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 ol organic carbon in separate areas, We report new data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate delta(13)C from Baltica. Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated delta(13)C values similar to those described previously from low-latitude sites, The new data demonstrate that the positive Hirnantian delta(13)C 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 or nic matter from Estonia provide the first indication of a synchronous shift in organic and inorganic delta(13)C in sediments from the same basin. This work provides new data of critical importance for constraining models of end-Ordovician palaeoceanography and climate change. (C) 1997 Elsevier Science B.V.