Progressive oxidation of anoxic and ferruginous deep-water during deposition of the terminal Ediacaran Laobao Formation in South China
Progressive oxidation of anoxic and ferruginous deep-water during deposition of the terminal Ediacaran Laobao Formation in South China
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DOI：
10.1016/j.palaeo.2012.01.019
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发表时间：
2012-03
期刊：
影响因子：
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通讯作者：
Huajin Chang;X. Chu;Lianjun Feng;Jing Huang
中科院分区：
文献类型：
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作者：
Huajin Chang;X. Chu;Lianjun Feng;Jing Huang
南华盆地埃迪卡拉纪末期老堡组为深水沉积的黑色硅质岩系。硅质岩具有精细的层状组构，没有明显的重结晶或交代残余结构。碎屑成分极其有限，无明显热液影响。因此，硅质岩似乎是一种自生的深水沉淀物。硅质岩中微量元素主要来源于海水，因此可以作为晚前寒武纪海水条件的一个相对独特的代用指标。利用氧化还原敏感性微量元素及其比值，重建了硅质岩序列的沉积环境。结合老堡组的铁形态分析结果，硅质岩中微量元素（U、V、Mo）富集，Mn亏损，Th/U比值很低（<2），V/Sc比值高（>7.9），V/（V+Ni）比值高（>0.6），表明硅质岩的沉积环境总体上是缺氧和富铁的。黄铁矿硫同位素组成在-15.4 ‰ ~ 11.0‰之间，且向剖面顶部呈递减趋势，可能反映了南华盆地埃迪卡拉纪末期硫酸盐浓度逐渐升高，这与其黄铁矿框架状直径特征的含义相一致。我们的数据证实，南华盆地深水仍然是缺氧和含铁的，它是在埃迪卡拉纪末期逐步氧化，这可能创造了必要的条件，多细胞后生动物的出现和快速进化。
The Laobao Formation, a black chert sequence, was deposited in deep-water in the terminal Ediacaran Nanhua Basin. The chert samples have finely laminated fabric, and have no distinct recrystallization or metasomatic relict texture. There is an extremely limited detrital component and no obvious hydrothermal influence. Therefore, the chert appears to be an authigenic deep-water precipitate. The trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late Precambrian seawater conditions. Redox-sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence. Combined with iron species results of the Laobao Formation, enrichments in redox-sensitive trace elements (U, V, Mo), depleted Mn, very low Th/U ratios (<2), high V/Sc (>7.9) and V/(V+Ni) (>0.6) ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert. The pyrite sulfur isotope compositions are between −15.4‰ and 11.0‰, and show a decreasing trend toward the top of the section, possibly implying sulfate concentration gradually increased in the terminal Ediacaran Nanhua Basin, which is consistent with implication of their pyrite framboid diameter characteristics. Our data confirm that deep-water in the Nanhua Basin was still anoxic and ferruginous, and it was progressively oxidized during the terminal Ediacaran period, which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans.