FENS IN KARST SINKHOLES ― ARCHIVES FOR LONG LASTING 'IMMISSION' CHRONOLOGIES

摘要

Fens in karst sinkholes are excellent archives for the reconstruction of vegetation, land use and emission rates over millennia. The reasons are the usually good preservation of pollen, the high portion of low density organic material with very low background concentrations of heavy metals, and the circum-neutral pH-values in most of these mires preventing migration of heavy metals. Immissions of dust and of harmful elements can easily be correlated with changes in vegetation ('immission' is a synonym for the deposition or impact of pollutants from the atmosphere on a receptor surface). One 13 m core from a ～5000 yr old karst sinkhole fen (Silberhohl, western margin of the Harz Mountains, central Germany) was investigated by geochemical analysis, pollen analysis and dated by ~(14)C and palynological data. The core consists of organic material with a few percent of CaCO_3 precipitated from groundwater and a small amount of atmospheric detritus. As early as the Iron Age (first pre-Christian millennium), slight but significant enrichments of Pb, Zn, Cu and Cd are observed. After 400 AD stronger enrichments occurred culminating in the High Middle Ages (～1200-1300 AD). Maximum values are 1250 μg g~(-1) Pb, 214 μg g~(-1) Cu, 740 μg g~(-1) Zn, and 3.8 μg g~(-1) Cd. The enrichments are caused by emissions during smelting of sulfidic lead-zinc ores from the adjacent Hercynian deposits to extract Ag and Cu. Except for cadmium, these values were never exceeded in modern times. Since the Iron Age 23 g technogenic Pb, 5.3 g Cu, 27 g Zn and 0.2 g Cd have been deposited per square meter. Palynological investigations show a strong correlation between decreasing red beech pollens (Fagus sylvatica) and increasing demand on wood for smelting in the Middle Ages. Simultaneously, the pollen share of pioneer trees such as birch (Betula pubescens) and of cereal grains (e.g. Secale) increases. Since the beginning of the 14th century, the decline of agriculture and population is reflected in the decreasing contents of Secale and heavy metals in the fen deposits.