Barite in Freshwater Sediments of the Baltic Sea Crystallises in a Diffusive Salinisation Gradient
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Barium plays a significant role in low-temperature geochemistry due to its connection with major biogeochemical cycles, such as carbon and sulfur. Barite, a common authigenic mineral in marine sediments, is a valuable proxy due to its low solubility and long-term stability. It is widely studied as an indicator of oceanic paleo-productivity but also forms in environments with steep gradients of dissolved barium and sulfate, such as hydrothermal vents, cold seeps, and during diagenesis in sediments. The focus of this research is on diagenetic barite crystallized in the freshwater sediments of the Baltic Sea, particularly in the Gotland Basin, where barite nodules are found in glacial varved clays beneath brackish Holocene muds. These barites formed after the Baltic Sea's post-glacial connection with the Atlantic during the Yoldia stage. Sulfate for barite precipitation originated from downward diffusion, while dissolved barium originated from ionic exchange between brackish water and the glacial clays. The sulfur isotope signatures of barite, marcasite, and pyrite reveal significant microbial sulfate reduction and suggest the porewater sulfur reservoir was initially closed to sulfate. The study also shows that sulfur isotopes in these diagenetic barites display a downward gradient at the boundary between brackish Yoldia sediments and Baltic Ice Lake clays, suggesting isotope discrimination during solid phase formation. Additionally, changes in barite surface texture, micro-morphology, Sr composition, and isotopic signatures (Ba, S, O) indicate variations in supersaturation, fluid composition, and crystal growth rates, supporting the idea of a paleo-salinization gradient and highlighting the modification of paleo-porewater components through diffusional processes.