DRIVERS OF THE LATE ORDOVICIAN MASS EXTINCTION: A MULTI-ARCHIVE APPROACH

The first mass extinction of the Phanerozoic Eon, the Late Ordovician Mass Extinction (LOME), marked a turning point in the evolution of marine biota. The abrupt onset of a severe glaciation occurred sandwiched between two extreme greenhouse phases. This rapidly changing climate resulted in the death of nearly 80% of marine organisms at the generic taxonomic level. Despite its evidence, the exact cause for the LOME event is still highly debated. Previous studies propose processes such as volcanism, redox shifts, deep- water anoxia, and/or excess CO₂ as possible drivers of the LOME. We analyse the diagenetic, chronologic, and glacial signatures of brachiopods, halite, and carbonates from multiple low paleo-latitudes (Anticosti Island, Hudson Bay Basin – Canada, South China, and Canning Basin – Australia) to paleo-reconstruct an Ordovician – Silurian world and distinguish primary drivers of the LOME.

The diagenetic potentials of halite and carbonate were assessed in comparison to brachiopod data of Veizer et al. (1999) via strontium isotope analysis. All sample results are within the ±0.000060 natural variation of ⁸⁷Sr/⁸⁶Sr measured in modern marine counterparts, providing robust evidence of primary material and accurate age assignments.

Trace element chemistry was conducted on halite samples from two separate localities. Previous studies proposed that the spikes of Hg, Mo, and U concentrations denote to the aftermath of a volcanic-related, greenhouse event that triggered the expansion of deep-water anoxia. However, our halite REE concentrations are extremely low, many below detection limits, corresponding to signatures of a glacial icehouse. Moreover, characteristics of an oxygenated marine environment were revealed via interpretation of our sedimentary cerium anomaly of evaporite samples.

Two-phase, primary, halite fluid inclusions preserve brine and atmospheric gas from its time of formation, allowing for a high-resolution paleo-reconstruction of climate trends using microthermometry. These homogenization temperatures reveal oscillations representative of daily/seasonal variation of inter-and/or glacial times, averaging approximately 24.2°C ± 0.5°C. Beyond these fluctuations, a minimum of 3 major cooling pulses are present. Further evidence of a glaciation driven LOME event.