Magnetic, geochemical and DNA properties of highly magnetic soils in England

Abstract

A range of mineral magnetic, Mössbauer, geochemical, microscopy and molecular biological techniques are applied to a small set of bulk and fine fractions of highly magnetic English topsoils that overlie weakly magnetic sedimentary geologies. Results show that the ferrimagnetic component of highly enhanced surface soils is dominated by superparamagnetic (SP) grains with a minor proportion of larger stable single-domain/pseudo-single-domain (SSD/PSD) grains that may derive from magnetosomes and magnetic inclusions. DNA screening of the soils by polymerase chain reaction (PCR) shows that the concentration of viable magnetotactic bacteria is too low (normally < 102 bacteria g−1) to explain the high concentrations of ferrimagnetic minerals observed. There does not appear to be any strong causative relationship between the presence or concentration of Magnetospirillum sp. and soil magnetic properties. Microcosm experiments were able to show that the destructive effects of waterlogging on secondary ferrimagnetic mineral (SFM) formation are rapid and associated with significant changes in bacterial populations. The combined results are used to examine alternative explanations for SFM formation and are consistent with previous findings (Dearing et al. 1996b, 1997) that ferrihydrite may be an important precursor of bacterially mediated magnetite in strongly magnetic temperate soils—a process driven by the rate of Fe flux to the biologically active surface soil.