Gamma ray burst hypothesis[edit]
A small minority of scientists have suggested that the initial extinctions could have been caused by a gamma ray burst originating from a hypernova within 6,000 light years of Earth (in a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis, to high levels of ultraviolet radiation.[13][14][15][16] Although the hypothesis is consistent with patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma ray burst ever happened.
Volcanism and weathering[edit]