A study of 28 deep-sea piston cores from high and low latitudes shows that during the last 2.5 m.y. eight species of Radiolaria became extinct. Prior to their extinction these species were widely distributed and became extinct isochronously throughout their geographic range. Six of the eight species disappeared in close proximity to magnetic reversals recorded in the sediment. Careful determination of the level of disappearance of these six species by counts of over 400,000 individuals shows that the correlation between reversal and extinction levels is indeed striking. The probability of six out of eight species extinctions showing this degree of correlation with reversals by chance is very small. The evidence is strongly suggestive that magnetic reversals either directly or indirectly exert a selective force.

Present data on the history of reversal frequency through the Phanerozoic indicate that there were several long intervals with few or no reversals. The best documented are the Permo-Carboniferous reversed interval (Kiamen) and the Cretaceous, primarily Upper Cretaceous normal interval. There are reversals in the uppermost Permian (Tartarian) reported from Russia and in the Maestrichtian reported from the United States. The mass extinctions of marine and land animals at the close of the Paleozoic and Mesozoic coincide in large part with this renewed reversal activity. If the magnetic field reversals exert a selective force then the removal of this force for tens of millions of years may allow the development of many potentially reversal susceptible species that will be vulnerable to the renewal of frequent reversals.

The evidence is mounting that Earth's magnetic field may have played an important role in the development of life on this planet.

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