Precambrian supercontinents,glaciations,atmospheric oxygenation,metazoan evolution and an impact that may have changed the second half of Earth history

查看全文 作　　者：Grant [1]M.Young

机构地区：[1]Department of Earth Sciences,Western University,1151 Richmond Street N,London,Ontario,Canada N6A 5B7

出　　处：《Geoscience Frontiers》索引2013年第4卷第3期,共15页

基　　金：supported by funds from the Natural Science and Engineering Council of Canada 摘　　要：<正>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 2.2 Ga.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 CO_2 from the atmosphere,initiating a cooling trend that resulted in continental glaciation.Ice cover prevented weathering so that CO_2 built 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 breakup 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 ~2023 Ma 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. 关键词：冰川沉积物地球化学前寒武纪后生动物地质演化地球历史超大陆大气

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Precambrian supercontinents,glaciations,atmospheric oxygenation,metazoan evolution and an impact that may have changed the second half of Earth history

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