地球历史中的巨型氧化作用事件:了解古地理背景演变的重要线索

Great Oxidation Event in history of the Earth:An important clue for the further understanding of evolution of palaeogeographical background

在塑造我们的星球环境的过程之中,分子氧起着关键的作用。大气圈和海洋中氧气的出现及其浓度随着时间的变化,与地球上的主要变化存在强烈关联,诸如构造重组、气候波动和生物进化。针对地球大气圈氧气含量的上升,多年研究的结果肯定了2个基本事实：（1）地球最早期的大气圈是缺乏氧气的;（2）今天的大气圈则为21%的氧气所组成。由于地质历史时期大气圈氧气水平的大多数地质标志,只是意味着存在与缺乏,这就为确定大气圈氧气含量上升的时间进程带来很多困难。即使如此,一系列地质证据已经表明,一个从缺氧的到含氧的大气圈的转变,大致发生在2.5—2.0Ga,这个转变被定义为巨型氧化作用事件（GOE）。近年来的深入研究发现,几个主要证据表明,在前寒武纪—寒武纪过渡时期的大约850—540Ma,发生了＂第二次巨型氧化作用事件（GOE-Ⅱ）＂,还被进一步定义为新元古代巨型氧化作用事件（NOE）。再者,大气圈氧气水平在显生宙还存在着一个特别的上升,这次变化在石炭纪晚期接近一个峰值为150%PAL（现代大气圈氧气含量水平）,所以,也可以定义为一次巨型氧化作用事件,即显生宙的巨型氧化作用事件（POE）。因为蓝细菌光合作用造成的氧气生产,曾经导致了大气圈与海洋的氧化作用,反过来为需氧呼吸作用和大型而且复杂的、最终富有智慧的生物进化,提供了基本条件;因此,大气圈氧气上升,是与地球动力学过程紧密相关的地球生物学过程的作用结果,从而成为了解漫长的地质历史时期古地理背景演变的重要线索。从古元古代的GOE,经过新元古代的NOE,到显生宙的POE,这些巨型氧化作用事件的内在特征、作用结果与基本属性,尽管存在着较大的差异,但是,从这些概念的出现到对它们的形成机理的探索性研究,涌现出了许多新概念和新认识;追索这些新概念和新认识,将为了解地球大气圈氧气上升的复杂历史所代表的一个特别的地球上生物学过程,提供一些有益的重要线索和思考途径。

Molecular oxygen has played a crucial role in shaping our planetary environment.The emergence of oxygen in the atmosphere and the oceans and its changing concentration through time is strongly linked to major changes on Earth such as tectonic reorganization,climatic perturbations and biological evolution.Two facts are known with certainty that are resulted from the long-term study on the rise of atmosphere oxygen,that are：（1） Earth＇s earliest atmosphere was essentially devoid of oxygen,and（2）today＇s atmosphere is composed of 21% oxygen.It should be emphasized that most geological indicators of atmospheric oxygen levels in geological time imply only presence or absence,which led to many difficulties on study of the rise of atmosphere oxygen.Even though,a battery of geological indicators suggest that a shift from an anoxic to an oxic atmosphere some time between 2.5 and 2.0 billion years ago,which is known and defined as the Great Oxidation Event（GOE）.Further research in resent years demonstrates that several lines of evidence point to a second ‘Great Oxidation Event＇（GOE ~ Ⅱ） during the Precambrian-Cambrian transition between about 850 and 540Ma,which is herewith referred to as the Neoproterozoic Oxygenation Event（NOE）.In practice,the considerable variation in atmospheric oxygen levels through Phanerozoic time peaked near 150% PAL in the late Carboniferous,which is herewith defined as the Phanerozoic Oxygenation Event（POE）.Because photosynthetic oxygen production by cyanobacteria led to oxygenation of the atmosphere and oceans,in turn provided an elementary condition for the aerobic respiration and the evolution of large,complex and ultimately intelligent organisms,the rise of atmosphere oxygen can be defined as the action product of geobiological process that is genetically related to the geodynamics,which become an important clue for the further understanding of the evolution of palaeogeographical background of the Earth.Furthermore,there are many intrinsically differences with respect to their consequences and merits among these Oxygenation Events,i.e.the GOE,the NOE and the POE,but the concept of themselves and the study on their forming mechanism demonstrate many new ideas and cognitions.Tracing these new ideas and cognitions will be helpful for the further understanding of this particular geobiological process represented by the rise of atmosphere oxygen,which can provide many useful thinking approaches and researching clues for the future research.