57Fe Mössbauer study of a sediment column in the Gdańsk Basin, Baltic Sea: Palaeoenvironmental application

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In search of a palaeoenvironmental reconstruction, iron minerals were identified in the <2 μm fraction of Late Pleistocene-Recent muds in the Gdańsk Basin (Baltic Sea). 57Fe Mössbauer spectroscopy (at room temperature, 110 K and 4.2 K) was used together with chemical analyses and X-ray powder diffraction. The sediment column of R.V. Meteor core 13404-1 represents 10,000 years of prodelta deposition and records a changing environment from the Yoldia (Y) Sea, through the Ancylus (A) Lake, through the Litorina (L)-Post-Litorina (PL) Sea. Two main sediment facies are distinguished in the core: the lower homogeneous clay to clayey mud with sulphidic bands and spots (Y-A facies) overlain by sapropelic mud (L-PL facies). In clays, ferrous iron was identified in octahedral sheets in chlorite and illite. Ferric ions are present both in M(1) and M(2) octahedral sites in illite, although the M(2) sites are more populated and uniform along the section. The distribution of iron in clay minerals is constant in the sediment column, which suggests that the clays have not been reduced or oxidized during early diagenesis. The constant quality of the clay matrix reflects the relative stability of the fluvial source of the sediment. Ferrous ions are also located in ferrous hydroxide, an unidentified Fe 2+ Fe 3+ hydroxide, monosulphides and authighenic siderite. Ferric ions are found in fine-crystalline ferrihydrite, α-and γ-FeOOH. Iron in non-clay magnetic minerals changes its valency and mineral form along the sediment column. The contents of total iron and Fe 3+ Fe 2+ measured in the <2 μm fraction of the fine bottom sediment are sensitive to the environmental change at the river mouth. Specific iron-mineral parageneses are ascribed to the ancient freshwater and brackish environments. The presence of lepidocrocite-ferrihydrite-Fe 2+ Fe 3+ hydroxide (and siderite and FeS) parageneses and the Fe 3+ Fe 2+ ratio above 2.0 indicate a brackish, stratified estuary-type basin. A ferrous hydroxide-goethite-dominated paragenesis with less FeS and a Fe 3+ Fe 2+ ratio of less than 1.0 testified to a freshwater environment. Transitional conditions to or from brackish environments are marked by total iron and siderite content peaks and abundant FeS. Iron-mineral assemblages in the prodelta muds yield much information. The salinity and water stratification history in the Gdańsk Basin during the Late Pleistocene to Recent is elucidated using the iron-mineral record.