KT-Boundary

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The K/T boundary event marks a monumental change in environmental conditions and in life forms. The "event" marks the transition from the Mesozoic Era with its final Cretaceous (K) period and the Cenozoic Era and its initial period, the Tertiary, in the Geological Timescale. The boundary occurred 65 million (6.5x10⁷) years ago.

One of the largest mass extinction events occurred at this boundary, which indeed makes it a most important "punctuation" in the history of life on earth and evolution.

The apparent catastrophic extinction of about 75% of all life forms over a relatively short time interval was a gigantic problem for students of earth history and evolution. The event marked the abrupt end of the Dinosauria (with the exception of the birds) along with multiple other species including 90% of oceanic phytoplankton and opened the field for the mammals and other survivors.

The cause for the event has a long history of study and speculation from volcanism to "mammals eating dinosaur eggs". In the 1970s Louis and Walter Alvarez while studying the thin layer of clay that typically marks the stratigraphic boundary between the Cretaceous and the Tertiary near Gubbio, Italy, noted an anomalous spike in iridium (Ir) content of the clay. The iridium content was about 30 times higher than that found in typical earth rocks. Iridium is much more common in asteroides and this observation led to the hypothesis of a large meteorite impact as the trigger for the extinctions of the K/T boundary. Further study of outcrops and drill hole intersections of the K/T boundary also contained this thin iridium rich clay seam. The clay also contained soot particles and strained quartz crystals as indicators of high temperature and shock pressures.

The hypothesis of a meteorite impact seemed to fit the data, but where was this large impact? Alvarez calculated that a chondritic asteroid approximately 10km in diameter would contain sufficient iridium to account for the anomalous worldwide K/T clay layer. Such a large bolide would create a large crater and should leave some evidence of its footprint.

Geophysical evidence of a large impact structure just off the north coast of the Yucatan in Mexico was recognized in 1990. This structure, the Chicxulub crater, was dated with the 40Ar/39Ar radiometric dating method as being 65 million years old right at the K/T boundary. Calculations based on the size of the crater were consistent with a large meteorite impact as proposed by Alvarez. It appeared the "smoking gun" had been located; however, more recent investigations on drill cores from the Chicxulub crater have cast new doubts on the connection between Chicxulub crater and the KT-impact. According to the results of this investigations, done by G. Keller et al., the Chicxulub crater predates the KT-boundary about 0.3 million years.

The shock of the impact, the tidal waves no doubt produced, the global firestorm possibly ignited by the high temperatures of the impact ejecta, the dust and smoke that blanketed the globe for an extended period and the acid and chemical interactions in the atmosphere and the ocean waters spelled catastrophe for any living thing on earth.

The impact evidence presented by Alvarez and others is well established and has seen considerable scientific support. However, some critics hold that it is not the whole story. At the same time period, the Deccan Traps, a gigantic volcano field in India, was being produced with an area estimated at greater than 2.6 million km² and as much as 2 km thick. D. M. McLean has proposed the extinction event was due to a K-T carbon cycle perturbation or intense "greenhouse effect" caused by the massive volcanism of the Deccan Traps. The iridium anomaly could also have been produced by a proposed deep mantle plume as the source of the Deccan volcanism. However, isotopic anomalies - e.g. the Cr-53 anomaly found in the KT boundary layer - are very difficult to explain by volcanism.

A large impact crater has been recently reported in the sea floor off the west coast of India. This, the Shiva crater, has also been dated at 65 million years right at the K/T boundary. The researchers suggest that the impact may have been the triggering event for the Deccan Traps. Two other craters, the Boltysh crater in the Ukraine and the Silverpit crater in the North Sea, also appear to have been formed at the K/T boundary. This suggests near simultaneous multiple impacts from perhaps a fragmented asteroidal object.

Another proposed cause for the K/T extinction event was radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The cosmic radiation from a supernova explosion should contain the plutonium isotope Pu-244, a relatively short lived isotope (half-life 81 myr) that is not found in earth rocks. However, analysis of the boundary layer revealed the absence of Pu-244 thus essentially disproving this hypothesis.

Others have pointed out that various species and groups were either in decline or had became extinct before the event. This suggests that the impact event and the massive volcanism were superimposed on and exacerbated the normal ebb and flow of evolution.

The concept of an impact as the prime cause of the extinction event is catastrophic and many geologists of the latter part of the twentieth century may find catastrophism distasteful for a couple of reasons. First, the principle of uniformitarianism of James Hutton and Charles Lyell has long been one of the mainstays of geologic education and major catastrophic events would seem to be at odds with this important concept. Second, creationists and other pseudoscience types have long used catastrophism to explain away the need for geologic time and to compress earth history into their picture of reality. Many geologists may be reluctant to support a concept that creationists may view as supporting their claims.

Catastrophic events do occur, witness the Shoemaker-Levy 9 comet collision with Jupiter - "the present is a key to the past" in this case, and uniformitarianism remains valid.