Late Cretaceous Biotic Response To Marine Dissolved Oxygen Content In Southern Tibet

In Earth sciences it had been a major topic for a long time to study the formation and evolution of the Neo-Tethys. Cretaceous is a special period during which the worldwide sea-level change, organic-rich black shales, volcanisms and relevant extreme warm greenhouse together with carbon cycle had been the hot spots attractiing geologists. Research on dissolved oxygen has substantial meanings to understand organic evolution. Meanwhile, dissolved oxygen content is the fundamental factor determining the Oceanic Anoxic Events (OAEs) and formation of black shales, the important marine source bed. Hence it had the important theoretical and practical meanings to discuss internal relation between dissolved oxygen and organic evolution. And Cretaceous in South Tibet was selected as the subject to investigate this relation in this article. Foraminifera and nanofossils are the most important fossils for marine stratigraphic classification and correlation after Jurassic and research on oceanography. Variation of growing ability is vital to comprehension of environmental transition. Planktonic foraminifera and calcareous nanofossil are two of the most primary producers, which grow mainly in upper euphotic zones and are main contributors of surface water growing abilities. Therefore, microfossils in submarine sediments can be an important indicator to surface water growing abilities of ancient ocean and can illustrate ancient marine environments to a certain extent. In South Tibet, beside the black shales that have been approved the results of OAE in Cretaceous, the overlying oceanic red beds deposited in high-oxidized circumstances have been discovered. Detailed research on stratigraphy and paleontology has provided the conditions for discussing the marine dissolved oxygen variations and organic evolvements. The paper "Late Cretaceous biotic response to marine dissolved oxygen content in southern Tibet"based on the study of stratigraphy, classified and counted the Late-Cretaceous fossils (including planktonic and benthic foraminifera & calcareous nanofossils) in southern Tibet, and initially reestablished the characteristic of organic evolution. This paper analyzed the relevance between Late-Cretaceous marine dissolved oxygen and organic characters in southern Tibet relying on various organic indexes had been acquired quantitatively, investigated the relation between marine dissolved oxygen content changes and phases of organic evolution, and also made a contrast to the geochemical analysis. Conclusions in this paper can be summarized as follows: 1) Based on the Zongshan Section of Gamba and the Chuangde Section of Gyangze in southern Tibet, three Upper Cretaceous units have been recognized as black, gray and red ones. The black unit was deposited during Cenomanian-Turonian; the gray unit was Turonian-Santonian and the red unit latest Santonian-Campanian. Each unitis of different biotic characters and displays different features of seawater composition during the time. 2) The black unit including a biotic extinction was caused by oceanic anoxic event. Crisis that deepwater keeled foraminifera encountered represents a ferocious anoxia in deep marine environment . Biotic recovery in gray unit reflects gradually improved marine circumstance. What the biotic extinction at the C/T boundary in black unit reflects an environmental mutation, whereas the gradual biotic change characterize the recovery of marine dissolved oxygen environment in a stepwise process. The red unit is the result of oxic deposition and characterized by extremely abundance of foraminiferal faunas, which reflects a high-oxidized marine circumstance. 3) As the most ordinary biotic index in paleoceanography researches, foraminifera represent anexcellent environmental indicator in this paper. For existing mainly in marine surface layer and tiny bodies, calcareous nanofossils did not response to marine dissolved oxygen changes clearly because of influences of numerous factors in the sedimentary process, but evolvements of which are coincident with foraminifera in general. 4) The comparative result between geochemical data and biotic character has displayed excellent conformability for research on Late Cretaceous sedimentary formation in southern Tibet and reflects a process of anoxic-improved gradually-oxic-return to normal evolvement.