Chicxulub crater
21.297222222222 - 89.594444444444Koordinaten: 21 ° 17 ' 50 "N, 89 ° 35' 40 " W
The Chicxulub crater ( Yucatec Maya, pronunciation: [ tʃikʃulub ] ) is a 180 km large and 65 million year old impact crater in the ground in the north of the Yucatán Peninsula in Central America ( Mexico). Since he is buried under thick sedimentary rocks and was not eroded, it is next to the Ries crater of the Swabian Alb in southern Germany, one of the best preserved large impact craters in the world. In connection with the Cretaceous - Tertiary boundary, he is associated with the extinction of the dinosaurs and most of the Mesozoic animal and plant life on the edge of the Cenozoic.
Location, size and identification of the crater
The center of the so-called Chicxulub impact crater is located on the Yucatecan coast, about under the eponymous village of Chicxulub Puerto, north of Mérida. While there is the southern part of the structure in the territory of the state of Yucatán, its northern part extends into the Gulf of Mexico. Depending on whether one is above the crater rim or inside the crater, it is from 300 to 1000 meters thick sedimentary layers of the Tertiary covered. The crater was. Means of measurement of magnetic and gravitational anomalies by Hildebrand et al (1991 ) detected and identified with the aid of petrographic analysis and the detection of shock - metamorphic minerals such as coesite or stishovite in Impaktit - rock samples from boreholes of the Mexican oil company PEMEX clearly identified as impact craters. It corresponds to a nearly circular basin with central mountain and inner ring structure whose diameter is estimated to be about 180-190 km. Sharpton et al. (1993) concluded from the gravitational anomalies that the crater rings has at least three, and probably still an additional outer ring to approximately 300 km in diameter. The diameter of the impactor ( asteroid or comet ) is estimated to be about 10-15 km.
On the surface of this third largest impact crater on earth little to notice, as the north of the Yucatan is very flat. However Analyses of Pope et al. ( 1996) found that mild elevations almost form semi-circular structures and the strength of the tropical soil formation also traces the former crater. In addition, at a radius of about 83 km ( diameter 166 km), there is a concentric beaded lacing her typical karst area cenotes. More recent data from the Shuttle Radar Topography Mission also showed clearly a semicircular topography in the area of the crater.,
Exploration of the Chicxulub crater
The coverage of the Chicxulub impact crater with mighty younger sedimentary rocks has hampered not only its discovery, but also makes it difficult to study and makes this extremely costly.
History
The story of Chicxulub dates back to the 1940s, when the state-owned Mexican oil company geophysicists during a systematic aircraft-based data collection for the first time discovered the unusual gravitational and magnetic anomaly in the area of Mérida. In the hope that this structure could prove to be a crude oil reservoir, several holes were drilled in the 1950s, which promoted Although no petroleum, but the Yucatán platform atypical, andesite -like rocks for days. Since most geologists were not yet familiar with the phenomenon of impact craters and their rocks at that time, this data from López Ramos (1975 ), this subsurface structure interpreted as a volcano, which had entered into the sedimentary rocks of the Cretaceous in the first internationally accessible publication. Two Mexican geophysicist ( Penfield and Camargo ) expressed for the first time in 1981 on a geophysical Congress the presumption, this might not also be a meteorite crater, which at that time was never addressed. At the same time worked in the 1970s, a working group from Berkeley to Walter Alvarez on the magnetostratigraphy of marine deposits of the Upper Cretaceous and the Paleogene in the Umbria -Marche Apennines in central Italy. In the global KP boundary layer occurring these discovered an extraordinarily high proportion of the extraterrestrial element iridium, which could be attributed only to a single large meteorite impact. This was followed by 10 years of mostly fruitless search for the source crater, which was not discovered until after it was found that the deposits are most powerful at the Cretaceous - Tertiary boundary in what is now the Gulf of Mexico and after the decades this data of the Mexicans had become aware. It is ironic that science thrillers (clearly in W. Alvarez's stated book), that the sample of Yucatán andesite, in which both the evidence of impact indicators as well as the dating of the crater (see below) succeeded over the years as a paperweight a geologist of the oil company PEMEX had served.
Geophysics
In order to draw conclusions on the energy released during the meteorite impact and to determine the angle of impact and size of the impactor, it is necessary to know the size and structure of the crater as accurately as possible. This is possible only through geophysical methods, in particular the geomagnetic, gravimetric and seismic. While for the first reconstruction of the Chicxulub crater, especially the PEMEX data were processed, more geophysical data could in the 1990s, particularly on land ( land seismics), by the Institute of Geophysics of the UNAM (Mexico) are collected. In January and February 2005 an even further seismic study aboard the R / V Maurice Ewing in the Gulf of Mexico was conducted, the results of which were recently unveiled at scientific meetings. In addition to the outcrops, which gives us the geophysics of the crater structure, it also enables, due to their clearly distinguishable from the surrounding sedimentary rocks geophysical characteristics to correlate the various available in bores Impactites each other and to determine their areal distribution in the subsurface in the north of the Yucatán.
Holes
The only way to examine the impactites of the Chicxulub crater directly, are usually quite elaborate and expensive drilling. The low accommodated in the 1950s and 1960s, holes, though sometimes very deep (up to 3500 m) extend into the ground, only limited meaningful as they - carried out with the aim of finding oil - no real core holes were only sporadic samples exist which are additionally largely lost. 1996 therefore initiated the UNAM flat drill program in the state of Yucatán, which could erbohren just outside the actual crater impactites of the ejecta of the crater because of its small depth ( max. 800 m), however.
In an international collaborative project led by the International Continental Deep Drilling Program at the GFZ Potsdam in 2002, the so-called Chicxulub Scientific Drilling Project in Yaxcopoil, south of Mérida, performed. The core hole Yaxcopoil -1 reached a depth of 1511 meters and promoted an almost complete core of tertiary sedimentary rocks ( 0-795 m ), several layers of impactites within the crater ( 794-896 m) and a sequence of Upper Cretaceous sedimentary rocks, probably to one that had slipped into the crater Megablock the subsoil belong (896-1511 m). Since the well-preserved Impactites this large impact crater on Earth are unique, since several research groups to investigate these rocks under a variety of aspects. First results were published in a special volume of the journal Meteoritics & Planetary Science, 2004. On the basis of geophysical explorations of 2005 are currently two other deep boreholes in the Chicxulub crater in planning.
Superficially occurring rocks
Although rocks of the Chicxulub crater themselves are not open to the surface, considerable progress has been made in recent years to discover the meteorite impact attributable to sedimentary rocks and on the earth's surface into ever closer proximity to the crater. In addition to sometimes several meters thick deposits in the southeastern United States, Haiti, Cuba, and in NO and Central Mexico, it is mainly chaotic breccia in southeastern Mexico and Guatemala who have experienced special attention. The most recent example of such rocks are at the surface in the southeast of the Yucatan discovered chaotic Kalksteinbrekzien which contain rare and fragments from the crater interior and the so-called continuous ejecta blanket of the Chicxulub crater, are comparable only with the debris masses of the Ries crater in Germany, in ( Ocampo. et al, 1996, Pope et al, 2005).. The study of this 280-365 km distance retrieved from the crater center sediments new insights both about the Chicxulub crater as well as for the Planetological basic research ( see below).
All these sedimentary rocks have in common is that they have as well as the crater rocks and the globally occurring KP boundary layer short-term phenomena, formed so -called event deposits formed within months, days, hours, or even just minutes after a strike. Such a formation period is orders of magnitude beyond the stratigraphic resolution capability both radiometric (here about 20,000-100,000 years) and biostratigraphic ( here about 50,000 years) datings. You are so catastrophically in the geological sense, greatly to the confusion about the exact age of the Chicxulub crater and the controversial discussions whether the possible connection of the crater to the mass extinction at the Cretaceous - Tertiary boundary contributes (see below). The case of Chicxulub is the main example of a new catastrophe theory, according to which geological processes have not, as requested by representatives of uniformity, always taken place slowly and uniformly and analogous to currently observable processes, but also in addition to these gradual processes in the Earth's abrupt changes ( cataclysms ) has given with some global impact.
Chicxulub, climate and the extinction of the dinosaurs
The coincidence of the global iridium anomaly occurring with the extinction event at the Cretaceous - Tertiary boundary was the starting point for the impact hypothesis for the mass extinction. In this early work, a crater of about 150-200 km in diameter and an impactor ( asteroid ) was called for at least 10 km to explain the worldwide occurrence of iridium in rocks this time. The cause of the mass extinction itself by the explosive release of a massive energy potential, five orders of magnitude higher than the entire nuclear arsenal of the earth, caused climate catastrophe was seen. Within a radius of up to one thousand kilometers around the impact zone almost all life, the shock wave and the impact following the tsunami was in outward to decreasing intensity by the heat wiped out. While 15 % of the mass of rocks thrown up the kinetic energy sufficient to overcome gravity and to let them escape into space, reaching 85% of the mass thrown up within 72 hours again the earth. It is believed that these highly heated rocks could have triggered global forest fires. Furthermore, the sunlight was globally distributed dust and gases blocked and triggered a Nuclear Winter comparable Impaktwinter of several months duration. This was followed by a cooling triggered by carbon and sulfur gases greenhouse effect, which then led to an anoxic event in the world's oceans. This along with intense acid rain by the vaporized sulfur-containing rocks of the Yucatan platform ( anhydrite) caused thus an almost complete collapse of the food chain, both on land and in the sea, making it the third largest mass extinction of Earth's history.
The age of the meteorite impact on the Yucatán was based on the PEMEX samples radiometrically dated at 65 million years ( 64.98 ± 0.05 Ma, . Swisher et al, 1992) and also with geochemically occurring at the boundary between Cretaceous and Paleogene Impaktgläsern (65, 01 ± 0.08 Ma and 65.07 ± 0.10 Ma; see a Blum et al, associated 1993).. This seemed early 90s of the iridium anomaly and mass extinction at the Cretaceous - Tertiary boundary (and thus the extinction of the dinosaurs) responsible crater to have been found.
Observations containing at Impaktgläsern sedimentary successions of the Upper Cretaceous - that is older than the Chicxulub crater -. Northern Mexico and the analysis of a sequence of sedimentary rocks above the impactites and below the Tertiary in the bore Yaxcopoil -1 brought some researchers ( Keller et al, 2004a, 2004b ) However, to the conviction of the crater do not play the role in the mass extinction at the end of the Cretaceous, which was attributed to him. He is, therefore, about 300,000 years older than the actual KT boundary layer, and also only one of several meteorite impacts that occurred at the Cretaceous-Tertiary boundary. The mass extinction was also caused in the opinion of Keller and her co-authors not only of meteorite impacts, but on a combination of these due to climate changes and the Deccan event.
Other publications bring the deposits in northeastern Mexico continue with the Chicxulub impact in conjunction ( Schulte and Kontny, 2005). The critical sequence above the impact rocks in this core represented by Smit and his colleagues ( Smit et al., 2004) not in the last 300,000 years of the Cretaceous sediments deposited gradually, but material which was washed back immediately after the impact into the crater.
A broad-based study led by the University of Erlangen -Nuremberg from 2010 suggests as the only plausible explanation for the extinction of the dinosaurs, the scientists involved in the Chicxulub impact.
The discussion about the role of the Chicxulub strike the mass extinction at the end of the Cretaceous continues unabated, the question can not be regarded as definitive clarified.