Sediment chronologies of atmospheric deposition in a precipitation-dominated seepage lake

Chronologies of Pb, polycyclic aromatic hydrocarbons (PAHs), Al, carbon, an d n-alkanes in pelagic sediments of Crystal Lake, a precipitation-dominated seepage lake in north-central Wisconsin, were determined to investigate th e geochemistry of sediments derived from atmospheric deposition and to eval uate the impact of environmental changes in the region on the geochemistry of this oligotrophic lake. Concentrations of Pb and combustion-derived PAHs in Crystal Lake sediments have increased by factors of 8 and 3, respective ly, over the past 150 years. In contrast, levels of perylene increased with depth in the sediment, indicating that postdepositional formation of this PAH might be occurring. Atmospheric fluxes of anthropogenic Pb and combusti on-derived PAHs were estimated to be 10,000 mu g m(-1) a(-1) and 34 mu g m( -1) a(-1), respectively. The settling sediment fluxes of planktonic n-alkan es (Sigma C-15, C-17, C-19) and terrestrial n-alkanes (Sigma C-25 C-27, C-2 9, C-31) in Crystal Lake were 4,400 mu g m(-2) a(-1) and 10,500 mu g m(-2) a(-1), respectively, whereas their accumulation rates in pelagic sediments were 270 mu g m(-2) a(-1) and 7,100 mu g m(-2) a(-1), respectively. The lar ge difference between the settling sediment flux and the accumulation rate of the planktonic n-alkanes in the sediments is ascribed to microbial degra dation during pr soon after deposition. In contrast, the terrestrial n-alka nes are incorporated in a wax matrix and are protected from degradation. Th e contribution of terrestrial n-alkanes to the organic carbon of the sedime nts has remained relatively constant over the past 150 ye,us. About 20% of the organic carbon that is incorporated in the present-day sediments of Cry stal Lake can be attributed to the deposition of pine pollen in the lake. D eforestation of the region in the late 1800s apparently caused terrigenous inputs and primary productivity of nonsiliceous plankton to increase, becau se planktonic and terrestrial n-alkane concentrations in the sediment incre ased at about the same time. However, since about 1900 concentrations of or ganic carbon and of terrestrial and planktonic n-alkanes in Crystal Lake se diments have decreased, and concentrations of Al and combustion-derived PAH have increased. These occurrences might reflect a decrease in pollen dispe rsal in the region and an increase in the atmospheric deposition of fly ash from coal-burning power plants.