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Tephrochronology: Methodology and Correlations, Antarctic Peninsula Area

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Tephrochronology: Methodology and Correlations, Antarctic Peninsula Area Tephrochronology: Methodology and correlations, Antarctic Peninsula Area Mats Molén Thesis in Physical Geography 30 ECTS Master’s Level Report passed: November 9 2012 Supervisor: Rolf Zale Tephrochronology: Methodology and correlations, Antarctic Peninsula Area Abstract Methods for tephrochronology are evaluated, in the following way: Lake sediments <500 years old from three small Antarctic lakes were analysed for identification of tephras. Subsamples were analysed for a) grain size, and identification and concentration of volcanogenic grains, b) identification of tephra horizons, c) element abundance by EPMA WDS/EDS and LA-ICP-MS, and d) possible correlations between lakes and volcanoes. Volcanogenic minerals and shards were found all through the sediment cores in all three lakes, in different abundances. A high background population of volcanogenic mineral grains, in all samples, made the identification of tephra horizons difficult, and shards could only be distinguished by certainty after chemical analysis of elements. The tephra layers commonly could not be seen by the naked eye, and, hence they are regarded as cryptotephras. Because of the small size of recent eruptions in the research area, and the travel distance of ash, most shards are small and difficult to analyse. Nine possible tephra horizons have been recorded in the three lakes, and preliminary correlations have been made. But because of analytical problems, the proposed correlations between the lakes and possible volcanic sources are preliminary. Table of contents 1. Introduction . 1 1.1. Advances and problems of tephrochronology . 1 1.1.1. General observations .. 1 1.1.2. Difficulties in tephrochronology work .. 2 1.1.3. The current research .. 4 1.2. Identification of shards and chemical analyses of ashes .. 4 1.2.1. Optical polarizing microscope . 4 1.2.2. Analyses of major, minor and trace elements .. 5 1.2.2.1. General description of instruments used in tephrochronology .. 5 1.2.2.2. Instrument limitations .. 6 1.2.2.3. Necessary data for chemical analyses .. 6 1.2.3. What elements are analysed? .. 7 1.2.3.1. Major elements . 7 1.2.3.2. Minor/trace elements . 7 2. Area description and earlier work .. 8 2.1. Geography of lakes and environment in the research area .. 8 2.2. Former research and correlations of the tephras in the research area .. 11 2.3. Volcanoes in the research area . 13 2.4. Tephras in other Antarctic lakes and marine sediments in the wind catchment area . 14 3. Materials and methods . 15 3.1. Pretreatment of samples . 15 3.2. Subsampling for grain size, grain and tephra identification, and chemical analysis . 16 3.2.1. General description of subsampling procedure . 16 3.2.2. Statistics . 16 3.2.3. Subsampling for chemical analysis .. 17 3.2.3. Analysis by WDS, EDS and LA-ICP-MS .. 17 3.2.3.1. Instruments and calculations . 17 3.2.3.2. Mounts and transferring of samples . 19 3.2.4. Step by step analyses of LA-ICP-MS-results . 20 4. Results .. 22 4.1. Tephra horizon identification .. 22 4.2. Chemical analysis - WDS followed by LA-ICP-MS, testing a method .. 24 4.3. Chemical analysis - EDS .. 34 4.3.1. Midge Lake . 34 4.3.2. Lake Boeckella .. 35 4.3.3. Hidden Lake .. 35 5. Discussion . 56 5.1. Measuring grain size and concentration of volcanogenic grains . 56 5.2. Identification of volcanogenic grains and tephra horizons . 57 5.2.1. Cryptotephras, statistics and background populations .. 57 5.2.2. Volcanogenic mineral grains . 58 5.3. Chemical analysis . 59 5.3.1. General discussion .. 59 5.3.2. WDS and LA-ICP-MS data .. 60 5.3.2.1. General, problems . 60 5.3.2.2. Single grain analysis .. 60 5.3.3. EDS data . 61 5.3.3.1. General .. 61 5.3.3.2. Midge Lake . 62 5.3.3.3. Lake Boeckella and Hidden Lake .. 62 5.3.3.4. FeO/MgO and SiO2 - alternative interpretations? .. 63 5.3.3.5. Pretreatment of samples and spread of data . 64 5.4. Hypothetical age correlations from depth of tephra horizons . 65 6. Conclusions .. 67 6.1. Current research . 67 6.2. Suggestions for future research . 68 7. Acknowledgments . 68 8. References .. 69 9. Appendices . 77 Appendix 1. Grain size analysis, tephra horizon identification, and statistical analysis . 77 Appendix 2. Subsample description for WDS, LA-ICP-MS and EDS analysis . 84 Detailed description on how to mount grains for chemical analysis . 86 Appendix 3. Grouping of WDS-analysis-data in shards/minerals . 87 Appendix 4. Instruments/methods used in tephrochronology for chemical analysis . 92 Methods for chemical analyses . 92 Non-chemical methods to identify tephras . 94 Appendix 5. Examples of earlier published geochemistry data from tephras, mainly from the research area .. 94 Deception Island, samples and age . 94 Published geochemistry analyses of volcanic eruptives . 95 Appendix 6. Different statistical methods used in tephrochronology work . 112 Supplementary material with raw data, in excel (CD) • Appendix 5. (Raw data for table A5:2 and table A5:3.) • EDS- final-after-recalc-CD. (All EDS-data, after conversion from atomic percent to weight percent.) • EDS-original-recalculated-1-26-CD. (Recalculation of EDS-data.
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