Balancing Redox Budgets: Mechanisms for Prolonging Anoxia During Major Carbon Burial Events

Widespread marine anoxic events occurred throughout the Phanerozoic, most notably the Mesozoic oceanic anoxic events (OAEs). They were likely the result of major climatic perturbations that resulted in the burial of significant quantities of organic matter-rich sediments. During OAEs, reduced carbon and sulfur are more efficiently sequestered, which, based on stoichiometric balances, should result in a net increase of oxygen counteracting marine deoxygenation. This relationship has been the fundamental basis of models for the long-term rise in oxygen across deep-time. However, the geologic record of OAEs indicate widespread marine anoxia persisted on million-year timescales while these processes occurred. Many OAEs occur contemporaneous with the emplacement of large igneous provinces (LIPs), which released volatile compounds that likely induced climatic perturbations that could, in turn, impact marine (de)oxygenation. These volcanic systems also released reducing compounds that may provide a mechanism for the temporal exacerbation of OAEs. Additionally, the climatic effects of LIPs are associated with enhanced chemical weathering intensities, which may have promoted increased oxidative weathering that also consumed oxygen. Here, forward box models provide first-order quantifications of the excess oxygen produced via organic carbon and pyrite sulfur burial along with the effects of introducing LIP-sourced reductants and oxidative weathering to the ocean-atmosphere system during OAEs. This study focuses on Oceanic Anoxic Event 2 (~94 Ma) and the Toarcian Oceanic Anoxic Event (~184 Ma) as the most well-studied OAEs. During both events, significant increases in oxygen are produced from the burial of reduced compounds, approximately 10–100% of modern atmospheric oxygen levels. The added LIP-reductants partially or even completely buffer this excess oxygen through the oxidation of reduced volatile compounds. Short-term increases in oxidative weathering show a similar magnitude of oxygen removal from the ocean-atmosphere system. This analysis highlights the importance of quantifying the oxygen budget during OAEs and similar events.