Evidence For The Meteor Impact Which Killed The Dinosaurs



Basaltic Spherules

Impact by a large body at high velocities into the planets surface ,as mentioned before, creates a massive amount of heat ( Smit, 1984 ); this shock-melts the rocks that are impacted and launches them up into the stratosphere where they rapidly cool before being deposited all over the world.
The spherules found at the K-T layer are basaltic equivalents of the more silicic tektites and microtektites ( Smit, 1981, Alessandro Montanari ) that we know to be results of small impacts. The spherules of the K-T are around 1mm in diameter and have been chemically altered by their interaction with the clay layer. The basaltic nature of these spherules points towards the collision occurring on oceanic crust. The case against mass volcanism ( the antithesis to the impact theory ) from spherules is that despite volcanoes being able to produce them, they can only do this with low level volcanism, this would prevent the distribution of the spherules around the world, which is the case at the K-T.

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Isotope Evidence

Isotopes are a useful way of examining the origins of a material, using the differences observed in nature we can examine K-T samples and deduce their possible origins, providing evidence for or against the impact theory.
For example, isotopic ratios of osmium 187 to 186 is higher in the continental crust than in the mantle or meteorites ( Jean-Marc Luck, 1983 ) as osmium is a decay product of rhenium, which is a lithophile and as such has partitioned into the crust. The ratios found at the K-T boundary are those associated with mantle or meteoritic composition.

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Elemental Ratios

Certain conditions favour certain elements and so if we examine the ratios of elements we can often identify a pattern that tells us what the conditions were. Meteorites have this 'fingerprint' and a characteristic set of elemental ratios. The ratios of the platinum-group of elements found in the boundary layer indicate an extraterrestrial origin. Work by Asaro and Bekov shows that the relative abundances of ruthenium, rhodium and iridium can differentiate terrestrial material from stony meteorites. Analysing K-T boundary samples from Stevns Klint, Denmark & Turkmenia, Soviet Union among other regions indicate that extraterrestrial input has had a large part to play in the formation of those strata.

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Crater, What Crater ?

Even by 1990 the scientists working on the impact theory had not found a suitable crater for the impact - the 'K-T Smoking Gun' - which would lend considerable weight to the argument. Several site were proposed ( e.g. Amirante Basin, Indian Ocean, Hartnady, 1986 ) but none of them fulfilled all the criteria needed. This must have been a source of great frustration as the crater may have been lost if it were on part of the 20% of the Earth's crust that has been destroyed at subducion zones since the Cretaceous, as well as being the piece of evidence that would convince the press and the world at large.
There was evidence for both oceanic crust ( e.g. basaltic spherules ) and continental impact ( e.g. shocked quartz, Shukolukov, 1988 ) so the areas to look for the impact site were still wide open, on top of this there was a wave of new theories coming through for multiple impact sites ( Shoemaker & Piet Hut in Alvarez & Asaro 1990, Shukolukov,1988 ) from either several impacts in close succession or by one object which broke up before impact.
The real brakthrough came in1989 when work was undertaken by NASA's Ames research center uncovered a 170km diameter semi-circle of sinkholes at Chicxulub, Yucatan Peninsula, Mexico. Ocampo then correlated the gravity, magnetic and stratigraphic evidence ( for example using tsunamites to determine the origin of impact )with the sinkholes to deduce the area had classic impact crater features, the discovery was published in 1991 and led to the discovery of the Chixculub crater - the 'K-T smoking gun'

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Site designed and maintained by Simon Lee,
at the University of Bristol
e-mail: sl4476@bristol.ac.uk