The rise in carbon dioxide was certainly dramatic. But it was utterly dwarfed by the fall in oxygen, which may have dropped by two-thirds from its high concentration maximum of as much as 35 percent in the early Permian, to perhaps as low as 12 percent in the early Triassic. The cause of the drop is agreed upon: carbon-rich material was no longer buried at the rates it had been in the late Carboniferous and early Permian. At the same time, the burial rate of pyrite-bearing sediment also dropped dramatically. With huge quantities of reduced carbon exposed to the atmosphere, oxidation set in, removing oxygen molecules from the air. But why did this happen?
The ultimate cause seems to be related to two events. First, the formation of the supercontinent Pangea was completed about the same time that the oxygen drop occurred. As the continents fused together, many of the sedimentary basins and swamps that had been the site of the rapid and profound burial of plant material that caused the rise of oxygen to the maxima of the Carboniferous-early Permian event were uplifted and thus could no longer serve as traps and reservoirs of reduced carbon. Second, the drop in carbon dioxide that culminated about 300 million years ago may have drastically reduced the amount of plant material through mass extinction of species, which in turn caused a significant reduction of plant biomass. There was less plant life to be deposited, and both of these events conspired to cause the oxygen crash.
What is called the Permian extinction is really a series of extinctions, beginning at the end of the Guadaloupian Stage of the Permian Period (about 254 million years ago), with a second and far more severe pulse at the Permian-Triassic boundary itself, dated at about 251 million years ago. This “double extinction” at the end of the Permian has been known for about a decade or more. But now we are finding that even between these larger events there were smaller extinction episodes as well. What caused this pattern?
The episode at 251 million years ago is one of the most controversial subjects in modern geological research. There are several scenarios in the controversy, and they are listed below in arbitrary order: