大气增氧机制:地球表层过程与深部氧化还原状态的耦合

Mechanisms for the rise of atmospheric oxygen:Bridging surface oxygenation processes and redox conditions of deep interiors

氧气是地球表面生命蓬勃发展的关键物质.目前的地球化学模型和矿物学证据认为,大气中的氧气主要源自生物作用,如通过藻类生物的光合作用.尽管源自生物作用的产氧机制占据相关研究的主流,氧元素实际占据了地球内部最大的摩尔分数,因此它在调整地壳和地幔的氧化还原状态方面发挥着重要作用,地球深部氧化还原水平的波动会对大气中的氧分压产生深远影响.本综述聚焦地质历史中的大气增氧机制,结合最新的生物地球化学和矿物学发现,讨论大气氧来自地球表层和深层的源与汇.越来越多的工作揭示,地球深层可能存在一个隐藏的氧储库,一些重大地质事件与这一深部氧储库可能密切相关.

Atmospheric oxygen is a key ingredient for life to flourish.Current geochemical models and mineral chemistry evidence indicate that the atmospheric oxygen mostly originated from biochemical processes,for example through marine macroalgae photosynthesis.Despite the popularity of biogeochemical sources,oxygen also occupies the largest mole fraction of Earth’s solid interiors and thus plays essential roles in tuning the redox state of the crust and mantle.The fluctuation of redox conditions stemming from Earth’s interiors may impose profound effects on oxygen fugacity in the atmosphere.Here,we review the state of knowledge on the rise of atmospheric oxygen in Earth’s geological history.We combine the latest biogeochemistry theories and mineral chemistry discoveries to present an extended picture for the production and consumption of oxygen throughout Earth’s surface and its interiors.This review contains five sessions in discussing the mechanisms for the rise of atmospheric oxygen.We begin with scientific background for the evolution of oxygen from the early to present-day Earth.We then go over established theories and known factors in controlling the production and consumption of oxygen,which invoke recent discoveries of potential oxygen sources from deep Earth’s interiors.While the solid Earth’s could be the greatest reservoir of oxygen,it is still a challenging task in detecting the behaviors of active oxygen and its influence on the oxidation state of mantle and crust.In the next session,the evolution of oxygen fugacity of Earth’s interiors is highlighted and we have paid particular attention to specific periods in Earth’s history that oxygen fugacity rose to high level.Those pioneering research leads to the theory that the deep Earth’s is a giant oxygen generator.In the following session,recent experimental and simulation works are introduced to show that water may play a key role to react with major mineral of the lower mantle and produce oxygen.The depth of~1800 km is critical in separating the chemical properties of minerals like ferropericlase,below which water becomes reactive and split itself into hydrogen and oxygen.For the superior mobility of hydrogen,the hydrogen fugacity is a more viable parameter in describing chemical states in Earth’s deep mantle.In the last session,we summarize current stage of research on the production and consumption of atmospheric oxygen.We also put forward scientific questions that require immediate attention to verify the hypothesis of Earth’s deep interiors as a hidden reservoir of active oxygen.Missing links,for instance a volatile shuttle that transport oxidized materials from the deep interiors to the surface,are proposed as future interests of research.The deep source of oxygen may link to atmospheric oxygen though large-scale surface events.