Diet and mobility in early medieval coastal Belgium: Challenges of interpreting multi-isotopic data

During the early medieval period (5th–9th century CE), the North Sea coastal societies were involved in long distance maritime trade relations, which resulted in a pronounced mobility of individuals throughout the North Sea area. This work presents the first isotope data from human remains on diet and mobility from early medieval Belgian coastal populations. A total of 23 out of 51 excavated individuals from the archaeological site of Koksijde, Belgium (7th–8th century AD), was selected for isotope and elemental analyses (δ13Ccol, δ15N, δ18OP, δ13Cap, δ18OC, 87Sr/86Sr and [Sr]). The high δ15N values of part of the individuals buried at Koksijde indicate that high trophic level ranked fish was included in their diets, suggesting an intensive exploitation of marine food sources. The δ18O values are not compatible with the predicted ‘local’ values while the strontium isotope ratios have values close to that of seawater (0.7092). Either the actual oxygen isotope values in early medieval Belgium are offset by 1 or2‰ compared to the current meteoric water predictions, or the population came from somewhere else. Whether or not this was linked to the population's mobility is difficult to assess based on their δ18O and 87Sr/86Sr values. Nonetheless, the study of the population at Koksijde provides new insights into the lives of the early medieval coastal Belgian societies.