OXYGEN AS A CONTROL ON THE PERMIAN-TRIASSIC EVOLUTION OF THE MARINE BIOSPHERE

The Permian through Triassic interval was a crucial 100 million years in the evolution of the Earth's biosphere. Three major extinction events (end-Guadalupian, Late Permian and Late Triassic) witnessed the decline of an enormous range of previously successful taxa, and ultimately enabled the rise to present dominance of the modern marine fauna. The Permian-Triassic interval also witnessed dramatic atmospheric, climatic, oceanographic and paleogeographic changes. One of the key factors influencing the evolution and extinction of the marine biosphere was oxygen. Inferred levels of atmospheric oxygen peaked in the Carboniferous and declined through the Permian to reach a minimum in the earliest Triassic. The coincident decline in body sizes of many marine taxa through this same interval are probably related to this decline in oxygen, exacerbated in the seas by global warming (reducing dissolved oxygen still further). The major Permian-Triassic Superanoxic event (end-Guadalupian to Middle Triassic) was probably the result of the combination of global warming (resulting in a sluggish ocean circulation) and a decrease in atmospheric oxygen levels. The importance of oceanic anoxia as a causal mechanism in the Late Permian and Late Triassic extinction events has become more equivocal in the light of recently published data. For example, mismatches in the timing of diversity decline, ecosystem collapse and the onset of anoxic conditions at several locations worldwide suggest that anoxia is not the root cause of the Late Permian event. Oceanic anoxia did, however, exert a degree of control on biotic recovery in the Early Triassic and Early Jurassic. Body sizes, biodiversity and patterns of marine recovery appear strongly linked to relative oxygenation, although other factors were also operating.