Upper ocean redox reconstructions of the Late Ordovician (Hirnantian) positive carbon isotope excursion: potential linkages to glaciation and mass extinction

The Late Ordovician (Hirnantian) mass extinction is the first of the 'big five' Phanerozoic extinction events and is characterized by two pulses occurring during major Gondwanan glaciation and a large positive carbon isotope excursion (Hirnantian carbon isotope excursion or HICE). The second extinction pulse has been attributed to marine anoxia based on its association with widespread organic-rich marine facies and geochemical proxies (e.g., δ²³⁸U and Fe speciation). This study investigates upper ocean redox changes using I/Ca ratios of marine limestones from two Upper Ordovician North American sections in the western U.S. (Copenhagen Canyon, Nevada) and Quebec, Canada (Anticosti Island). Preliminary results showed that the I/Ca values are relatively lower in the Nevada section than those of the Anticosti Island section. The I/Ca record from Nevada generally decreased during the HICE, suggesting expanded low oxygen upper waters, at least locally. These redox trends need to be further confirmed in the Anticosti Island section to evaluate the spatial variability of ocean redox conditions during peak glaciation, mass extinction, and global changes in the carbon cycle.