Walking commodities: a multi-isotopic approach (δ13C, δ15N, δ34S, 14C and 87/86Sr) to trace the animal economy of the Viking Age town of Birka

Carbon (δ13C), nitrogen (δ15N), and strontium (87/86Sr) isotope analyses have been applied widely over the past four decades to reconstruct human and animal dietary and mobility patterns. Sulfur (δ34S) has recently shown great promise to further enhance isotope analyses of the geologic and hydrologic contexts in which organic material formed. For this case study we applied this suite of multi-isotopic analyses to a dataset of 45 animal bones and teeth from the urban Viking Age town of Birka located in present-day Sweden. This research falls in line with previous studies as a potential way to bridge the understanding of relationships between centers and hinterlands by tracing socioeconomic networks of subsistence and food provisioning utilizing the animal economy as a proxy. The utilization of δ34S values enables terrestrial, marine and freshwater food niches to be disentangled when δ13C and δ15N values may be overlapping between each of the niches. The incorporation of five 87/86Sr samples further allowed us to carefully interpret the movement of animals across the landscape. We identified cattle potentially originating > 180 km from Birka during the earliest stages of occupation (early 8th century CE), while pigs and ovicaprids were more locally reared, indicating the dimensions of the early market economy in the Viking period was complex and multifaceted