Wetland Sulfur Isotope Signals and Dynamic Isotope Baselines: Implications for Archaeological Research

Sarah K Wexler^*Rhiannon Stevens

• Institute of Archaeology, UCL, London, United Kingdom

Sulfur isotopic composition (δ³⁴S) is used in archaeological research to reconstruct past mobility patterns and diet using environmental baselines. Human and faunal collagen δ³⁴S is generally interpreted as reflecting environmental baseline bioavailable δ³⁴S derived from local geological sulfur and marine sulfur near the coast. However, recent studies have highlighted that the δ³⁴S of bioavailable sulfur is modified in wetlands and waterlogged environments as a result of microbial sulfur cycling. Microbial sulfate reduction and sulfide oxidation, driven by fluctuating redox conditions from flooding and draining, can significantly alter bioavailable δ³⁴S baselines in the biosphere, and subsequently in animal tissues, through the food chain. This challenges conventional approaches that assume local geology and coastal proximity alone determine bioavailable baseline δ³⁴S.This study considers sulfur cycling within terrestrial ecosystems and its implications for archaeological interpretation, emphasising the need to consider the effects of coeval microbial sulfur cycling on environmental δ³⁴S baselines derived from geological and coastal sulfur. We explore how plant δ³⁴S is influenced by anoxic soil conditions, hydrological controls and plant waterlogging adaptations, integrating these factors into a theoretical framework and simple numerical models of plant δ³⁴S in wetland and waterlogged environments. The models simulate the effect of different hydrological regimes on plant δ³⁴S (in flow-through, standing water, drained post flooding, and simultaneously flooded and drained landscapes).We suggest how δ³⁴S could change for crop plants grown using different past water management techniques, and discuss effects of wetland utilisation and resource use on animal and human collagen δ³⁴S. This study demonstrates that different systems can result in multiple 'wetland' sulfur isotope signals that can deviate both positively and negatively from local baselines that are driven by geological and marine sulfur. δ³⁴S has the potential to be a useful tool for exploring ancient wetland