Uranium lead dating meteorites
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The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: (1) End Ordovician (Ordovician-Silurian extinction), (2) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction), (4) End Triassic (Triassic-Jurassic extinction), and (5) End Cretaceous (Cretaceous-Tertiary extinction).(See geologic time scale for an overview of these time periods.) These and a pair of other extinction events acting as "book ends" for the Big Five are highlighted below: Other hypotheses, such as the spread of a new disease or simple competition following an especially successful biological innovation are also considered.
The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters.However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.The Ordovician-Silurian extinction (about 444 mya), which may have comprised several closely spaced events, was the second largest of the five major extinction events in Earth history in terms of percentage of genera that went extinct.Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within 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, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation.These changes in the isotopic ratios may indicate distinct events or particular phases within one event.
At that time, all complex multicellular organisms lived in the sea, and of them, about 100 marine families covering about 49 percent of genera (a more reliable estimate than species) of fauna became extinct (Rohde 2005).Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.Anoxic conditions in the seabed of late Devonian ocean basins produced some oil shale.The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms.(The only larger one was the Permian-Triassic extinction (about 251 mya).) The End Ordovician extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician period and the following Silurian period.