白垩纪黑色页岩与大洋红层:缺氧到富氧的过程与机制

白垩纪是地球历史中一个重要的阶段,期间发生了以黑色页岩为特征的缺氧事件和以大洋红层为特征的富氧环境等许多重大地质事件,从白垩纪大洋缺氧到富氧转化的过程与机制,提出上述两个典型事件是同一原因形成的两个不同结果。一方面,白垩纪大规模的岩浆活动,引起大气中CO2气体浓度的升高和地球内部大量热能释放,并且改变了海陆面积的对比,最终导致大气温度的升高。海水温度的升高和CO2浓度的增加导致海洋环境中溶解O2的降低,缺氧事件随之而产生。另一方面,剧烈的岩浆活动在海底产生大量的富含铁元素的基性和超基性岩,在与海水发生反应时,岩石中的铁元素进入海水中。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CO2,并且产生等量的O2。随着海水中O2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。然而,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。

CRETACEOUS BLACK SHALE AND OCEANIC RED BEDS:PROCESS AND MECHANISM OF OCEANIC ANOXIC EVENTS AND OXIC ENVIRONMENT

The Cretaceous is an important period in which occurred many geological events,especially the OAEs(Oceanic anoxic events) characterized by black shales,and the oxic 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 caused by the same reason.On the one hand,the Cretaceous large-scale magmatic activities caused CO2 to be concentrated in air,the inner energy of the Earth to release and the ocean-land area to change.Finally,they caused the increase in atmospheric temperature.This change presented the same trend as the oceanic water temperature,and caused the decrease of O2 concentration in the Cretaceous ocean,and then the OAEs occurred.On the other hand,the violent volcanoes supplied lava containing Fe for the seafloor.When the seawater reacted with the lava,the element Fe became dissolved in seawater.Iron is a micronutrient essential for the synthesis of enzymes required for photosynthesis in oceanic environment,and it could spur phytoplankton growth rapidly.The growth of phytoplankton which can consume carbon dioxide is commonly found in 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 rich in Fe3 appeared.The anoxic and oxic conditions in the Cretaceous ocean were caused by magmatic 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.