Palaeoclimate perturbations before the Sheinwoodian glaciation: A trigger for extinctions during the ‘Ireviken Event’

Telychian to Sheinwoodian conodonts (Pterospathodus eopennatus ssp. n. 1 Zone through the Ozarkodina sagitta rhenana Superzone) were studied in detail for their oxygen isotope composition. The Upper Kockelella ranuliformis and Ozarkodina sagitta rhenana Zones represent the peak interval of the Sheinwoodian positive δ18Oapatite excursion reflecting the coolest conditions with marine sea–water temperatures decreased by more than 6 °C in the subtropics. The δ18Oapatite data cover pre- through post-‘Ireviken Event’ strata. The study includes also material from younger levels up to the late Wenlock Ctenognathodus murchisoni Zone. However this record is incomplete with one major hiatus reflecting the regression associated with the Middle Silurian Mulde Event. The stratigraphic levels of the analyzed conodont samples from Estonia can be correlated with high precision to the datum points of the ‘Ireviken Event’ strata in the Lower and Upper Visby formations on Gotland, Sweden. Inferred short-term climatic changes in the upper Telychian part of the studied interval may have been caused by phases of major volcanic activity along the Caledonian front. Across the ‘Ireviken Event’ we observe variations in oxygen isotope ratios which allow a direct comparison to the stepwise extinctions and faunal reorganisations in different groups. Our data suggest that faunal extinctions are connected to time intervals of warming before the establishment of more stable and cooler conditions during the main Sheinwoodian glacial. The most severe extinctions and faunal turnovers at datum points 2 and 4 occurred during the early, warmer interval of the event. After the shift into the Sheinwoodian icehouse in the Lower Kockelella ranuliformis Zone, reflected by rapidly decreasing sea–water temperatures, faunas restabilized and reefal communities started to flourish. Sedimentary evidence for the glaciation is discussed for different palaeolatitudes.