Barite precipitation in freshwater limnic sediments: a proxy for salinization

The diagenetic precipitation of barite (BaSO4) in sediments requires the mobilisation and sources of dissolved barium and sulfate, the latter often limited in the sulfur cycling of lacustrine systems. In this study, we investigate the origin and proxy potential of barite that has crystallised in freshwater sediments of the Baltic Sea. Barite nodules with up to millimetre-scale grain sizes are found in the glacial varved clays of the limnic Baltic Ice Lake phase (>16 to 11.7 ka BP), underlying brackish Holocene muds. We have comprehensively analysed the solid phase of the host sediments and the barite, and the porewaters in the respective sediments, both, geochemically and isotopically for the signatures of sulphur, barium, oxygen, and also the related carbon cycling. The sulphur isotope signatures preserved in the barites display a remarkable downward gradient from the lithological boundary between the brackish Holocene sediments and the preceding limnic varved clay deposits. The sulphur isotope signature of different mineral components (marcasite, pyrite and barite) shows that the porewater sulphur reservoir was initially affected by microbial sulphate reduction. Aside from the smaller importance of bacterial activity in the glacial clays, the observed trend sustains an isotope discrimination upon solid phase formation, or minor fractions of isotopically light sulphur that may have been incorporated upon crystallisation at depth. It had been hypothesised that sulphate for barite precipitation originated from the postglacial connection of the Baltic Sea with the Atlantic Sea, that has led to brackish waters flowing into the different Baltic Sea basins and downward diffusion of sulphate and other dissolved constituents through the sediment column. Taken together, the observed changes in barite surface texture and Sr composition, as well as isotope signatures (Ba, S, O isotopes), indicate changes in the supersaturation and composition of the paleo-porewater fluids and the crystal growth rate, supporting the concept of a paleo-salinisation gradient that is geochemically imprinted in the barites up to date. Moreover, we explore the oxygen isotope signature in the barite as a proxy for the parent porewater fluids, and show that the pore waters at this site with low sedimentation rates have been completely modified to date by diffusional processes, in contrast to sites with higher sedimentation rates (IODP cores) that still retain the original porewater signature.This investigation outlines that diagenetic barites in limnic sediments can evidence past salinization events, and furthermore, how the isotope signature of individual barite constituents can be used infer the parental fluid composition. This abstract summarises a detailed investigation recently published in a Special Publication (Roeser et al., 2025).Roeser P., Böttcher M.E., Lapham L.L., Halas S., Pretet C., Nägler T., Prieto M., Struck U., Huckriede H. (2025) Barite in Baltic freshwater sediments crystallises in a diffusive salinisation gradient, 370-395; In: Nucleation and Growth of Sedimentary Minerals (Eds P.H. Meister, C. Fischer and N. Preto), International Association of Sedimentology, Special Publication, 5