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Formation of authigenic sulfides and carbonates in Holocene 14C-dated sediments is interpreted based on analyses of cations, FeSS, FeS2S, Corg and sulfur isotopes. Mineral distribution was studied in thin sections and mineral morphology with SEM-EDS. The sediments were deposited in the Gotland Deep, Baltic Sea, during freshwater and brackish stages. Calcium-rich rhodochrosite (MnCO3) occurs in concentrations up to 17%dw in brackish water sediments and are mainly confined to 0.02–0.5-mm-thick laminae. The frequency with which these laminae occur suggests that they formed with a periodicity of one to a few years, and their purity suggests that formation took place in a few weeks. We propose that these carbonate laminae resulted from oxic water inflow to the euxinic basin. Mn(II) dissolved in the water column was rapidly oxidized and large amounts of Mn-oxides reached the sediment. Reduction of these Mn-oxides in a high-alkalinity environment facilitated rhodochrosite precipitation at the sediment surface. Pyrite in the brackish water sediments formed at or above the sediment-water interface, in an iron-limited environment. Due to alternating oxic and euxinic conditions FeS was oxidized, resulting in a high FeS2SFeSS ratio (median = 170). Formation of iron sulfides in the freshwater sediments occurred below the sediment-water interface, in a sulfate-limited environment. As a result of the rapid exhaustion of reactive iron in the brackish water sediments, excess H2S diffused downwards to pyritize the uppermost part of the underlying freshwater sediments. |
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