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Title: Assessing the composition and timing of Late Quaternary volcanic eruptions in southern South America using tephra layers from lacustrine and peat sequences
Author: Smith, Rebecca
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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Volcanic ash (tephra) deposits from large, explosive events travel great distances and can form visible or invisible (cryptotephra) layers in sedimentary archives. These layers have unique chemical (glass) compositions which are useful for stratigraphic correlations; correlating tephra over wide regions can provide temporal constraints on varying magma chemistry, eruption style, frequency and magnitude. Eruptive behavior has been proposed as partially controlled by ice-unloading at the end of the Last Glacial Maximum, ~18 ka. This thesis aims to compile detailed eruption records from southern South America over the last 80 ka to improve event frequency datasets, facilitate stratigraphic correlations, and enable the evaluation of climatic influences, particularly ice-unloading, on explosive volcanism. The visible tephra layers of two sedimentary archives, Lago Puyehue (Chile) and Laguna Potrok Aike (Argentina) were examined. Tephra componentry (via microscopy), and geochemical characterisation of glass-shards (via WDS-EPMA major elemental analysis) were used to investigate temporal changes in the frequency and composition of tephra deposits. A reference chemistry database was developed by analysing glass-shards from proximal deposits to produce robust tephrocorrelations. These records provide evidence for numerous eruptions in southern South America over the past 80 ka. The records do not show any systematic changes in eruption composition or frequency over time, suggesting the effect of changing ice loads is limited in this context. These records often have tephra layers that appear to be primary but are actually reworked, with low glass concentrations, rounded minerals, and/or heterogenous glass compositions. This reworking is likely caused by wind, basin morphology, and tephra loading. Cryptotephra records from three Falkland Islands peat archives were also examined to better understand the dispersal of Andean volcanic deposits. Moreover, extensive age-modelling was undertaken in this thesis to refine the ages of 49 explosive Quaternary eruptions from southern South America. The eruption stratigraphies of particular volcanoes were also modelled using novel Kernel Density Estimates to better constrain temporal variation in frequency and the age-uncertainties.
Supervisor: Wastegård, Stefan ; Smith, Victoria ; Fontijn, Karen Sponsor: Natural Environment Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Quaternary Science ; Tephrochronology