A broad band magnetic susceptibility test study — The magnetic spectroscopy of a Neolithic ditch

Abstract

The magnetic characteristics of soil, sediments and archaeological deposits are environmentally sensitive, and can therefore be used to understand formation processes at archaeological sites. The magnetic susceptibility (MS) of a material indicates the concentration of magnetic inclusions of a sample. This depends on the concentration of magnetic grains but also on the composition of the magnetic mineralogy as the size of those grains. The grain size relates to the size-dependent magnetic domain state, which vary from thermally unstable ultrafine superparamagnetic (SP) grains to stable single-domain (SD) and pseudo-single-domain (PSD) grains to large multi-domain (MD) grains. Standard dual frequency measurements (χFD), magnetic susceptibility measurements at two different frequencies, are applied to semi-quantitatively evaluate a sample's SP inclusion. Other methods are used to quantitatively evaluate the magnetic grain size distribution (GSD) of a sample. Recently, magnetic susceptibility meters are available permitting broad-band magnetic susceptibility (BBMS) measurements to be made over a large spectrum of frequencies. This allows quantifying a narrow GSD of SP grains based on their frequency-dependency. Although novel in archaeology, such measurements have been applied in a handful of environmental studies in recent years concerning loess deposits in China and Bulgaria as well as ceramics in a study from the Czech Republic. To assess the applicability of BBMS in archaeology, soil samples were taken from a ditch at the middle-Neolithic Circular Ditched Enclosure, Hornsburg I, Lower Austria. The samples were measured with the University of Toronto Electromagnetic Induction Spectrometer (UTEMIS II). Measurements at different frequencies, ranging between 140 Hz to 63 kHz, allowed the calculation of the samples' GSD's, the grain sizes' mean, standard deviation and range as well as the frequency dependence. These can be used to understand a sites or features soil formation history, as archaeological deposits often exhibit enhanced magnetic properties that are distinct from surrounding non-anthropogenic soils, sediments and deposits. Therefore understanding how these contrasts relate to the changes in magnetic concentration, composition and grain size can provide indispensable information towards the formation processes and post-depositional changes at an archaeological site. The results of the study show promise for BBMS studies being a useful tool for differentiating different soils, sediments and archaeological deposits and thus reflecting the different archaeological phases and soil formation processes within the examined ditch. This provides for a better understanding of the depositional and post-depositional processes of the ditch deposits allowing for further archaeological assessment and offering additional applicability for a wide range of archaeological fields.