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

¹ Institute of Geology, Tallinn Technical University, 7 Estonia Blvd., 10143 Tallinn, Estonia; e-mail : kaljo{at}gi.ee

Abstract

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 ¹⁸O and 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 ¹³C excursions are recorded in the Baltic area (peak values in parentheses) : middle Caradoc (2.2 ), early Ashgill (2.5 ), Hirnantian (6 ), early Aeronian (3.7 ), early Telychian (2.7 ), early Wenlock (5.2 ). 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.

Key Words: Baltica • Carbon isotopes • Glaciations • Gondwana • Ordovician • Palaeoclimates • Silurian

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