Application of high resolution Mobile Metal Ion (MMI) soil geochemistry to archaeological investigations: an example from a Roman metal working site, Somerset, United Kingdom

An innovative application of Mobile Metal Ion (MMI) partial extraction soil geochemistry is used to identify below-surface archaeological features, using a previously incompletely surveyed Roman metal-working site at St. Algar’s Farm, Somerset, as a case study. Soil samples were taken and analysed for 53 elements by the MMI geochemical method. Lead, Tl, Ba and Zn were found in very high concentrations and the sensitivity of the technique also enabled Ag, Au and Sn to be measured in anomalous concentrations. Elemental maps accurately outlined known metal working areas. Principal component analysis and bivariate correlations identified two suites of associated elements: Pb, Ba, Tl, Ag, Au, Cu, Sb, the base and noble metal group (BNM), and Fe, Ti, Nb, Mn, Co, Cu, P, Li, Rb, Sc, Cs, K, Ga, P, Zr, Th and Sn, the pegmatite (PEG) group. These were used to form indices which delineate the metal working area and areas possibly related to the processing of pegmatite containing Sn. The high sensitivity MMI data were compared with strong-acid digest results from a limited number of the MMI samples; the MMI data showed better geochemical contrast than the strong-acid results. Multi-element statistical similarity comparisons with off-site samples suggest likely sources for the Pb and Sn used at the St Algar’s site. The increased sensitivity of MMI soil analysis combined with the multi-element capacity allows a more detailed archaeological interpretation