Katian

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.

Abstract: The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated δ13C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive δ13C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian δ13C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric p CO2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level—which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p CO2 by suppressing shelf-carbonate production—signals the transition to a Late Ordovician icehouse climate ∼10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.

Abstract: The Late Ordovician (Katian-Hirnantian) through earliest Silurian (Rhuddanian) interval was a time of varying climate and sea level, marked by a peak glacial episode in the early-mid-Hirnantian. Synthesis of recently published data permits global correlation of at least two cycles of glacial advance and retreat with a distinct interglacial period that is recognizable in sequence-stratigraphic and chemostratigraphic records in many parts of the world. A period of warming and sea-level rise during the late Katian is marked by the widespread occurrences of oceanic anoxia in paleotropical and subtropical localities, mostly confined to regions of inferred upwelling and semirestricted marine basins. Nitrogen isotope data show that the regions of oceanic anoxia were marked by intense water-column denitrification in which cyanobacteria were the principal source of fixed N. In the overlying peak glacial interval of the Hirnantian, sedimentary successions from localities representing a wide range of water depths and paleolatitudes indicate that anoxia was restricted during the early-mid-Hirnantian. The shift to more positive N isotope values also suggests less intense water-column denitrification. In the overlying late Hirnantian and early Rhuddanian, the distribution of black shales reaches its greatest extent in the studied interval. Localities showing evidence of anoxia are globally spread over all paleolatitudes and water depths for which data are available, indicating a Rhuddanian ocean anoxic event comparable to examples from the Mesozoic. It is accompanied by a return to intensely denitrifying conditions within the water column, as indicated by the shift to negative N isotope values. The two phases of Hirnantian mass extinction coincide with rapid, climate-driven changes in oceanic anoxia. The first extinction occurred at the onset of glaciation and with the loss of anoxic conditions at the end of the Katian. The second extinction occurred at the demise of glaciation and coincided with the return of anoxic conditions during the late Hirnantian–early Rhuddanian. Integration of our N isotope data with graptolite biodiversity records suggests that the extinctions were profoundly influenced by changes occurring at the base of the marine food web, i.e., redox-driven changes in nutrient cycling and primary producer communities.