Dimitri Kaljo

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.

Application of carbon isotope stratigraphy to dating the Baltic Silurian rocks

Abstract The potential of carbon isotope excursions as a stratigraphic correlation tool has been proved by several earlier studies, but partly indistinct biostratigraphic definition hinders their usage. Six positive carbon isotope excursions have been established in the Baltic Silurian (values of the first four in the East Baltic): the early Aeronian excursion (peak δ13C values reach +3.7‰); the early Telychian excursion (+2.7‰); the early Sheinwoodian excursion (+5.2‰); the middle to late Homerian double-peaked excursion (+4.6‰); the late Gorstian excursion (+1.1‰, in Gotland) and the middle Ludfordian excursion—the most prominent shift in the Phanerozoic (maximum values reach in the East Baltic +8.2‰, in Scania +11.2‰).The biostratigraphic dating of three Wenlock and Ludlow excursions (the early Sheinwoodian, mid to late Homerian and mid-Ludfordian) is discussed and refined in the paper and despite minor problems, these could successfully serve as a tool for geological correlation and as chronostratigraphic markers. Using the mid-Ludfordian shift as a marker, it is suggested that the upper part of the Hamra-Sundre interval on Gotland correlate with the lower part of the Kuressaare Stage in Estonia.

Carbon isotope stratigraphy in the latest Ordovician of Estonia

Abstract On the basis of bulk carbonate carbon isotope data from six drill cores and the type section of the Porkuni Stage of Estonia, δ13C curves were compiled, which altogether cover nearly the whole succession of the latest Ordovician (Hirnantian) —well known as a time of short-lived glaciation and mass extinction. In pre-Porkuni and Silurian rocks, δ13C values were low (from −0.2‰ to 1.7‰). For the Porkuni Stage three intervals of the curve were established: (1) lower — a positive shift up to 6.7‰, (2) middle part with variable values from 3.3‰ to 5.4‰, and (3) upper phase with slightly decreased values from 3.2‰ to 3.6‰. The correlation with the Dobs Linn (Scotland) kerogen δ13C curve shows that the first two intervals coincide with the Normalograptus extraordinarius graptolite Biozone, the third one with the N. persculptus Biozone.

Terminal Ordovician stratigraphy of the Siljan district, Sweden

Integration of new isotopic data and earlier biostratigraphic information from eight sections through the terminal Ordovician (Pirgu and Porkuni stages) of the Siljan district, Sweden, allows a more precise correlation of sections in terms of biostratigraphy and carbon isotope dating. Four levels with positive δ13C excursions are identified (from bottom) – the Moe, an unnamed excursion, Paroveja and Hirnantian Carbon Isotope Excursion (HICE). The δ13C values through the Boda Limestone are 1–2‰ higher than usual in Baltica, only the values for the HICE remains within what is expected. Background values increase from 1.5‰ in the bottom of the core of the Boda Limestone up to 3‰ in the top of it. The HICE is identified in five of eight sections and the main peak falls according to inferred correlation within the Metabolograptus persculptus Biozone, at or close to the Hindella beds in the Upper Boda Member. The late Katian (Pirgu) age of Holorhynchus in the Siljan district is clear and its co-occurrence with the chitinozoan Belonechitina gamachiana in Estonia supports a Katian age for this zone. The base of the Ozarkodina hassi Biozone may occur within units B–C of the Upper Boda Member and in the upper part of the Loka Formation and most likely is correlated with the M. persculptus Biozone. The Hirnantia–Dalmanitina faunas reported from the lowermost part of the Loka Formation and units B–D of the Upper Boda Member seem to range through all the Hirnantian, but detailed morphological studies allow to distinguish an older ( = extraordinarius) and a younger ( = persculptus) fauna.

Upper Silurian stratigraphy of Podolia revisited: carbon isotopes, bentonites and biostratigraphy

Nowadays carbon isotopes and bentonites have become new authorities in stratigraphy besides traditional biostratigraphy. Three positive carbon isotope excursions have been identified in the Upper Silurian of Podolia: the mid-Ludfordian excursion, the late Přídolí excursion at the Trubchyn/Dzvenygorod junction and the SIDE at the Silurian–Devonian boundary. Fourteen bentonite beds are known from this upper Ludlow–Přídolí interval. Some of these are better studied and are helpful for correlation of sections. Using the stage slices as a framework, we show that the Přídolí can be subdivided into “natural” stage rank units based on the integrated bio-, litho- and chemostratigraphy.