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

Abstract

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 100 cm. 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 C28 and C30 acids. At all depths below 40 cm, as well as at the surface, C16 and C18 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.