High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation

Abstract

The two phases of the Late Ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the Gondwanan glaciation. In this paper we argue that the distinctive Late Ordovician carbon isotope profile provides a chronostratigraphic "ruler" against which a sequence of environmental and biotic events may be located. The ruler also allows regional and global high-resolution correlation of successions representing very different environments. Cores from the Upper Ordovician succession of Estonia and Latvia record a large δ 1 3 C c a r b o n a t e excursion (up to 6‰), with a similar profile shape. The consistent relationship between the chemostratigraphy and biostratigraphy in the Baltic region suggests that the isotope profile has a regional chronostratigraphic value. The presence of similar profiles in Nevada, United States, suggests that the excursion is a global chronostratigraphic signal. This interpretation enables a detailed correlation to be made between Upper Ordovician shallow-marine and basinal sequences that have wholly different faunas. Successions in the Baltic area and in Canada that do not display the model profile are interpreted as incomplete. Reinter-pretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction. Key levels of environmental change have been located against the carbon isotope profile. New oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events. Cooling and sea-level fall started at the same stratigraphic level as the start of the carbon isotope excursion. The later rise in sea level and fall in oxygen isotope values record the end of the glaciation. These restrict the duration of the main glaciation to only 1.5 graptolite zones. We propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling, sea-level fall, and temperature change documented here. The chronostratigraphic "ruler" provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high-resolution correlation of biotic events at different locations. This approach identifies regional similarities and differences in the patterns of extinction. The main phase of graptolite extinction in the Monitor Range, Nevada, for example, is synchronous with the chitinozoan extinction in the Baltic region, but chitinozoan taxa survive to higher levels in Nevada. The benthic faunas in the Baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the Hirnantian-the level at which the marine environment started to change rapidly-but that there were further extinctions of species within the early Hirnantian. The cold adapted Hirnantia fauna did not appear immediately after the extinction in this area. The relationship between the second phase of extinction and the carbon isotope excursion is less clear, but the available data suggest that the extinction coincides with a time of rapid environmental change, but not at the inception of environmental change, as happened in the first phase.