and increase in their diversity is attributed to the late Permian, when Pangaea's global effect was thought to have peaked. While Pangaea's formation certainly initiated a long period of marine extinction, its impact on the "Great Dying" and the end of the Permian is uncertain. Interstellar dust John Gribbin argues that the Solar System last passed through a spiral arm of the Milky Way around 250 million years ago and that the resultant dusty gas clouds may have caused a dimming of the Sun, which combined with the effect of Pangaea to produce an ice age. Combination of causes Possible causes supported by strong evidence appear to describe a sequence of catastrophes, each worse than the last: the Siberian Traps eruptions were bad enough alone, but because they occurred near coal beds and the continental shelf, they also triggered very large releases of carbon dioxide and methane. The resultant global warming may have caused perhaps the most severe anoxic event in the oceans' history: according to this theory, the oceans became so anoxic, anaerobic sulfur-reducing organisms dominated the chemistry of the oceans and caused massive emissions of toxic hydrogen sulfide. However, there may be some weak links in this chain of events: the changes in the 13C/12C ratio expected to result from a massive release of methane do not match the patterns seen throughout the early Triassic; and the types of oceanic thermohaline circulation that may have existed at the end of the Permian are not likely to have supported deep-sea anoxia. See also Carbon dioxide Extinction event Climate change List of possible impact structures on Earth Silurian hypothesis References Further reading (editor), Understanding Late Devonian and Permian–Triassic Biotic and Climatic Events (Volume 20 in series Developments in Palaeontology and Stratigraphy (2006)). The state of the inquiry into the extinction events. (editor), Permo–Triassic Events in the Eastern Tethys : Stratigraphy Classification and Relations with the Western Tethys (in series World and Regional Geology) External links Podcast available. . . Events that forced the climate Extinction events Hypothetical impact events Meteorological hypotheses Permian events Permian life Triassic events Triassic life