Wet feet: developing sulfur isotope provenance methods to identify wetland inhabitants

The stable isotopes of sulfur provide a distinctive signature for marine proximity and interaction. Exploring coastal proximity has been the principal application of sulfur isotopes in archaeology and palaeoecology, but this deals only with high (greater than 14‰) isotope values, meaning little interpretation has been gained from lower values. Progress has been hindered by issues with biosphere mapping. Air pollution can impact modern landscapes, significantly lowering sulfur isotope baselines, leading to the assumption that modern vegetation-based sulfur maps are not reliable. This research explores the potential of previously undiagnostic low, and often, negative sulfur isotope values for identifying wetland dwellers. Impervious clays that support wetlands are distinctive ecosystems and this study tests the hypothesis that they will produce low isotope values owing to both the underlying substrate and to redox conditions. Primary mapping of targeted areas using modern plants highlights zones with natural negative sulfur values and demonstrates that this constitutes a distinctive wetland signature. Analysis of modern and archaeological fauna demonstrates that these distinctive isotope compositions are transferred into the food chain. These findings propel the interpretative potential of sulfur isotopes forward and add to the growing knowledge to provide means for identifying archaeological humans and animals raised in wetlands.