Contributions to the Neoproterozoic Geobiology
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Contributions to the Neoproterozoic Geobiology Bing Shen Dissertation submitted to the faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy In Department of Geosciences Shuhai Xiao (Chair) Robert Bodnar Michal Kowalewski J. Fred Read November 29, 2007 Blacksburg, Virginia Key words: Neoproterozoic, Ediacaran, Ediacara fossils, China, Disparity, Sulfur isotope, Carbon isotope Copyright 2007, Bing Shen Contributions to the Neoproterozoic Geobiology Bing Shen Abstract This thesis makes several contributions to improve our understanding of the Neoproterozoic Paleobiology. In chapter 1, a comprehensive quantitative analysis of the Ediacara fossils indicates that the oldest Ediacara assemblage—the Avalon assemblage— already encompassed the full range of Ediacara morphospace. A comparable morphospace range was occupied by the subsequent White Sea and Nama assemblages, although it was populated differently. In contrast, taxonomic richness increased in the White Sea assemblage and declined in the Nama assemblage. The Avalon morphospace expansion mirrors the Cambrian explosion, and both may reflect similar underlying mechanisms. Chapter 2 describes problematic macrofossils collected from the Neoproterozoic slate of the upper Zhengmuguan Formation in North China and sandstone of the Zhoujieshan Formation in Chaidam. Some of these fossils were previously interpreted as animal traces. Our study of these fossils recognizes four genera and five species. None of these taxa can be interpreted as animal traces. Instead, they are problematic body fossils of unresolved phylogenetic affinities. Chapter 3 reports stable isotopes of the Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Our new data indicate that carbonate associated sulfate (CAS) abundance decreases rapidly in the basal 34 cap dolostone and δ SCAS composition varies between +9‰ and +15‰ in the lower 2.5 34 m. In the overlying interval, CAS abundance remains low while δ SCAS rises ~5‰ and 34 34 varies more widely between +10‰ and +21‰. δ Spy is typically greater than δ SCAS measured from the same samples. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin. 13 18 34 34 Chapter 4 studies δ C, δ O, δ SCAS, and δ Spy of the Zhoujieshan cap carbonate that overlies the Ediacaran Hongtiegou glaciation. The Zhoujieshan cap dolostone shows 13 34 positive δ C values (0 –1.7‰). δ SCAS shows rapid stratigraphic variations from +13.9 34 to +24.1‰, probably due to relatively low oceanic sulfate concentrations. δ Spy shows a 34 34 steady stratigraphic trend. Thus, the δ SCAS and δ Spy trends are decoupled from each 34 34 other. The decoupling of δ SCAS and δ Spy trends suggests that CAS and pyrite were derived from different sulfur pools, which were probably due to the postglacial basin stratification. Acknowledgments This work would not have been possible without the efforts and dedication of my advisor, Shuhai Xiao. I thank Shuhai for introducing me to this fascinating field of Neoproterozoic geobiology, spending numerous hours discussing fossils, geochemistry, and various other questions with me, and being patient in correcting my naïve grammatical errors. Many thanks to my committee members: Bob Bodnar, Michal Kowalewski, and J. Fred Read, for providing continuous support and inspiration. I also thank Juan Liu and Nizhou Han and Kathleen McFadden for helping me with geochemistry laboratory experiments. Clayton Loehn, James Schiffbauer, and Viktoras Liogys helped me in sample preparation, scanning electron microscopy, and elemental mapping. Rich Krause, John Huntley, Ying Zhang, and Zhengrong Li provided valuable suggestions in writing SAS codes. Susan Barbour Wood, Jen Stempien, Peter Voice, Matthew Bychowski, Troy Dexter, Yurena Yanes, Jing Zhao, Fang Lin, and many other fellow graduate students, have always been helpful whenever I need their expertise. I would also like to thank Chuanming Zhou, Xunlai Yuan, Guoxiang Li, Zhe Chen, Yaosong Xue, Leiming Yin, Jinlong Wang, Jie Hu, Guwei Xie, and Fanwei Meng, from Nanjing Institute of Geology and Palaeontology, Yunshan Wang from Geological Survey of Qinghai Province, Luyi Zhang from Xi’an Institute of Geology They assisted me in my field work in China and made my trips much more enjoyable. My data collection would not have gone as smoothly without their help. I want to thank my parents for their love, understanding, and morale support for such a long time. Finally, special thanks to my wife, Lin Dong, for her useful comments, discussions, arguments, and suggestions on my research, and for all of her love and support in the past four years. iii TABLE OF CONTENTS Abstract ……………………………………………………………………..ii Table of Contents …………………………………………………………..iii List of Figures ……………………………………………………………...vi List of Tables ……………………………………………………………..viii Introduction and Overview of the Research ……………………………….ix Acknowledgements ……………………………………………………….xvi Chapter 1 The Avalon Explosion: Evolution of Ediacara Morphospace Abstract ………………………………………………………………1 1. Introduction ……………………………………………………….1 2. Materials and Methods ……………………………………………2 2.1. Data Acquisition ……..……………………………………3 2.1.1. Morphological data ……………………………...….3 2.1.2. Abundance and preservational quality ………………...5 2.2. Data Analysis……………………………………………6 2.2.1. Taxonomic diversity ………………………………...6 2.2.2. Morphological analysis ……………………………...7 2.3. Potential Bias ………………………………………….10 2.3.1. Temporal resolution ……………………………….10 2.3.2. Spatial resolution ………………………………….10 2.3.3. Paleogeographic map ……………………………...11 3. Discussion ……………………………………………………….11 4. Conclusions ……………………………………………………...14 References …………………………………………………………..15 Chapter 2 Problematic Macrofossils from Ediacaran Successions in the North China and Chaidam Blocks: Implications for Their Evolutionary Roots and Biostratigraphic Significance ……………………………34 Abstract ……………………………………………………………..34 1. Introduction ……………………………………………………...34 2. Geological Settings ……………………………………………...36 2.1. Suyukou Section …………………………………………36 2.2. Quanjishan Section ………………………………………37 2.3. Interpretation and Correlation of Diamictite ………………...38 3. Systematic Paleontology ………………………………………...39 Genus HELANOICHNUS Yang in Yang and Zheng, 1985 ………….39 Genus HORODYSKIA Yochelson and Fedonkin, 2000 …………….42 iv Genus PALAEOPASCICHNUS Palij, 1976 ……………………….44 Genus SHAANXILITHES Xing, Yue, and Zhang in Xing et al., 1984...49 4. Taphonomy ……………………………………………………...52 5. Interpretations and Affinities ……………………………………53 5.1. Helanoichnus helanensis ……..…………………………...53 5.2. Horodyskia ……………………………………………...53 5.3. Palaeopascichnus ………………………………………..54 5.4. Shaanxilithes …………………………………………….55 6. Implications for Regional Correlation …………………………..56 7. Conclusions ……………………………………………………...57 References …………………………………………………………..57 Chapter 3 Stratification and Mixing of a Post-glacial Neoproterozoic Ocean: Evidence from Carbon and Sulfur Isotopes in a Cap Dolostone from Northwest China ……………………………………………………82 Abstract ……………………………………………………………..82 1. Introduction ……………………………………………………...83 2. Geological Background …………………………………………84 3. Methods ………………………………………………………….86 3.1. Carbon and Oxygen Isotopes ……………………………...86 3.2. Sulfur Isotopes …………………………………………..87 3.3. Elemental Geochemistry ………………………………….88 4. Results …………………………………………………………...89 4.1. Carbon and Oxygen Isotopes ……………………………...89 4.2. Sulfur Isotopes …………………………………………..89 4.3. Elemental Geochemistry ………………………………….90 34 34 5. Validity of CAS Concentration, δ SCAS, and δ Spy Data ………90 5.1. Pyrite oxidation during laboratory preparation or outcrop weathering ………………………………………………90 34 5.2. Diagenetic alteration of CAS concentration and δ SCAS ……...91 34 5.3. Using δ SCAS as a proxy for seawater sulfate ……………….92 34 5.4. Evaluation of δ Spy values ………………………………..93 6. Implications for Sulfur Cycling in the Quruqtagh Basin ………..94 6.1. Oceanic stratification model ………………………………95 6.2. Sulfate minimum zone model ……………………………..99 7. Conclusions ……………………………………………………100 References …………………………………………………………101 Chapter 4 v The Neoproterozoic Quanji Group in the Chaidam Basin, carbon and sulfur isotopes of a cap carbonate associated with an Ediacaran glaciation …………………………………………………………..124 Abstract ……………………………………………………………124 1. Introduction …………………………………………………….125 2. Regional Geology ……………………………………………...126 3. Litho- and Biostratigraphy of the Quanji Group ……………….126 3.1. The Mahuanggou Formation ……………………………..127 3.2. The Kubaimu Formation ………………………………...127 3.3. The Shiyingliang Formation ……………………………..128 3.4. The Hongzaoshan Formation …………………………….128 3.5. The Heitupo Formation ………………………………….129 3.6. The Hongtiegou Formation ………………………………129 3.7. The Zhoujieshan Formation ……………………………...130 4. Methods ………………………………………………………...130 4.1. Carbon and Oxygen Isotopes …………………………….131 4.2. Sulfur Isotopes …………………………………………131 4.3. Elemental Geochemistry ………………………………...133 5. Results ………………………………………………………….133 5.1. δ13C and δ18O …………………………………………..133 34 34 5.2. δ SCAS and δ Spy ……………………………………………...134 5.3. Elemental Geochemistry ………………………………...134 6. Discussions …………………………………………………….134 6.1. Diagenetic alteration of δ13C values ………………………134 6.2. Fidelity of sulfur isotope signatures ………………………135 6.3. Age of the Hongtiegou Glaciation ………………………..136 6.4. Geochemical Cycle after the Hongtiegou Glaciation ………..138 6.4.1. Interpretation of δ13C in the Zhoujieshan Cap Dolostone