Supplying the Augustean conquest of the Alps — investigating the origins of Roman slingshot projectiles by lead isotope analysis

Lead isotope analyses of 40 lead slingshot projectiles and two lead plummets, recovered from a conflict landscape proximal to the Crap Ses Gorge in the Surses Valley, Switzerland, and thought to relate to the central marching route of Augustus’ Alpine Campaigns (late 1st c. BCE), are presented. Results point to a centralised production using a mixed lead stock primarily derived from the Almería-Cartagena Volcanic Belt of the southeastern Iberian Peninsula, but with an input from the Ossa Morena/Central Iberian Zones (OMZ/CIZ), Iberian Pyrite Belt (IPB), and/or Massif Central of Gaul. Seven projectiles discovered in Brigantium/Bregenz and Tasgetium/Eschenz to the north are isotopically distinct from those found in the Surses Valley, deriving from a separate metal stock comprised only of OMZ/CIZ, IPB, and/or Massif Central lead. With the potential of using the lead isotope signature of lead projectiles to trace Roman marching routes highlighted, those discovered in Brigantium and Tasgetium likely relate to a different advance than projectiles found in the Surses Valley.