252-4 Isotopic Constraints on Ordovician Cooling and Glacial Dynamics

Session: Climate Transitions in the Paleozoic

Presenting Author:

Kristin Bergmann

Authors:

Bergmann, Kristin¹, Zimmt, Joshua Ben², Finnegan, Seth³, Swanson-Hysell, Nick⁴, Macdonald, Francis A.⁵, Present, Theodore Michael⁶, Desrochers, Andre⁷

(1) MIT, Cambridge, MA, USA, (2) National Park Service, Cave Junction, OR, USA, (3) UC Berkeley Integrative Biology, Berkeley, CA, USA, (4) University of Minnesota, Minneapolis, Minnesota, USA, (5) UC Berkeley, Berkeley, CA, USA, (6) Caltech, Pasadena, CA, USA, (7) University of Ottawa, Ottawa, Canada; Société du Patrimoine Mondial Anticosti, Port-Menier, Canada,

Abstract:

The end-Ordovician glaciation marks Earth's first Phanerozoic icehouse, yet the timing, drivers, and magnitude of cooling and glaciation remain poorly constrained. Using compiled global conodont δ¹⁸O data with refined age models, we document continuous tropical cooling of ~15°C from Early Ordovician (~40°C) to Late Ordovician (~25°C). This protracted cooling, and its magnitude, parallels the marine transition from early Eocene greenhouse to Pleistocene icehouse conditions. We attribute this trend to changes in paleogeography that controlled the geological sources and sinks of carbon. To resolve glacial onset timing and magnitude, we analyzed fossils from the Ellis Bay Formation on Anticosti Island, which preserves a rare Ordovician-Silurian boundary section. Our approach combines stable isotope geochemistry (δ¹³C, δ¹⁸O; n = 81), carbonate clumped isotope paleothermometry (Δ₄₇; n = 45), and trace element analysis on rugose corals. Diagenetic screening, using combined isotopic and elemental criteria, identified fossils that underwent predominantly closed-system alteration. Well-preserved fossils reveal two intervals of increasing δ¹⁸O values: a moderate increase (1.0-1.5‰) in the Fraise Member and a larger excursion (2.5-4.5‰) at the base of the Laframboise Member. Only the latter shows Δ₄₇ evidence for substantial tropical cooling (~10°C), indicating that maximum cooling and ice sheet growth occurred well after the Katian-Hirnantian boundary. These results deconvolve ice volume growth and temperature changes. The synthesis of glacial deposits in paleogeographically reconstructed Gondwanan basins, alongside glacioeustatic records, indicates that the ice volume during the Hirnantian glacial maximum was comparable to or exceeded that of the Last Glacial Maximum, with an associated sea-level fall of 125-150 m.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5902

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.