| 大气圈氧气含量水平上升的时间进程:一个与地球动力学过程紧密相关的地球生物学过程 |
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| 引用本文： | 梅冥相,孟庆芬.大气圈氧气含量水平上升的时间进程:一个与地球动力学过程紧密相关的地球生物学过程[J].古地理学报,2016,18(1):1-20. |
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| 作者姓名： | 梅冥相 孟庆芬 |
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| 作者单位： | 中国地质大学(北京)地球科学与资源学院,北京 100083 |
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| 基金项目： | Financially supported by the National Natural Science Foundation of China(Grant No.:41472090,40472065,49802012) |
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| 摘 要： | 两种气体,氮气和氧气,以压倒优势的状态主导着地球的大气圈。氮气是原生的,而且其存在和丰度不是生物过程所驱动的;相反,氧气是生物通过水的氧化作用而连续产生的,这个氧化作用得到了太阳光的能量驱动。氧气,一种对动物生命进化最为关键的气体,是如何变成大气圈中丰度第2的气体?问题并非以前所设想的那么简单;为了了解大气圈氧化的时间进程,我们不但要知道氧气是什么时候而且是如何第1次出现的,而且还要知道氧气是如何在大气圈中保持一个高浓度的。可以肯定的2个事实是:地球最早期的大气圈是缺乏氧气的,而今天的大气圈则为21%的氧气所组成。需要特别强调的是,大多数古代大气圈氧气水平的地质标志,只是意味着存在与缺乏,而且发生在以下2个时间点的大多数事件是高度不肯定的;但是,一系列地质证据已经表明,大气圈氧气含量水平上升的时间进程发生在2个时间点上:(1)一个从缺氧的到含氧的大气圈的转变,大致发生在2.0~2.5,Ga期间,这个转变就是著名的巨型氧化作用事件(GOE);(2)发生在前寒武纪-寒武纪过渡时期的大约540~850,Ma的第2次巨型氧化作用事件(GOE-Ⅱ),被进一步命名为新元古代氧化作用事件(NOE)。GOE与NOE,就得出了地球大气圈氧气含量水平上升三段式的盛行图像。随着研究的深入,得到了以下重要认识:如果说大气圈氧气含量的总体增加,从太古宙微不足道的水平增加到今天21%,是由于氧气生产作用增强的结果而代表了一个复杂的地球生物学过程的话,那么,这个过程则发生在随着侵蚀作用与沉积作用相对于火山活动而变得更加重要的状况下,更进一步讲,叠加在这个总体趋势下的则是一系列的阶梯式的氧气含量水平上升,这与超大陆聚合作用之后异常高的沉积作用周期是相联系的,从而进一步说明了大气圈氧气含量水平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。大气圈氧气含量水平一系列的阶梯式的上升,被总结为7个事件而与超大陆汇聚事件得到了良好的对比,从而提供了一个更加清晰的图像;也就是说,在超大陆汇聚作用之后,得到增强的沉积作用促进了大部分有机碳和黄铁矿的埋藏,因而阻碍了它们与自由氧的反应,结果就是大气圈氧气含量水平的实质性上升。新颖的观点和重要的认识,为深入理解地球大气圈氧气含量水平上升这一个重要的地球生物学过程,提供了重要的思考途径和研究线索;追索这些研究进展,将有助于揭开地球大气圈演变历史的神秘面纱并寻找出更多的科学研究生长点。
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| 关 键 词： | 时间进程 氧气含量水平上升 大气圈 研究进展 |
| 收稿时间： | 25 June 2015 |
Timing of the rise of atmospheric oxygen content level:A geobiological process that is closely and genetically related to the geodynamics |
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| Mei Mingxiang,Meng Qingfen.Timing of the rise of atmospheric oxygen content level:A geobiological process that is closely and genetically related to the geodynamics[J].Journal of Palaeogeography,2016,18(1):1-20. |
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| Authors: | Mei Mingxiang Meng Qingfen |
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| Institution: | School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083 |
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| Abstract: | Earth’s atmosphere is overwhelmingly dominated by two types of gas: N2 and O2. The nitrogen(N2)is primordial,and its presence and abundance are not driven by biological processes. In contrast,the oxygen(O2)is continuously produced biologically via the oxidation of water driven by energy from the Sun. How did O2,a gas critical to the evolution of animal life,become the second most abundant gas on Earth?The problem is not as simple as it might first appear. In order to understand this problem,we must know not only how and when O2 was first generated,but also how it came to persist in high concentrations in the atmosphere. Two facts are known with certainty,that are,Earth’s earliest atmosphere was essentially devoid of oxygen;and today’s 21% of atmosphere is composed of oxygen. It should be emphasized that most geological indicators of ancient atmospheric oxygen levels imply only presence or absence and that most of the events that took place between those following two time points are highly uncertain. A battery of geological indicators suggests the timing of the rise of atmosphere oxygen occurred at following time points:(1)A shift from an anoxic to an oxic atmosphere some time between 2.5 and 2.0 billion years ago,which is known as the Great Oxidation Event(GOE);(2)A second‘Great Oxidation Event’ during the Precambrian-Cambrian transition between about 0.85 and 0.54 billion years ago(GOE-Ⅱ),which is herewith referred to as the Neoproterozoic Oxygenation Event(NOE). Both the GOE and the NOE lead to a prevailing view of atmospheric oxygen evolution over time that is marked by a three durations for the atmospheric oxygen level. Following important cognitions are gained with the further research. If the overall increase in atmospheric O2,from negligible in the early Archaean to 21% today,was due to the increasing oxygen production as erosion and sedimentation became increasingly important relative to volcanic activity,so, the series of step increases in O2, associated with periods of anomalously high sedimentation following supercontinent amalgamation,superimposed on this overall trend, which demonstrates that the rise of atmospheric oxygen is resulted from the sophisticated geobiological process that is closely genetically related to geodynamics. The stepwise rise for the atmospheric oxygen is further grouped into 7 events that can be correlated with the period after the supercontinent amalgamation in the history of the Earth,which provides a more clear view;importantly,enhanced sedimentation during these periods promoted the burial of a high fraction of organic carbon and pyrite,thus preventing their reaction with free oxygen,and leading to sustained increases in atmospheric oxygen. Both novel viewpoints and important cognitions provide many important thinking approaches and researching clues for the further understanding of the geobiological process for the rise of atmospheric oxygen. Therefore,tracing these research advances on the study of rise of atmospheric oxygen is helpful to drive new research that will unveil new truths about the evolutionary history of the atmosphere of our planet. |
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| Keywords: | timing rise of oxygen content level atmosphere research advance |
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