Chemical and magnetic surveydata as indicators of ancient human activity: the suburban Sagalassos case (SW Turkey)

Several studies have suggested that multi-element soil geochemical surveying holds potential as an archaeological survey method, complementing more common techniques such as magnetometry and fieldwalking. Despite this, only limited research has been carried out concerning the joint interpretation of these techniques. In this study, we combined geochemical and magnetic survey data from an archaeological site comprising a suburban area of 1.5 ha, situated in the Roman to Byzantine city of Sagalassos (Taurus Mountains, SW-Turkey). For the geochemical survey, a total of 120 soil samples were collected in a grid with cell sizes of 20 m². Al, As, Ba, Ca, Cu, Co, Cr, Fe, K, Mn, Mg, Na, Ni, Pb, P, Sr, Ti, V and Zn were measured by inductively coupled plasma optical emission spectrometry (ICP-OES), using a Varian 720-ES apparatus after Aqua Regia digestion. The magnetic survey was performed using a Geometrics G-858 magnetometer in gradient mode, along 0.5 m spaced transects. The chemical data were analysed for patterns using a contiguity constrained spatial clustering algorithm. The main difference with more standard clustering methods is that this method takes into account the spatial dimensions of the dataset, resulting in clusters that are not only statistically, but also spatially homogeneous The resulting spatial clusters revealed that an area characterised by high-frequency magnetic anomalies was spatially associated with a chemical cluster rich in Cu, K, P, Pb and Zn. We interpreted this as resulting from the accumulation and decomposition of occupational debris. Increased Al, As and Ba concentrations helped interpreting an area with a low magnetic signal as a region where limestone bedrock was located close to the soil surface. Finally, two zones of enhanced magnetization were shown to spatially overlap with two clusters of soils containing elevated levels of Co, Cr, Mg, Mn, Ni, (Fe) and Ti, V, (Fe) respectively, indicating the presence of two different types of mafic to ultramafic ophiolitic bedrock near the soil surface. This study illustrates that multi-element geochemistry has potential as a survey technique, because it offers direct information on soil bedrock or ancient human disturbance, thereby adding an extra dimension to the interpretation of geophysical survey data.status: publishe