Precambrian supercontinents, glaciations, atmospheric oxygenation, metazoan evolution and an impact that may have changed the second half of Earth history
Precambrian supercontinents, glaciations, atmospheric oxygenation, metazoan evolution and an impact that may have changed the second half of Earth history
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DOI：
10.1016/j.gsf.2012.07.003
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发表时间：
2013-05
影响因子：
8.9
通讯作者：
G. M. Young
中科院分区：
文献类型：
--
作者：
G. M. Young
在超过4亿年的地质演化过程中，地球经历了两次极端的气候扰动，当时发生了广泛的冰期，以及伴随着大气氧化作用的交替暖期。这两次气候震荡中较年轻的一次发生在寒武纪后生动物生命形式“大爆发”之前，但类似的极端气候条件在大约2.4至2.2Ga之间存在。在很长一段时间内，太阳亮度和地幔温度的变化在调节地球气候方面发挥了重要作用，但这两个时期的气候剧变都与超级大陆有关。在造山带和热浮力的超级大陆上，增强的风化作用会从大气中剥离二氧化碳，引发冷却趋势，导致大陆冰川作用。冰盖阻止了风化作用，使二氧化碳再次积聚，导致冰盖崩塌，迎来了温暖的气候。这种负反馈循环为早、晚元古代的多次冰期及其与温暖气候下形成的沉积岩的密切联系提供了一种合理的解释。在每一次冰期循环之间，营养物质被冲入世界海洋，刺激光合作用并引起大气的氧化。裂谷作用为许多古冰川沉积物提供了容身之处，而脱离气候循环则是基于超大陆的分裂，当时被淹没的大陆边缘对风化有缓和作用。新元古代盖帽碳酸盐的地球化学特征带有强烈的热液信号，表明它们在冰川结束时从深海中沉淀，翻覆并溢出到大陆架上。埃斯帕诺拉组的古元古代（休伦）碳酸盐岩可能是在受热液影响的有限裂谷环境中形成的。为什么后生动物在古元古代大氧化事件后没有开始进化？答案可能就在2023年前南非弗里德堡撞击留下的巨大伤疤中，以及世界范围内富含有机碳的Shunga事件沉积物中，这些沉积物证明了生命几乎灭绝，并可能彻底改变了地球历史进程。
In more than 4 Ga of geological evolution, the Earth has twice gone through extreme climatic perturbations, when extensive glaciations occurred, together with alternating warm periods which were accompanied by atmospheric oxygenation. The younger of these two episodes of climatic oscillation preceded the Cambrian “explosion” of metazoan life forms, but similar extreme climatic conditions existed between about 2.4 and 2.2Ga. Over long time periods, changing solar luminosity and mantle temperatures have played important roles in regulating Earth's climate but both periods of climatic upheaval are associated with supercontinents. Enhanced weathering on the orogenically and thermally buoyed supercontinents would have stripped CO2from the atmosphere, initiating a cooling trend that resulted in continental glaciation. Ice cover prevented weathering so that CO2built up once more, causing collapse of the ice sheets and ushering in a warm climatic episode. This negative feedback loop provides a plausible explanation for multiple glaciations of the Early and Late Proterozoic, and their intimate association with sedimentary rocks formed in warm climates. Between each glacial cycle nutrients were flushed into world oceans, stimulating photosynthetic activity and causing oxygenation of the atmosphere. Accommodation for many ancient glacial deposits was provided by rifting but escape from the climatic cycle was predicated on break-up of the supercontinent, when flooded continental margins had a moderating influence on weathering. The geochemistry of Neoproterozoic cap carbonates carries a strong hydrothermal signal, suggesting that they precipitated from deep sea waters, overturned and spilled onto continental shelves at the termination of glaciations. Paleoproterozoic (Huronian) carbonates of the Espanola Formation were probably formed as a result of ponding and evaporation in a hydrothermally influenced, restricted rift setting. Why did metazoan evolution not take off after the Great Oxidation Event of the Paleoproterozoic? The answer may lie in the huge scar left by the ∼2023Ma Vredefort impact in South Africa, and in the worldwide organic carbon-rich deposits of the Shunga Event, attesting to the near-extirpation of life and possible radical alteration of the course of Earth history.