NEW CLUMPED ISOTOPE CONSTRAINTS ON THE TEMPO AND MAGNITUDE OF HIRNANTIAN (LATE ORDOVICIAN) GLACIATION FROM ANTICOSTI ISLAND, QUÉBEC, CANADA

The Late Ordovician glaciation stands out among major Phanerozoic glaciations for its comparatively short duration and its association with a mass extinction. Many open questions about the causes of the glaciation and its potential role in driving this marine extinction event cannot be resolved without more precise and higher-resolution constraints on the sequence and magnitude of Late Ordovician-Early Silurian climatic events. Oxygen isotopic (δ18O) records provide high-resolution records of ancient climate change, but Late Ordovician oxygen isotope archives have been variably altered by diagenetic and metamorphic processes. Clumped isotope paleothermometry provides a tool for distinguishing primary and secondary isotopic signals to better reconstruct climatic events.

Anticosti Island (Québec, Canada) exposes an expanded and well-preserved Late Ordovician-Early Silurian mixed carbonate-siliciclastic boundary section. Along the western end of Anticosti, the Ellis Bay Formation contains two positive carbon isotope excursions of increasing magnitude, representing a lower and upper Hirnantian carbon isotope excursion (HICE), and therefore indicating the western Ellis Bay Formation is Hirnantian in age. Previous work on Anticosti has demonstrated that biogenic carbonates (brachiopods and rugose corals) from the uppermost Ellis Bay Formation preserve plausible surface temperatures and an apparent mid-late Hirnantian tropical cooling event. Here, we analyze carbonates from the Ellis Bay Formation at a higher resolution and apply a framework for evaluating the primacy of δ18O signatures using uXRF elemental mapping.

Rugose corals associated with the Upper HICE contain enriched δ18O values and a range of clumped isotope temperatures. Corals with higher clumped isotope temperatures are generally associated with more depleted δ18O values, suggesting some open system alteration. Constraining our analysis to corals that preserve plausibly primary temperatures and evidence of closed system alteration indicates a 2-3 per mil δ18O excursion during the Upper HICE. Expanding the analysis to the entire Ellis Bay Formation reveals a much smaller ~0.5 per mil δ18O excursion during the Lower HICE, suggesting that maximum ice sheet volume and cooling was not reached until the mid-late Hirnantian.