The Cenomanian-Turonian boundary is marked globally by the extinction of marine invertebrates and an increase in the accumulation and preservation of organic carbon-rich sediments (black shales). The timing of this boundary also coincides with large-scale volcanism from at least three large igneous provinces: the Caribbean and Ontong-Java oceanic plateaus and the Madagascar flood basalts. In this paper, we assess the possibility that hydrothermalism associated with large-scale submarine magmatism was responsible for the reduction of dissolved O 2 in the oceans. We investigate two potential mechanisms: the oxidation of reduced material in hydrothermal effluents and the stimulation of primary productivity in the water column due to the injection of hydrothermal Fe into surface waters. With the first mechanism, we find that a 10,000-km 3 submarine basalt eruption (three orders of magnitude larger than recent midocean ridge eruptions) could release enough reduced hydrothermal material to consume at least 6 percent of the dissolved O 2 in seawater in a well-ventilated ocean (such as the present ocean). With the second mechanism, we calculate that even a small percentage of the hydrothermal Fe released from a single large flow could have led to a significant increase in primary productivity in areas where Fe was the biolimiting nutrient (such as open-ocean areas in the Pacific). The potential impact of both of these O 2 -reducing mechanisms at the Cenomanian-Turonian boundary would have been significantly greater if Cretaceous oceans were less well ventilated than the present oceans.