Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China
Isotopic and REE evidence for the paleoenvironmental evolution of the late Ediacaran Dengying Section, Ningqiang of Shaanxi Province, China
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DOI：
10.1016/j.precamres.2013.12.011
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发表时间：
2014-03
影响因子：
3.8
通讯作者：
Pu Zhang;H. Hua;Weiguo Liu
中科院分区：
文献类型：
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作者：
Pu Zhang;H. Hua;Weiguo Liu
弱矿化类群最有可能是寒武纪大爆发期间存在的高等生物的祖先，因此代表了简单和复杂生命之间重要但最不了解的进化联系。在埃迪卡拉纪晚期，在弱矿化类群出现的时间跨度的海水温度和碳循环的调查，有助于更深入地了解动物骨骼矿化的早期阶段存在的环境条件。为此，我们对陕西宁强碳酸盐岩的碳氧同位素组成、岩相学、常量元素和微量元素进行了分析，并对稀土元素的变化进行了研究。灯影组（Fm.）包括高家山段和北湾段。此外，高家山段拥有迄今为止发现的最多样化的生物群之一，具有弱矿化骨骼。结果表明，高家山段具有碳正偏移、高δ 18 O值、δ 13 C与δ 18 O值正相关、高稀土元素含量、明显的MREE富集模式、高Fe含量和轻微的Ce负异常。这些证据表明，一个有限的温暖的海洋环境与河水或地下水的流入。北湾段低碳同位素、高δ 18 O值及其不均一性、碳氧不相关性、海水型稀土元素分布模式等特征表明，北湾段处于一个受周期性水平衡扰动影响的狭窄、浅、微冷和/或干旱的海洋环境。据此推测，高家山段与北湾段的环境差异可能是灯影组中段保存了大量动物骨骼化石的原因。
Weakly mineralized taxa are most likely the ancestors of higher organisms that existed during the Cambrian explosion, thereby representing an important yet least understood evolutionary link between simple and complex life. An investigation of the seawater temperature and the carbon cycle during the late Ediacaran, in the time span of the emergence of weakly mineralized taxa, contributes to the deeper understanding of the environmental conditions existing during the early stages of animal skeleton mineralization. Towards this end, we conducted the analyses on the carbon and oxygen isotope composition, petrography, major elements, and trace elements, involving the rare earth elements (REEs) variations in the carbonate rocks of Ningqiang County in Shaanxi Province of China. The samples from the Dengying Formation (Fm.) include the Gaojiashan and Beiwan Members, in ascending order. In addition, the Gaojiashan Member hosts one of the most diverse collections of biota with weakly mineralized skeletons discovered to date. Our data shows that a positive carbon excursion, high δ18O values, positive correlation between δ13C and δ18O values, high REE concentrations, a distinctive MREE-enriched pattern, high Fe concentrations and slightly negative Ce anomalies in the Gaojiashan Member. These evidences indicate a restricted warm sea environment with an influx of river water or groundwater. Conversely, the low carbon isotope values, high δ18O values and its heterogeneity, the absence of correlation between carbon and oxygen, combined with a seawater-like REE distribution pattern in the Beiwan Member suggested that a restrict, shallow, slightly cool and/or arid sea environment condition that was subject to periodic perturbations in the hydrologic balance. Therefore, we surmised that the environmental differences between the Gaojiashan and the Beiwan Members probably resulted in the preservation of a number of animal skeleton fossils in the middle section of the Dengying Fm.