Oxygen and carbon isotopic composition of Dinantian brachiopods: Paleoenvironmental implications for the Lower Carboniferous of western Europe

A total of 193 Lower Carboniferous (Dinantian) brachiopod shells from a variety of western European locations have been analysed for their δ18O and δ13C values. For 168 of these shells, the degree of their diagenetic alteration has been established by trace-element criteria. The concentrations of Ca, Mg, Sr, Mn and Fe were routinely measured by ICP–AES utilizing the diluted phosphoric acid that remained after carbonate dissolution for stable-isotope gas preparation. As a result, the trace-element and stable-isotope data originate from the same piece of the shell. The Dinantian δ18O and δ13C records, based on the means for single formations, appear to display cyclic higher-order fluctuations with a periodicity of ×106 years and amplitudes of 2–5‰, with an increase of ∼5‰ during the lower Tournaisian as the most conspicuous feature of both records. For most Dinantian stages, the structure of the δ18O record is mirrored by the δ13C oscillations. Most δ18O cycles coincide with the Dinantian record of third-order global sea-level stands as well as with the local record of transgressions/regressions. Model calculations, based on sea-level stands that may result from combined ice mass and temperature effects, yield a δ18O trend that reproduces the features of the overall δ18O record in Dinantian biogenic calcites. Yet, the absence of glacial phenomena for the major portion of the Dinantian sedimentary record is difficult to reconcile with the model solution. The tentative short-term δ13C peaks may be a reflection of increased organic productivity, resulting from a bloom of marine biomass that may follow an increased release of nutrients to the ocean from erosion of shelf sediments during low sea-level stands.