Variability in the mineral magnetic properties of soils and sediments within a single field in the Cape Fold mountains, South Africa: Implications for sediment source tracing

Abstract

When tracing the sources of sediment, reference to potential source groups is a necessity. However, the range of uncertainty present in source fingerprinting outputs has been shown to be increased by a high within-source group variability. The environmental factors controlling the magnetic properties of samples within a single field were examined on a particle size specific basis to continue building on the recent studies of the spatial variability associated with different tracer types. The study area was in the Eastern Cape of South Africa over a Quartzite geology and was covered by rough grassland.Topsoils and gully walls were intensively sampled within the study field. Further samples were collected from weathering bedrock exposures, burnt soil and wetland soil in order to explore potential controlling factors on source magnetism. Each sample was fractionated to 125–63μm, 63–32μm, 32–25μm, 25–10μm and <10μm.Topographic position was a major controlling factor on soil magnetism, with saturated lowland gully walls and wetland topsoils being less magnetic than dry topsoils, especially within the <25μm size fractions. The magnetism of the fields soils was primarily controlled by the concentrations of super paramagnetic and single domain grains formed through pedogenesis and combustion. The magnetism of samples increased with decreasing particle size. Dissolution of fine magnetic grains in the <25μm fractions provided a potential basis for surface – subsurface source discrimination but also increased within-source group variability. The 25–10μm fraction was a good compromise between good discrimination and a low within-source variability.Little difference was found between the low frequency magnetic susceptibility of quartzite topsoils and the shale and sandstone topsoils found elsewhere in the Eastern Cape. However, igneous sources such as dolerite are far more magnetic than the quartzite topsoils. Discrimination between sediment sources can be highly variable within the <63μm fraction. For example, the <10μm fraction of soils from the study site had a higher magnetic susceptibility than Karoo shales (sieved to <63μm), whilst the 63–32μm fraction had a lower susceptibility than the Karoo shales.