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Isotope analyses help map Iron Age trade routes

Elemental and lead isotope analyses of Iron Age copper ingots are unlocking secrets of early trade routes and show how indigenous Mediterranean communities functioned from as far back as about 2,600 years ago.

For the first time, a scientific team led by Flinders University archaeologists, working with the Institute of History (CSIC) in Spain, has examined the origins of metal items from an Iron Age archaeological site in southwest France and found they were sourced from a variety of Mediterranean locations.

metal ingot archaeology
Rochelongue’s underwater site artifacts are shown in-situ during the campaign of 1964. Photo courtesy of Cap d’Agde Museum Archive.

The study, “Shipping Metal: Characterisation and Provenance Study of the Copper Ingots from the Rochelongue Underwater Site (Seventh–Sixth Century BC), West Languedoc, France,” by Enrique Aragón, Ignacio Montero-Ruiz, Mark Polzer, and Wendy van Duivenvoorde has been published in the Journal of Archaeological Science: Reports.

The underwater site of Rochelongue, believed to be four small boats located west of Cap d’Agde in southwestern France and discovered in 1964, dates to about 600 BCE. Its cargo included 800 kilograms of copper ingots and about 1,700 bronze artifacts. They contain very pure copper with traces of lead, antimony, nickel, and silver.

Isotopes tracked to specific geographic regions

Flinders University maritime archaeology researcher Dr. Enrique Aragón Nunez has stated that the isotope analysis showed the composition of different ingots in the cache is consistent with Iberian and also eastern Alpine metalliferous sources, and possibly some Mediterranean sources. This illustrates that water trade and movement were active in this period between Atlantic, Continental, and Mediterranean circuits.

Enrique Aragón preparing materials to be sampled. Photo courtesy of Frédèrique Nicot.

This now provides a key to investigate the coastal mobility and cultural interactions between the Languedoc area in France and the broader Western Mediterranean basin in 600 BCE – before permanent Greek settlement occurred in this region.

Trade for metals, especially with seafaring people from the Levant, Aegean, and Greek mainland, influenced these indigenous communities with the introduction of their foreign cultural goods and practices.

While the various sizes, shapes, and composition of the various ingots found at Rochelongue show they originated from diverse geographical sources, the elemental and lead isotope analyses provide much more comprehensive knowledge, showing that a broad and diverse exchange network existed in this period for metals that includes continental and maritime routes.

Ingots made from unalloyed copper

“These metallic objects are important diagnostically because they lend themselves to source tracing of geological components and technological studies of their processing and manufacture,” said van Duivenvoorde, a Flinders University Maritime Archaeology Associate Professor. “The copper ingots were made of unalloyed copper with low levels of impurities, and more than half can be linked to the Iberian Peninsula. This points to the circulation of metal through the wider Mediterranean region, but also to local and western alpine mining and manufacture, and possibly northwestern Sardinia. Therefore, the Rochelongue items speak of indigenous agency rather than maritime intervention.”

Maps showing metal trade routes and origins. Image courtesy of Flinders University/Elsevier.

All mass spectrometry work on the copper ingots was performed at SGlker Lab from the University of the Basque Country (UPV) Bilbao, Spain.

The Rochelongue ingot metal provides a window into expansive networks of trade contacts that must be evaluated alongside all typological and cultural data.

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