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Magnetic investigations of framboidal greigite formation: a record of anthropogenic environmental changes in eutrophic Lake St Moritz, Switzerland

Magnetic minerals found in sediments can either be transported to the basin from the catchment area or can be authigenically formed in the sediments. SEM studies, x-ray diffraction spectra and magnetic mineralogical analyses performed on magnetic extracts from freeze-dried sediments of Lake St Moritz, Switzer land, undoubtedly identified the presence of greigite. The occurrence of this magnetic iron sulphide (Fe3S4) is associated with changing environmental conditions that can be interpreted from historical data available for the area. Results of the combined XRD and SEM study show that authigenically formed greigite in Lake St Moritz sediments displays a predominant framboidal crystal arrangement. Amorphous, irregular shapes and spherical aggregates, most often associated with this mineral, are present in lower concentration. These results provide a natural example of framboidal iron sulphide formation and support previous experimental results that have shown that oxidation state and iron availability are critical factors for greigite formation. Despite its metastable character, greigite can also be preserved in the geological record. Its magnetic signature may, in some cases, be representative of a well-defined depositional environment.