Biogeochemical processes in sandy beach sediments of Spiekeroog Island and Majorca

以波浪和潮汐为主的沙滩构成了世界海岸线的主要部分，在碳、营养和金属循环中发挥着重要作用。在涨潮期间，海水渗透到沉积物中，同时有机物也被结合在一起。底栖微生物降解这种有机物，在退潮期间，富含营养的水排放到沿海海水中，在那里回收的营养物质被用于初级生产。微生物的降解过程导致氧化还原梯度，这对孔隙水的组成有影响。海滩还可以位于淡水和海水的混合区(地下河口)。在这里，盐度梯度可能会影响沉积物-孔隙水的相互作用，而淡水地下水流动是陆源溶质的来源。因此，为了了解海滩系统在碳、营养和金属循环方面的全球作用，必须在世界各地详细研究海滩沉积物中的生物地球化学过程。由于在这一领域的研究还很少，特别是对这些体系对氧化还原敏感的痕量金属的源或汇功能的研究，本项目将对目前的研究状况做出重要贡献。我们计划研究Spiekeroog群岛(德国西北部，中潮，硅质碎屑)和马略卡岛(西班牙，小潮，钙质)上两个截然不同的海滩系统地下河口的生物地球化学过程。我们打算分析海滩孔隙水中的主要离子、溶解有机碳、氧、硫化氢、营养物质(N、P、C、Si)和微量金属(Mn、Fe、U、Mo、V、Re)以及Fe同位素比值。我们的进一步目标是表征沉积物的组成，这对孔隙水的组成有显著的影响。高分辨率采样将沿着横岸断面(沙丘至低水位线)进行，分别至5米(Spiekeroog)和2米(Majorca)。该项目的重点是确定氧化还原和盐度梯度及其对孔隙水组成的影响。将根据所获得的数据进行水化学模拟，以帮助更好地了解不同化学成分地下水混合的影响。我们将能够评估海滩的源或汇功能，涉及必要的浮游植物营养物质和对氧化还原敏感的痕量金属。铁同位素比值将扩展关于地下河口铁循环的有限知识，并提供从这些系统排放孔隙水的更好的铁同位素特征。此外，这项研究将为模拟不同孔隙水组分从可渗透沉积物进入沿海海洋的通量提供深刻的数据基础。

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.