Evidence For The Meteor Impact Which Killed The Dinosaurs

1980: The Iridium Layer

In 1980 scientists called Walter and Luis Alvarez wrote a paper on a 1cm layer of clay they had discovered in Gubbio, Italy at what was known to be the zone that marked the time the dinosaurs became extinct. This layer was found to be rich in a rare element called iridium ( Ir ). The presence of this element in much greater abundance than would be normally expected caused great confusion. Most of the Earth's store of Ir has been concentrated into the centre of the planet where it has been alloyed with hydrogen. At Gubbio layers have been found to contain an 'iridium spike' of 20, 30 and 160 times the amount of Ir normally present ( see image, right ). These levels are normally only found in either material from deep within the Earth or from primitive stony meteorites called 'carbonaceous chondrites' Alvarez & Asaro, 1990 ). At the time the only one site had been found with this anomaly, yet in a bold statement the 1980 paper Walter and Luis Alvarez claimed the entire K-T extinction was due to a massive meteor colliding with the Earth.

Ir found in sediments is from cosmogenic fallout and constitutes around 0.03 ppb. The levels found at Gubbio are in the order of 500ppb. Higher concentrations can be attributed to longer periods of deposition, in this case the death of many calcium carbonate producers stopping the production of limestone which surrounds the clay layer, thus slow clay deposition could result in higher Ir. But this amount of Ir would require a depositional time of around 500,000 years. The high resolution records at the K-T in Caravaca, Spain ( Smit, 1981 ) however indicate a period of no more than 50 years ( other sites resolve the length of time to roughly 1,000 years ).
Combined with this is that the ratio of Ir to platinum, osmium, ruthenium, rhodium and gold are the same ( within 5% ) in the boundary layer as they are in type one carbonaceous chondrite meteorites ( Asaro,1990 ). From this evidence it was clear that at least some form of extraterrestrial influence had taken part in the extinction.

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Footprints Of Fire

A large impact such as the K-T meteor creates huge amounts of heat merely by friction with the atmosphere, as well as that with the surface when it impacts. This causes an intense wall of fire to spread out from the crater the same way ripples do from a stone dropped into water. The fire burns any vegetation in it's way and leaves a 'footprint' layer of burnt plant remains just below the layer of debris ejected from the crater by the impact, which is still in the air as the firewall destroys the plant life.

Work by Anders ( 1988 ) showed that at least 5 site ( Europe & New Zealand ) are between 100 and 10000 fold richer in isotopically uniform carbon that is thought to be derived from a single global fire. The fuel seems to have been mainly trees rather than fossil fuel. The carbon isotopes and sheer volume (7000 teragrams )of carbon indicate that the source was coniferous trees and that most or all of the Cretaceous forests were destroyed. This alone would cause immense darkness ( Turco et al, 1980 ) and prolong the impact winters cold spell, followed by the carbon dioxide from the burnt material enhancing the global warming, which would take place once the dust and soot left the atmosphere, by around 9 degrees ( Anders, 1988; O'Keefe & Ahrens, 1982 & various others ).

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Shocked Quartz

Quartz is a mineral made from Silica ( pure sand ) and has a structure that can be deformed if it undergoes high pressures. The high velocity impact of a meteor would not only create extreme levels of heat ( see above ) but also high pressures. The type of pressure applied by an impact causes what is known as shock metamorphism, this deforms the structure of the quartz and creates parallel laminar ripples. This form of quartz has been found around the world and is a very strong argument for the meteor in two respects:
  1. It is found all over the world, not just locally.
  2. No other process on the planet creates this type of quartz.

The shock-deformation causes the grains to have multiple intersecting planar lamellae ( Foord et al ), which are attributed to hypervelocity shock that is seen only in laboratories, nuclear test sites and impact craters. Although quartz is the mineral most often cited for this effect other minerals such as plagioclase also show this deformation.
The major 'antithesis' to the meteor hypothesis is for massive volcanic activity, this can create shocked quartz ( and many other features of the K-T boundary ) from explosive eruptions, but the form of the quartz is different from that observed. To compound this a form of high pressure quartz called stishovite has been recorded; the pressures needed to form stishovite far exceed those capable of being produced in a volcano ( McHone ).

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at the University of Bristol
e-mail: sl4476@bristol.ac.uk