Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China
Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China
复制标题
DOI：
10.1016/j.chemgeo.2016.12.010
复制
发表时间：
2017-02-05
期刊：
影响因子：
3.9
通讯作者：
Liu, Xiao-Ming
中科院分区：
文献类型：
--
作者：
Cui, Huan;Kaufman, Alan J.;Liu, Xiao-Ming
埃迪卡拉纪的特点是地球历史上最深刻的负碳同位素（δ C-13）漂移，舒兰漂移。各种假设-包括海洋中溶解有机碳（DOC）的大量氧化，陆地有机碳的风化，或甲烷水合物和/或从地下排出的石油的释放和氧化-已被提出作为C-13贫化碳的来源。最近，有人提出，全球规模的早期自生碳酸盐沉淀，在松散沉积物中的厌氧微生物代谢的驱动下，可能导致了舒兰远足，但缺乏经验证据。本文对中国南方陡山沱组最上部的一个舒兰相关层段进行了综合分析。我们的研究揭示了甲烷衍生的自生方解石（形成早期成岩水泥和结核）的岩相学证据，显着亏损的C-13-这表明通过甲烷的厌氧氧化（AOM）的孔隙流体中的碱度的积累-和系统亏损O-18相对于共生白云岩。这些矿物的早期自生作用可能是由微生物硫酸盐还原作用的增加所驱动的，这是在大气氧水平显著上升的背景下由大陆风化作用的增强所引发的。鉴于中岭甲烷源自生碳酸盐的发现，并根据我们的盆地规模的地层对比，我们假设，在Shuram远足显着的C-13和O-18耗尽（包括其共同的变化在世界范围内指出）可能反映了一个插曲的硫酸盐-甲烷过渡带（SMTZ）内发生的自生。如果是真的，那么Shuram远足是全球海洋地球化学反应，以增强海水硫酸盐浓度在埃迪卡拉海洋驱动的新元古代氧化的表面环境。因此，这种古海洋学的转变可能为后来的动物进化和多样化铺平了道路。我们的研究突出了一个综合的方法，结合岩相学，矿物学，和纹理特定的微钻井地球化学在化学地层学研究的意义。(C)© 2016 Elsevier B. V.版权所有。
The Ediacaran Period is characterized by the most profound negative carbon isotope (delta C-13) excursion in Earth history, the Shuram Excursion. Various hypotheses - including the massive oxidation of dissolved organic carbon (DOC) in the oceans, the weathering of terrestrial organic carbon, or the release and oxidation of methane hydrates and/or expelled petroleum from the subsurface - have been proposed as sources of the C-13-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobic microbial metabolism in unconsolidated sediments, may have caused the Shuram Excursion, but empirical evidence is lacking. Here we present a comprehensive analysis of a Shuram-associated interval from the uppermost Doushantuo Formation in South China. Our study reveals petrographic evidence of methane-derived authigenic calcite (formed as early diagenetic cements and nodules) that are remarkably depleted in C-13- suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) - and systematically depleted in O-18 relative to co-occurring dolomite. Early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction, triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels. In light of the finding of methane-derived authigenic carbonates at Zhongling, and based on our basin-scale stratigraphic correlation, we hypothesize that the marked C-13 and O-18 depletion (including their co-variation noted worldwide) in the Shuram Excursion may reflect an episode of authigenesis occurring within a sulfate-methane transition zone (SMTZ). If true, the Shuram Excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the Ediacaran ocean driven by the Neoproterozoic oxidation of surface environments. This paleo-oceanographic transition may have therefore paved the way for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specific micro-drilling geochemistry in chemostratigraphic studies. (C) 2016 Elsevier B.V. All rights reserved.