Paleogeography of the Middle Ordovician (~470 Ma)[citation needed]
TheOrdovician meteor event was a dramatic increase in the rate at whichL chondrite meteorites fell to Earth during theMiddle Ordovician period, about 467.5±0.28 million years ago,[1][2] lasting for about 40 million years.[3] This rate increase is indicated by abundant fossil L chondrite meteorites in a quarry in Sweden and enhanced concentrations of ordinary chondriticchromite grains in sedimentary rocks from this time.[1][4][5][6][7]
According to a 2019 study, this temporary increase in the impact rate could have been caused by the destruction of the L chondrite parent body that was 150 kilometers (93 mi) in diameter and orbited in theasteroid belt betweenMars andJupiter.[8] This occurred around 468 ± 0.3 million years ago having scattered fragments into Earth-crossing orbits, a chronology which is also supported by shock ages in numerous L chondrite meteorites that fall to Earth today.[9]
A 2024 study found that all of 21 studied craters from this event were at the time within 30° of the equator. Impactors directly from the asteroid belt would be expected to produce a random distribution of craters, so this suggests that the event may have been caused by an asteroid that passed within Earth'sRoche limit and broke up intoa ring system, material from which then deorbited and formed the craters. It is also speculated that the shading of Earth by this ring may have contributed to theHirnantian glaciation.[3]
^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).
^LINDSKOG, Anders; SCHMITZ, Birger; CRONHOLM, Anders; DRONOV, Andrei (2012-07-30). "A Russian record of a Middle Ordovician meteorite shower: Extraterrestrial chromite at Lynna River, St. Petersburg region".Meteoritics & Planetary Science.47 (8):1274–1290.Bibcode:2012M&PS...47.1274L.doi:10.1111/j.1945-5100.2012.01383.x.ISSN1086-9379.
^Cronholm, Anders; Schmitz, Birger (2010-07-01). "Extraterrestrial chromite distribution across the mid-Ordovician Puxi River section, central China: Evidence for a global major spike in flux of L-chondritic matter".Icarus.208 (1):36–48.Bibcode:2010Icar..208...36C.doi:10.1016/j.icarus.2010.02.004.ISSN0019-1035.
