Hirnantian sea-level changes in the Baltoscandian Basin, a review

Correlation of Hirnantian sections of the Baltoscandian Basin reveals four major transgressive-regressive sea-level cycles related to waxing and waning of ice sheets at high Southern Hemisphere paleolatitudes. The correlation between stratigraphic members in Estonia and Latvia shows transgressions-regressions in the first half of the Hirnantian, when sea level fluctuated between shallow and deep shelves. During the second half of the Hirnantian, the sea regressed into the former deep shelf, leaving North Estonia as a mainland. After that, the sea retreated to the Central Baltoscandian Basin and the whole Baltic Shelf stayed subaerial. This final regression brought the coastline to the vicinity of Scania in the west and of Lublin Basin in the east. In the latest Hirnantian, during ice-cap melting, sea level began to ascend. The Baltic shelf submerged again in the Early Silurian. This transgression of latest Hirnantian–earliest Silurian indicates fast melting of ice in Gondwanan ice sheets. We are proposing that the destabilisation of great amount of gas hydrates due to diminishing of water column pressure during the last deepest regression acted as a trigger for rapid warming of climate. The ascertainment of sea-level changes presumes correlations between different parts of the Baltoscandian Basin. Comparison of published data on stratigraphy, lithology, faunal finds, chemical elements and stable carbon isotopes was the ground for tracking Hirnantian changes.