Diagenetic overprint to a negative carbon isotope anomaly associated with the Gaskiers glaciation of the Ediacaran Doushantuo Formation in South China
Diagenetic overprint to a negative carbon isotope anomaly associated with the Gaskiers glaciation of the Ediacaran Doushantuo Formation in South China
复制标题
华南埃迪卡拉系陡山沱组Gaskiers冰川期负碳同位素异常的成岩叠印
DOI：
10.1016/j.precamres.2016.01.004
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发表时间：
2016-05-01
影响因子：
3.8
通讯作者：
Wang, Wei
中科院分区：
文献类型：
--
作者：
Furuyama, Seishiro;Kano, Akihiro;Wang, Wei
埃迪卡拉碳同位素记录包含显着且持久的负异常，但尚未给出令人信服的解释。我们在中国南方湖南省杨家坪埃迪卡拉系陡山沱组上部发现了成岩作用叠印，导致了 120 米厚的负异常区间。扬子地台外陆架沉积的粗粒碳酸盐岩为通过测量微钻子样本来检查成岩成分提供了机会。此异常区间内散装样品的 Delta C-13 值（+5 至 -14 ppm）在此归因于高度 C-13 贫化（高达 -33 ppm）方解石胶结物在 Delta C-13 负值较小的白云石沉积颗粒上的二次沉淀。在异常上方或下方的区间中没有观察到这种成岩叠印。在这些区间中，水泥和颗粒之间的 Delta C-13 差异微不足道。该异常区间的锶同位素变化与三峡地区报道的陡山沱组中部δ C-13负异常区间一致，推测与Gaskiers冰川作用有关。该关联方案表明异常层位与杨家坪上覆灯影组之间存在长期间断（类似于 35 密尔）。我们认为，通过硫酸盐还原对还原碳（有机碳或甲烷）进行厌氧氧化是导致 C-13 贫化的次生方解石的最有可能的过程，它逐渐填充了粒间孔隙。埃迪卡拉纪和加斯基尔斯冰川期间大气中氧气的增加增强了氧化风化作用，并将海水硫酸盐浓度提高到足以发生成岩蚀变的水平。这与中埃迪卡拉纪 Sr-87/Sr-86 从 0.7080 增加到 0.7085 同时发生。我们还发现杨家坪陡山沱组三角洲C-13剖面与贵州省瓮安的三角洲C-13剖面相似，这是另一个记录外陆架沉积环境的剖面。虽然还需要进一步研究，但杨家坪3单元与瓮安三角洲C-13散点区间之间的相关性表明，瓮安含胚磷岩很可能是在Gaskiers冰川作用之前形成的。 (C) 2016 Elsevier B.V. 保留所有权利。
Ediacaran carbon isotope records contain significant and long-lasting negative anomalies for which convincing explanations have not been given. We recognized a diagenetic overprint contributing to the 120-m-thick negative anomaly interval in the upper part of the Ediacaran Doushantuo Formation at Yangjiaping, Hunan province in South China. Coarse-grained carbonate fades deposited in an outer shelf setting of the Yangtze platform provided an opportunity to examine diagenetic components by measuring micro-drilled sub-samples. Scattered delta C-13 values of bulk samples (+5 to -14 parts per thousand) in this anomaly interval are here attributed to the secondary precipitation of highly C-13-depleted (up to -33 parts per thousand) calcite cement on dolomite sedimentary grains of less negative delta C-13. This diagenetic overprint was not observed in the intervals above or below the anomaly. In these intervals the delta C-13 difference between the cements and the grains is insignificant. Strontium isotopic variation in the anomaly interval is identical to that of the interval with negative delta C-13 anomaly in the middle Doushantuo Formation reported from the Three Gorges area, which is presumably correlated with the Gaskiers glaciation. This correlation scheme indicates a long-term hiatus (similar to 35 myrs) between the anomaly horizon and the overlying Dengying Formation at Yangjiaping. We suggest that the anaerobic oxidization of reduced carbon (organic carbon or methane) by sulfate reduction was the most likely process responsible for C-13-depleted secondary calcite, which progressively filled intergranular porosity. An increase in atmospheric oxygen during Ediacaran and the Gaskiers glaciation enhanced oxidative weathering and elevated seawater sulfate concentrations to a level sufficient for diagenetic alteration. This co-occurred with an increase in Sr-87/Sr-86 from 0.7080 to 0.7085 in middle Ediacaran. We also show that the delta C-13 profile of the Doushantuo Formation at Yangjiaping is similar to that at Weng'an in Guizhou province, another section recording an outer shelf depositional environment. Although further study is needed, correlation between the Yangjiaping Unit 3 with the delta C-13-scattered interval in Weng'an suggests that the embryo-bearing phosphorites at Weng'an were formed likely before the Gaskiers glaciation. (C) 2016 Elsevier B.V. All rights reserved.