What, if anything, are mass extinctions?

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Abstract
Many phenomena that have traditionally been called ‘mass extinctions’ are in fact clusters of extinction episodes roughly associated in geological time. This is the case with the latest Ordovician, late Devonian, mid-Cretaceous, latest Cretaceous and Late Eocene-Oligocene extinctions. Several of these clusters are caused, each episode by a different causal factor. Such mass extinctions are then due to the coincidence of various processes in the environment, and they can hardly be considered as individual events. The latest Permian mass extinction, however, is caused by a single process that affected the global ocean-atmosphere system. In the late Permian, the world ocean was full of deposits rich in organic matter, which enhanced nutrient recycling. After oxygen was brought to the sea floor (by whatever process), nutrients began to sink to the sea-bottom, and the resulting nutrient deficiency must have caused mass extinction in the sea. Oxidation of huge amounts of organic matter and associated sediments at the sea bottom must have drawn oxygen from the atmosphere, and the resulting fall in atmospheric oxygen must have contributed to extinctions on land.