This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
The flyby of asteroid 2012 DA14 and especially the past and present Russian meteors are impressive reminders that the terrestrial biosphere can be affected also by extraterrestrial forces. However contrary to headlines by the general media the connection between mass extinctions and large meteor impacts is still poorly understand.
The Scaglia Variegata and Scaglia Cinerea are two geological formations deposited during the late Eocene and early Oligocene (36 and respectively 33 million years ago). This period of transition is characterized by profound climatic changes and a biological crisis at a global scale. In less than a million years about 20% of genera of marine organisms became extinct. The causes of this mass extinction are not yet entirely clear. The gradual hypothesis invokes the slow drift of continents (especially the drift of Antarctica to the South Pole and subsequent global cooling effect) and the resulting climatic and environmental changes; the catastrophic hypothesis invokes the impact of a meteorite as the main cause of the sudden biological crisis.
In fact two very large impact craters are dated into the Eocene: Chesapeake Bay (located in North- America) and Popigai (Siberia). Popigai Crater is with a diameter of 100 km one of the largest impact craters on our planet (and the largest formed in the last 65 million years), followed by the 85 km large Chesapeake Bay Crater.
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Some clues to solve the mystery about the biological crisis at the end of the Eocene are found in the sediments exposed in the quarry of Massignano, named after a small village near the Italian city of Ancona.
Fig.1. The Eocene-Oligocene transition in the quarry of Massignano (Marche - Italy), above sediment bed 17 (top of the quarry).
At the base of the outcrop with the Scaglia Variegata and Scaglia Cinerea formations two thin layers have been identified, characterized by abnormal concentrations of the element Iridium and the isotope Helium-3. Both substances are depleted in the rocks of earth's crust, but quite concentrated in extraterrestrial material. These layers also contain spherules of the minerals spinel and quartz, displaying a lamellar and shattered structure. Such grains can be formed only by high temperatures and high pressure as experienced during an impact.
It seems almost certain that two objects of extraterrestrial origin hit earth during the transition from the Eocene to the Oligocene, but it appears also that the effects of both impacts on terrestrial ecosystems were very limited. Complicating the question how large impacts affect the life on earth are also temporal uncertainties. According to some dating results the two mentioned impacts do not coincide with the Eocene extinction phase (between 37 to 38 million years ago) or the Eocene-Oligocene boundary (35,5 to 36,0 million years ago), but occurred 1 to 2 million years before and respectively after these periods.
Fig.2. "Raup´s Kill Curve", named after paleontologist David M. Raup. The "kill curve" (blue) of RAUP 1991 was originally fit to the Cretaceous - Palaeogene impact data (60% of species wiped out with the Chicxulub crater of about 180 kilometer in diameter), and it predicted that much smaller impacts should cause significant extinctions. However, when the late Eocene impacts (which caused almost no extinctions) are plotted, the "kill curve" takes a different, S-like shape, and suggests that only the impacts above a certain threshold value have the potential to cause mass extinctions. But even one of the best studied event - the Chicxulub-impact - is not unequivocal.
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PROTHERO, D. (2006): After the Dinosaurs: The Age of Mammals (Life of the Past). Indiana University Press: 384