The biogeochemistry and paleolimnology of sediments from Lake Tahoe, California-Nevada

Ultra-oligotrophic Lake Tahoe is one of the largest, deepest, and clearest subalpine lakes in the world. It is valued as a unique environmental resource, although currently threatened by urban development and regional transport of atmospheric pollution. Paleolimnology was applied in this study for a reconstruction of anthropogenic perturbations in the Tahoe basin and lake response.; An important tool in the study of modern sediments is {dollar}sp{lcub}210{rcub}{dollar}Pb dating, which yields a sediment chronology for the last 130 years. Various methods of {dollar}sp{lcub}210{rcub}{dollar}Pb data interpretation were explored in this work. The analyses revealed dramatic changes in mass sedimentation rates at Lake Tahoe, associated with two episodes of watershed disturbance over the last 150 years; rapid urbanization since the late 1950s, and clear-cut logging of virgin timber from the Tahoe basin in the late 1800s.; Sediment chronologies constructed from event related markers, {dollar}sp{lcub}14{rcub}{dollar}C dates, and {dollar}sp{lcub}210{rcub}{dollar}Pb dating indicate that logging era sedimentation rates increased approximately seven to twelve fold over pre-disturbance rates. During recent urbanization they were four times higher than pre-disturbance rates. The low sedimentation rates observed between these two disturbance events indicate that erosion decreased rapidly after logging ended.; Since Lake Tahoe is dominated by diatoms, biogenic silica was used as an indicator of historical primary production. Sediment concentrations of biogenic silica decreased with the onset of both disturbance events; a consequence of dilution by terrigenous material, as indicated by increasing concentrations of titanium. The flux of biogenic silica increased with mass sedimentation rates, indicating that primary production probably responded to enhanced nutrients from watershed disturbance. During the intervening period, with watershed recovery, concentrations of biogenic silica decreased. This suggests the lake responds quickly to reduced nutrient inputs, despite its long hydraulic retention.; Historical atmospheric loading of lead and mercury at Lake Tahoe was reconstructed from concentration profiles and the sediment core chronologies. Modern sediments were enriched five fold for Hg and six fold for Pb. The average flux ratio (21) of modern to baseline deposition rates for Hg was higher than has been reported in any other study from North America.