Silicon analysis and dissolved cation concentrations in the sandy beach ecosystem Spiekeroog, German North Sea

Dissolved silicon isotope compositions (d30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736 µM, but homogeneous d30Si of +1.0 ± 0.15 per mil. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3 µM, and consequently characterised by high d30Si of +3.0 per mil. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23-192 µM), and concentrations increase with depth at all sampling sites. The corresponding d30Si values are highly variable (+0.3 per mil to +2.2 per mil) and decrease with depth at each site. All groundwater d30Si values are lower than seawater and most values are lower than dissolved d30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5 µmol Si g). Silica discharge from the beach to the coastal ocean is estimated as approximately 210 mol Si yr per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.