Fluctuation in the Level of Pluvial Lake Lahontan During the last 40,000 Years

摘要: Samples of algal tufa, gastropods and calcite-cemented sand were collected from the Walker and Pyramid Lake areas of the Lahontan Basin, Nevada. X-ray diffraction petrographic and radiocarbon analyses show that massive forms of tufa such as the dendritic variety contain secondary carbon-bearing material and therefore yield unreliable radiocarbon dates. Dense coating of tufa (lithoid), however, gave radiocarbon ages in agreement with dates on coexisting aragonite gastropods. Radiocarbon data from the study were combined with previously dated noncarbonate materials [Born, S. M. (1972). “Lake Quaternary History, Deltaic Sedimentation, and Mudlump Formation at Pyramid Lake, Nevada”, Center for Water Resources, Desert Research Inst., Reno, Nevada] to give an internally consistent record of lake level fluctuations for the past 40,000 years. The main features of the Lahontan chronology are (1) extreme high stands (1330 m above sea level) 13,500 to 11,000 and 25,000 to 22,000 B.P., (2) a moderate high stand (1260 m above sea level) 20,000 to 15,000 B.P., (3) a low stand of unknown elevation 40,000 to 25,000 B.P., (4) an extremely low stand 9000 to 5000 B.P., and (5) an overall increase in the size of Walker and Pyramid Lakes during the past 5000 years, until the late 19th century. Pore fluid data indicate that Walker Lake desiccated sometime during the period 9050 to 6400 B.P. Salts deposited as a result of this dessication are still undergoing dissolution causing a flux of chloride, carbon, and other solute species from the sediments to the overlying lake water. Pore fluid data obtained from Pyramid Lake sediments do not indicate the presence of a concentrated brine at depth. This suggests that Pyramid Lake did not dry completely during this period although it may have been severely reduced in size. There has been considerable disagreement regarding the occurrence of extreme arid conditions (altithermal period) since 10,000 B.P. [Mehringer, P. J. (1977). “Models and Great Basin Prehistory”. Desert Research Inst. Pub, Reno, Nevada]. The data of this study suggest that such a climatic regime did occur in the western Great Basin during the period 9000 to 5000 B.P.