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 determ ination of stable C- and O-isotopic compositions in Paleozoic sediments. In the present study, critical precautions for obtaining reliable isotope val ues and maximum possible reliability are discussed. Samples were taken in t he tectonically undisturbed and exceptionally well-preserved Silurian seque nce 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 ultrastruc ture are distinguished and attributed either to biological differences or t o diagenetic alterations. For isotope analyses, only largely unaltered area s of the secondary, fibrous shell layer should be used. Shell layers with o ther types of ultrastructure (primary, tertiary, laminar) may have isotopic compositions different from the assumed seawater value even when diageneti cally unaltered. Only brachiopod taxa with a shell succession that includes a sufficiently thick secondary shell layer (e.g., Spiriferida. Rhynchonell ida, Orthida) give reliable, mutually corresponding isotope values, indicat ing 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, ostr acodes) give non-equilibrium isotopic results and therefore should not be u sed. By restricting the samples to one species or to a group of closely rel ated species, even minor differences in primary isotopic composition can be excluded. In the Silurian of Gotland, the stratigraphically widely distrib uted spiriferid Atrypa reticularis was selected for study. Considering the type of record, four different taphonomic assemblages are d efined. In autochthonous assemblages of specimens, which lived contemporari ly, original values of carbon as well as oxygen isotopes can be evaluated w ith a reliability of < +/- 0.4 parts per thousand. This allows the recognit ion of minor, hydrographically produced differences between neighbouring, c ontemporary facies areas and the determination of small (< 1 parts per thou sand) isotopic excursions. Thus. besides being indicators of hydrographic c onditions in ancient oceans, stable carbon- and oxygen-isotope compositions can be used as a stratigraphic tool that provides correlations, sometimes even within biozones. (C) 2001 Elsevier Science B.V. All rights reserved.