THE EARLY TRIASSIC AND THE SUPERCONTINENT CYCLE

The Earth has witnessed the formation and breakup of five supercontinents over the past three billion years, in a process known as the supercontinent cycle. The Phanerozoic has been dominated by the amalgamation and breakup of Pangea, which existed from 300 to 200 million years ago. The evolution of life on Earth in the mid-Phanerozoic was strongly affected by the existence of Pangea. In fact, three of the biggest mass extinctions in the Phanerozoic occurred during this 100 million-year-long time interval. Pangea, which spanned from pole to pole, was bordered by the enormous Panthalassia Ocean, which had much different circulation and geochemical dynamics than the modern ocean. Because a supercontinent acts as an insulator that traps mantle heat, once Pangea formed the mantle began heating up and producing basaltic magma that rose to the surface. Formation of flood basalts and large igneous provinces was common from the later part of Pangea’s existence into the subsequent breakup, and this time interval was also marked by significant increases in atmospheric carbon dioxide. Four mass extinctions marked this later part of Pangea’s history (end-Guadalupian, end-Permian, end-Triassic, and early Toarcian), and each is correlated with a large Pangean igneous province. Size of these mass extinctions varied in extent and their recoveries varied in duration. The Triassic began about 251 million years ago with the eruption of the Siberian traps and the largest mass extinction of the Phanerozoic. The effects of this extinction lingered until the beginning of the Middle Triassic, 4-7 million years later. The Early Triassic contains a unique fossil and stratigraphic record, when compared to the rest of the Phanerozoic. Such features include extreme cosmopolitanism of the benthic fauna, widespread abundance of benthic opportunists, a metazoan reef gap, proliferation of microbial sedimentary structures, an anomalous carbon isotope record, a chert gap, and a coal gap. It is likely that the effects of the eruption of the Siberian Traps were amplified by the configuration of Pangea and Panthalassia. Thus the end-Permian mass extinction and Early Triassic represent the most severe of the Pangean biotic crises, and the bottleneck which the effects of this supercontinent produced on metazoan life is still manifest today.