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In the Fennoscandian Shield the fluctuations in arid–humid climate conditions are inferred from the Al2O3/TiO2 ratio of the clay fraction of Estonian carbonate rocks of the Baltoscandian sedimentary Basin. The Late Ordovician climate of the Fennoscandian Shield was mainly semi-humid with several humid pulses in the Sandbian, mid-Katian and Hirnantian. The post-Hirnantian climate was fluctuating between humid and arid until stable humid conditions were established in the Middle and early Late Llandovery. Starting from the latest Llandovery, the climate turned semi-arid and arid. The Ordovician–Silurian long-term trend toward aridity was caused by the equator-ward drift of the Baltica plate. Shorter-term fluctuations of Al2O3/TiO2 reflect climate changes of the global extent.

The occurrence of black shale in deep shelf environments, chertification in shoreward carbonate facies, barite occurrences, positive δ 13 C excursions in carbonate sections and enhanced micritic limestone accumulation give evidence of a primary bioproductivity rise suggesting upwelling in the Aeronian of the East Baltic. The end-Rhuddanian onset of the rise in primary bioproductivity corresponds to the beginning of black shale formation on the deep shelf and micritic limestone formation on the shallower shelf. The bioproductivity maximum was in the mid-Aeronian, judging from the highest values of δ 13 C. Upwelling ceased and primary productivity decreased at the end of Aeronian.