本文選題：新元古代　切入點：寒武紀　出處：《中國地質(zhì)大學》2016年博士論文　論文類型：學位論文

【摘要】：新元古代至寒武紀是地球海洋化學與生命演化的關(guān)鍵時期。沉積Mo同位素組成對探索早期地球海洋氧化還原狀態(tài)及其與生物演化之間關(guān)系具有重要作用。然而,已有的研究對于Mo在早期地球分層海洋中的生物地球化學循環(huán)認識尚較為有限,這使得在應用其進行水體化學研究時存在較多問題。因此,我們開展了新元古代至寒武紀早期海洋Mo化學循環(huán)的研究；在此基礎(chǔ)上,通過準確厘定Mo同位素分餾的局部和全球控制因素,從而對這個時期海洋局部和全球兩個尺度上的氧化還原狀態(tài)予以約束。分層海洋中Mo的循環(huán)強烈受到局部Fe-Mn氧化物和H2S的控制。通過對寒武紀牛蹄塘組楊家坪剖面(湖南省石門縣)的分析,我們發(fā)現(xiàn)Fe-Mn搬運機不僅會導致Mo可能相對U產(chǎn)生額外富集,其與低H28水體的交互還會導致沉積物中形成異常低的同位素組成。在硫化水體中,沉積物中Mo同位素組成具有受H2S濃度梯度控制的空間差異性。這兩種機制導致沉積物Mo同位素在空間上規(guī)律性分布的特征。因此,在對某一時期海水的Mo同位素組成進行重建時,通過空間上的對比有可能有效的排除局部因素的影響。Mo在硫化環(huán)境中的轉(zhuǎn)化為MoS42-并進入沉積物需要硫化持續(xù)足夠時間。因此,Mo同位素的分餾可以用來探索地質(zhì)歷史中的短暫氧化事件。通過對寒武紀荷塘組藍田剖面(安徽省休寧縣)含海綿化石石煤層Mo同位素的研究,我們在含層狀海綿化石的層位中發(fā)現(xiàn)了多次Mo同位素的負偏移,并伴隨更高的Mo、U和V的富集。該特征指示在沉積南華盆地內(nèi)多次的氧化水體的涌入過程,但盆地內(nèi)水體的氧化可能只持續(xù)了有限的時間。由于海綿對氧氣的需求量很低,短暫氧化的水體為海綿的發(fā)育創(chuàng)造了重要的條件。這項研究展示了Mo同位素技術(shù)在記錄古海洋海水氧化還原狀態(tài)的快速波動方面具有重要潛力。埃迪卡拉紀藍田生物群的出現(xiàn)指示生物系統(tǒng)的復雜程度相對之前顯著增加,而海洋氧化在其中所起的作用還尚不清楚。通過對該時期中嶺剖面和袁家剖面的研究,我們發(fā)現(xiàn)這個時期海洋的Mo同位素組成仍然只有+1.2‰,與之前的海洋無明顯差異,表明該時期海洋的硫化相對之前發(fā)生降低,但氧化水平仍然很低。因此,我們認為海洋氧化對此次生物事件的發(fā)生作用不大。新元古代至寒武紀海洋氧化還原狀態(tài)的演化歷來受到眾多的關(guān)注,但已有研究多針對于某個時段而難以對整個進程給出完整約束。我們對這個時期海洋Mo同位素的演化曲線進行了重建,結(jié)果顯示Mo同位素在750 Ma至560 Ma持續(xù)較低的同位素組成(+1.1‰-+1.2‰)且波動較小,而在約551 Ma時出現(xiàn)顯著的增加,達到+2.0‰并在之后的寒武紀接近現(xiàn)代海洋,對應Kimberella—最早的三胚層動物的出現(xiàn)。這指示海洋的整體氧化程度可能在551 Ma前后發(fā)生顯著改變。之前海洋較低的氧化程度可能限制了動物的演化,導致其演化速度和分異程度均有限。
[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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