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Extraction and Measurement of Cosmogenic in Situ 14C from Quartz

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Extraction and Measurement of Cosmogenic in Situ 14C from Quartz EXTRACTION AND MEASUREMENT OF COSMOGENIC IN SITU 14C FROM QUARTZ PHILIP NAYSMITH Thesis presented for the degree of Master of Science University of Glasgow Scottish Universities Environmental Research Centre East Kilbride © P. Naysmith 2007 TABLE OF CONTENTS ACKNOWLEDGEMENTS ........................................................................................ iv DECLARATION......................................................................................................... v ABSTRACT .............................................................................................................. vi CHAPTER 1: INTRODUCTION................................................................................. 1 1.1. Introduction ..................................................................................................... 1 1.2. History of Surface Exposure Dating ................................................................ 4 1.3. Formation of Cosmogenic Nuclides ................................................................ 5 14 1.4. Advantages of In Situ C for Surface Exposure Dating .................................. 7 14 1.5. Previous Studies of In Situ C........................................................................ 9 14 1.6. Production Rates for In situ C..................................................................... 18 1.7. Measurement Techniques............................................................................. 20 14 1.8. Applications using In Situ C........................................................................ 24 CHAPTER 2: METHODS ........................................................................................ 29 14 2.1. Quartz Preparation for In Situ C Measurements......................................... 29 2.1.1.Kohl and Nishiizumi Method.................................................................... 29 2.1.2.The Edinburgh Method ........................................................................... 30 2.1.3.SUERC Method ..................................................................................... 31 2.2. Vacuum System ............................................................................................ 32 2.3. Combustion and CO2 Collection Section....................................................... 32 2.4. Purification and Measurement Section.......................................................... 38 2.5. Calculations................................................................................................... 41 2.6. Graphitisation Section ................................................................................... 43 2.7. Improvements to SUERC In Situ System...................................................... 46 2.8. Combustion Recovery Test........................................................................... 51 2.9. System Blanks .............................................................................................. 52 2.10. SUERC AMS Systems .................................................................................. 53 2.11. Summary of Extraction Method ..................................................................... 55 CHAPTER 3: RESULTS AND DISCUSSION.......................................................... 57 3.1. Calculations................................................................................................... 57 3.1.1.Step One................................................................................................. 57 3.1.2.Step Two................................................................................................. 60 3.2. Results .......................................................................................................... 63 3.2.1.Quality Assurance Standards.................................................................. 63 3.2.2.Performance Results .............................................................................. 65 3.2.3.PP-4 Results........................................................................................... 70 3.2.4.Shielded quartz results ........................................................................... 72 CHAPTER 4: CONCLUSIONS................................................................................ 75 REFERENCES ........................................................................................................ 79 i LIST OF TABLES Table 1: Half-lives of cosmogenic isotopes..........................................................8 14 Table 2: C activity in “dead” CO2 dilution gas..................................................14 14 Table 3: Recovery yields and C atom content of “dead” CO2 ..........................15 14 -1 Table 4: No. of C atoms g SiO2 extracted from 5 g shielded quartz samples ..............................................................................................................16 14 Table 5: C atoms extracted per gram of PP-4 quartz ......................................18 14 Table 6: Calculated in situ C production rates.................................................19 Table 7: Typical beam currents used in AMS sources .......................................23 Table 8: Results of CaCO3 hydrolysis for finger calibration................................43 Table 9: Graphite conversion test results...........................................................45 Table 10: Recovery tests results using 1 ml and 0.1 ml of CO2 .........................49 Table 11: Single combusted Oxalic II vs. average Oxalic II standards...............52 Table 12: Results for Barley Mash samples.......................................................64 Table 13: Results for Belfast Cellulose samples................................................65 Table 14: Results for 1 ml Doublespar blank samples .......................................65 Table 15: Results from recovery tests on 0.1 ml and 1 ml blank samples .........66 Table 16: Results for system blanks for the in situ system ................................67 Table 17: Results for system blanks from the literature .....................................68 Table 18: CO2 volumes from Lake Bonneville PP-4 quartz samples .................70 14 -1 Table 19: PP-4 results in C atoms g SiO2 ......................................................71 Table 20: Final results for PP-4 quartz...............................................................72 Table 21: Shielded quartz sample collected from a depth of 250 m...................73 Table 22: PP-4 sample using a shielded quartz as procedural blank.................74 ii LIST OF FIGURES Figure 1: Schematic diagram of 14C extraction system (Lifton, 1997) ................12 Figure 2: Generalized time altitude diagram of Lake Bonneville, Utah...............17 Figure 3: NEC tandem pelletron accelerator......................................................22 Figure 4: SUERC in situ extraction system .......................................................34 Figure 5: Quartz sleeve before (upper) and after (lower) the combustion stage ...................................................................................................................35 Fiqure 6: Quartz sleeve being removed from the furnace after combustion.......36 Figure 7: The combustion section of the in situ vacuum system........................37 Figure 8: The capacitance manometer and measuring finger............................39 Figure 9: Hydrolysis unit ....................................................................................40 Figure 10: CO2 measurement and graphite section of the extraction line ...........44 Figure 11: Change in background (F measured) against mass of graphite .......45 Figure 12: The new ‘O’ ring joint ........................................................................48 Figure 13: Variable temperature trap designed by Lifton (Lifton, 1997) .............50 Figure 14: CO2 Extraction Process ....................................................................55 iii ACKNOWLEDGEMENTS There are lots of people I would like to thank for all their help and encouragement during this study. Firstly, I would like to thank my two supervisors, Dr Gordon Cook and Dr Sheng Xu, for all their advice, support and encouragement over the last four years. I would also like to thank our previous Director, Professor Tony Fallick, for the opportunity to carry out this part time research project. I would also like to thank my colleagues in the radiocarbon laboratory Gordon, Bob, Elaine, Graham and Lesley, who allowed me the time to complete this study. Building this vacuum system proved a challenge to our glassblowers. I would like to thank Willie and Gayle for their skill in building the vacuum system and for fixing the small breaks in the vacuum line quickly. Over the four years of this study I have received help, encouragement, and advice from Nat Lifton, I would like to thank him for all his help. It was good to know that there is someone else in the world that could maybe explain some of the problems I was having with my extraction system. Finally, I would like to thank my wife Fiona for taking the time to read this thesis and for her encouragement and support during this study. To Ian and Angus - they always had the ability to make me laugh which helped a lot at the times when the writing became a struggle. iv DECLARATION I declare that this thesis is my own composition and that the work of which it is a record was carried out by me at the Scottish Universities Environmental Research Centre, and
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