Cretaceous black shale and the oceanic red beds:Process and mechanisms of oceanic anoxic events and oxic environment |
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| Authors: | Zhenguo ZHANG Nianqiao FANG Lianfeng GAO Baoling GUI Muhua CUI |
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| Institution: | (1) Hebei Polytecnic University, Tangshan, 063009, China;(2) China University of Geosciences, Beijing, 100083, China |
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| Abstract: | The Cretaceous is an important period in which many geological events occurred, especially the OAEs (oceanic anoxic events)
which are characterized by black shale, and the oxic process characterized by CORBs (Cretaceous oceanic red beds). In this
paper, the causative mechanism behind the formation of black shale and the oceanic red beds are described in detail. This
may explain how the oceanic environment changed from anoxic to oxic in the Cretaceous period. It is suggested that these two
different events happened because of the same cause. On the one hand, the large-scale magma activities in Cretaceous caused
the concentration of CO2, the release of the inner energy of the earth, superficial change in the ocean-land, and finally, the increase of atmospheric
temperature. These changes implied the same tendency as the oceanic water temperature show, and caused the decrease in O2 concentration in the Cretaceous ocean, and finally resulted in the occurrence of the OAEs. On the other hand, violent and
frequent volcanic eruptions in the Cretaceous produced plenty of Fe-enriched lava on the seafloor. When the seawater reacted
with the lava, the element Fe became dissolved in seawater. Iron, which could help phytoplankton grow rapidly, is a micronutrient
essential to the synthesis of enzymes required for photosynthesis in the oceanic environment. Phytoplankton, which grows in
much of the oceans around the world, can consume carbon dioxide in the air and the ocean. Meanwhile, an equal quantity of
oxygen can be produced by the phytoplankton during its growth. Finally, the oxic environment characterized by red sediment
rich in Fe3+ appeared. The anoxic and oxic conditions in the Cretaceous ocean were caused by volcanic activities, but they stemmed from
different causative mechanisms. The former was based on physical and chemical processes, while the latter involved more complicated
bio-oceanic-geochemical processes.
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Translated from Marine Geology & Quaternary Geology, 2007, 27(3): 69–75 译自: 海洋地质与第四纪地质] |
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| Keywords: | anoxic event black shale oceanic red bed oxic process oceanic volcanic activity Cretaceous |
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