Oceanic hypoxia and mass extinction at the Cenomanian-Turonian boundary triggered by a flare-up of arc volcanism

The Cenomanian-Turonian Boundary (CTB) at 93.5 Ma was characterized by global depletion of oxygen in the deep oceans as evidenced by global deposition of black shales and the mass extinction of benthic organisms. These conditions have been attributed to a combination of greenhouse-induced oceanic stagnation and high biological productivity. Because the CTB overlapped in time with the emplacement of the submarine Caribbean flood basalt province (95-88 Ma), the flood basalt-mass extinction coincidence is generally thought to be causal. The hypothesis is that oxygen was immediately consumed in the deep ocean by direct oxidation of reduced Fe in the basalt or, alternatively, such Fe was leached out and transported to surface waters, enhancing biological productivity. However, oxygen consumption by direct or indirect oxidation of Fe in submarine flood basalts is shown here to be difficult, complicating this causal link. We present here an alternative hypothesis based on the under-appreciated coincidence in time between the CTB and a flare-up of continental arc volcanism along the North America margin, which extended from southern Mexico up through Canada. Unlike submarine flood basalt volcanism, arc volcanoes generate ash, which is erupted into the stratosphere and deposited around the world. We show that the magnitude of the ash flux was sufficient enough to fertilize surface waters with reduced Fe, thereby generating a global phytoplanktonic bloom, which exceed the threshold of biological productivity above which bottom waters would be depleted of oxygen and benthic organisms suffocated. We conclude that although the CTB was coincident with both submarine flood basalt volcanism and subaerial arc volcanism, the latter process may have been more effective at triggering oxygen depletion and mass extinction. Mass extinctions are often thought to be triggered by catastrophic and episodic processes, such as flood basalts and meteor impacts, hence our hypothesis, if correct, suggests that more mundane plate tectonic processes, such as subduction and arc volcanism, may occasionally cause mass extinctions.