Understanding multi-scalar exchange networks along the Tanzanian Swahili Coast through the geochemical examination of copper-based metals, 7th-15th centuries CE

Abstract

Throughout the Global Middle Ages, Swahili Coast towns were key in articulating long-distance and regional trade that linked Eastern and Southern Africa with triangular Indian Ocean networks extending to India and the Persian Gulf and Red Sea. Copper and copper-based metal alloy objects were likely imported through these trade networks and reworked locally, but the role these metals played in local Swahili economies as well as global exchange systems remains unclear. Therefore, the examination of copper-based metals can shed light on the impact of multi-scalar trade connections within Swahili communities. To that end, this thesis used a combination of solution-mode inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector ICP-MS (MC-ICP-MS) to perform geochemical and lead isotopic analyses of a collection of 28 copper-based artifacts from five different sites along the Tanzanian Swahili Coast, dating from the 7th-15th centuries CE. The results of the bulk elemental analysis reveal a variety of metal types, including pure coppers, brasses, bronzes, as well as ternary, quaternary, and highly leaded alloys, attesting to the prevalence of mixing and recycling practices surrounding the production of these copper-based metals. Trace element compositions were examined alongside lead isotope ratios in order to discern possible geological provenances of the copper and added lead, revealing likely connections with ore sources in Southwest Asia, India, and possibly the African Interior as well. The results of this research demonstrate the potential of combined isotopic and geochemical analyses to aid in understanding copper exchange in the medieval African and Indian Ocean worlds, the Swahili Coast’s role in these historic multi-scalar economic networks, and the role of nonlocal trade items such as copper within Swahili communities.