MESOZOIC OXYGEN DROP: EVOLUTION AND EXTINCTION IN THE MARINE REALM

Much of the Mesozoic was dominated by processes linked to Pangea and its breakup. Because a supercontinent acts as an insulator that traps mantle heat, once Pangea had formed the mantle began producing basaltic magma that rose to the surface, which led to extensive volcanism through the development of large igneous provinces (LIPS). This produced times with high CO2 levels, a large greenhouse effect, sluggish ocean circulation and resulting oceanic oxygenation events (OAEs). All of this also resulted in a steep drop of atmospheric oxygen concentrations in the latest Paleozoic with resulting low concentrations continuing into the Cretaceous. Mid-Phanerozoic mass extinctions were caused by environmental conditions related to Pangea and it's breakup. Widespread oceanic euxinia triggered by the eruption of the Siberian Traps and resultant global warming led to the end-Permian mass extinction with continued related environmental stress throughout the Early Triassic. Similar widespread anoxia related to the eruption of CAMP is also implicated as the cause of the end-Triassic mass extinction, as is eruption of the Karoo and Ferrar Traps for the early Toarcian (Jurassic) extinction. In the Cretaceous much of the production of LIPS shifted to the deep ocean, with formation of submarine volcanic plateaus and the development of several OAE's, which also represent times of biotic change in marine environments. Evolution of marine organisms was strongly affected by the widespread distribution of dysoxic water masses through this time. Stressful conditions caused by the shallowing of anoxic deep waters allowed bivalves to ecologically displace brachiopods before and after the Permian-Triassic boundary, through the Early Triassic. Widespread black shale biofacies were dominated by flat clams, including in the Triassic Claraia, halobiids, and monotids, abundant Bositra in the Jurassic and inoceramids in the Cretaceous. A variety of Mesozoic reef environments were dominated by bivalves adapted to stressed environments, such as lithiotids. In addition, the expanded oxygen minimum layer (OML) common to Mesozoic oceans harbored diverse ammonoids adapted to dysoxic conditions. Thus the production of LIPS and resultant effects upon oxygen concentrations strongly shaped the faunas which inhabited Mesozoic oceans.