Visible near-infrared spectroscopy as an aid for archaeological interpretation

We present a pilot study using visible near-infrared spectroscopy (vis-NIRS) to investigate ancient geochemical activity signals from an excavation of the Viking-Age Ribe Emporia, Denmark. Our aim is to study whether vis-NIRS is applicable to archaeological soil analysis and if it can provide additional insight into the use of space inside houses. Using 1059 bulk soil samples sampled in a 0.25 cm grid, we test if vis-NIRS is useful for predicting magnetic susceptibility and anthropogenic elements such as Ca, Cu, P and Sr. Furthermore, we test both portable X-ray fluorescence (pXRF), magnetic susceptibility and bulk chemistry (by inductively coupled plasma mass spectrometry, ICP-MS) as reference data for training the vis-NIRS models. Predictions were performed using a partial least squares (PLS) regression with a non-linear iterative partial least squares (NIPALS) algorithm, and the best cross-validation predictions for each element based on both portable pXRF and ICP-MS as well as predictions for magnetic susceptibility were analysed spatially using ordinary kriging. The spatial analysis showed reduced precision for all elements and magnetic susceptibility compared to reference maps. Additionally, the spectral predictions had difficulties in predicting the entire elemental and magnetic susceptibility range and especially struggled with the prediction of hotspot areas. Elemental predictions based on the two different reference datasets, collected with pXRF and ICP-MS, performed equally well. The more cost-efficient approach was pXRF for calibration data. Further development is needed for the application of vis-NIRS to be fully implemented together with geochemical tools as a regular part of archaeological soil analysis on site.