Mineralogy and Multi-Isotope Geochemistry of a high-energy Subterranean Estuary in the temperate Climate Zone
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Subterranean estuaries (STE) serve as critical interfaces between terrestrial water and element cycles and the marine coastal environment. They are characterized by steep biogeochemical gradients that influence processes such as the transport and transformation of carbon, sulfur, and metal compounds. Besides oxygen and nitrate, sulfate availability often plays a key role in carbon mineralization. Interactions with mineral phases within the aquifer contribute to the hydrochemical properties of solutions existing in the sediment, with minerals acting as elemental sources or sinks during biogeochemical cycling. Within the research group DynaDeep, it is aimed to investigate processes within a STE built of permeable sands located in the southern part of the North Sea in the temperate climate zone. The dynamic beach system in the north of Spiekeroog Island offers a unique setting to study interactions between fresh and saltwater under present and future environmental conditions. Vertical observations of sediments, and seasonal changes of pore water composition are made possible through boreholes and multi-level wells. This study presents findings from multi-isotope (COSH) analyses of water, dissolved carbon species, sulfate, and iron sulfides, alongside hydrochemical gradients and sedimentary mineral phase analyses. Water isotope ratios reveal freshwater mixing, while carbon isotope signatures in dissolved inorganic carbon highlight sources such as organic matter degradation and carbonate corrosion. Sulfate and iron sulfide isotopes indicate formation of reduced phases from microbial sulfate reduction, with evidence of sulfide oxidation near the surface or contributions from non-marine sulfate at lower salinity. Microscopic and phase analyses identify iron sulfides of in-situ formations, and calcite, apatite, and barite in the sediments, primarily of in-situ or detrital origin. Dolomite, another potential alkalinity source, is identified as detrital.