Stable isotope geochemistry of siderite concretions from Jurassic sedimentary rocks on Bornholm (Denmark)

Abstract

The diagenetic history of siderite concretions in Mesozoic sedimentary rocks on the island of Bornholm has been investigated using stable isotope-, AAS- and XRD-analysis and determination of carbonate content. Siderite concretions and bands, found in non-marine as well as marine environments, are investigated. The siderite concretions were precipitated from fresh to brackish pore-water prior to compaction. 8'3CPDB values from -14.2 to +1.4 o/oo indicate ne ar-surface precipitation affected by bacterial oxidation of organic matter. 8180 values between +22.1 and +28.7 o/oo 5180SMOW indicate concretion precipitation in pore-water of fresh to brackish origin. Weathering effects are seen, but temperature fractionation is not observed.