Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada)

A detailed carbon isotope analysis of Ordovician-Silurian boundary strata on Anticosti Island, Quebec, reveals that the Hirnantian positive carbon isotope excursion—characteristic of global records—peaks at ~4.5‰ in carbonate rocks. This excursion, coupled with glacioeustatic sea-level fluctuations, suggests that sea-level drawdown and subsequent rise were not perfectly synchronized with the isotopic shift. The study proposes that the entire Hirnantian Stage on Anticosti is confined to the Laframboise and lower Fox Point Members, supported by a combined litho- and chemostratigraphic approach. Discontinuities in the isotope curve across proximal-to-distal transects further indicate diachroneity in sedimentary deposits, with oncolite beds transgressing westward while Becscie limestones prograded eastward. The findings align with the 'weathering' hypothesis for the Hirnantian carbon isotope excursion, providing insights into terminal Ordovician climate dynamics.

Jones, D. S., Fike, D. A., Finnegan, S., Fischer, W. W., Schrag, D. P., & McCay, D. (2011). Terminal Ordovician carbon isotope stratigraphy and glacioeustatic sea-level change across Anticosti Island (Quebec, Canada). Geological Society of America Bulletin, 123(7-8), 1645–1664. https://doi.org/10.1130/b30323.1