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Biogeochemical Cycling Through the Neoproterozoic-Cambrian Transition in China: an Integrated Study of Redox-Sensitive Elements

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Biogeochemical Cycling Through the Neoproterozoic-Cambrian Transition in China: an Integrated Study of Redox-Sensitive Elements Biogeochemical cycling through the Neoproterozoic-Cambrian transition in China: an integrated study of redox-sensitive elements Ph.D.-Thesis by Lawrence Michael Och Supervised by Dr. Graham A. Shields-Zhou and Prof. John McArthur University College London (UCL) Department of Earth Sciences April 5, 2011 1 I, Lawrence Och confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I conform that this has been indicated in the thesis. 2 Abstract I investigated changes in biogeochemical cycling during the Precambrian-Cambrian transition on the Yangtze Platform in South China by analyzing about 350 predominantly black shale samples from several sedimentary successions deposited during the interval from the Late Cryogenian to the Lower Cambrian. I focused on redox-sensitive trace-metal concentrations in these sediments, whereby special attention is paid to molybdenum, vanadium and uranium to try to pinpoint the onset of increasing atmospheric oxygen levels and the transition from widespread euxinia to a pervasively oxygenated deep ocean. The measurements have been carried out using X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Besides that, total organic carbon and total sulphur contents have been measured for all samples. This approach is completed by iron speciation analysis which is considered to be a reliable redox proxy. I also conducted extended literature research on trace-metal enrichments in anoxic sediments throughout Earth history as well as a major review (which will be included as an extended introduction) on all currently available lines of evidence for a major Neoproterozoic Oxygenation Event, including carbon, sulphur, strontium, molybdenum and chromium isotope studies. I show that the well preserved sedimentary succession from the Precambrian-Cambrian transition on the Yangtze Platform might represent a unique archive of ancient geochemical conditions on the Earth’s surface, based on a significant increase of Mo, V and U enrichment in black shales across the Pc-C boundary. The evidence for predominantly anoxic-ferruginous and even intermittently euxinic conditions in the water column across the Pc-C boundary and significant regional variations in geochemical parameters unravel complex interactions between ocean chemistry, platformal configuration and paleontology. 3 Contents Outline 10 1. Introduction 11 1.1. The oxygenation of the Earth’s surface: an overview of the last 4 billion years 11 1.2. Tectonics and climate during the Neoproterozoic - Cambrian Transition 24 1.2.1. Tectonic reorganization from the mid-Neoproterozoic to Early Cambrian (825 - 530 Ma) 24 1.2.2. Climatic shifts in the Neoproterozoic 750 – 580 Ma 30 1.3. Biological innovations during the Neoproterozoic – Cambrian Interval 35 1.3.1. The origin of life on Earth, the advent of photosynthesis and the animal revolution 35 1.3.2. The oxygen requirements of early animals 41 1.4. Isotopic evidence in support of the Neoproterozoic Oxygenation Event 45 1.4.1. Carbon isotopes 45 1.4.1.1. The biogeochemical carbon cycle 45 1.4.1.2. The carbon isotopic record 47 1.4.1.3. The carbon isotopic record during the Neoproterozoic 48 1.4.1.4. Total organic carbon content in sedimentary rocks throughout Earth’s history 53 1.4.2. Strontium isotopes 55 1.4.2.1. The 87Sr/86Sr ratio of the ocean 55 1.4.2.2. Changes in the seawater 87Sr/86Sr ratio during the Neoproterozoic 57 4 1.4.3. Sulphur isotopes 60 1.4.3.1. The sedimentary sulphur cycle 60 1.4.3.2. Sulphur isotope fractionation 61 1.4.3.3. The sedimentary sulphur isotope record 66 1.4.3.4. The sulphur isotopic record during the Precambrian 67 1.4.3.5. Temporal changes in total sulphur and pyrite content in sedimentary rocks 71 1.4.4. Chromium isotopes 73 1.4.5. Molybdenum isotopes 75 1.5. Rare Earth Element (REE) patterns as paleoredox proxies: the cerium anomaly 77 2. Redox-sensitive trace-metals and iron speciation 80 2.1. Redox-sensitive trace metals 80 2.2. The redox-sensitive trace metals Mo, V and U as paleoredox proxies for the Neoproterozoic - Cambrian transition 82 2.2.1. Uranium 84 2.2.2. Molybdenum 86 2.2.3. Vanadium 90 2.2.4. Other trace-elements and biological productivity 92 2.3. Iron speciation: A proxy for the oxygenation of the deep ocean 96 2.4. Redox-sensitive trace-metals and iron speciation in black shales on the Yangtze Platform 100 3. Geological setting 101 3.1. The geological evolution of the Yangtze Platform during the Neoproterozoic and Early Cambrian 101 5 3.2. The Cryogenian 103 3.2.1. The interglacial Datangpo Formation 103 3.3. The Ediacaran 104 3.3.1. The Doushantuo Formation (ca. 635 – 551 Ma) 104 3.3.2. The Dengying and Liuchapo formation (ca. 551 – 542 Ma) 112 3.4. The Early Cambrian 116 3.4.1. The Zhujiaqing Formation (ca. 542 – 526 Ma) 116 3.4.2. The Niutitang Formation (ca. 542 – 520? Ma) 118 3.4.3. The Jiumenchong Formation (ca. 542 – 520? Ma) 119 3.4.4. The Yanjiahe and Shuijingtuo formations (ca. 542 – 521 Ma) 122 3.4.5. The Xiaotan section: probably the best Precambrian – Cambrian succession in the world! 124 4. Materials and Methods 128 4.1. Instrumental analysis 128 4.1.1. X-ray fluorescence spectrometry (XRF) 128 4.1.2. Inductively coupled plasma mass spectrometry (ICP-MS) 130 4.1.3. Laser Ablation ICP-MS (LA-ICP-MS) 132 4.2. A comparison between XRF and ICP-MS analytical results 136 4.3. Iron speciation analysis 140 4.3.1. The procedure 140 4.3.2. Comparing total iron measurements (XRF and AAS) 142 4.4. Carbon and sulphur analysis 143 4.4.1. C/S analyser 143 4.4.2. Sulphide isotopes 144 4.5. Trace-metal normalisation 144 6 5. Results and interpretation of the investigated sections 146 5.1. The Cryogenian 146 5.1.1. The Datangpo Formation: a profile from the manganese mine at Changxingpo, Guizhou Province 146 5.1.2. Discussion 148 5.2. The Ediacaran and Precambrian – Cambrian boundary sections 150 5.2.1. The Maoshi section, Guizhou Province: A Doushantuo/Dengying boundary succession 150 5.2.2. The geochemistry of the Doushantuo Formation in the Three Gorges Area, Hubei Province 151 5.2.2.1. Member II of the Doushantuo Formation at Jiulongwan 151 5.2.2.2. The Miaohe Member (Mb. IV) at Jiulongwan 153 5.2.2.3. Member II and the Miaohe Member at Jijiawan 156 5.2.3. Ocean chemistry and platform evolution during the Ediacaran 157 5.2.4. A high-resolution profile from the top of the Miaohe Member at Jiulongwan 161 5.2.4.1. Results 161 5.2.4.2. Discussion 164 5.2.5. The Miaohe Member at Baiguoyuan, Hubei Province: A comparison 166 5.2.5.1. Results 166 5.2.5.2. Discussion 168 5.2.6. The Dengying and Liuchapo formations 170 5.2.6.1. The geochemical profile at Wuhe (Shibantan Mb. of the Dengying Fm.) 170 5.2.6.2. The Huanglian section (Liuchapo and Jiumenchong formations) 171 5.2.6.3. The Longbizui section (Liuchapo and Jiumenchong formations) 173 5.2.6.4. Discussion 175 7 5.3. The Early Cambrian 179 5.3.1. The Early Cambrian in the Three Gorges Area (Shuijingtuo and Yangjiahe formations) 179 5.3.1.1. The redox geochemistry during the Early Cambrian at the Jijiawan section 179 5.3.1.2. The redox geochemistry during the Early Cambrian at the Wuhe section 180 5.3.1.3. Ocean chemistry and platform evolution during the Early Cambrian: a sequel 182 5.3.2. The Zhongnan section (Niutitang Formation): A highly condensed succession on the platform margin 186 5.3.2.1. Geochemical analysis 186 5.3.2.2. Discussion 187 The Phosphorite 187 The Ni-Mo sulphide ore horizon 188 5.3.3. The Xiaotan section 190 5.3.3.1. The redox geochemistry of its black shale successions 190 1st black shale (lower Shiyantou Fm.) 190 2nd black shale succession (upper Shiyantou Fm.) 192 3rd black shale succession and beyond (Yuanshan Fm.) 195 5.3.3.2. High resolution sampling: results and discussion 195 5.3.4. Other Early Cambrian sections on the southwestern platform (Yunnan Province) 197 5.3.4.1. The Deze Section (Zhongyicun Fm., Dahai Mb.) 197 5.3.4.2. The Meishucun Section (lower Shiyantou Fm.) 198 5.3.5. Discussion: Biogeochemical cycling during the Early Cambrian on the southwestern Yangtze Platform 199 5.4. Biogeochemical cycling across the Precambrian – Cambrian Transitions on the Yangtze Platform: summary and global perspective 209 8 6. Conclusions 222 6.1. The biogeochemical cycling of redox-sensitive trace-metals during the Precambrian-Cambrian transition on the Yangtze Platform 222 6.2. A multi-proxy approach to investigate ancient paleoredox conditions 223 6.3. Future Challenges 224 Appendices Formal Acknowledgements 226 Personal Acknowledgements 226 References 228 References used for trace-metal, TOC and S compilations 262 Supplement (CD-ROM attached at the back) Electronic version of the present PhD thesis Stratigraphic correlation of the analysed sections Correlation coefficients (R2) in some black shale successions Raw elemental and isotopic data Full LA-ICP-MS dataset High resolution geochemical profiles from the LA-ICP-MS analysis 9 Outline For the present study, I investigated the biogeochemical cycling of redox-sensitive trace-metals focussing on molybdenum, vanadium and uranium alongside the redox conditions during the Precambrian – Cambrian transition on the Yangtze Platform, South China, within the framework of the second great Oxygenation Event, herewith termed the Neoproterozoic Oxygenation Event.
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