Ordovician–Silurian extinction event

The Ordovician mass extinction was based on the percentage of the extinguished species as well as relative to the total loss of individuals, the second largest of the five major mass extinction of Earth's history. In the period between 450-440 million years ago today probably occurred two extinctions that occurred at an interval of one million years. In relation to life in the sea, it was the second largest mass extinction after the extinction event at the Permian-Triassic boundary. At this time the habitat of all known forms of life was confined to the seas and lakes. About 85 % of species, 60 % of genera and 26% of the families of all meeresbewohnenden species died out, including two -thirds of all brachiopods and bryozoans. Particularly affected were also molluscs, echinoderms and corals. The direct cause of the mass extinction appears to be the movement of Gondwana to have been that moved into the region of the South Pole. This resulted in a global cooling and glaciation and a resulting sea-level waste. The falling sea level interrupted or eliminated habitats along the Flachseebezirke along the continental shelf. In deposits that were discovered in the Sahara, they found evidence of this icing. A combination of the drop in sea level and a slowdown caused by the icing are the likely reasons for the mass extinction of the Ordovician.

Context

The extinction event took place 443 million years ago; it was the time of the most significant diversification of the earth's history and marks the boundary between the Ordovician and the following Silurian. During the extinction event more pronounced changes took place in the isotopic composition of oxygen and carbon, which indicate changes biologically caused. This complexity may indicate several individual, close to successive events or to multiple phases within an event. At this time lived the most complex multi-cellular organisms in the seas. About 100 marine families became extinct, which corresponds to about 49 % of genera of fauna. Were decimated molluscs, echinoderms, trilobites, conodonts and graptolites. Statistical analyzes suggest that the occurring at this time loss of maritime life was caused more by an increase in the extinction rate than by a decrease in speciation.

Possible Causes

These extinctions are the subject of intense research activity. The extinction highlights correspond to the beginning and end of the most severe ice age of the Phanerozoic. It marks the end of a long cooling trend in Hirnantium at the end of the Ordovician, prevailed in the typically greenhouse conditions.

The event was preceded by a drop in atmospheric carbon dioxide concentration, which primarily affected the shallow water of the oceans, where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it.

The event associated with the rock layer was found in North Africa and in the then adjacent northeastern South America in rock sequences of the late Ordovician, which at that time was located at the South Pole. By freezing water of the oceans is bound, during interglacials it is released. This caused a repeated rise and fall of sea level; the extensive, intra- continental Ordovician shallow-water seas disappeared, what many ecological niches eliminated. On their return, they were colonized by founder populations lacking many organisms families. With the next pulse Vergletscherungs these disappeared, what the biological diversity each time further reduced ( Emiliani 1992 p. 491 ). In the layer sequences of North Africa reported Julien Morneau five Vergletscherungspulsen seismic sections.

This was accompanied by a shift in the locations of the ground -water development. From greenhouse warm low latitudes they were moved to higher latitudes, but they were characterized by colder conditions. Thus, the deep-water currents and the enrichment of soil water with oxygen enhanced. For a short time there flourished a corresponding adaptations of the fauna before anoxic conditions recurred. The collapse of oceanic circulation patterns brought nutrients from the deep ocean to the top. Survivors species were those that deal with the changing conditions and were able to fill the left by the extinctions ecological niches.

Gamma-ray burst

A small minority of scientists has suggested that the initial extinction events were caused by a gamma-ray burst, which came from a hypernova within a distance of 6000 light years from Earth ( from an adjacent spiral arm of the Milky Way ). A ten -second outbreak would have half the ozone layer of the Earth abruptly destroyed. Organisms that lived near the earth's surface - including plants - would have been subjected to intense ultraviolet radiation. Although the hypothesis matches the pattern of the beginning of the extinction event, there is no clear evidence that there ever was a gamma-ray burst in the vicinity of the earth.

Volcanism and weathering

Recent research results suggest a greater role of the greenhouse gas CO2. Outgassing due to intense volcanism in the Ordovician were offset by strong weathering of the elevating Appalachians, which carbon dioxide was removed from the atmosphere. In Hirnantium volcanism returned and the continued weathering caused a significant and rapid reduction in carbon dioxide concentration. This correlates with a short and Ice Age.

End of the event

The end of the second event took place when melting glaciers again caused a rise in sea level and stabilized this. The recovery of the diversity of life with the re- flooding of continental shelves at the beginning of the Silurian also brought an increased diversity within the surviving orders.

Swell

• Emiliani, Cesare. (1992). Planet Earth: Cosmology, Geology, & the Evolution of Life and the Environment. Cambridge University Press. (Paperback Edition ISBN 0-521-40949-7 )