SEDIMENTARY RECORD OF SOURCES AND ACCUMULATION OF ORGANIC-MATTER IN PYRAMID-LAKE, NEVADA, OVER THE PAST 1,000 YEARS

We have examined the organic matter contents of two Pb-210-dated cores of sediment from Pyramid Lake for consequences of the historic enviro nmental and limnological changes that have occurred since modern settl ement of this region began in the 1850s. Agricultural diversion of Tru ckee River water has diminished the volume of this saline, terminal la ke by one-fourth since 1905. Organic C:N ratios, organic delta(13)C va lues, and terrigenous:aquatic n-alkane ratios indicate that the propor tion of land-derived organic matter in sediments has decreased as rive r how has decreased. Algal productivity also appears to have decreased since 1930. Some prehistorical variations in the delivery of organic matter are evident in older sediments. Elevations in the proportion of land plant wax and in C:N ratios in sediments deposited A.D. 1200-140 0 suggest that contributions of land-derived organic matter were enhan ced at this time. Proportions of biomarker n-alkanes indicate that gra sses were more important than trees and shrubs as the dominant source of land plant waxes during this period, suggesting an interlude of reg ionally wetter climate.