Early–Middle Ordovician biotic and sedimentary dynamics in the Baltoscandian paleobasin

Abstract: The Baltoscandian region forms part of the paleocontinent Baltica, which was largely covered by a shallow epeiric sea throughout much of the Ordovician (c. 485.5–444 Ma). This ancient sea is today recorded by a thin succession of sedimentary rocks. During the Early–Middle Ordovician (c. 485.5–457.5 Ma), Baltica was situated in mid-latitudes on the southern hemisphere and cool-water carbonates formed across large areas of the Baltoscandian paleobasin. The so-called orthoceratite limestone is the most widely distributed rock type from the Early–Middle Ordovician in Sweden. It developed in a time-transgressive manner geographically, but much of the Lower–Middle Ordovician succession in the mainland of Sweden is typically characterized by this lithology. The depositional environment of the ‘orthoceratite limestone’ has long remained poorly understood in many respects. This is in large part due to a lack of analogous depositional environments in modern seas, but also due to remaining gaps in our knowledge about the rock type in general. It has generally been agreed upon that the ‘orthoceratite limestone’ is a cool-water deposit formed in a sediment-starved epeiric sea, but interpretations have differed widely with regards to prevailing water depth.The eight papers appended to this doctoral dissertation are based on various investigations of the ‘orthoceratite limestone’ in Sweden and coeval rocks in surrounding countries. Detailed macroscopic and microscopic studies of the biotic and sedimentary characteristics have added information about the Baltoscandian paleobasin and the biotic and paleoenvironmental development during the Early–Middle Ordovician. It is concluded that the depositional environment of the ‘orthoceratite limestone’ varied considerably through both space and time; it spanned from intertidal areas to settings many tens of meters deep. Variations inthe overall characteristics and fossil content of the ‘orthoceratite limestone’ and coeval regional rocks appear to mainly record variations in (relative) sea level.The collective results indicate that sea level varied significantly throughout the Early–Middle Ordovician, likely in large part as a response to variations in climate and related changes in global marine water volume. The inferred variations are consistent across multiple different proxies – abiotic and biotic alike – and cyclic patterns occur in the datasets. Geochemical data suggest that the global climate changed considerably during the Early–Middle Ordovician, and that the climate ultimately entered an ‘Icehouse’-like state. The onset of the latter phase is recorded as a distinct drop in sea level during the Middle Ordovician. The aforementioned changes reverberated through the marine realm and likely contributed to the rapid diversification that is seen among fossils during the so-called Great Ordovician Biodiversification Event (GOBE). Based on a refined absolute and relative time scale for the Middle Ordovician, the GOBE can be confidently shown to be unrelated to a prolonged meteorite bombardment that occurred during this time interval.The papers in the dissertation collectively show that a combination of approaches and analytic techniques leads to maximal information output and confidence in interpretations. The use of state-of-the-art analytic and imaging techniques further allows for the discovery of previously undocumented rock characteristics and fossils, and better description and understanding of such documented before.