ペルム紀-三畳紀境界の遠洋域深海堆積岩の古生物学的地球化学的研究

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  • 高橋 聡
    東京大学地球惑星科学専攻地球惑星システム科学講座

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タイトル別名
  • Palaeontological and geochemical studies on Permian-Triassic pelagic deep-sea sedimentary rocks
  • ペルムキ : サンジョウキ キョウカイ ノ エンヨウイキ シンカイ タイセキガン ノ コ セイブツガクテキ チキュウ カガクテキ ケンキュウ

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The most severe mass extinction in Earth's history occurred at the end of Permian Period, and recovery from this mass extinction extended until the subsequent Early Triassic. Organic geochemical works from many studies of shallow water platform sedimentary strata have indicated that increases in green sulphur bacterial and cyanobacterial biomasses as well as terrigenous organic material occurred in end-Permian to earliest Triassic shallow-marine environments. Additionally, recent studies on pelagic deep sea sedimentary rocks in accretionary complexes of Japan have increased understanding of the low latitudinal Panthalassic record through the Late Permian and into the Early Triassic. Correlations of conodont fossil occurrences and stable carbon isotope compositions of organic matter in the continuous deep-sea Permian-Triassic boundary section indicate that the mass extinction horizon is located at the onset of black claystone above microfossil-rich siliceous claystone beds. Although this black claystone bed suggests oxygen-poor conditions, bacterial blooms and terrigenous inputs in the pelagic region were not especially significant based on the detected smooth organic carbon isotope curve, which does not include any isotopically heavier signals. Subsequent recovery of radiolarian chert deposition occurred at the late Early Triassic-early Middle Triassic transition. Organic molecules have been detected from these strata. Considering their producing processes in the pelagic deep sea, highly concentrated dibenzothiophene in the latest Early Triassic black chert indicate sulfur-rich depositional conditions. Also, high concentrations of polycyclic aromatic hydrocarbons (PAHs) suggests a high flux of marine organic matter during the latest Early Triassic. As these occurrences coexist with changes in perhaps bacterial community and decreases in radiolarian fossil diversity, this event may be related to the delay in the recovery of life and environments after the end-Permian mass extinction.

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