Cretaceousâ€“Paleogene extinction event
The Cretaceous-Tertiary boundary, usually called KT boundary, is the time a geological event million before the age of 66, which defines the transition from the Cretaceous to Paleogene (until 2000 " tertiary "). It was the beginning of one of the largest mass extinction of Earth's history, which ended in particular the era of the dinosaurs. This geological cardinal point forms the boundary between the Mesozoic and Cenozoic of.
However, this is not, this is a time in the traditional sense. The geological event consists of serious environmental changes in the geologically short period of about a million years. Due to the impact of one or more meteorites ( KT- Impact) and increased volcanic activity, there was an extreme faunal and floral change. Geological feature of the KT boundary is the iridium anomaly, which suggests a meteorite impact, as well as high amounts of ash and rock beads, which must have formed at high heat. Whether these events are related to the extinction causally, is not currently understood.
Since the ICS has the " tertiary " is replaced by " Paleogene ", one would speak of the KP boundary since 2000 actually, but it has become the term KT boundary enforced.
Evidence of meteorite impacts
Although an entry extraterrestrial material by several meteorite impacts on the Earth's surface is occupied at this time by geological research results or a, is the question whether this event is actually responsible for the mass extinction at this time, not yet fully understood. As a possible point of impact is called the Gulf of Mexico near the Yucatan peninsula often the Chicxulub crater. Controversial studies on cores from the crater area by G. Keller (2004) and recently M. Harting (2004) suggest, however, that the Chicxulub crater could be about 300,000 years older than the Cretaceous - Paleogene boundary layer. That impact could not have caused the mass extinction in this case, which must be in the case of a global devastating strike expired in a much shorter period than the above 300,000 years. Alternatively, therefore, it is theorized that it could be at the KT impact to the impact of several asteroids and comets within a shorter period of time.
An important indication of the hypothesis of one or more strikes is the unusually high iridium content of many rocks near the Cretaceous - Paleogene boundary. Since the mantle compared to stony meteorites is low in iridium, one suspects that finds itself in these layers of dust whirled up by the impact. Strong support is replaced by the hypothesis of a meteor impact caused by an abnormality of the chromium isotope distribution in the same layer, which also contains the iridium anomaly. The chrome isotope distribution is usually homogeneously on the earth. While still argue with the iridium anomaly, that volcanic activity could cause an iridium enrichment, the isotope anomaly can only be explained by the admixture of extraterrestrial material for chromium. Further mineralogical traces of the impact consist of results of the pressure wave and the high temperatures, such as altered quartz features (PDFs ), stishovite, zircon, diamond crystals and glass beads ( microtektites ). These structures occur worldwide and accept quantitatively proportional to the distance from the crater. A possible candidate for the impactor is an asteroid with a composition similar to carbonaceous chondrites; the latter have the same chromium isotope distribution as the KT or KP boundary layer. As a comet probably made of ice and dust particles whose composition is similar to carbonaceous chondrites, a comet can not be excluded as a possible impactor.
The impact of a meteorite with a diameter of several kilometers causing a huge explosion which may have the entry of dust particles into the atmosphere and a consequent global climate change result. Due to the attenuation of solar radiation, the temperatures, plant growth returns - effects that are transmitted to the entire food chain. It is therefore presumed that the extinction of the dinosaurs could be related at the end of the Cretaceous period with this event. If confirmed, that the origin of the Chicxulub crater is not identical in time with the mass extinction, which is associated with the adopted KT impact event, however, it is questionable whether even strikes of this magnitude sufficient to cause a corresponding mass extinction. Climate change could also be caused by changes in the Earth's atmosphere during a period of increased volcanism.
In general, the causes of mass extinctions that occurred several times in Earth's history, yet little understood. Ecology, a sub-discipline of biology, points out that single events in complex habitats can indeed take a serious impact, but which are carried away by conventional evolutionary processes. Accordingly explain a cosmic event, especially the offense, but not the biological process of revolution in the world of life itself. The representatives of the meteorite hypothesis, however, argue that such a rarely occurring event may actually be effective as a transformative force in terms of a rapid physical destruction of large parts of the earth's surface with pressure, fire and water waves, poisoned atmosphere, rapid temperature fluctuations, blackout etc. assuming an arbitrarily scaled- impact event such a scenario is described scientifically conceivable but currently can not be proven that it actually occurred at the KT boundary. An ecological upheaval, however, as it is fossil proves is possible even without such a disastrous sequence.
In addition to a single - strike a number of alternative processes of the KT- Inzession be considered. So is also suspected that the impact could be just the end or the catastrophic part of a chain of independent events, which resulted in their entire sequence to the frame of the mass extinction. Also conceivable is a series of small and large meteorite impacts that hit the earth in a geologically short period of time. This seems especially likely when one around the center of the galaxy pulls the motion of the solar system into consideration, it being assumed that they occasionally crosses areas with higher density of cosmic debris, which are astronomically unobservable at the present time. Also, the gravitational influence of a moving past other stars can lead to changes in behavior of the already existing railway in the solar system objects.
In addition, can be triggered by a sufficiently large impact even volcanic activity. The pulse generated by the impact is made geotektonisch in the rock layers continues and may cause by superposition of locally significant transformations and areal leakage of magma.
Be explained also another great mass extinction at the transition between the Palaeozoic and Mesozoic eras ( Paleozoic and Mesozoic ), could by a meteorite impact, the " PT- impact " ( " Permian-Triassic impact "). But no high iridium shares or isotopic anomalies for this limit in the corresponding strata previously determined yet. Climate change or climate disasters ( and consequently the mass extinction ) could of course also have causes in the context of internal geodynamic processes on Earth, such as increased volcanic activity. At least for the Permian - Triassic boundary can be an extraterrestrial cause for the mass extinction currently do not use.