Deposition or diagenesis? Probing the Ediacaran Shuram excursion in South China by SIMS
Deposition or diagenesis? Probing the Ediacaran Shuram excursion in South China by SIMS
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DOI：
10.1016/j.gloplacha.2021.103591
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发表时间：
2021-09-14
影响因子：
3.9
通讯作者：
Valley, John W.
中科院分区：
文献类型：
--
作者：
Cui, Huan;Kitajima, Kouki;Valley, John W.
埃迪卡拉纪（Ediacaran）635-541 Ma）见证了最早的宏观动物古生物学证据（即，埃迪卡拉生物群）和地球历史上最大的碳循环异常的地球化学观测（即，Shuram Excursion，东南部）。对于东南部的成因，人们提出了许多假说，从原生海水异常到同沉积或沉积后成岩作用。尽管密集的地球化学和理论工作发表在过去的十年中，经验证据，严格基于基本的岩相结果在微米尺度仍然是有限的。为了评价东南部沉积物的组成和成岩作用，采用阴极发光、二次离子质谱和扫描电镜等方法对华南陡山沱组EN 3段进行了研究。详细的岩相学观察表明，EN 3石灰岩主要由方解石微晶石，少量但数量可变的弥散状带状白云石晶体。前者可能是由沉积泥晶灰岩的新变作用形成的，而后者则是沉积后进行性泥晶化作用的结果。成对的西姆斯三角洲13 C方解石和三角洲13 C白云石组合物的平均值在每个样品中是不可区分的，并与已发表的微钻散装粉末三角洲13 C值一致，我们解释为代表海水溶解无机碳（DIC）的沉积“背景”信号。我们还观察到MUM规模的变化西姆斯三角洲13 C数据，可能反映了一个变量成岩叠加沉积后。我们的综合岩相学和地球化学结果是一致的沉积起源的SE和提供的证据很少假设同位素蚀变的流星和混合带成岩作用或晚埋藏成岩作用。根据这项研究，我们建议，SE确实代表了海洋碳循环异常，承担了密切的时间联系埃迪卡拉纪地表环境。
The Ediacaran Period (ca. 635-541 Ma) witnessed the earliest paleontological evidence for macroscopic animals (i.e., Ediacara biota) and geochemical observations of the largest carbon cycle anomaly in Earth history (i.e., Shuram Excursion, SE). Numerous hypotheses have been proposed for the origins of the SE, ranging from primary seawater anomaly to syn- or post-depositional diagenesis. Despite intensive geochemical and theoretical work published in the past decade, empirical evidence that is strictly based on fundamental petrographic results at the micrometer scale is still limited. To evaluate depositional compositions and diagenetic effects on samples from the SE, we investigated the EN3 interval in the Doushantuo Formation of South China via integrated cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and scanning electron microscope (SEM). Detailed petrographic observations reveal that the EN3 limestone is dominated by calcite microspar, with minor but variable amounts of disseminated zoned dolomite crystals. The former likely formed via neomorphism of depositional micrite, while the latter was the result of progressive post-depositional dolomitization. The mean values of paired SIMS delta 13Ccalcite and delta 13Cdolomite compositions are indistinguishable in each sample and consistent with published micro-drilled bulk-powder delta 13C values, which we interpret to represent depositional "background" signals of seawater dissolved inorganic carbon (DIC). We also observed mu m-scale variability of SIMS delta 13C data that may reflect a variable diagenetic overprint after deposition. Our integrated petrographic and geochemical results are consistent with a depositional origin of the SE and provide little evidence for the hypothesized isotope alteration by meteoric and mixing-zone diagenesis or late burial diagenesis. In light of this study, we propose that the SE indeed represents a marine carbon cycle anomaly that bears a close temporal link to the Ediacaran surface environment.