A MAJOR ISSUE WITH OCEANIC ANOXIC EVENTS: OXYGEN PRODUCTION AND POSSIBLE MEANS TO REDUCE IT

Oceanic Anoxic Events (OAEs) and similar events throughout the Phanerozoic are a product of significant environmental upheaval that are generally well-recognized in the geologic record by extensive marine organic-rich black shales. These events bury considerable amounts of reduced materials such as organic carbon and pyrite, causing a net release of oxygen. Therefore, the processes and ultimate burial products of OAEs are counterintuitive as the excess oxygen production should limit the duration of OAEs to only a few thousand years instead of being hundreds of thousands to millions of years long. This decoupling implies probable additional mechanisms involved in the removal of oxygen to help alleviate the imbalance in the redox budget.

There are many OAEs and OAE-like events, though not all, that are associated with volcanism, specifically large igneous provinces (LIPs). The dominant anoxia-inducing mechanism is nutrient-related delivery to drive increased primary production and a cascade of consequences. It is suggested that the release of volatile gases from LIPs drive climatic perturbations, inducing these bioproductivity increases, thus connecting these phenomena. Excess reductants in gases, solid-phase aerosols, and/or particles released during volcanism can also provide a mechanism to remove oxygen from the ocean-atmosphere system.

A forward box model was assembled to quantify, on a first-order, the excess oxygen produced via organic carbon and pyrite sulfur burial as well as the effectiveness of LIP reductants to remove this oxygen. The two most well-studied OAEs, Oceanic Anoxic Event 2 (OAE-2) of the Cretaceous and the Toarcian Oceanic Anoxic Event (T-OAE) of the Jurassic, were both analyzed with this model under various parameters to determine the effectiveness of reductants released from the LIPs in the removal of oxygen, though there are limits to both due to poorly constrained LIP (OAE-2) or size of organic carbon burial (T-OAE), requiring further study. Overall, a range of oxygen produced during these OAEs was removed through the introduction of various reductants that may have accompanied LIP emplacement, all showing partial to complete effectiveness of LIP reductants to limit oxygen production. Thus, this poorly studied aspect of LIP eruptions can provide a means to remove excess oxygen.