Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions – insights from molybdenum and uranium isotopic global redox proxies

A global oceanic sulfidic anoxia occurred during the Late Ordovician Mass Extinction (LOME2), significantly contributing to the 85% loss of marine animal species. This finding is supported by molybdenum and uranium isotopic proxies, with δ²³⁸U in carbonates from Anticosti Island serving as a key indicator. The study combines existing paleoredox data from four continents with new Mo isotope data, revealing that ocean anoxia was both spatially extensive and temporally linked to extinction phases. Additionally, periodic sulfidic anoxia in well-ventilated shallow oceans during warmer periods may have driven high seawater δ⁹⁸Mo values observed during LOME1. These insights suggest that oxygen loss played a causal role in both extinction phases.

Dahl, T. W., Hammarlund, E. U., Rasmussen, C. M. Ø., Bond, D. P. G., & Canfield, D. E. (2021). Sulfidic anoxia in the oceans during the Late Ordovician mass extinctions – insights from molybdenum and uranium isotopic global redox proxies. Earth-Science Reviews, 220, 103748. https://doi.org/10.1016/j.earscirev.2021.103748