Abstract
The occurrence of pyrite (FeS2) and iron sulfide in surficial sediments of Piburger See was compared with thermodynamic calculations based on chemical analyses of iron and hydrogen sulfide in the interstitial water. The area below 17 m, where black spots were found in the sediments, showed ion products (log KFeS=aFe2·aHS/aH+) between−3.11 and −4.01. In areas with no visible FeS concretions logKFeS values were in the range of −4.74 to −5.77, thus thermodynamic calculations seem to be in accordance with the appearance of iron sulfide. Nevertheless pyrite framboids, composed by more than 1,000 single crystals, coul be found even in shallow parts of the lake. Therefore the formation of pyrite is assumed to occur in microniches (diatom frustules, testacean shells). Inside these microcompartments high concentrations of hydrogen sulfide are reached due to the anaerobic decomposition of organic matter, whereas iron and additional sulfur are supplied by the diffusion of ferrous iron and sulfate from the anoxic environment.
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