A MULTI-ARCHIVE LOOK INTO THE LATE ORDOVICIAN ICEHOUSE AND FIRST MASS EXTINCTION OF THE PHANEROZOIC

The Late Ordovician marks the first mass extinction event of the Phanerozoic, which wiped out a wide range of marine organisms at the generic level. Despite its detrimental impact on Ordovician life, it is highly debated as to which factors contributed to the Late Ordovician Mass Extinction (LOME). Previous studies propose processes such as redox shifts, volcanism, deep- water anoxia, and/or excess CO2 as significant influencers. We evaluate the stratigraphic, chronologic, and diagenetic signatures of brachiopods, halite, and carbonates from multiple low paleo-latitudes (Anticosti Island, Hudson Bay Basin – Canada, South China, and Canning Basin – Australia) that contain the Ordovician – Silurian boundary to determine plausible drivers of the LOME.

The diagenetic integrities of halite and carbonate were assessed in comparison to brachiopod data of Veizer et al. (1999) through strontium isotope analysis. Since the strontium isotope remains in equilibrium with seawater, we can cross-analyze the preservation potentials of three different marine archives. All sample results lay within the ± 0.00006 ‰ natural variation of 87Sr/86Sr measured in modern marine counterparts, providing robust evidence of primary material and accurate age assignments.

Halite samples of two separate localities underwent trace element chemistry. Previous studies suggested that the spikes of Hg, Mo, and U concentrations signify the aftermath of a volcanic-related, greenhouse event that triggered the expansion of deep-water anoxia. However, our halite Hg, Mo and U concentrations are extremely low, many below detection limits, which correspond to icehouse glacial signatures. As well, interpretation of our sedimentary cerium anomaly of evaporite samples reveal characteristics of an oxygenated marine environment.

Microthermometry was conducted on primary halite fluid inclusions to determine temperature trends during the LOME. These homogenization temperatures reveal oscillations representative of daily/seasonal variation of inter-and/or glacial times, averaging approximately 24.2°C ± 0.5°C. Outside these fluctuations, we noticed a minimum of 3 major cooling pulses associated with the major glaciation characterizing latest-Ordovician – earliest-Silurian time.