An integrative biostratigraphic, chemostratigraphic, and sequence stratigraphic perspective of the Ordovician–Silurian boundary on Anticosti Island (Canada)

Anticosti Island, Canada, has long been recognized as an exceptional OrdovicianâSilurian boundary succession with the potential to serve as one of the best records of climatic, oceanographic, and biological events associated with the Late Ordovician mass extinction. However, differing interpretations as to the position of the Hirnantian Stage within the stratigraphic succession due to the paucity of diagnostic graptolites, the apparent absence of a typical Hirnantia fauna within the Upper Ordovician Ellis Bay Formation, and lateral facies variability among outcrops has hindered the study of the OrdovicianâSilurian boundary on the island, particularly in the eastern half of the outcrop belt. Definitively identifying the stratigraphic position of the Hirnantian Stage within the succession is therefore critical for understanding this classic OrdovicianâSilurian boundary section, as well as for the integration of data from Anticosti into our global understanding of the Late Ordovician mass extinction. Here, we take an integrative approach to studying the Ellis Bay and lowermost Becscie formations, combining new paleobiological, geochemical, radiometric, and sequence stratigraphic constraints from ongoing fieldwork with existing biostratigraphic, geochemical, and palynological studies in the context of newly measured stratigraphic sections. These formations record six depositional sequences bounded by regionally traceable but subtle unconformities, often mantled by thin siliciclastic veneers reworked into transgressive lag facies. Many of these unconformities have gone unrecognized despite more than a century of work at certain localities. Furthermore, despite previous controversy, multiple lines of evidence favor a Hirnantian age for the entire Ellis Bay and lowermost Becscie formations, including newly recognized occurrences of Hirnantia and Hindella in the lower Ellis Bay Formation, a two-phased positive carbon isotope excursion, with the second phase reaching ~6â° in the Laframboise Member of the Ellis Bay Formation, and a U-Pb TIMS age of 443.61 ± 0.52 Ma from zircons in a bentonite from the mid-Ellis Bay Formation. While graptolite and conodont biostratigraphy support this age model, determination based on chitinozoan biozonation is more equivocal but may be controlled by facies preferences. Conodont, brachiopod, and chemostratigraphic data additionally suggest that the Hirnantian Stage may extend slightly into the lower Becscie Formation on the western end of Anticosti and well into the lower Becscie Formation in the eastern part of Anticosti. Our reappraisal of a classic OrdovicianâSilurian boundary section has important implications for understanding the sequence of climatic, environmental, and biological events throughout the Late Ordovician mass extinction. Given that the Ellis Bay and lowermost Becscie formations are indeed Hirnantian in age (encompassing ~2 My), these formations record six fourth-order depositional sequences of approximately ~333 ky. Furthermore, comparison of the Hirnantian of Anticosti to coeval exposures suggests that other regions may be incomplete at the level of the fourth-order cycles that occur in the Ellis Bay Formation. Resulting uncertainties in correlations based on unconformities and interpretations of stratigraphic architecture may therefore greatly complicate global correlation of Hirnantian records. Further study of this issue is critical, as stratigraphic architecture is expected to be an overarching control on the expression of oceanographic, climatic, and biotic events at a regional scale, complicating the interpretation of the pattern and drivers of the Late Ordovician mass extinction

A long-term record of early to mid-Paleozoic marine redox change

The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic

Evolution of the retiolitid Neogothograptus [Graptolithina] and its new species from the upper Wenlock of Poland, Baltica

Neogothograptus reticulatus sp. nov. from the upper Homerian Colonograptus praedeubeli Biozone, and N. thorsteinssoni and N. alatiformis from the Lobograptus progenitor Biozone, are described for the first time from three localities: borehole, Baltic erratic boulder of East European Platform and Holy Cross Mountains of Poland. N. reticulatus, presently the oldest known species of Neogothograptus, is also tentatively identified from upper Homerian strata of southeastern Australia. The two other species are previously known only from Arctic Canada, and possibly China. The morphology of the Neogothograptus reticulatus rhabdosome, its appendix, thecal profile, densely reticulated rhabdosome and genicular hoods suggest a close relationship to N. eximinassa from Colonograptus ludensis Biozone. N. reticulatus and N. eximinassa are most similar to Gothograptus nassa, the earliest−known retiolitid to appear immediately following the Cyrtograptus lundgreni extinction event. The biostratigraphic position of N. reticulatus suggests it might be considered as a potential ancestor to all younger (Ludlow) species of Neogothograptus. Cladistic analysis, however, provides no direct support for such an interpretation and, instead, suggests that Baculograptus batesi may be the ancestor. The occurrences of Neogothograptus, as well as G. nassa, from a number of Silurian terranes mostly from low paleolatitude regions, but also from high paleolatitudes, demonstrate their tolerance to a broad range of paleoenvironments

New stable isotope data and fossils from Hirnantian stage in Bohemia and Spain: implications for correlation and paleoclimate

Rocks in the Late Ordovician paleotropics commonly exhibit the now well-known, positive Hirnantian Isotopic Carbon Excursion (HICE). This event is coincident with both continental scale glaciation in Gondwana and dramatic extinction across the marine realm (Delabroye and Vecoli, 2010). Both the proximate cause of the isotopic excursion and the ultimate drivers of large scale cooling remain the subject of debate. Suggestions range from tectonic effects on weathering or changes in biological productivity, through large basaltic eruptions to gamma ray bombardment. Discussion of these alternative models is beyond the scope of this short paper, however. Our intent is to briefly report the direct association of a new high resolution δ13Corganic record from rocks at a high latitude site that also bears biostratigraphic and sequence stratigraphic data needed to link Hirnantian oceanographic changes (especially those recorded in the paleotropics) with glacial events in the peri-Gondwanan realm (Delabroye and Vecoli, 2010; Young et al., 2010).Peer reviewe