The Cretaceous period is generally considered to have been a time of warm climate. Evidence for cooler episodes exists, particularly in the early Cretaceous period, but the timing and significance of these cool episodes are not well constrained. The seasonality of temperatures is important for constraining equator-to-pole temperature gradients and may indicate the presence of polar ice sheets; however, reconstructions of Cretaceous sea surface temperatures are predominantly based on the oxygen isotopic composition of planktonic foraminifera that do not provide information about such intra-annual variations. Here we present intra-shell variations in delta18O values of rudist bivalves (Hippuritoidea) from palaeolatitudes between 8 degrees and 31 degrees N, which record the evolution of the seasonality of Cretaceous sea surface temperatures in detail. We find high maximum temperatures (approximately 35 to 37 degrees C) and relatively low seasonal variability (< 12 degrees C) between 20 degrees and 30 degrees N during the warmer Cretaceous episodes. In contrast, during the cooler episodes our data show seasonal sea surface temperature variability of up to 18 degrees C near 25 degrees N, comparable to the range found today. Such a large seasonal variability is compatible with the existence of polar ice sheets.

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A biostratigraphic correlation of the Devonian/Carboniferous (D/C) boundary sections from the Carnic Alps, the Graz Palaeozoic, the Montagne Noire and the Pyrenees resulted in a high-resolution record of the carbon isotopic composition of micrites (d 13 C carb ), of sedimentary organic matter (d 13 C org ) and of oxygen isotope ratios of conodont apatite (d 18 O phosph ). The studies focused on the interval between the Upper postera Zone (Late Famennian) and the sandbergi Zone (Lower Tournaisian). For the first time, weak but significant positive carbon isotope excursions in micrites and in the sedimentary organic matter is reported from the Middle and Upper expansa zones of the Carnic Alps. They coincide with a decrease in the oxygen isotope values of conodont apatite. The excursions indicate changes in the global carbon cycle during an episode of high seawater temperatures, and correlate with sedimentary change and a stepwise eustatic rise in the Rhenish Massif. High carbon isotope values were also measured in limestones from the Graz Palaeozoic in the Upper praesulcata Zone, which were previously reported from the Rhenish Massif, Carnic Alps, Montagne Noire and the North America continent. The change from a palmatolepid-polygnathid conodont biofacies to a palmatolepid-bispathodid-branmehlid biofacies in the expansa Zone in the Carnic Alps is obviously influenced by anoxic conditions and repeated transgressive phases. The protognathodids and the polygnathids start to radiate in the Upper praesulcata Zone, after the main end-Famennian extinction episode. This is connected with the disappearance of the palmatolepids, and environmental stress created by worldwide anoxic conditions, climate change and sea-level changes, stimulated the radiation of both protognathodids and the polygnathids. The regional correlation of the geochemical records as well as interpretations of these records on a global scale indicate that changes in conodont biofacies of Late Famennian-Early Tournaisian limestones were caused by a complex pattern of environmental changes.

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