Middle and Upper Ordovician carbon isotope chemostratigraphy in Baltoscandia: a correlation standard and clues to environmental history

The Baltoscandian epeiric sea was characterised by relatively slow but continuous carbonate deposition from the Middle Ordovician up to the Silurian. Relatively good preservation of Ordovician and Silurian carbonate rocks in Baltoscandia makes this region unique for chemostratigraphical and palaeoenvironmental studies. Previous Palaeozoic carbon isotope studies have revealed isotopic events in different lithologies over the Baltoscandian palaeobasin and some of them on different palaeocontinents. We propose a composite carbon isotope curve for the Baltoscandian region, a new subdivision of the Middle and Upper Ordovician succession into seventeen chemostratigraphic zones, and an improved nomenclature of isotopic excursions, based on analyses of δ13Ccarb data from 25 drillcore and outcrop sections of Estonia, Latvia and Sweden. The composite carbon isotope curve is compared with biostratigraphical data on conodont and chitinozoan distribution. The Katian–Hirnantian interval with highly fluctuating isotopic values shows climatic cyclicity of about two million years in Baltoscandia. No abrupt changes occur in the Dapingian–Sandbian part of the Baltoscandian isotope curve, but the mid-Darriwilian Excursion nearly coincides with the onset of cooler temperatures similar to modern-like equatorial sea surface temperatures and related global biodiversification increase, suggesting a climatic background for this event.