Isotopic constraints on the peak of the Early Paleozoic Icehouse

While the intensification of the Early Paleozoic Icehouse is commonly cited as the main driver of the Late Ordovician mass extinction, we lack high-resolution, stratigraphically constrained climate records to test this hypothesis. Here, we develop a high-resolution climate record for the Late Ordovician by applying stable isotope geochemistry (δ¹⁸O, δ¹³C; n = 81) and carbonate clumped isotope paleothermometry (Δ₄₇; n = 45) to fossils from the stratigraphically expanded Ellis Bay Formation on Anticosti Island (Canada). Focusing our analysis on fossils that primarily experienced closed-system alteration, we identify two distinct phases of increasing fossil δ¹⁸O_VPDB values: a moderate increase of 1.0‰–1.5‰ across the Katian/Hirnantian boundary and a larger increase of 2.5‰–4.5‰ in the middle to late Hirnantian. Only the latter is associated with Δ₄₇ evidence for cool tropical sea surface temperatures, and based on its sequence stratigraphic context immediately overlying a regional subaerial unconformity, we interpret this excursion as reflecting the earliest stages of transgression during the waning of the Hirnantian Glacial Maximum. This revises the conclusions of previous paired stable and clumped isotope studies, which argued that the major drop in sea surface temperatures occurred at the Katian/Hirnantian boundary. When integrated with patterns of faunal turnover from Anticosti Island, our conclusion that major tropical cooling and maximum ice volumes did not occur until the middle to late Hirnantian suggests that an apparent pulse of extinction near the Katian/Hirnantian boundary may be in part a stratigraphic artifact generated by widespread glacio-eustically forced unconformities and facies shifts.

We thank the David and Lucile Packard Foundation’s support to both Bergmann and Finnegan for field and laboratory expenses; the Fonds de recherche du Québec (FRQ-Anticosti program) support to Zimmt; Roger Creel, Matt Braun, Jon Husson, and Mariko Cappello for their expertise and assistance with fieldwork; Diane Erwin for her assistance with the preparation and mounting of fossils; Fran Meyer for her work on the clumped isotope analyses at the Massachusetts Institute of Technology; Bryce Barney for his help in comparing our results to those of previous studies; and the community of Port-Menier for their support and assistance throughout this project.