Uranium lead dating meteorites

23-Feb-2016 23:28

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.Evidence of these has been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.Glaciation locks up water from the oceans, and the interglacials free it, causing sea levels repeatedly to drop and rise.The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.The Late Devonian extinction was one of the five major extinction events in the history of the Earth's biota.The event was preceded by a fall in atmospheric CO, which selectively affected the shallow seas where most organisms lived.As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it.

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The shifting in and out of glaciation stages incurred a shift in the location of bottom water formation—from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water.

The estimates of species loss depend on surveys of marine taxa that are perhaps not known well enough to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases.

Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms (armored fishes).

A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes (the jawless fishes) suddenly disappeared. Although it is clear that there was a massive loss of biodiversity toward the end of the Devonian, the extent of time during which these events took place is still unclear, with estimates as brief as 500 thousand years or as extended as 15 million years, the full length of the Famennian.

Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions.

(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.

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