Magnetic susceptibility of soil: an evaluation of conflicting theories using a national data set

TLDR

The study evaluates competing theories of soil magnetism by combining magnetic susceptibility measurements of topsoils across England with data on soil type, geochemistry, and magnetotactic bacteria concentrations. A new mechanism is proposed in which secondary ferrimagnetic minerals form through abiotic weathering and biotic fermentation, with iron supply—potentially climate‑driven—as the fundamental driver. Strongly magnetic soils in unpolluted areas arise from ultrafine superparamagnetic grains over weak substrates; magnetotactic bacteria and crop burning are insufficient explanations, while pollution or primary ferrimagnetic minerals account for occasional high values, and the results support climate‑dependent associations in loess‑paleosol sequences.

Abstract

Magnetic susceptibility values for topsoils across England arc combined with data for soil type, geochemistry and concentrations of magnetotactic bacteria in order to evaluate different theories for explaining soil magnetism. Strongly magnetic soils in unpolluted areas are found over weakly magnetic substrates and are dominated by ultrafine superparamagnetic grains. Magnetotactic bacteria are present in insufficient concentrations to account for strongly magnetic soils, and crop burning is discounted as a major factor. A small number of samples show high values associated with either airborne magnetic particulates from pollution or residual primary ferrimagnetic minerals from igneous substrates. The results are used to construct a new mechanism for the formation of secondary ferrimagnetic minerals that links abiological weathering and biological fermentation processes. The fundamental driving force in the mechanism is Fe supply, which may be linked to climate. Observed causative associations between climate and the magnetic susceptibility of loess-palaeosol sequences are supported by the findings.