"It was nothing of this earth, but a piece of the great outside; and as such dowered with outside properties and obedient to outside laws."
"The Colour Out of Space", by H.P. Lovecraft (1927)
In the night of November 8, to November 9, a 400m large asteroid, named 2005 YU55, will pass close to (and miss) earth. This rare event is a good motive to shortly discuss the role of asteroids and especially meteorites on the geology of earth and especially on the history of life. It was a discovery in the mid of the 20th century that started a controversy not yet settled today.
Fig.1. "Cretaceous diary", guest book in the little inn of the gorge of Gubbio.
Until 50 years ago it seemed that a characteristic peculiarity of the Cretaceous-Palaeogene (C/Pg) transition, famous for the mass extinction event that "killed" off the dinosaurs, was the apparent lack of a complete stratigraphic record. However in the decade 1960-1970 the American geologist Walter Alvarez discovered a homogeneous and complete succession of bedded limestone- and marl-layers in the gorge of Gubbio (also Gola del Bottaccione, located in the far north-eastern part of the Italian province of Perugia, Umbria), called the "Scaglia rossa"-formation.
Alvarez attempted to calculate the rate of deposition of this formation by analysing the concentration of rare earth elements, also called rare earth metals, found in the sediments. A constant rain of micrometeorites, enriched in these metals, coming from outer space causes a constant concentration of rare earth metals in the sediments. A sudden change therefore can indicate that the rate of deposition of the sediment also suddenly changed.
It was during this research that Alvarez discovered the today well-known Iridium anomaly. At first the origin of this anomalous concentration remained unclear - in 1980 Water Alvarez and his father, the nobel-prize physicist Luis W. Alvarez (1911-1988), proposed two possible explanations: a very slow sedimentation rate of the Scaglia Rossa, resulting in an apparent concentration of micrometeorites, or the impact of a large mass of extraterrestrial material.
But Alvarez could not provide further evidence to confirm or disprove both hypotheses.
Fig.2. The famous transition between Cretaceous (right) and Paleocene (left) in the Scaglia Rossa; Gorge of Gubbio.
In 1981 the geologists Antonio Camargo-Zanoguera and Glen Penfield presented during a geophysical conference their research on a geological mystery discovered 30 years earlier during surveys on the Yucatan peninsula (south-eastern Mexico). The two researchers proposed a new interpretation of a circular structure revealed by seismic investigations and buried under 300 to 1.000 meters of sediments, considered until then of volcanic origin. Zanoguera and Penfield suggested that the circular structure was a weathered and buried crater, formed by the impact of a large meteorite millions of years ago. But their hypothesis aroused little interest.
Only 10 years later some researchers from the University of Arizona started to study the crater and obtained the first absolute age - 65 millions of years. Further research in 1993 revealed that the structure was in fact an impact crater with a total diameter of 180km. The crater was named after the beach town of Chicxulub - meaning "the devil's tail".
Alvarez, meanwhile, continued his research on the impact hypothesis and noted the temporal coincidence of the Iridium anomaly, the Chicxulub-impact and the mass extinction at the end of the Cretaceous. The impact could explain the observed rare earth elements anomaly, was an important time marker and also appeared to be the main culprit to blame for the extinction of the dinosaurs. In 1995 the limit between the Cretaceous and the Palaeogene was therefore defined at the stratotype (the GSSP) of El Kef (Tunisia), coinciding with the peak of Iridium and the mass extinction of foraminifera at the base of a clay layer (also referred as C/Pg boundary clay) deposited after the impact.
Today more than 350 sites are worldwide known to record the C/Pg transition. Within a radius of 500km the debris layer of the impact is very thick; around the crater it reaches a thickness of 100 to 80m! In a radius of 500 to 1.000km the sediments are typical tsunami deposits - layers containing debris and spherules (molten rocks that subsequently cooled to form droplets) transported by the waves from the impact site.
With increasing distance the layer thins out to form the known Iridium rich clay overlying a layer with small spherules. In a distance over 5.000km the impact is represented by a single layer of red clay that still contains traces of the material ejected from the crater (it is at the basis of this layer that the C/Pg limit of the El Kef section is defined).
The geological evidence supports the hypothesis that a large extraterrestrial mass impacted on earth, but it's a whole other problem how Chicxulub is related to the extinction of the Mesozoic fauna.
According to the most popular scenario the mass extinction was caused mainly by the consequences of the impact: the shock wave and fire storms were soon followed by the release of large quantities of gas from the vaporized rocks, rich in carbonates and sulphates. The gases reacted with the vapour to form acid rain and the dust in the atmosphere blocked solar irradiance, causing a "nuclear winter".
Depending on the locality and the ecological niche that a species occupied - and a good dose of luck - these changes decided the specie's fate - to survive or become extinct.
Most of this scenario however is speculative and despite the strong media presence of the crater of doom the biotic effects of impacts and the response of organisms and ecosystems to such an event are still unknown or at best controversial (to be continued…).
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