PATTERNS OF EUSTASY, PALEOCEANOGRAPHY AND CONODONT BIOTIC CHANGE ASSOCIATED WITH THE TERMINAL ORDOVICIAN GLACIATION

Several critical issues remain to be resolved with regard to the terminal Ordovician glaciation, particularly the duration of the glaciation, the number of major glacial phases, its cause(s), the drawdown of CO2, and whether the glaciation was the only dominant reason for the associated mass extinction of the biota. Perhaps the best stratigraphic record of the associated eustatic, oceanographic and biotic events is contained in marginal basins of Laurentia; detailed studies from Anticosti Island, the Canadian Arctic Islands, and the northern Canadian Cordillera with massive collecting for conodonts has yielded much new data. These results are combined with other work on peri-Gondwana material, climate modeling and isotope geochemistry (Sr and Nd). Several lines of evidence suggest that the initial glaciation started in the late Carodocian-early Ashgillian, developing its climax of development in the Hirnantian/Gamachian, with three minor waning phases extending to the Llandovery/Wenlock boundary. This is supported to some degree by detailed reconstruction of the eustatic record from the marginal basins based on lithology, sedimentary structures, and conodont and brachiopod community patterns. Climate modeling suggests that the Gondwanan ice sheet could have been established by reducing the high level of CO2 in the Ordovician by half (to 8-10X PAL). Stratigraphic, faunal and Nd isotope data indicate that the lowered sea levels exposed most of the cratons, destroyed the structure of the epeiric seas and their hypersaline environments, and dramatically altered the pattern of faunal realms/provinces. In particular, conodont extinction was severe, with progressive recovery in four cycles through the Llandovery.