An insight from the sea: provenance studies on Roman lead artefacts from the Arade River, Portimão (Portugal)

In the present study, 37 lead artefacts were characterised to identify possible lead sources allowing to establish trade fluxes concerning food and textile products during Roman times. These artefacts were uncovered by dredging works at the Arade River estuary (Portimão). The city of Portimão (Lusitania province) was an important harbour, where several fish-processing factories were installed, and Arade River provides major access to the hinterland, both displaying an important commercial activity during the Late Antiquity. The methodology includes the typological and chemical (elemental and Pb isotopes) characterisation of artefacts. Samples were divided into the following: (i) rectangular plaques with decorations in relief such as tridents, fishes, or palms leaf, an iconography known to be displayed in some African amphora handles; (ii) small plaques with one perforation and incised Roman numerals, probably related with textile products; and (iii) fishing net weights, smooth plaques of unknown functionality, and a small rectangular prismatic plaque, perhaps an ingot. Elemental analysis was performed by ICP-MS, and results were interpreted by multivariate statistical analysis, which suggested different processes to obtain raw materials, namely lead obtained by the reduction of litharge or smelting of silver-poor galena. Cluster analysis grouped most of samples with motif depictions, which were further analysed by MC-ICP-MS to determine Pb isotope ratios. The possible sources of lead were identified by combining archaeological data with the nearest Euclidean neighbours using a large database comprising the Iberian Peninsula and Mediterranean region. The Pb isotope signatures suggested lead sources located not only in the Iberian Peninsula but also in North Africa, evidencing a long-distance trade between those Roman provinces.