Some numerical considerations in the geochemical analysis of distal microtephra |
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| Institution: | 1. Department of Electrochemical and Surface Engineering, SURF Research Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium;2. Department of Applied Physics and Photonics, Brussels Photonics Team B-PHOT, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium;3. Department of Art Sciences and Archaeology, MARI Research Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium;1. Faculty of Earth Sciences, University of Iceland, Reykjavík, Iceland;2. INSTAAR and Department of Geological Sciences, University of Colorado Boulder, CO, USA;1. Department of Earth Science and Bjerknes Centre for Climate Research, University of Bergen, 5007, Bergen, Norway;2. NORCE Norwegian Research Centre and Bjerknes Centre for Climate Research, 5007, Bergen, Norway;3. School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, United Kingdom;4. Institute of Geological Sciences and Oeschger Center for Climate Research, University of Bern, 3012, Bern, Switzerland;5. Physics of Ice, Climate and Earth, Niels Bohr Institute, University of Copenhagen, 2200, Copenhagen, Denmark;6. Research School of Earth Sciences, Australian National University, Canberra, ACT, 2601, Australia;1. State Key Laboratory of Isotope Geochemistry and CAS Center for Excellence in Deep Earth Science, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China;2. Department of Geography, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK;3. Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK;4. Scottish Universities Environmental Research Centre (SUERC), University of Glasgow, East Kilbride, G75 0QF, UK;1. Institute of Geology, Czech Academy of Sciences, Rozvojova 269, Prague, Czech Republic;2. Department of Applied Geophysics, Charles University, Albertov 6, Prague, Czech Republic;3. Geophysical Institute, University of Alaska Fairbanks, 903 N Koyukuk Drive, Fairbanks, AK, USA;4. Institute for Environmental Studies, Charles University, Benatska 2, Prague, Czech Republic;5. Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern, Altenbergrain 21, CH-3013, Bern, Switzerland;6. Department of Conservation and Research, Bavarian Forest National Park, 94481 Grafenau, Germany;7. Chair of Wildlife Ecology and Wildlife Management, University of Freiburg, Tennenbacher Straße 4, Freiburg, Germany;1. Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK;2. School of Archaeology, 1 South Parks Road, Oxford, OX1 3TG, UK;3. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA, 94709, USA;4. Department of Geography, University of Cambridge, Cambridge, CB2 3EN, UK;5. School of Earth Sciences, Addis Ababa University, P. O. Box 1176, Addid Ababa, Ethiopia;6. Department of Mining and Geological Engineering, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana;7. Department of Geosciences, University of Arizona, Tucson, AZ, 85721, USA;8. Institute of Geography Education, University of Cologne, Gronewaldstraße 2, 50931, Köln, Germany;9. Botany Department, School of Natural Sciences, Trinity College Dublin, Ireland;10. Institute of Geosciences, University of Potsdam, Karl-Liebknecht-Str. 24-25, 14476, Potsdam, Germany;11. Institute for Geography and Geology, University of Greifswald, Friedrich-Ludwig-Jahn Straße 16, 17489 Greifswald, Germany |
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| Abstract: | The use of Late Quaternary tephra horizons as isochronous markers for high resolution correlation between records is of growing importance in a number of scientific fields. Particular advancements have been made through the identification of microtephra deposits (very distal deposits of volcanic ash invisible to the naked eye). The successful correlation of ash layers and volcanic eruptions depends on the acquisition of the major element chemistries of a tephra layer, derived from microprobe analyses of individual glass shards. This is particularly important for microtephras where aeolian fractionation has removed much of the mineral phase of the ash deposit, which often aids tephra discrimination. There are, however, difficulties in distinguishing precisely between different eruptions of the same volcano using major element composition and this may not always be resolved by the use of trace elements. Given the potential of tephrochronology as a correlative tool in many disciplines it is important to resolve this difficulty. One way forward is to perform more robust statistical analyses on the geochemical data, as in general, analyses are confined to a series of bi-plots of major elements. Here the authors explore some of the problems associated with dealing with microprobe data generated for individual tephras and pay particular attention to the ‘unit sum problem’. Using a subset of data generated as part of a EURODELTA project on tephra distribution in the Adriatic, the authors demonstrate that this problem is detectable in major element data from tephras and have consequently applied the logratio method before further analyses of the data. It is demonstrated that the use of logratios combined with discriminant functions analysis provides a more robust assessment of likely chemical correlations between tephras, and are superior to the use of bi-plots alone; and obviate the need for any data normalization, a particular point of contention between tephrochronologists. |
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