater creeks. Since the early 1940's, part of this inflow has been diverted, causing the shoreline to fall from 1,957 m to 1,943 m, and the loss by evaporation of almost 50 percent of the lake's pre-1940 volume. The rate of decline closely reflects the regional climate. The concentration of major ions (Na+, HCO3, CO32–, Cl, SO42–) is remarkably conservative, showing an increase in salinity almost inversely proportional to the decrease in volume.

The first Holocene sedimentary column from Mono Lake has been obtained by coring and sedimentary facies have been mapped. Studies included the sedimentology, mineralogy and paleontology of the sediments by optical and SEM-EDAX methods and absolute dating by paleomagnetic, tephrochronology and 14C methods, including AMS.

The results show: (1) a drastic recession from 350 m-deep Lake Lahontan to a 38 m-deep early Mono Lake sometime before 11,000 yr BP, and reaching a possible Holocene minimum about 10,000 yr BP; (2) a high stand beginning at least 9,000 years ago, when the lake was at least 16 m deeper than today; (3) a gradual recession starting about 6,500 yr BP in which shorelines eventually dropped to below the present level, reaching a minimum about 200 BP; meanwhile, the nearby Mono Craters were contributing large amounts of rhyolitic volcanic ash to the sediments at about 2,000 yr BP; (4) another high stand after 600 yr BP indicating a climatically induced influx of freshwater runoff peaking at about 300 yr BP, which probably caused long-term meromixis; and (5) a gradual recession, eventually resulting in overturn and the initiation of a monomictic lake, punctuated by brief interludes of meromixis—a regime that has prevailed until the present.

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