Magnetometer prospection is now considered a standard method of archaeological prospection. Not seldom, however, the measurement results (magnetograms) of this quick and often efficient prospecting method are interpreted without required...
moreMagnetometer prospection is now considered a standard method of archaeological prospection. Not seldom, however, the measurement results (magnetograms) of this quick and often efficient prospecting method are interpreted without required validation in the lab, which may lead to incorrect assessments of the magnetic field anomalies in archaeological and geoarchaeological contexts. This study seeks to address this problem and is dedicated to the aim of improving upon interpretations of magnetometer measurements and further developing processes that can be applied towards rapid and sufficient clarification of magnetic field anomalies. For this purpose, magnetometric and environmental magnetic measurements, combined with pedological and sedimentological parameters, are evaluated in several survey areas.
The survey areas and environmental archives in focus are the brown plaggen soils at the transition between the lower Weser Uplands and the Dümmer Geest Lowland, as well as the semi-terrestrial sediments of the Fossa Carolina at the transition between the Franconian Swabian Keuper-Lias Plains and the Franconian Jura.
The magnetometric prospecting is conducted using a vector gradiometer (Bartington Grad601 dual), deployed frequently on archaeological prospecting missions. Magnetic susceptibility is used as a geophysical indicator for the identification and initial characterization of magnetically conspicuous and magnetic field anomaly generating soil horizons and sedimentological layers reviewed in environmental archives. Based on the Curie temperature measurements and magnetic domain investigations, the environmental magnetic minerals and their particle sizes are characterized in magnetically conspicuous layers and horizons.
Findings from the combination of environmental magnetic measurements with soil characteristics of brown plaggen soils indicate that the ferrimagnetic PSD iron sulfide greigite detected in the plaggen layers must have been induced anthropogenically. Furthermore, there are indications that these greigites may have been modified and in some cases mobilized by seepage and groundwater. A comparison with greigites from literature shows that the plaggen soil PSD greigites are found in a magnetic domain in which no natural greigites have been previously measured. This magnetic state is put forth for discussion as an oxidation stage of the ferrimagnetic iron sulfide.
In the trench fills or paleochannels of the Fossa Carolina, authigenic SD greigites were discovered in layers exhibiting magnetic field anomalies, thus confirming the supposition by Stanjek et al. (1994) that greigites can be of significance for magnetometer prospecting in semi-terrestrial environments. By contrast, highly-heated, thermoremanent PSD titanomagnetites, causing magnetic field anomalies, were discovered in flanking accompanying structures of the Fossa Carolina. Their origin remains unresolved, yet a natural formation of these magnetic minerals on site is ruled out.
Considering the objective of this research, the methodical approach at the Fossa Carolina proved to be an especially efficient and relatively economical method for verified clarification of magnetic field anomalies in the magnetograms.
