Subproject P4 will study trace element and metal isotope dynamics related to biogeochemical processes in the STE on Spiekeroog Island. We hypothesise that overlapping redox zones, dynamic changes in microbial activity and spatially and temporally changing redox and salinity interfaces create distinct trace element and isotope records in high-energy beach systems. P4 will measure trace element concentrations (Fe, Mn, Co, Mo, Re, Tl, U, V, rare earth elements) and Fe and Mo isotope signatures in (pore)water and sediment samples. Regular field sampling will give insight into spatio-temporal variations of trace element and isotope patterns under varying boundary conditions. Laboratory incubation experiments will be used to determine trace element mobilisation, retention and isotope fractionation rates in order to understand the physico-chemical and microbial changes driving these reactions in the dynamic deep to shallow subsurface of the STE in detail. Trace element and supplementary major ion, nutrient and total alkalinity data will be provided for mathematical modelling (P1, P6) and rate measurements (P2) and biogeochemical investigations carried out in P3 and P5 will be used to collaboratively assess how trace element and metal isotope transformation and fractionation are interconnected with water source and age, redox conditions and organic matter and microbial community characteristics.