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Belemnite Palaeo-Proxies and Dating of Mesozoic Carbonates

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Belemnite Palaeo-Proxies and Dating of Mesozoic Carbonates Belemnite Palaeo-proxies and Dating of Mesozoic Carbonates Qiong Li Department of Earth Sciences UCL A thesis submitted to University College London in the Faculty of Mathematics and Physical Sciences for the degree of: Doctor of Philosophy (PhD) Earth Sciences April 2011 I, Qiong Li confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Signed ____________________________________ Date ____________________________________ 2 Abstract This PhD project contains two related parts, both linked by a study of belemnites. The first part is to test the validity and quality of belemnite palaeo-environmental proxies, and the second to investigate the feasibility of U-Pb dating of Mesozoic carbonates. In the first part, a wide range of well-preserved Mesozoic belemnites was analyzed for bulk stable isotope and trace element compositions, so as to access species- specific effects on compositions and to evaluate belemnite palaeo-proxies (notably δ18O, δ13C, Mg/Ca and Sr/Ca) as palaeo-temperature indicators. Then, high- resolution profiles of trace elements (Mg, Ca, Sr, Na, etc.) were obtained from those best-preserved specimens by electron microprobe and laser ablation-ICP-MS, and compared with the bulk analysis for a better understanding of intro-rostral variations. Results of four different belemnite polulations with differing ages and from differing stratigraphic levels show that Mg/Ca ratios in bulk analysis neither correlate with δ18O within one single species nor within one belemnite population. Mg/Ca is therefore not considered as a palaeo-temperature indicator for belemnites. Variations in Mg profiles along belemnite cross-sections are then interpreted to likely reflect the changes of ontogeny and growth rate. Part two is to apply U-Pb dating method to Mesozoic belemnites and other carbonates. An innovative approach of dating carbonates in deep time has been developed, using both laser-ablation MC-ICP-MS and isotope dilution measurements; and ages with the highest achieveable precision (±1%) and accuracy for Mesozoic calcite cements from three separate ammonites have been produced. Part I contains chapters 1 to 8. Chapter 1 gives a brief account of belemnites (e.g. the morphology, biogeography, palaeo-proxies and palaeo-ecology) and problems that need addressing in belemnite studies; and chapters 2-8 then focus on the geochemistry (isotopic ratios and elemental compositions) of belemnites from four different Mesozoic stratigraphic levels of Europe (Pliensbachian of Yorkshire, Toarcian of Dorset, Callovian of Oxfordshire and Valangigian of Vergol, SE Fance). Part II only contains chapter 9 which presents the U-Pb dating work of a range of Mesozoic carbonate samples. 3 Acknowledgements I would first and foremost like to express my great thanks to my principal supervisor, Professor John McArthur, for his guidance, support, and encouragement throughout the period of this PhD. The conversations and meetings with him always brought me with fresh ideas and plans to take my work forward. I have also learned through his teaching never to be afraid of errors but to find solutions. I could not have asked for more in a supervisor like him. This PhD project was funded by two overseas scholarships at UCL and NIGL of British Geological Survey. I would particularly like to thank my second supervisor Professor Randy Parrish at NIGL, not only for providing me with the funding and lab facilities for sample dating, but also for his great contributions on my data scrutinizing and his enthusiastic support in writing scientific papers. His care to my work and life has kept me going through so many difficult times. I would also like to thank Dr Matt Horstwood, Dr Steve Noble and Vanessa Pashley for their assistance in radioactive isotope analyses and data processing. My particular thanks also go to Professor Melanie Lang and Hilary Sloane for arranging and running stable isotope analyses of my belemnite samples very quickly. The whole NIGL team has brought me an open and international working environment that I have enjoyed with my heart. I am grateful to Professor Tim Atkinson for revising this thesis and his contributions to our submitted paper based on some work in this PhD research. I would also like to thank Dr Wolfgang Muller for his support during profiling elemental concentrations at Royal Holloway; and Tony Osborn and Dorinda Ostermann for their help during chemical and isotopic analyses at UCL. Jim Davy at UCL and Hank Sombroek at Birkberk are also thanked for help with thin/thick section making. I would especially like to thank my best friend Bethan Hallett who was working with me in the same office over the past three years. Her solid friendship and company have kept me working hard in many late evenings. Last but not least, I would like to thank my family especially my husband Dr Yongliang Sun, who was supporting me overwhelmingly throughout my PhD. His 4 patience, love and willingness to look after our family have ensured me a comfortable home to work and relax. And finally, I would love to dedicate this thesis to my coming baby girl. She might even read it one day! 5 Contents Contents .................................................................................................................. 6 List of Figures ......................................................................................................... 13 List of Tables .......................................................................................................... 21 Chapter 1 Introduction ........................................................................................... 24 1.1 Belemnite morphology ................................................................................. 25 1.1.1 The structure and terminology ................................................................ 25 1.1.2 Belemnite growth pattern ....................................................................... 27 1.1.3 Belemnite differentiation characteristics ................................................. 27 1.2 Belemnite biostratigraphy ............................................................................. 29 1.3 Belemnite palaeo-proxies.............................................................................. 30 1.3.1 Belemnites palaeotemperature indicator: δ18O ........................................ 32 1.3.2 Trace elements & δ18O: the combined palaeo-proxies ............................. 34 1.3.3 Belemnite δ13C ....................................................................................... 35 1.4 Problems to address ...................................................................................... 36 1.4.1 Inter-species differences in belemnite composition ................................. 36 1.4.2 Intra-rostral variations ............................................................................ 38 1.4.3 The palaeo-ecology of belemnites .......................................................... 39 Chapter 2 Samples and Methodology ..................................................................... 42 2.1 Samples ........................................................................................................ 42 6 2.2 Sample preparation of belemnites ................................................................. 42 2.3 Visual preservation judgement and microstructure analysis by SEM ............. 44 2.4 Bulk sample analysis .................................................................................... 44 2.4.1 Stable isotope analysis ........................................................................... 45 2.4.2 Trace element measurement ................................................................... 46 2.5 Intro-rostral analysis ..................................................................................... 46 2.5.1 Microsampling and stable isotope analysis within a rostrum ................... 46 2.5.2 Multi-element analysis by Electron microprobe ...................................... 47 2.5.3 Multi-element analysis by LA-ICP-MS .................................................. 47 2.5.4 Method validation .................................................................................. 49 Chapter 3 Sample Preservation .............................................................................. 51 3.1 Belemnite thin section observation ............................................................... 51 3.2 Visual preservation degree ............................................................................ 54 3.3 Belemnite microstructure .............................................................................. 55 3.4 Summary of preservation assessment ............................................................ 56 Chapter 4 Pliensbachian Belemnites from Dorset ................................................... 59 4.1 Introduction .................................................................................................. 59 4.2 Sampling ...................................................................................................... 60 4.3 Results .......................................................................................................... 63 4.3.1 Belemnite section observation ................................................................ 63 7 4.3.2 Laser ablation profiles of different species ............................................. 66 4.3.3 Electron microprobe
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