Contrasting `Oceanic Anoxic Events' in Earth's History

Oxygen is fundamental to the evolution of life and a lack of oxygen in marine environments is often cited as a causal mechanism for mass extinctions. Anoxia was likely a main contributor to the Permo-Triassic extinction and also important for causing the delayed and complex recovery that took place during the Early Triassic. Despite strong evidence for anoxia, at various places, times and water depths, the extended Permo-to-Early Triassic interval was exceptionally complex and multiple environmental pressures occurred simultaneously. Each kill-mechanism holds a unique position in the carbon cycle and yet they are all inter-related. For example, oceanic anoxia is intimately linked to changes in weathering and nutrient availability, biological productivity and temperature increases. It also represents one of many powerful mechanisms that can help stabilize the global climate, where carbon burial under anoxic conditions can drive dramatic cooling. Here we examine the details of anoxia development for the Permo-Triassic extinction and Early Triassic recovery period, calling upon comparisons to the well-studied Mesozoic Oceanic Anoxic Events (OAEs). The OAEs represent discrete intervals of anoxia that may offer a framework for understanding oxygenation around the Permo-Triassic. In particular, we question whether the Early Triassic represents a series of separate carbon-cycle perturbations, like the OAEs, or a complex internal response to extreme environmental change. We present independent evidence for the global extent of anoxia during OAE2, using 238U/235U (δ238U). These data are placed into the wider carbon-cycle context in order to unravel the driving mechanisms and potential feedback responses of anoxia development. This conceptual OAE model will then be applied to the complexities of the extended Permo-Triassic interval to examine the details of carbon cycle behaviour.