New Mexico Geological Society Annual Spring Meeting — Abstracts

The Ordovician witnessed an explosion in marine biodiversity followed by the first of the ‘big-five’ Phanerozoic mass extinctions, the Late Ordovician mass extinction (LOME). The LOME consists of two discrete faunal turnovers; the first coincident with the onset of Hirnantian glaciation and the second with end-Hirnantian deglaciation. Lithologic and geochemical evidence suggests widespread marine anoxia triggered the second faunal turnover; however, these redox proxies only speak to the bottom water or porewater conditions present at the site of deposition. Uranium isotopic ratios preserved in marine carbonates record global seawater conditions due to the fact that the ocean residence time for uranium (~500 ky) is significantly longer than ocean mixing times. Bulk carbonate samples from the Upper Ordovician of Anticosti Island, Canada were analyzed to evaluate global marine redox patterns across the LOME. The Anticosti section was chosen because of its well-studied sequence and biostratigraphy and the fact that it has not been subjected to deep burial or tectonic processes. Isotopic analysis of bulk carbonates record relatively uniform values of ~0.1 per mil across the Katian-Hirnantian boundary and into the mid-Hirnantian. The values then exhibit an abrupt negative shift to ~-0.4 per mil in the Late-Hirnantian, followed by a return to values of ~0.1 per mil in the early Rhuddanian. This negative shift is roughly coincident with the second faunal turnover and is similar in magnitude to the shift reported across the end-Permian extinction. These results support earlier interpretations of widespread marine anoxia associated with the second faunal turnover; however they are at odds with recent interpretations of an intense anoxic event in the late Hirnantian-early Rhuddanian.