峡东地区埃迪卡拉系黑色页岩的微量元素和Fe同位素特征及其古环境意义

The Ediacaran Trace Elements and Fe Isotopes of Black Shale in the Three Gorges Area:Implications for Paleooceanography

通过分析宜昌峡东地区九龙湾剖面埃迪卡拉系陡山沱组黑色页岩的微量元素和Fe同位素组成,对埃迪卡拉纪海洋的氧化还原状态进行了制约。黑色页岩氧化还原敏感元素的富集系数以及U/Th、V/(V+Ni)、V/Cr值等在陡山沱期存在着明显的演化趋势。这些趋势显示陡山沱组二段下部水体处于氧化的状态,陡山沱组二段上部水体处于还原状态,陡山沱组四段黑色页岩发育在氧化环境中。同时,陡山沱组二段下部黑色页岩富集重的Fe同位素,表明黑色页岩来自于海水的铁主要是以Fe3+的氧化物或氢氧化物形式沉淀的,进一步说明这个时期的水体处于氧化状态。综合黑色页岩微量元素和Fe同位素信息,显示在埃迪卡拉纪陡山沱早期海水经历了先氧化后还原的过程,陡山沱晚期海水变得氧化了。这种特征与埃迪卡拉纪陡山沱期生物群的出现和演化相对应,说明海洋的氧化为埃迪卡拉纪生物的出现和演化提供了前提条件。

The Ediacaran trace elements and Fe isotopes of the black shales of the Doushantuo Formation from the Jiulongwan section in the Three Gorges area have been used to contrain redox conditions of the Ediacaran ocean.The enrichment factor of redox-sensitive element and U/Th,V/（V＋Ni）,V/Cr values of the black shales exist obvious evolutional trend during the Ediacaran period,which show the ocean of the lower of the Doushantuo Member Ⅱ was in an oxic condition, the ocean of the upper of the Doushantuo Member Ⅱ was in a reduced state,and the black shales of the Doushantuo Member Ⅳdeposited in an oxic environment.Meanwhile,the black shales in the lower of the Doushantuo Member Ⅱare enriched in heavy isotope,which demonstrates that iron from seawater is mainly controlled by the Fe3＋minerals,and further illustrates that the ocean was in oxidized state.Comprehensive on trace elements and Fe isotopes show the ocean experienced first oxidation then reduction process during the early Ediacaran, and was in a relatively oxidizing environment during the late Ediacaran （Doushantuo Member Ⅳ）. This feature is corresponding to evolution of biota during the Ediacaran period,impying that the oxidation of ocean could create prerequisite conditions for the emergence and evolution of multicellular metazoans.