新元古代至寒武纪化学分层海洋中的钼生物地球化学循环

发布时间：2018-03-09 05:21

  本文选题：新元古代　切入点：寒武纪　出处：《中国地质大学》2016年博士论文　论文类型：学位论文

【摘要】：新元古代至寒武纪是地球海洋化学与生命演化的关键时期。沉积Mo同位素组成对探索早期地球海洋氧化还原状态及其与生物演化之间关系具有重要作用。然而,已有的研究对于Mo在早期地球分层海洋中的生物地球化学循环认识尚较为有限,这使得在应用其进行水体化学研究时存在较多问题。因此,我们开展了新元古代至寒武纪早期海洋Mo化学循环的研究；在此基础上,通过准确厘定Mo同位素分馏的局部和全球控制因素,从而对这个时期海洋局部和全球两个尺度上的氧化还原状态予以约束。分层海洋中Mo的循环强烈受到局部Fe-Mn氧化物和H2S的控制。通过对寒武纪牛蹄塘组杨家坪剖面(湖南省石门县)的分析,我们发现Fe-Mn搬运机不仅会导致Mo可能相对U产生额外富集,其与低H28水体的交互还会导致沉积物中形成异常低的同位素组成。在硫化水体中,沉积物中Mo同位素组成具有受H2S浓度梯度控制的空间差异性。这两种机制导致沉积物Mo同位素在空间上规律性分布的特征。因此,在对某一时期海水的Mo同位素组成进行重建时,通过空间上的对比有可能有效的排除局部因素的影响。Mo在硫化环境中的转化为MoS42-并进入沉积物需要硫化持续足够时间。因此,Mo同位素的分馏可以用来探索地质历史中的短暂氧化事件。通过对寒武纪荷塘组蓝田剖面(安徽省休宁县)含海绵化石石煤层Mo同位素的研究,我们在含层状海绵化石的层位中发现了多次Mo同位素的负偏移,并伴随更高的Mo、U和V的富集。该特征指示在沉积南华盆地内多次的氧化水体的涌入过程,但盆地内水体的氧化可能只持续了有限的时间。由于海绵对氧气的需求量很低,短暂氧化的水体为海绵的发育创造了重要的条件。这项研究展示了Mo同位素技术在记录古海洋海水氧化还原状态的快速波动方面具有重要潜力。埃迪卡拉纪蓝田生物群的出现指示生物系统的复杂程度相对之前显著增加,而海洋氧化在其中所起的作用还尚不清楚。通过对该时期中岭剖面和袁家剖面的研究,我们发现这个时期海洋的Mo同位素组成仍然只有+1.2‰,与之前的海洋无明显差异,表明该时期海洋的硫化相对之前发生降低,但氧化水平仍然很低。因此,我们认为海洋氧化对此次生物事件的发生作用不大。新元古代至寒武纪海洋氧化还原状态的演化历来受到众多的关注,但已有研究多针对于某个时段而难以对整个进程给出完整约束。我们对这个时期海洋Mo同位素的演化曲线进行了重建,结果显示Mo同位素在750 Ma至560 Ma持续较低的同位素组成(+1.1‰-+1.2‰)且波动较小,而在约551 Ma时出现显著的增加,达到+2.0‰并在之后的寒武纪接近现代海洋,对应Kimberella—最早的三胚层动物的出现。这指示海洋的整体氧化程度可能在551 Ma前后发生显著改变。之前海洋较低的氧化程度可能限制了动物的演化,导致其演化速度和分异程度均有限。
[Abstract]:Neoproterozoic to Cambrian is a critical period for the evolution of marine chemistry and life. The composition of sedimentary Mo isotopes plays an important role in exploring the early redox state of the Earth's oceans and its relationship with biological evolution. The previous studies have limited understanding of the biogeochemical cycle of Mo in the early stratified oceans of the earth, which leads to many problems in the application of Mo in the study of water chemistry. We have carried out a study of the marine Mo chemical cycle from Neoproterozoic to early Cambrian. On this basis, the local and global control factors of Mo isotopic fractionation have been accurately determined. The cycle of Mo in stratified ocean is strongly controlled by local Fe-Mn oxides and H2S. The Yangjiaping profile of the Cambrian Niutetang formation was carried out. Analysis of face (Shimen County, Hunan Province), We found that the Fe-Mn conveyer not only resulted in additional enrichment of Mo relative to U, but also resulted in the formation of unusually low isotopic composition in sediments. The composition of Mo isotopes in sediments has the spatial difference controlled by H _ 2S concentration gradient. These two mechanisms lead to the regular distribution of Mo isotopes in sediments. When reconstructing the Mo isotopic composition of seawater at a certain time, It is possible to effectively exclude the influence of local factors by spatial comparison. Mo can be converted into MoS42- in vulcanized environment and it takes enough time for sulfidation to enter the sediment. Therefore, fractionation of Mo isotopes can be used to explore the geological calendar. Brief oxidation events in history. A study of Mo isotopes in coal seams containing spongy fossil stones in the Lantian section of the Cambrian Heitang formation (Xiuning County, Anhui Province), We have found several negative migration of Mo isotopes in stratiform spongy fossils, accompanied by higher Mo U and V enrichment. This feature indicates the process of multiple influx of oxidized water bodies in the sedimentary South China Basin. However, the oxidation of water bodies in the basin may have lasted only a limited time. The study shows that Mo isotope technique has important potential in recording the rapid fluctuation of redox state of paleo-marine seawater. The appearance of biota indicates a significant increase in the complexity of biological systems, However, the role of ocean oxidation in it is still unclear. By studying the Zhongling and Yuanjia sections in this period, we find that the Mo isotopic composition of the ocean in this period is still only 1.2 鈥，

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