地球圈层之间相互作用对白垩纪大洋缺氧与富氧过程的制约

白垩纪诸多地质事件中,以黑色页岩为特征的大洋缺氧事件和以红层为特征的大洋富氧环境尤其引人关注。本文探讨了白垩纪大洋从缺氧到富氧转化的过程与机制,认为上述沉积事件是地球圈层之间相互作用的结果。白垩纪岩石圈剧烈的岩浆活动,是缺氧、富氧事件发生的源动力,水圈、大气圈、生物圈的共同作用是沉积事件发生的结果。具体过程为:白垩纪大规模的火山喷发,改变了海陆面积的对比,并引起地球内部大量热能释放和大气中CO_2气体浓度的升高,最终导致大气温度的升高。海水温度的升高和CO_2浓度的增加导致海洋环境中溶解O_2的降低,缺氧事件随之而产生。同时,海底岩浆喷发在海底产生大量的富含铁元素的基性和超基性岩石,通过海底风化和热液活动,铁元素从岩石圈进入水圈。海水中的铁元素是海洋浮游植物宝贵的营养盐类,其含量的增加可激发浮游植物的大规模繁盛,而这一生命过程可以吸收海水中大量的CO_2,并且产生等量的O_2。随着海水中O_2浓度的不断升高,以富含Fe3+的红色沉积物为特征的海洋富氧环境出现。藏南和深海钻探、大洋钻探典型剖面的数据证实大洋缺氧和富氧发生的韵律性,即缺氧事件之后往往伴随富氧环境的出现。研究认为,白垩纪大洋缺氧和富氧事件是同一原因导致的不同结果,地球圈层相互作用是其根本制约因素。由岩浆活动引起的缺氧事件和同样由其造成的富氧环境,其机制存在明显的差异,前者以物理、化学过程为主,后者除此之外还演绎了更为复杂的生物-海洋地球化学过程。

Interactions between the Earth Sphere and its constraint on the progress of anoxic-oxic in the Cretaceous Ocean

The Cretaceous is an important period in which occurred many geological events, especially the OAEs (Oceanic Anoxic Events) characterized by black shales, and theoxic process characterized by CORBs (Cretaceous Oceanic Red Beds). This paper describes the causative mechanism which explains how the oceanic environment changed from anoxic to oxic in Cretaceous. Two typical events show different results that caused by interactionsoftheEarthSpheres. Here we propose that the rise of atmospheric CO₂ occurred because the enhanced submarine volcanism-was abruptly and permanently diminished during the Cretaceous. The Cretaceous large-scale submarine volcanism caused the concentration of CO₂. The releasing of the inner energy of the lithosphere and thedistribution oflandwhich caused the increasing of atmospheric temperature. This change presented the same trend as the oceanic water temperature, and caused the decreasing of O₂ concentration in the Cretaceous ocean, and then the OAEs occurred. The lithosphere produced volume of lava in the upper oceanic crustwhich contained Fe in the seafloor. When thehydrothermal fluids alteration of oceanic crust and the seawater/basalt interactions (including microbes alteration of submarine basaltic glass), the element Fe dissolved in seawater. Iron is a micronutrient essential for the synthesis of enzymes required for photosynthesis in oceanic environment, it could spur phytoplankton growth rapidly. The photosynthesis of phytoplankton which can consume carbon dioxide is in much of the world''s oceans, wherever they are in atmosphere or in ocean. This process could produce equal oxygen. And then, the oxic environment characterized by red sediment which is rich in Fe³⁺ appeared. The data show rhythm of the anoxic and oxic from south Tibet and DSDP/ODP section, which the anoxic is often accompanied by the occurrence of oxygen rich environment.Undoubtedly, the anoxic andoxic in the Cretaceous Ocean were controlled by the mutually dependent processes of the Earth system which included lithosphere, hydrosphere, atmosphere and biosphere. An important conclusion of this study is that the black shalesand the oceanic red beds are caused by the same reason, but led different results. The anoxic and oxic in the Cretaceous ocean were caused by volcanic activities, but they were of different causative mechanisms. The former was based on physical and chemical process, while the latter involved more complicated bio-oceanic-geochemistry process.