GLOBAL CARBON ISOTOPIC EVENTS ASSOCIATED WITH MASS EXTINCTION AND GLACIATION IN THE LATE ORDOVICIAN

Mass extinctions and glacioeustatic sea-level changes in the lower par t of the Hirnantian (final stage of the Ashgill) are accompanied by sh ifts in marine stable-isotope compositions. Previously published stabl e-isotope changes have been used to identify the onset and demise of t he Gondwana glaciation and to suggest relationships between biotic cha nges and carbon cycling within the oceans. However, the existing isoto pic data set had limitations because it was derived from Ordovician lo w-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 set ting in Argentina have elevated delta(13)C values similar to those des cribed previously from low-latitude sites, The new data demonstrate th at the positive Hirnantian delta(13)C excursion, previously only recog nised from low-palaeolatitude areas, was widespread and probably globa l in extent, The poor preservation state of the brachiopods unfortunat ely prevented the determination of a reliable oxygen isotopic value fr om 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 fro m the same basin. This work provides new data of critical importance f or constraining models of end-Ordovician palaeoceanography and climate change. (C) 1997 Elsevier Science B.V.