Viive Viira

Implications of Gondwana glaciations in the Baltic late Ordovician and Silurian and a carbon isotopic test of environmental cyclicity

Four glaciations -- the extensively studied Hirnantian episode and three in the Llandovery-earliest Wenlock -- are well established on the Gondwana palaeocontinent. New data [Hamoumi, 1999] shift the beginning of the glacial epoch to the early Caradoc when Baltica was moving from middle to low latitudes of the southern Hemisphere. Despite the rather considerable distance between polar areas of Gondwana and subtropical Baltica all four glacial events are reflected via global climatic mechanisms in the East Baltic. It is generally accepted that glaciations are marked by positive excursions of {delta}18O and {delta}13C values caused by increase of the polar ice caps, bioproduction and decrease of oceanic water temperature, etc. Based on these relationships, the Gondwana glacial events are correlated with coeval isotopic shifts established in sections of Baltica. In addition, agreement of the oceanic processes and corresponding carbon isotopic trends predicted by Jeppsson [1990] to real measured values is analysed. The following positive {delta}13C excursions are recorded in the Baltic area (peak values in parentheses) : middle Caradoc (2.2 {per thousand}), early Ashgill (2.5 {per thousand}), Hirnantian (6 {per thousand}), early Aeronian (3.7 {per thousand}), early Telychian (2.7 {per thousand}), early Wenlock (5.2 {per thousand}). Most of these shifts correlate well with glacio-eustatic sea level lowstands and biodiversity changes, as shown by the most extensive Oandu crisis in the Caradoc, Hirnantian mass extinction and the Wenlock Ireviken Event. Analysis of data allows the following conclusions : (1) all four Gondwana glaciations identified by tillites, microconglomeratic clays, etc. and dated biostratigraphically are recognised in the Baltic area through clear positive carbon isotope excursions at the same levels; (2) three smaller carbon isotope excursions in the Caradoc and Ashgill together with algal abundance data suggest the presence of several colder climate episodes during the late Ordovician. This may support the idea of the earlier onset of the glacial epoch on Gondwana; however, correct biostratigraphic dating of supposed glacial sediments is required; (3) the carbon isotopic testing of the oceanic model by Jeppsson reveals too many contradictions between model predictions and measured values. This means that the environmental background of isotopic events and relationships with oceanic events should be revised ; (4) for delimitation of the climatic --oceanic episodes, a more general marker identifying environmental change via a basinal approach seems useful. For this purpose lithological, geochemical or palaeontological criteria can be used.

An Early to Middle Ordovician succession of conodont faunas at Mäekalda, northern Estonia

The Maekalda section presents a c. 12 m thick succession of mainly limestones, dolomites and siltstones of late Tremadoc to late Llanvirn age, exposed on the eastern outskirts of Tallinn, northern Estonia. The condensed sequence is rich in well-preserved conodont elements that provide the basis for detailed biozonation. In the Baltoscandian zonation, beds from the upper Varangu Paltodus deltifer Zone up to the lower Aseri Eoplacognathus foliaceus Subzone have been identified. In all, the conodont faunas exhibit a close resemblance to Swedish faunas. Many intervals can also be compared with coeval strata in western Russia. The presence at some levels of representatives of Laurentian conodont lineages provides an unusual opportunity for correlating Baltoscandian sequences with sequences in North America, especially in the lower Arenig part of the succession. The middle Volkhov (upper middle Arenig) Paroistodus originalis Zone at Maekalda is remarkably complete bearing in mind the highly condensed nature of the section as a whole. The details of this zone can be correlated with the equivalent intervals in Swedish sections. In the upper Volkhov–lower Kunda beds there is a considerable stratigraphic gap in the section, but the upper Kunda has provided an excellent series of successive populations of Eoplacognathus , indicating that in this part the section is virtually complete.

Cambrian–Ordovician Boundary beds at Tônismägi, Tallinn, North Estonia

The Cambrian–Ordovician Boundary beds at the Tonismagi section in Tallinn, North Estonia, are represented by siltstones and sandstones with interlayers of clay and kerogenous argillites. In this section, three conodont zones – Cordylodus proavus, C. intermedius and C. rotundatus-angulatus – were established. Graptolites of the Rhabdinopora flabelliforme group were found in the upper part of the C. proavus Zone. This section demonstrates that in suitable facies, species of Rhabdinopora may appear earlier than usually considered.

New data on Ordovician stable isotope record and conodont biostratigraphy from the Viki reference drill core, Saaremaa Island, western Estonia

The Viki drill core is one of the reference sections for the Estonian bedrock succession, characterizing strata of Mid Ordovician through early Silurian age in the eastern part of the Baltoscandian basin. In this article, we present the hitherto missing Ordovician stable carbon isotope curve and Middle Ordovician conodont biostratigraphy from the Viki core to complement the previous lithological, geochemical, geophysical and biostratigraphic studies. For the first time, the Hirnantian sulfur isotope excursion is reported from the eastern Baltic region, showing high amplitude and tight coupling with carbon cycling.

Conodont biostratigraphy and sedimentary history in the upper Tremadoc at Uuga, Cape Pakri, NW Estonia

Abstract The upper Tremadocian boundary beds at Cape Pakri, NW Estonia, consist of an extremely friable glauconitic sandstone, which presents a challenge to detailed biostratigraphy. A combination of sedimentological and biostratigraphical criteria has served to clarify the tempo and mode of the processes that formed the sandstone and explain its relationships to strata immediately below and above it. Apatitic conodont elements, which abound in all these sediments, are particularly well suited to tracing the geological history of the surrounding sediment, since they can be repeatedly included in the sediment, eroded and redeposited, often leaving telltale marks on the elements which are nevertheless identifiable. By separating the indigenous elements from those that had been redeposited, we could place the local upper boundary of the Tremadocian at slightly more than 1 m above the base of the c. 4 m-thick sandy deposit. We showed that the sandstone, where 58-97% of the conodont elements have been redeposited, had been formed during four successive phases of sand deposition. The entire sandstone unit belongs to the Paroistodus proteus Zone. In the sandy and clayey Varanguan beds of the Paltodus deltifer Zone that underlie the sandstone, less than 50% of the conodont elements had been redeposited. The upper part of the section consists of limestone beds belonging to the Oepikodus evae Zone, where the redeposited portion of the conodont elements decreases upwards.

A Cordylodus fauna from the SW Gulf of Bothnia, Sweden

Abstract A conodont fauna including several species of Cordylodus was retrieved from two glacial siltstone boulders found on the shore of Lilltuppen, a small island immediately off the Swedish coast in the Gulf of Bothnia at c. 61°N, 17°E. The erratics are remnants of a poorly known level in the Swedish stratigraphic succession. They probably derive from a nearby point on the floor of the Baltic Sea and constitute an example of an “eastern facies”, reminiscent of part of the Kallavere Formation in northern Estonia. The conodont fauna closely resembles that of the Maardu Member of the Kallavere Formation, and indicates a position in the Cordylodus lindstromi conodont zone just at the Cambrian–Ordovician boundary.

Conodonts in Silurian hypersaline environments: Specialized and unexpectedly diverse

Hypersaline environments are commonly assumed to be barren of metazoans and therefore are avoided by paleontologists, yet a number of early Paleozoic jawless vertebrate groups specialized to live in such settings. Sampling bias against restricted settings resulted in substantial underestimation of their diversity. Rare studies venturing into such environments yielded multiple new species of conodonts, suggesting that the diversity and habitat range of these hyperdiverse predators of the early oceans are equally underestimated. We describe here autochthonous conodont fauna from evaporite-bearing horizons from the middle Silurian of Estonia that provide evidence for efficient osmoregulation in this group. Based on a global compilation of coeval conodont assemblages, we show that marginal-marine, periodically emergent environments were characterized by higher conodont diversity than open-marine shallow settings. This diversity is due to a high number of species occurring in these environments only. The high degree of specialization is also reflected by the highest within-habitat variability (β diversity) in marginal settings. Most conodont species had narrow environmental niches and, unlike in marine invertebrates, extreme environments were inhabited by the most specialized taxa. Such environments represent a large proportion of early Paleozoic tropical epicratonic basins. Our analysis allows quantification of the degree to which mid-Silurian conodont diversity is underestimated as a result of sampling bias against marginal-marine settings.