Enigmatic Belgian oxygen values are the highest in north-western Europe

Oxygen isotopes in biological apatite are commonly used as a proxy in mobility studies to identify ‘locals’ vs. ‘non-locals’ and, as a further step, to possibly identify compatible regions for those individuals that are characterized as ‘non-locals’. However, factors like temperature, warmer vs. colder climatic phases,
air pressure, altitude, cooking practices..., affect δ18O values rendering interpretations difficult. Different methods to identify outliers within a population have been proposed and include 3*MAD or Turkey IQR, and most studies primarily base their ‘non-local’ oxygen interpretations on the offsets with expectations compared to oxygen isotope maps. However, recent oxygen results from Belgium highlight potential issues using these maps.
Oxygen isotope results measured on tooth enamel carbonates from different Flemish cemeteries (Koksijde, and Ypres) ranging from the early till late medieval period are inconsistent with the existing oxygen isotope maps. Ranging up to -3.4 ‰ (VPDB), the δ18O values observed in these 2 sites are among
the highest in Europe. The exact reason for these differences with the rest of north-western Europe *remain unclear. We argue that methodological issues can be ruled out as the results from samples from other locations in Europe, prepared with the same pre-treatment and measured in the same lab during
the same runs, are compatible with the oxygen isotope maps.
Further research is needed to understand the reasons for the large difference in oxygen isotope ratios between Belgian populations and the rest of Europe. The implications of these findings could significantly impact mobility studies within Europe and their interpretations.