Bacterial magnetite in Swedish varved lake-sediments: a potential bio-marker of environmental change

Detailed mineral magnetic measurements were carried out on three varved lake-sediment sequences in Sweden, which extend to ca. 7000 BC. The comparison of the magnetic properties of the organic rich varved lake-sediments with their respective catchment materials indicates that the magnetic properties of the sediments are dominated by relatively high concentrations of single-domain magnetite magnetosomes produced by magnetotactic bacteria. The dimictic nature of the lakes, which help form and preserve the varves due to the weakly oxic or suboxic environment, also appear to be suitable for a high degree of magnetosome preservation. The concentration of the magnetosomes in two of the sequences, as determined by mass specific magnetic measurements, exhibits a positive linear relationship with the total organic carbon content and suggests that magnetic remanence measurements reflect lake productivity, via processes of organic matter accumulation and decomposition. Further research should focus on the potential to use the fossil magnetosomes as proxy-climate/environmental change indicators and as recorders of geomagnetic field variations and behaviour. (C) 2002 Elsevier Science Ltd and INQUA. All rights reserved.