This study investigates the sedimentological and geochemical changes
that occurred during the last 2200 years in the meromictic Lake Lucerne
(Switzerland), one of the largest freshwater lakes of Central Europe.
The stable isotope composition (delta C-13 and delta O-18 values) of
bulk carbonates is compared to changes in grain-size distribution (clay
and silt fraction), natural trace element input (titanium and thorium
concentrations), and organic material abundance (C-org, nitrogen and
phosphorus) and composition (C/N ratios and hydrogen and oxygen
indexes). A drop in carbonate accumulation and in the delta O-18 values
of sediments between ca. AD 500 and 700 followed a large and consistent
rise in chemical weathering, marked by increases in the silicate-clay
fraction and in crustal element concentrations. During the following
millennium, there was a long-term decreasing trend in the lithogenic
trace element input and in the phosphorus loading, suggesting decreasing
terrigeneous input from runoff water. The major sedimentological change
over the studied period occurred after ca. AD 1800 with a significant
increase in the erosion-driven silt-fraction and in the sedimentation
rate. During the last century, human-induced increase in nutrient input
to the lake highly enhanced the accumulation of organic matter in
sediment. Changes in nutrients and oxygen conditions in the hypolimnion
of Lake Lucerne during the eutrophication period (i.e., the last 40
years) highly modified the geochemical fluxes compared to the relatively
stable oligotrophic conditions that prevailed during the previous 2000
years. Before the 19th century, climate driven meromixis had a limited
impact on the organic matter flux to the sediments, but the accumulation
of carbonate considerably decreased during periods of lower mechanical
erosion rates and high chemical weathering rates. (C) 2012 Elsevier B.V.
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