Global Anoxia and Mass Extinction at the Cenomanian-Turonian Boundary Triggered by Subduction Zone Volcanism

The mid Cretaceous (120 to 80 million years ago) was one of the warmest periods during the Phanerozoic, and was punctuated by several oceanic anoxic events (OAEs) characterized by globally distributed black shales. One OAE termed the Cenomanian-Turonian Boundary (CTB) event at 93.5 Ma coincided with mass extinction of benthic foramnifera and a perturbation in the global carbon cycle. The causes of the CTB event have been attributed to either oceanic stagnation or high productivity, but the environmental or climatic triggers have not been resolved yet. Here, we show that the CTB event coincides with a flare-up of continental arc volcanism along the North American Cordillera. The enhanced flux of volcanic ash inputted micro-nutrient iron into the ocean, fertilizing surface waters and generating a global phytoplanktonic bloom, which in turn depleted oxygen in deep ocean. Under the greenhouse conditions of the Cretaceous, the ocean system was vulnerable to anoxia—only a 50% increase in biological productivity was sufficient to trigger global ocean hypoxia (oxygen concentration <120 µM) or expand the oxygen minimum zone to depths of 2200 m. We show that the increase in ash flux associated with the magmatic flare-up sufficiently exceeds this threshold, suggesting that the CTB event may have been directly linked to subduction zone volcanism. If so, the CTB event represents one example, where plate tectonics, mass extinctions, and the evolution of life were intimately linked.