ジオカタストロフィー

生物の絶滅と地球磁場の逆転

抄録

One of the most interesting paleontological contributions to general knowledge is evidence that once-dominant giant reptiles died out in the end of the Cretaceous period. Recently, on the basis of the iridium enrichment in Cretaceous-Tertiary (K-T) boundary layers, the Berkeley research group suggested that the process of mass extinction of species is extraterrestrial in origin, probably caused by the impact of large meteorites on the Earth. Since this suggestion, it has become fashionable for geochemists to discover an iridium abundance at K-T boundary sites or at other mass-extinction horizons around the world.
Many Earth- and planet-scientists have eagerly discussed on the possibilities of a global catastrophe due to bolide impacts. The probability of striking the Earth by an Earth-crossing asteroid is estimated to be not no small. As a matter of fact, in January 18, 1991, astronomical observations found a small asteroid crossing the Earth's orbit within half the distance to the Moon. Earth-crossing comets are even more plentiful. The probability of comet collisions to the Earth is supposed to increase extremely when the Solar System encounters with a dense molecular cloud in the Galaxy.
Some volcanologists protest against the impact hypothesis, insisting that a bolide impact is not the only way to cause a global catastrophe. Despite that a large-scale volcanic eruption can produce iridium enrichments, the presence of shock quartz at K-T boundary sites remains a problem for proponents of volcanism. It seems that the extraterrestrial-origin hypothesis has advantage over the volcanic hypothesis. Various hypotheses of mass extinction are reviewed in the first half of this paper.
It is a well-known fact that the Earth's magnetic field has repeatedly flip-flopped. The great circulation of molten iron in the Earth' core generates the geomagnetic field. The sophisticated theory of electromagnetic fluid dynamics explains the mechanism of the geomagnetic generation, but there still remain uncertainties in the mechanism of geomagnetic reversal. Some paleomagneticians speculated that the asteroid impact is an immediate cause of geomagnetic reversal, but others rejected the idea of such a linkage from standpoints of microtektite studies.
Recently, a Japanese research group has analyzed samples of sedimentary rock, whose sedimentation rate was 3 m/ka during the Matuyama-Brunhes reversal period, and discovered changes in both oxygen and carbon isotope ratios coincident with reversal. The decrease in carbon isotope ratio of 1 permill corresponds supposedly to the 40 % reduction of biomass, which indicates a large amount of CO₂ gas flows into the ocean and the atmosphere from the biosphere. The concurrent decrease in oxygen isotope ratio can be explained as the CO₂-induced greenhouse effect.
The coincidence of a biomass changes with a geomagnetic reversal implies that the reversal may be one of possible causes of mass extinction. The biosphere is protected by the shield effect of magnetic field against solar high-energy particles. During the time of reversal when the shield effect disappears, the biosphere may be directly exposed to harmful ultraviolet ray and further affected by the concurrently-occurred global climate change. On the basis of observational facts that the geomagnetic dipole moment has been rapidly decreasing, many geomagneticians give a warning that the Earth is now standing at the starting point of a magnetic reversal. If the decrease will continue as it is, the geomagnetic field will completely disappear in one thousand years or later.