Hirnantian (Late Ordovician) chemostratigraphy of Anticosti Island, eastern Canada: Comparison of stable carbon and oxygen isotope compositions from brachiopod shells and bulk carbonate

Brachiopod shells from Anticosti Island's Ellis Bay Formation preserve pristine stable carbon (C) and oxygen (O) isotope signatures, offering a more sensitive and faithful archive of ancient seawater composition than bulk carbonate. Intra-shell isotopic variations reveal consistent trends across ten species, with δ13C increasing by ~0.5–1‰ from early to late growth stages, indicating minimal diagenetic alteration. Inter-species comparisons show small differences in average δ13C and δ18O at given stratigraphic levels, supporting their comparability for chemostratigraphy. Notably, brachiopod shells exhibit significantly higher δ13C than bulk micrite during the peak Hirnantian Isotopic Carbon Excursion (HICE), reinforcing their utility as reliable proxies for paleoenvironmental reconstruction, particularly during the Hirnantian glaciation event.

Schwarz, D. M., Longstaffe, F. J., Jin, J., & Desrochers, A. (2026). Hirnantian (Late Ordovician) chemostratigraphy of Anticosti Island, eastern Canada: Comparison of stable carbon and oxygen isotope compositions from brachiopod shells and bulk carbonate. Palaeogeography, Palaeoclimatology, Palaeoecology, 681, 113382. https://doi.org/10.1016/j.palaeo.2025.113382