Paleoecological and paleoenvironmental variations reconstructed by organic geochemical investigations in the mid-Cretaceous oceanic anoxic event (OAE) 1a and 1b

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  • Okano Kazuki
    Department of Natural History Sciences, Faculty of Science, Hokkaido University
  • Sawada Ken
    Department of Natural History Sciences, Faculty of Science, Hokkaido University

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  • 有機地球化学からみた白亜紀中期,海洋無酸素事変1aおよび1bにおける古生態・古環境変動
  • ユウキ チキュウ カガク カラ ミタ ハクアキ チュウキ カイヨウ ムサンソ ジヘン 1a オヨビ 1b ニ オケル コセイタイ コ カンキョウ ヘンドウ

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Abstract

Oceanic Anoxic Events (OAEs) occurred several times during the mid-Cretaceous. The black shales that have laminated structure and contain abundantly organic carbon were deposited during the OAEs. OAEs were related to drastic environmental changes and turnovers of marine biota, which were reconstructed by organic geochemical investigations such as the biomarker analyses. OAE1a (early Aptian) was characterized by high contribution of cyanobacteria as a marine producer to sedimentary organic matter revealed by analyses of 2-methylhopanoids. In OAE1a, the high productivity of eukaryote in surface ocean was also considered from high concentrations of steroid hydrocarbons, and other specific biomarkers such as alkenone and dinosterane were identified. On the other hand, OAE1b (late Aptian〜early Albian) was characterized by high contribution of archaea such as methanogen as a marine producers to sedimentary organic matter up to 80% revealed by the high concentrations of archaea-derived isoprenoids. Moreover, high concentrations of terrestrial biomarkers such as dibenzofuran and retene were recognized during OAE1b. This fact indicates that a large amount of terrigenous matter was more efficiently transported to ocean during OAE1b.

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