Pv. Doskey et Tw. Talbot, Sediment chronologies of atmospheric deposition in a precipitation-dominated seepage lake, LIMN OCEAN, 45(4), 2000, pp. 895-904
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.