Trace element chemostratigraphy of two Ediacaran-Cambrian successions in South China: Implications for organosedimentary metal enrichment and silicification in the early Cambrian
Trace element chemostratigraphy of two Ediacaran-Cambrian successions in South China: Implications for organosedimentary metal enrichment and silicification in the early Cambrian
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DOI：
10.1016/j.palaeo.2007.03.016
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发表时间：
2007-10-08
影响因子：
3
通讯作者：
Yang, Xinglian
中科院分区：
文献类型：
--
作者：
Guo, Qingjun;Shields, Graham A.;Yang, Xinglian
对华南的两个埃迪卡拉纪-早寒武世沉积层序进行了微量元素研究：四川省沙滩剖面代表浅层台地环境，以及贵州省松桃剖面对应较盆地的沉积环境。在整个华南地区，这个时代的沉积地层在普遍富含有机物、地层相关的海侵单元中表现出特征性的痕量金属富集。这里使用微量元素（包括稀土元素）数据来确定早寒武世金属清除期间的古环境和成岩条件，导致灰色页岩富集 Mo > Cd > V > U > Ni > Ag > Zn > Cu > Pb。在沙滩，高有机碳含量（高达 5% TOC）与钼富集相关，这表明金属清除与有机生产力和腐烂有关。低 Th/U 比值与金属清除过程中的高度还原条件一致，而极高的 V 含量意味着在极度缺氧时底层水可能是常氧的。在松涛，非常低的Th/U比与高V/Sc和低δC-13(org)密切相关，这意味着缺氧、有机分解和金属封存也与深水相相关。没有明确的微量元素证据表明热液活动有助于早寒武世华南金属富集。与海洋海侵的系统关系表明，富集主要是由有机生产力和埋藏的海洋学变化引起的，而不是沉积后流体流动造成的。负 Cc 异常和类似海水的稀土元素分布模式是松桃九门冲组六岔坡燧石和下部黑色页岩的一个特征。 Ce 消耗表明这些沉积单元的自生成分是在含氧条件下形成的，这与其他地球化学氧化还原指标相矛盾。我们认为，这种燧石主要源自硅质浮游生物的生物二氧化硅，硅质浮游生物在水柱内的氧化还原边界上方形成。与一些已发表的模型相反，硅化作用和热液流体在金属传输中都没有发挥重要作用。 (c) 2007 Elsevier B.V. 保留所有权利。
A trace element study is presented of two Ediacaran-Early Cambrian sedimentary successions in South China: the Shatan section, Sichuan Province, which represents a shallow platform setting, and the Songtao section, Guizhou Province, which corresponds to a more basinal, depositional environment. Across South China, sedimentary strata of this age exhibit characteristic trace metal enrichments in generally organic-rich, stratigraphically correlative, transgressive units. Trace element (including REE) data are used here to determine the palaeoenvironmental and diagenctic conditions during this Early Cambrian interval of metal scavenging that resulted in enrichments over grey shale of Mo > Cd > V > U > Ni > Ag > Zn > Cu > Pb. At Shatan, high organic carbon contents (up to 5% TOC) correlate with Mo enrichment, which indicates that metal scavenging was linked to organic productivity and decay. Low Th/U ratios are consistent with highly reducing conditions during metal scavenging, while extremely high V contents imply that bottom waters were possibly euxinic at times of extreme anoxia. At Songtao, very low Th/U ratios are strongly associated with high V/Sc and low delta C-13(org), implying that anoxia, organic decomposition and metal sequestration were associated in the deepwater facies, too. There is no unambiguous trace element evidence that hydrothermal activity contributed to metal enrichment in South China during the Early Cambrian. The systematic relationship with ocean transgression suggests that enrichment was caused largely by oceanographic changes to organic productivity and burial rather than post-depositional fluid flow. Negative Cc anomalies and seawater-like REE distribution patterns are a feature of the Liuchapo chert and lower black shales of the Jiumenchong Formation at Songtao. Ce depletion indicates that authigenic components of these sedimentary units formed under oxic conditions, which contradicts the other geochemical redox proxies. We argue that this chert derived primarily from biogenic silica from siliceous plankton, which formed above a redox boundary within the water column. Neither silicification nor hydrothermal fluids, contrary to some published models, played a significant role in metal transport. (c) 2007 Elsevier B.V. All rights reserved.