Changes in the Early Holocene lacustrine environment inferred from the subfossil ostracod record in the Varangu section, northern Estonia

The Varangu section is located on the southern slope of the Pandivere Upland in northern Estonia. A silty clay bed formed in the study area at 11 ;200–9300 ;cal ;yr ;BP, according to ostracod subfossils (e.g. Tonnacypris estonica, Leucocythere mirabilis, Limnocytherina sanctipatricii) in an oxygen-rich cool and oligotrophic profundal lacustrine environment, with an inflow of surface waters through springs. The record of specific ostracods (e.g. Cyclocypris ovum, Cypridopsis vidua, Metacypris cordata) reflects littoral environments, ongoing eutrophication, temperature increase and a progressive shallowing of the lake in the early Holocene (9300–7400 ;cal ;yr ;BP) when the tufa bed accumulated. A slight cooling and productivity decrease at 9100–8600 ;cal ;yr ;BP preceded further temperature rise and water level lowering, leading to the development of a eutrophic lake and cease of ; ;tufa precipitation (8600–7400 ;cal ;yr ;BP)

Age of the Ordovician sedimentary succession in Lumparn Bay, Åland Islands, Finland

Depression of the ancient Lumparn meteorite impact structure in the Ãland Islands is partly infilled with the lower Palaeozoic sediments, lying presently below sea level. The Cambrian and Ordovician sedimentary cover is distributed in the area of 15 km2, with a total thickness of up to 70 m. The Ordovician carbonate rocks of this site, known by the erratic boulders and by the drill­ing project in the late 1950s, are particularly interesting because of the isolated distant position of this outlier from other distribution areas of the Ordovician sediments preserved in the Baltoscandian Palaeobasin. Ordovician sections in the Lumparn Bay are com­posed of two distinct lithological units. The older, argillaceous wackestone-packstone unit has been tradi­tionally named âOrthoceras limestoneâ or âOrthoceratite Limestoneâ, and the younger micritic limestone unit is addressed as âÃstersjö Limestoneâ (or âBaltic Limestoneâ). Here we present new data on ostracod biostratigraphy and stable carbon isotope chemo­stratigraphy derived from three old drillcores from the Tranvik area of the Lumparn Bay. The comparison of ostracod distribution with its succession in Estonia generally supports the previous Darriwilian and Sandbian age interpretations for the Orthoceras Limestone. Considering the ostracod distribution together with earlier acritarch and conodont datings, stable carbon isotopic data and limestone lithology, the Orthoceras Limestone unit might correspond to the Kunda, Aseri, Lasnamägi, Uhaku, Kukruse, Haljala, and Keila regional stages (RSs). The Ãsterjö Limestone comprises almost the entire Katian, corresponding to the Rakvere, Nabala, Vormsi and Pirgu RSs in the Baltoscandian stratigraphic chart. The uppermost sample yielded the most abundant and diverse assemblage of ostracods that suggests it to correspond to the Vormsi or lower-middle Pirgu (late Katian) RSs. The global Middle Darriwilian Carbon Isotopic Excursion (MDICE) and Guttenberg Carbon Isotopic Excursion (GICE) as well as the Katian Rakvere and Saunja carbon isotopic excursions are recorded in the studied succession. The δ13Ccarb bulk rock curve of the Orthoceras Limestone shows a positive excursion up to 1â° in the lower part of the interval followed by a negative excursion up to â1â° in the middle part and another positive excursion reaching 2â° in the upper part of the interval. This curve resembles well the carbon isotope curve from the Darriwilian and Sandbian in the core sections of the Hiiumaa Island, NW Estonia, and could be interpreted as the MDICE and the rising limb of the GICE. The δ13C curve of the Ãstersjö Limestone interval shows two positive excursions. The lower one comprises most likely the peak of the Rakvere Excursion (Rakvere RS) in Estonia, which is also supported by the ostracod data. The upper carbon isotopic excursion may represent the Saunja Excursion correlated with the upper part of the Nabala RS. The ostracod biofacies characterized by faunal associations of both the Orthoceras and Ãstersjö limestone units, and the lithological succession of these units suggest more similarities with the Estonian Shelf facies (North Estonian Confacies) than Scandinavian Basin facies of the Baltoscandian Palaeobasin

Searching for the Ordovician–Silurian boundary in Estonia, Latvia and Lithuania

The present study focuses on determining the position of the lower boundary of the Silurian System in the eastern Baltic region. To achieve this, we conducted a comprehensive analysis of stable isotopic curves, combined with previously published data on the graptolite record. Our isotopic correlations are primarily based on the δ13Corg curve of the Dobâs Linn section, the GSSP of the Silurian System, and the δ13Ccarb curve of the Monitor Range section in Nevada. Our results provide robust evidence for correlating the basal Varbola Formation, the lower part of the Ãhne Formation and the StaÄiÅ«nai Formation, and suggest their latest Ordovician age. The integration of stable isotopic data and graptolite records allows for a more accurate characterisation of the OrdovicianâSilurian boundary in this region

Towards a revised Sandbian conodont biozonation of Baltica

One of the key objectives of the studies proposed by the Ordovician Subcommission is the improvement of regional stratigraphy for further advancements in global correlation. The results of this work can be found in numerous updates and reviews published in the recent Geological Society, London, Special Publications âA Global Synthesis of the Ordovician Systemâ. Several of these papers refer to the Baltic Ordovician conodont biozones. While different schemes share many common features, their correlation with stages and biozones differ in detail. Considering the recent developments in the studies of the Sandbian stratigraphy in the Baltoscandian region, it is possible to complement the current conodont biozonation. Pygodus anserinus, Amorphognathus tvaerensis and A. superbus conodont zones are recognized in the Sandbian Stage in Baltica. The lower boundary of the stage is located within the Pygodus anserinus Zone, and the main part of the stage corresponds to the Amorphognathus tvaerensis Zone. In Scandinavia, the upper boundary of the stage correlates with a level within the A. superbus Zone or lies in a conodont-poor interval in the eastern Baltic region. The lower boundary of the A. inaequalisSubzone is tentatively correlated with the base of the Sandbian in Scandinavia but is located in the uppermost Darriwilian in the eastern Baltic region. In both areas, the A. tvaerensis Zone is subdivided into the Baltoniodus variabilis, B. gerdae and B. alobatus conodont subzones. The A. inaequalis Subzone â in some cases indicated as a zone â has been included in the regional stratigraphic charts for more than a decade. However, so far, A. inaequalis (Rhodes) has been reliably identified and also illustrated only from Avalonia, i.e. outside the palaeocontinent Baltica. A recent restudy of collections from the Fjäcka main section and the Smedsby GÃ¥rd drillcore (both from Sweden), as well as from several Estonian sections, did not prove the occurrence of A. inaequalisin these areas. The A. tvaerensis Zone comprises almost the whole Sandbian, both in Scandinavia and the eastern Baltic areas. During this long age, the morphology of the P and M elements of A. tvaerensis (Bergström) gradually changed, and elements of distinct morphology appeared and were assigned to a new species, Amorphognathus viirae Paiste, Männik et Meidla, 2022, in the upper part of the range of the species. In succession, A. viirae appears in the upper part of the B. gerdae Subzone. Cur­rently, A. viirae has been identified in numerous Estonian sections, as well as in the Fjäcka main section and the Smedsby GÃ¥rd drillcore in Sweden. Based on published figures, it occurs evidently in the Mójcza Formation of the Holy Cross Mountains (Poland) and the Black Knob Ridge section in Oklahoma (USA), in the GSSP for the base of the Katian Stage. A. inaequalis has also been reported and an eponymous zone identified in two other sections located on the palaeocontinent Baltica, in the Bliudziai-150 drillcore (Lithuania) and the Kovel-1 drillcore (Ukraine). During the restudy of collections from these sections, no elements of A. inaequalis were found in either of them. The earliest recorded elements of the genus Amorphognathus have been assigned to A. tvaerensis, and the specimens from the upper range of this species reidentified as A. viirae. Analysis of the new material and revision of previous collections has revealed problems related to the Sandbian conodont biozonation of the palaeocontinent Baltica and demonstrated the need for its updating. The new proposed zonation excludes the A. inaequalis Subzone, as the occurrence of its nominal taxon on the palaeocontinent Baltica could not be proved. Additionally, a new unit, the A. viirae Zone, has been included in the zonation. It corresponds to the upper part of the former B. gerdae Subzone, which is now treated as a zone. Also, all subzones based on the succession of Baltoniodus species have been elevated to the rank of zones

Recent advances in the Ordovician stratigraphy of the Baltic Palaeobasin and Tornquist margin of Baltica

Ordovician rocks are widely distributed in the Baltoscandian region as well as in Poland, Belarus, Ukraine and Moldova. The Ordovician studies in this area were initiated in the 19th century in the outcrop belt in northern Estonia. The strata are well accessible here and fossils and sedimentary structures are excellently preserved. Further south, in other countries, the succession is lying progressively deeper (up to 2500 m) in the subsurface, except for limited exposure in Ukraine. The thickness of the Ordovician within the Estonian outcrop area reaches about 100 m but exceeds 200 m in several parts of the subsurface area. Since the 1960s, several Ordovician correlation charts have been compiled for this area. Recent developments in the stratigraphy are summarised in the volume âA Global Synthesis of the Ordovician System: Part 1â (Geological Society, London, Special Publication, 532). The system of bio-, litho- and chronostratigraphic units is highly detailed in the area. The regional stages defined in Estonia were introduced for the western part of the East European Platform in the 1980s. The correlation of the regional succession to the global stratigraphic standard is generally well constrained, although it still needs to be refined in some details. A novel element of the stratigraphic standard, the isotopic zones, is based on secular variations of stable carbon isotopic composition of bulk carbonates and allows amendments to the correlation of strata. The application of a regular timescale is based on a well-dated system of biostratigraphic marker levels that were traced into the Baltic Palaeobasin and further to the south. The dated boundaries were tied to the regional succession mainly based on the correlation of conodont, chitinozoan and graptolite zones, but also using chemostratigraphic events. Correlation of formations to the chronostratigraphic standard in ten subregions (North and Central Estonia, South Estonia together with West Latvia and West Lithuania, Kaliningrad Region, East Latvia, Central Lithuania, East Lithuania together with northwestern Belarus, southwestern Belarus, West Volyn and Podillya together with East Volyn and Moldova) is summarised in an emended correlation chart. Development of the subregional correlation charts was well coordinated before the 1990s and the charts were based on a unified nomenclature of lithostratigraphic units for major facies zones that crossed the national borders. Trends in the development of nomenclature and correlation of formations have been different in different countries after 1991. This resulted in increasing differences in nomenclature and rank of lithostratigraphic units in subregions and led to an increase of the number of subregions. The development towards a more detailed stratigraphic classification in Lithuania has elevated the rank of many former units (several formations are now ranked as superformations, etc.). In 2011, a completely new system of formations and members replaced the formerly applied standard in the Kaliningrad Region. The climatic history of the region presented in papers of the last decades is modified in the light of the newest results of isotope-geochemical studies on Baltoscandian sections, which do not support the idea of gradual warming throughout the Middle and Late Ordovician in the region. The global cooling trend was also influencing Baltica despite the continental drift towards the lower latitudes. The richly fossiliferous regional succession has been extensively studied, but analyses with a broader view have been sparse. According to the general understanding, backed by data on different fossil groups, the main origination episodes in the early Darriwilian and DarriwilianâSandbian transition led to the peak of regional diversity in the early Sandbian. Remarkable extinction events known from the early Darriwilian, early Sandbian and early Katian are expressed to a different degree in different fossil groups. The major extinction event in the latest KatianâHirnantian, which impacted all major invertebrate groups, has been ascribed to the Hirnantian glaciation, the related glacioeustatic sea-level fall and the repeated rapid rearrangement of facies. A recovery that started in the latest Ordovician was relatively slow. Significant spatial differences in the dynamics of biodiversity within the eastern Baltic area and between this area and Scandinavia are considered partly due to uneven data coverage

The Ordovician System: From overlapping unit stratotypes to Global Boundary Stratotype Sections and Points

For nearly a century the Ordovician System was hidden as Murchison and Sedgwick tussled over the overlapping ground between their Silurian and Cambrian systems. The Ordovician is, in fact, one of the longest of the geological periods, characterised by major magmatic and plate tectonic activity; the roles of microcontinents and volcanic archipelagos were significant in shaping the Ordovician planet and the evolution of its biotas, associated with an immense biodiversification, significant fluctuations in climate and sea levels, and the first Phanerozoic mass extinction of marine invertebrates. The period was unique in being thalassocratic; epicontinental seas had a wider reach than during any other geological period. The land areas were restricted to isolated microblocks of archipelagos of various sizes with low relief, with rivers traversing gentle gradients, carrying sparse terrigenous material seaward. It is an ancient world with few parallels elsewhere in the Phanerozoic, and little in common with Holocene ecosystems and environments. The Ordovician System was introduced by Charles Lapworth as a solution to the stratotypes of overlapping units loosely defined by Adam Sedgwick for the Cambrian and by Roderick Murchison for the Silurian. Following a period of intensive research into all the key regions of the globe, unit stratotypes in the type areas of England and Wales have been replaced by seven global stages and three series based on Global Boundary Stratotype Sections and Points, enhancing the definition of these chronostratigraphic units and facilitating global correlation. As a consequence, the biological and geological events during the period can be recognised, and the magnitude and significance of originations and extinctions understood. A global synthesis of successions in Europe (Geological Society, London, Special Publications, 532) and the rest of the world (Geological Society, London, Special Publications, 533) has emphasised the importance of a universal language for Ordovician chronostratigraphy and its dividends

Lower Silurian biostratigraphy of the Viirelaid core, western Estonia

The distribution of five groups of fossils in the upper Llandovery (Telychian) and Wenlock of the Viirelaid core section, Estonia, is presented and discussed in terms of their biozones (conodonts and chitinozoans) and mutual positions (scolecodonts, ostracods, and brachiopods). Graphical correlation of the Viirelaid and Paatsalu sections shows a stronger linear correlation for zonal conodonts than for chitinozoans and scolecodonts. In the given case, this is caused by different nature of zones: chitinozoan zones are based on appearances and/or disappearances of ordinal taxa whereas conodont zonation corresponds, as a rule, to evolutionary succeeding species in the Pterospathodus lineage. At the same time, the positions of chitinozoan zones in the Viirelaid, Ruhnu, Aizpute, and Paatsalu cores are oscillating with respect to conodont zones

Thallophytic algal flora from a new Silurian Lagerstätte

Algae have always been the most diverse and numerous eukaryotic autotrophs in marine ecosystems. Both fossil andmolecular data point to an early Proterozoic origin for algae. Although known for their long evolutionary history, the fossil recordof algae is sparse and mainly remains of taxa with heavily calcified thalli are preserved. However, fossils of noncalcareous algae,known as extremely sporadic and occasional finds from different parts of the world, provide important insights into thepalaeobiology of algal flora in the Earth's past. Here we describe highly diverse flora of noncalcareous thallophytic algae from aSilurian age deposit

Stratigraphic age of the Ordovician sedimentary succession in Lumparn Bay, Åland Islands, Finland

The depression of the ancient Lumparn meteorite impact structure is partly infilled by Cambrian and Ordovician sediments, lying nowadays below the seawater. The Ordovician carbonate succession at this site, recognised through erratic boulders and by the drilling project in the late 1950s, is of particular interest due to its isolated and distant location from other areas with Ordovician sediments in the Baltoscandian Palaeobasin. In this study, we present new data on ostracod biostratigraphy and stable carbon isotope chemostratigraphy obtained from three old drillcores. Comparison of ostracod distribution with sections in Estonia generally supports the previous interpretations of a Darriwilian and Sandbian age for the lowermost Tranvik Limestone (which we propose renaming the “orthoceratite limestone” in the Åland Islands area). The uppermost Östersjö Limestone spans almost the entire Katian age. We also observe the global MDICE and GICE carbon isotopic excursions, as well as Katian Rakvere and Saunja excursions, within the Lumparn succession. The Ordovician succession in Lumparn Bay is facially resembling the Estonian Shelf facies of the Baltoscandian Palaeobasin.</p