Linda Hints

Global carbon isotopic events associated with mass extinction and glaciation in the late Ordovician

Mass extinctions and glacioeustatic sea-level changes in the lower part of the Hirnantian (final stage of the Ashgill) are accompanied by shifts in marine stable-isotope compositions. Previously published stable-isotope changes have been used to identify the onset and demise of the Gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans. However, the existing isotopic data set had limitations because it was derived from Ordovician low-latitude settings and from carbonates or organic carbon in separate areas. We report new data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate δ13C from Baltica. Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated δ13C values similar to those described previously from low-latitude sites. The new data demonstrate that the positive Hirnantian δ13C excursion, previously only recognised from low-palaeolatitude areas, was widespread and probably global in extent. The poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material. Preliminary carbon isotopic data from thermally immature organic matter from Estonia provide the first indication of a synchronous shift in organic and inorganic δ13C in sediments from the same basin. This work provides new data of critical importance for constraining models of end-Ordovician palaeoceanography and climate change.

Ordovician reef and mound evolution: the Baltoscandian picture

The widespread growth of reefs formed by a framework of biogenic constructors and frame-lacking carbonate mounds began on Baltica during Ordovician time. Previously, Ordovician reef and mound development on Baltica was considered to be sporadic and local. A review of all known bioherm localities across the Baltic Basin reveals a more consistent pattern. Ordovician bioherms grew in a wide E–W-aligned belt across the Baltic Basin and occur in several places in Norway. Substantial reef development began simultaneously across the region during the late Sandbian – early Katian interval and climaxed during the late Katian Pirgu age. The current spatiotemporal distribution of bioherms is a result of interdependent factors that involve original drivers of reef development such as relative sea level, climate during the time of deposition and effects of post-depositional erosion. Oceanographic conditions were likely more favourable during times of cooler global climates, low sea level and glacial episodes. At the same time, the likelihood that bioherms are preserved from long-term erosion is higher when deposited during low sea level in deeper parts of the basin. A main factor controlling the timing of the reef and mound evolution was Balticas shift toward palaeotropical latitudes during Late Ordovician time. The time equivalence between initial reef growth and the Guttenberg isotope carbon excursion (GICE) suggests that global climatic conditions were important.

The Hirnantian (Late Ordovician) brachiopod fauna of the East Baltic: Taxonomy of the key species

Rhynchonelliformean brachiopods, belonging to 17 genera, are described from the East Baltic Porkuni Regional Stage, correlated with the global Hirnantian Stage. The brachiopod genera Paromalomena, Proboscizambon?, Kinnella, Drabovia, and Coolinia, which are described from the region for the first time demonstrate together with characteristic Hirnantian species of the genera Hirnantia, Dalmanella, Plectothyrella, Eostropheodonta, and Hindella a greater than previously thought commonality of the Baltic fauna with the terminal Ordovician Hirnantia brachiopod fauna of the Kosov Province. The samples containing brachiopods were collected from 43 drill core sections in Central East Baltic. The study area belongs to the Livonian Tongue of the Central Baltoscandian Facies Belt in the Baltic Basin. The brachiopods occur mainly in the skeletal and silty limestone of the Kuldiga Formation within the lower half of the Porkuni Stage. Few brachiopods are known from the sandy or oolitic limestone of the Saldus Formation in the upper part of the stage. Due to excellent preservation some brachiopod species (e.g., Cliftonia psittacina and Dalmanella testudinaria) yield key morphological information, relevant to their classification and phylogeny. This taxonomic study of the East Baltic brachiopods presents essential groundwork for analysis in progress on the distribution and onshore-offshore successions of the Hirnantia brachiopod fauna within both a Baltoscandian and global context.

Discovery of the Ordovician Kinnekulle K-bentonite at the Põõsaspea cliff, NW Estonia

A previously unknown outcrop of the Kinnekulle K-bentonite (metabentonite) is reported from the Põõsaspea cliff, NW Estonia. The bed has a sharp lower and a gradational upper contact and comprises ca 28 cm of clay overlain by ca 10 cm of hard K-feldspar-rich variety. The latter contains a layer of breccia, which indicates early onset of recrystallization and hardening of volcanic material. The discovery shows that the Põõsaspea cliff section is younger than previously thought and includes rocks of both Haljala and Keila stages.