Climate changes in the Middle and Late Ordovician of the tropical belt: The δ18O record from the Tunguska Basin of Siberia and its palaeogeographical relation

Oxygen isotope studies on Darriwilian to Katian conodonts were performed from two sections of the south-western Tunguska Basin (Siberia). The new δ18Ophos records are compared with data from drill cores in Estonia, located in the Baltoscandian Basin. From this epicontinental basin on Baltica, it has been shown that, besides the general cooling trend in the pre-Hirnantian, the Late Ordovician climate was unstable. Major cooling events are reflected by sea-level lowstands, which are also recognized in the Tunguska Basin. The new δ18Ophos records from Siberia show differences but also similarities to the δ18Ophos trends from Baltica. In contrast to the general cooling trend characterizing the Upper Ordovician on Baltica, δ18Ophos data from the Tunguska Basin suggest that, beside prominent cooling events recorded as positive excursions in the δ18O curves from both palaeocontinents, the sea surface temperature stayed relatively stable on Siberia. Several of the cooling episodes (CE's) known from this time interval on Baltica were identified in the succession of Siberia: the Early Nabala CE is observed in the upper Mangazea Formation, and the Vormsi CE in the uppermost part of the formation, the Haljala and Keila cooling episodes probably correspond to the intervals in the lower Mangazea Formation, and the Early Pirgu CE to a small δ18Ophos peak in the lowermost Dolbor Formation. The most pronounced cooling episode on Siberia is recorded in the upper Darriwilian, within the lower Ust'Stolbovaya Formation, and reflects the middle Darriwilian glaciation. The recognition of the same cooling episodes on two palaeocontinents provides clear evidence that they represent global climate changes but also contributes to an improvement in the dating of the corresponding Siberian strata and of their intercontinental correlation. The offset of δ18Ophos records between Baltica and Siberia can be tracked back to oceanic circulation patterns where cold-water currents in the southern hemisphere bring colder waters northwards into the Baltoscandian Basin at the western side of Baltica while Siberia remained stable in the southern subtropics affected only by tropical warm water currents.