Isotopic and geochemical compositions of Silurian calcitic brachiopod shells, carbonate host rocks, and calcite cements from two northern European sites with different diagenetic histories are compared (Gotland, Sweden; Oslo Graben, Norway). The sequence exposed on Gotland is characterized by repeated synsedimentary subaerial exposure and meteoric influence but only insignificant thermal alteration during shallow burial diagenesis. In contrast, Silurian sediments exposed in the Oslo area show only few signs of early exposure but were severely affected by thermal alteration during burial of the succession.
The preservation potential of brachiopod shells in each basin is different. Most nonluminescent brachiopods from Gotland show well-preserved microstructures and trace-element contents very similar to those of modern brachiopod shells. Tightly clustered isotope values of individual brachiopod populations do not suggest significant meteoric alteration of primary marine isotope signatures.
In contrast, there is strong evidence that all brachiopod shells from the Oslo area contain diagenetically altered δ18O values. Alteration of Norwegian brachiopods is not always reflected in their trace-element contents and cathodoluminescence characteristics, but under SEM the shells often display corrosion and recrystallization. Scattered oxygen-isotope values in nonluminescent brachiopods (δ18O −18 to −5.5‰) together with low δ18O values (<−10‰) and unusual trace-element enrichments (Sr, Mn, Fe) of associated carbonate matrix samples and fracture-filling calcite cements can be related to contact metamorphism and infiltration of hydrothermal fluids during Permian time. Extensive fluid/rock exchange was facilitated by brittle deformation, as evidenced by numerous luminescent microfractures and macrofractures crosscutting brachiopod shells and surrounding sediments.
The case study from the Oslo Graben demonstrates that high fluid/rock ratios and thermal overprinting during burial diagenesis may result in pervasive diagenetic alteration of δ18O values in nonluminescent brachiopod shells. This alteration can be detected by combined petrographic and geochemical investigations on brachiopod shells, diagenetic cements, and carbonate host rocks. Brachiopods from deeply buried and hydrothermally altered sites should not be used for deciphering marine δ18O signals. However, the data presented here argue against a general rejection of lower Paleozoic brachiopods as indicators of marine isotope signatures.