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Ordovician meteor event
The Ordovician meteor event is a proposed shower of L chondrite meteors that occurred during the Middle Ordovician period, roughly 470 million years ago. This theory was proposed by Swiss and Swedish researchers based on the comparatively tight age clustering of L chondrite grains in sediments in southern Sweden and at the Neugrund crater in Estonia.〔H. Haack et al. ''Meteorite, asteroidal, and theoretical constraints on the 500-Ma disruption of the L chondrite parent body'', Icarus, Vol. 119, p. 182 (1996).〕〔Korochantseva et al. ("L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar-39Ar dating" ) Meteoritics & Planetary Science 42, 1, pp. 3-150, Jan. 2007.〕 They proposed that a large asteroid transferred directly into a resonant orbit with Jupiter, which shifted its orbit to intercept Earth.
In addition to the northern European evidence, there is circumstantial evidence that several Middle Ordovician meteors fell roughly simultaneously 469 million years ago in a line across North America, including the Ames crater in Oklahoma, the Decorah crater in Iowa, the Slate Islands crater in Lake Superior, and the Rock Elm crater in Wisconsin. It is hypothesized that this shower was associated with, or possibly caused, the Great Ordovician Biodiversification Event.〔(【引用サイトリンク】url=http://all-geo.org/metageologist/2013/09/the-great-ordovician-meteor-shower/ )〕
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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