Source of strontium in archaeological mobility studies—marine diet contribution to the isotopic composition

The strontium isotope composition of human tissues is widely used in archaeological mobility studies. However, little attention is paid to the relative contributions of terrestrial versus marine sources of strontium in these studies. There is some debate over the role of a solid diet versus drinking water as the most important source of strontium for the human body, with related possibilities of misinterpretation of the archaeological record if only strontium isotope compositions of the biosphere are studied. However, there is a third component, marine strontium, which is commonly not assumed to contribute towards the strontium isotope composition of archaeological skeletal remains, especially in locations that are not directly coastal. To illustrate the potentially obfuscating effects of mixed Sr sources in a human population, we present a case study of twelve individuals from the medieval Finnish site Iin Hamina with a known dietary history. Our study shows that marine consumption is a significant factor explaining the strontium isotope composition of the Iin Hamina human remains, with implication of erroneous conclusions about immigration without prior knowledge of diet composition. Thus, future studies should always incorporate a rigorous analysis of dietary history, with special regard to potential consumption of aquatic resources, when strontium isotope analysis is used as a method in the study of palaeomobility.