Shell succession, assemblage and species dependent effects on the C/O-isotopic composition of brachiopods — examples from the Silurian of Gotland

Brachiopods are the most widely used biological constituents for the determination of stable C- and O-isotopic compositions in Paleozoic sediments. In the present study, critical precautions for obtaining reliable isotope values and maximum possible reliability are discussed. Samples were taken in the tectonically undisturbed and exceptionally well-preserved Silurian sequence on Gotland (Sweden). The state of preservation of brachiopod shells was examined by cathodoluminescence (CL) microscopy and by comparison of shell ultrastructures with those of recent brachiopods. Nine types of ultrastructure are distinguished and attributed either to biological differences or to diagenetic alterations. For isotope analyses, only largely unaltered areas of the secondary, fibrous shell layer should be used. Shell layers with other types of ultrastructure (primary, tertiary, laminar) may have isotopic compositions different from the assumed seawater value even when diagenetically unaltered. Only brachiopod taxa with a shell succession that includes a sufficiently thick secondary shell layer (e.g., Spiriferida, Rhynchonellida, Orthida) give reliable, mutually corresponding isotope values, indicating that the shell calcite precipitated at or near equilibrium with ambient seawater. Brachiopods with other kinds of shell successions (Strophomenida, Pentamerida) as well as other calcite-shelled organisms (trilobites, ostracodes) give non-equilibrium isotopic results and therefore should not be used. By restricting the samples to one species or to a group of closely related species, even minor differences in primary isotopic composition can be excluded. In the Silurian of Gotland, the stratigraphically widely distributed spiriferid Atrypa reticularis was selected for study. ering the type of record, four different taphonomic assemblages are defined. In autochthonous assemblages of specimens, which lived contemporarily, original values of carbon as well as oxygen isotopes can be evaluated with a reliability of <±0.4‰. This allows the recognition of minor, hydrographically produced differences between neighbouring, contemporary facies areas and the determination of small (<1‰) isotopic excursions. Thus, besides being indicators of hydrographic conditions in ancient oceans, stable carbon- and oxygen-isotope compositions can be used as a stratigraphic tool that provides correlations, sometimes even within biozones.