Alkane and alkanoic acid variations with depth in modern sediments o f Pyramid Lake

-Pyramid Lake, a salt lake in Nevada, has undergone a 30~ reduction in volume over the past 100 yr due to diversion of river inflow for irrigation of farmland. This has altered the depositional-benthic environment and possibly has changed the overall aquatic environment of Pyramid Lake as indicated by changes in the hydrocarbon and monocarboxylic acid content in a sediment core. In the top 30 cm, total organic carbon levels decrease from 2.5 to 1.8% dry weight but change little over the next 100cm. A decrease in n-alkane concentrations also occurs in the top 30 cm, although no change in chain-length distributions is found. Most significantly, a 6-fold increase in n-alkanoic acid concentrations exists between the sediment surface and a core depth of 20-30 cm and is due primarily to high amounts of Cza and C3o acids. At all depths below 40cm, as well as at the surface, Ct6 and Cta acids dominate the distributions, and overall concentrations are much lower than at 20-23 cm. These changes in concentration and in n-alkanoic acid chain-length distributions appear to coincide with the beginning of lake volume reduction and may demonstrate the effect of increased sedimentation rate upon organic matter preservation. In addition, changes in n-alkane distribution throughout this 135 cm sediment thickness indicate fluctuations in the relative contributions of lake-derived and land-derived organic materials. I N T R O D U C ~ O N Organic matter in subaqueous sediments is derived from biological sources in the overlying water and in the benthic environment, from biological sources in nearby land areas, and from detrital matter eroded from geological deposits. Study of specific organic compounds isolated from different depths in modern sediments can provide information about changes in organic matter sources and about postdepositional alterations of the organic matter itself. Geolipids, particularly aliphatic hydrocarbons and fatty acids, have been used widely as biological and diagenetic indicators. Investigation of distributions of n-alkanes in sediments of 3 Washington state (U.S.A.) lakes has led to the conclusion that terrigenous sources contribute most of these hydrocarbons (WAKErtAM, 1976). Total aliphatic hydrocarbon concentrations indicate an additional, anthropogenic source of hydrocarbons to modern sediments of lakes in urban a r e a s (WAKEHAM, 1976; WAKEHAM and CARPENTER, 1976). In a survey of sediments from several oligotrophic English lakes, CRANWELL (1973a) found n-alkane compositions to vary with the type of vegetation present in the drainage basins. Fatty acid compositions have been interpreted as giving evidence of short-term postdepositional changes as well as of geolipid sources. CRANWELL (1974) used chain length distributions of n-alkanoic acids and relative amounts of branched/cyclic and alkenoic acids to distinguish between periods of eutrophy and oligotrophy as recorded in sediments of English lakes. However, such interpretations are complicated by diagenetic modifications of sedimented geolipids. Microbial activity may contribute significantly to fatty acids in sediments (BOON et al., 1975), and acids shorter than C2o can experience more degradation than longer-chain o n e s (MATSUDA and KOYAMA, 1977). In addition, unsaturated acid concentrations decrease faster than those of saturated acids in lake ~Now at: Graduate School of Oceanography, University of Rhode Island, Kingston, Rode Island 02881. U.S.A. 2 Now at: National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario L7R 4A6. Canada.