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Náttúrufræðingurinn - 1991, Síða 20

Náttúrufræðingurinn - 1991, Síða 20
the length of time necessary for the depos- ition of the boundary clay that marks the Cretaceous/Tertiary (K/T) boundary in a very well known sedimentary section at Gubbio, a small town in central Italy. Their method was to determine the iri- dium content of the clay and compare that with the estimated annual downfall of cosmic dust. They chose iridium because it is about 1000 times more abundant in meteorites than in crustal materials. The analytical results of their work were quite unexpected and the group felt compelled to visit other well documented sections from the same geological period and col- lected samples from Stevns Klint in Denmark and Woodside Creek in New Zealand. Chemical analyses of a band of clay at the boundary in each of the three localities showed similar if not identical compositional patterns. The abnormally high quantities of iri- dium and osmium were of most interest. They were much too higli to be explained by influx rates of cosmic matter. The Al- varez group proposed that exactly 65 million years ago an asteroid, 10 ± 4 km in diameter, struck the earth and exploded. Such an impact would result in a crater that would be at least 150 km in diameter. On impact, a vast cloud of dust was thrown up into the air and shut out the sunlight for several years. During the time of darkness photosynthesis was supp- ressed and food chains collapsed, causing mass extinction on a large scale. Further- more, the existence of soot in many K/T boundary sections is considered evidence of wide-ranging, even global, wildfires raging at the time possibly caused by an asteroid impact (cf. Wolbach et al. 1985, Melosh et al. 1990). To begin with, the Al- varez group drew attention to volcanism and pointed out that atmospheric effects of the asteroid impact had been about 1000 times greater than those at the enor- mous eruption at Krakatoa in 1883. As there was no suitably large volcanic erupt- ion candidate known in the geological record they rejected the idea of volcanic activity having been the cause of the ext- inctions. Numerous other workers all over the world followed up on the work of the Alvarez group and came to similar con- clusions. Very soon it became quite clear tliat the iridium anomaly was a worldwide phenomenon and there were several other elements that showed a similar pattern. The same year that the Alvarez group came forward with their hypothesis an al- ternative scenario was suggested in which a cometary nucleus dived into the ocean, warming up the atmosphere somewhat and causing widespread toxicity in the oceans when it dissolved (Hsu 1980). A third alternative was proposed in which the earth met with a shower of comets (Hut et al. 1987). While many meteoric craters and meteoric impact structures round the world have been proposed as candidates, they are either too small, too far away from the major iridium sites, or of the wrong age (cf. Grieve 1987). Another group of American earth scientists (Officer & Drake 1983 and 1985) has been in the forefront for a terrestrial school of thought. They favour causes of terrestrial origins having led to the mass extinctions and insist there is no need to look for extraterrestrial causes. Analytical work on the airborne products of the 1983 Kilauea eruption on Hawaii showed the presence of a small quantity of iridium (Zoller et al. 1983), thereby lending credence to the claim that the iridium an- omaly at the K/T boundary could have been caused by fall-out from a gigantic volcanic cloud that enveloped the earth. Officer and Drake and their co-workers reviewed published data and demonstrat- ed that the K/T iridium-anomaly at a num- ber of locations from Spain and Italy to the Pacific is not as sharp temporally as is often assumed and the iridium could have been deposited over periods of up to at least 100,000 years (Officer and Drake 1983 and 1985, Officer et al. 1987). It has also been shown that while the iridium- concentrations are significantly higher than the crustal average but comparable to meteorite sources the concentrations of arsenic and antimony at the K/T boundary 14
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