Cronholm Anders

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  • Preprint
    Asteroid breakup linked to the Great Ordovician Biodiversification Event
    ( 2007-09-26) Schmitz, Birger ; Harper, David A. T. ; Peucker-Ehrenbrink, Bernhard ; Stouge, Svend ; Alwmark, Carl ; Cronholm, Anders ; Bergstrom, Stig M. ; Tassinari, Mario ; Xiaofeng, Wang
    The rise and diversification of shelled invertebrate life in the early Phanerozoic took place in two major steps. During the Cambrian Explosion at ca. 540 Ma a large number of new phyla appeared over a short time interval. Biodiversity at the family, genus and species level, however, remained low until the Great Ordovician Biodiversification Event (GOBE) in the mid-Ordovician. This event represents the most intense phase of species radiation during the Paleozoic and the biological component of planet's seafloors was irreversibly changed. The causes of the GOBE remain elusive mainly because of a lack of detailed data relating faunal to environmental change. Here we show that the onset of the major phase of the GOBE coincides at ca. 470 Ma with the disruption in the asteroid belt of the L chondrite parent body, the largest documented asteroid breakup event during the last few billion years. The precise coincidence between an event in space and on Earth is established by bed-by-bed records of extraterrestrial chromite, osmium isotopes and invertebrate fossils in mid- Ordovician strata in Baltoscandia and China. We argue that frequent impacts on Earth of kilometer-sized asteroids accelerated the biodiversification. This is supported also by abundant mid-Ordovician fossil meteorites and impact craters.
  • Article
    A fossil winonaite-like meteorite in Ordovician limestone : a piece of the impactor that broke up the L-chondrite parent body?
    (Elsevier, 2014-06-06) Schmitz, Birger ; Huss, Gary R. ; Meier, Matthias M. M. ; Peucker-Ehrenbrink, Bernhard ; Church, Ross P. ; Cronholm, Anders ; Davies, Melvyn B. ; Heck, Philipp R. ; Johansen, Anders ; Keil, Klaus ; Kristiansson, Per ; Ravizza, Gregory E. ; Tassinari, Mario ; Terfelt, Fredrik
    About a quarter of all meteorites falling on Earth today originate from the breakup of the L-chondrite parent body ∼470 Ma∼470 Ma ago, the largest documented breakup in the asteroid belt in the past ∼3 Ga∼3 Ga. A window into the flux of meteorites to Earth shortly after this event comes from the recovery of about 100 fossil L chondrites (1–21 cm in diameter) in a quarry of mid-Ordovician limestone in southern Sweden. Here we report on the first non-L-chondritic meteorite from the quarry, an 8 cm large winonaite-related meteorite of a type not known among present-day meteorite falls and finds. The noble gas data for relict spinels recovered from the meteorite show that it may be a remnant of the body that hit and broke up the L-chondrite parent body, creating one of the major asteroid families in the asteroid belt. After two decades of systematic recovery of fossil meteorites and relict extraterrestrial spinel grains from marine limestone, it appears that the meteorite flux to Earth in the mid-Ordovician was very different from that of today.