Biogeochemical processes in sandy beach sediments of Spiekeroog Island and Majorca

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Wave- and tide-dominated sandy beaches comprise a major part of the Worlds coastline and play an important role in carbon, nutrient and metal cycling. During rising tide seawater penetrates into the sediment and simultaneously organic matter is incorporated. Benthic microorganisms degrade this organic matter and during ebb tide nutrient-enriched water discharges into coastal seawater where the recycled nutrients are utilized for primary production. Microbial degradation processes lead to redox gradients, which have an impact on the pore water composition. Beaches can further be located in a mixing zone of fresh groundwater and seawater (subterranean estuary). Here, salinity gradients may influence sediment-pore water interaction and fresh groundwater flow serves as a source for terrestrial-derived solutes. To understand the global role of beach systems regarding carbon, nutrient and metal cycling, it is therefore vital to investigate biogeochemical processes in beach sediments in detail and at several locations worldwide. Since there are still few studies in this field, especially with a focus on the source or sink function of these systems regarding redox sensitive trace metals, this project will form an important contribution to the present state of research. We plan to investigate biogeochemical processes in the subterranean estuaries of two contrasting beach systems on the Islands Spiekeroog (NW Germany, mesotidal, siliciclastic) and Majorca (Spain, microtidal, calcareous). We intend to analyze major ions, dissolved organic carbon, oxygen, hydrogen sulfide, nutrients (N, P, C, Si) and trace metals (Mn, Fe, U, Mo, V, Re) as well as Fe isotope ratios in beach pore waters. We further aim to characterize the sediment composition, which significantly influences the pore water composition. High resolution sampling will be conducted along cross shore transects (dune to low water line) down to 5 m (Spiekeroog) and 2 m (Majorca), respectively. This project focuses on the identification of redox and salinity gradients and their effects on the pore water composition. Hydrochemical modeling on the basis of the obtained data will be applied to contribute to a better understanding of the effects of mixing of chemically different groundwaters. We will be able to evaluate the source or sink function of beaches concerning essential phytoplankton nutrients and redox sensitive trace metals. Fe isotope ratios will extend the limited knowledge of Fe cycling in subterranean estuaries and provide a better substantiated Fe isotopic signature of discharging pore waters from these systems. Furthermore, this study will provide a profound data basis for modeling flux rates of distinct pore water constituents from permeable sediments into the coastal ocean.