A composite Tethyan Late Jurassic–Early Cretaceous carbon and oxygen isotope curve is presented. C-isotope data provide information on the evolution and perturbation of the global carbon cycle. O-isotope data are used as a palaeotemperature proxy in combination with palaeontological information. The resulting trends in climate and in palaeoceanography are compared with biocalcification trends and oceanographic conditions favouring or inhibiting biocalcification. Positive C-isotope anomalies in the Valanginian and Aptian correlate with episodes of increased volcanic activity regarded as a source of excess atmospheric carbon dioxide. A major warming pulse accompanies the Aptian but not the Valanginian C-isotope event. The observed change in Early Aptian temperatures could have triggered the destabilization of sedimentary gas hydrates and the sudden release of methane to the biosphere as recorded as a distinct negative carbon isotope pulse preceding the positive excursion. Both C-isotope anomalies are accompanied by biocalcification crises that may have been triggered by pCO2-induced changes in climate and in surface water chemistry. Elevated nutrient levels in river-influenced coastal waters and in upwelling regions further weakened marine calcification. These conditions contrast with ‘normal’ trophic conditions prevailing in the latest Jurassic and favouring biocalcification. The C- and O-isotope curves record a stable mode of carbon cycling and stable temperatures. We conclude that biocalcification is mostly triggered (and inhibited) by CO2 conditions in the atmosphere–ocean system.