原生代-古生代境界の地球システム変動と多細胞動物の出現

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  • Large Scale Fluctuations of the Earth System during the Late Proterozoic and Early Cambrian and Its Implications for the Appearance of Metazoa
  • ゲンセイ ダイ コセイダイ キョウカイ ノ チキュウ システム ヘンドウ ト

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The abrupt appearance of metazoa at the Precambrian and Cambrian boundary has been regarded as one of the most important events in the history of life on Earth. Recent progress in stratigraphical studies of the Late Proterozoic strata and their correlations has revealed that the prelude of early metazoan evolution involved several severe climate changes, the deposition of banded iron formations and phosphorites, large isotopic fluctuations of carbon, strontium and sulfur, and the rifting and collision of continents. These data imply that a significant disturbance in the biogeochemical cycle resulted in large fluctuations of climate and ocean circulation patterns, which in turn triggered biospheric evolution. The sequence of events might have been ultimately caused by mantle dynamics. Conversely, it is highly likely that the appearance of metazoa was a driving force for biogeochemical reorganization.<br>In this review, the current methodology of the Late Proterozoic stratigraphy is described, with the temporal correlations of major events during the Late Proterozoic to the Early Cambrian summerized. The interesting features discussed in connection with the Cambrian explosion are, (1) the close temporal correlations among ice ages, banded iron depositions, and negative excursions of the carbon isotopic curve, (2) negative excursions of the Sr isotopic curve which probably resulted from the rifting of former continents, and (3) the rapid rise of the Sr isotopic curve as correlated to the Pan-African orogeny and the deposition of massive phosphorites, which is well correlated to the Cambrian explosion.<br>A working stagnant ocean model is proposed to explain the above apparent correlations in a unified framework. It is postulated that the ocean was basically stagnant during the Late Proterozoic, the result of a massive supply of ferrous iron from hydrothermal activities. This stagnant ocean removed carbon dioxide as trapped organic carbon in sediments, utlimately leading to the ice ages. The anoxic ocean separated organic carbon in the deeper water and molecular oxygen at the site of photic zone bio-synthesis. This resulted in the gradual increase in the partial pressure of oxygen in the shallow water and atmosphere, as well as the storage of organic matter within sediments. Ubiquitous reducing agents, such as ferrous ion in the ocean or volcanic gases from the mantle, however, prevented a rapid increase in oxygen pressure during the Riphean and Sturtian eras. With the Earth's aging, the supply of reducing agents from the Earth's interior diminished. After the final removal of ferrous iron through oxidation, coupled with glacially induced vertical oceanic circulation at the onset of the Vendian, a rapid rise in atmospheric oxygen pressure occurred. This may be the major reason for the appearance of large body-sized metazoa. Since studies of Precambrian-Cambrian boundary events should be one of the most important topics in the Decoding Earth Evolution Program, future problems and research directions discussed here should stimulate efforts to improve the present crude model and create a deeper understanding of the causes and consequences of the appearance of metazoa.

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  • 地震　第２輯

    地震　第２輯 48 (4), 487-508, 1996

    公益社団法人 日本地震学会

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