Titel: Towards a Holocene tephrochronology for the Azores
Handledare: Stefan Wastegård, Otto Hermelin (IGV) och Ewa Lind
Opponent: Christine Lane, School of Environment, Education and Development, The University of Manchester, UK
Examinator: Gunhild Rosqvist
Abstract:
The Azores is situated in the North Atlantic Ocean and is one of the most active volcanic regions in the Northern Hemisphere. The volcanic history of the islands is fairly well known and several explosive trachytic eruptions have been reported but the geo-chemical compositions of the glass component of the tephra as well as the dispersal of tephras to distal areas are less well known. The purpose of this study is twofold: (1) to present major element geochemistry of the glass component from several historic as well as prehistoric tephras, including the trachybasaltic Capelinhos AD 1957-58 eruption on the island of Faial, and the trachytic explosive eruptions of Sete Cidades (c. AD 1440), Fogo A (c. 5000 BP), Fogo AD 1563 and Furnas AD 1630 on the island of São Miguel; (2) to present a refined tephrostratigraphy for the island of Pico. Analyses of major element geochemistry suggest that tephras from the three active stratovolcanoes on São Miguel can be separated in biplots showing e.g. FeOtot vs.TiO2 and FeOtot vs. CaO.
The tephrostratigraphy of Caveiro bog on the island of Pico is based on a radiocarbon dated core with eight tephra layers extending back to c. 7000 BP. All tephras are of trachybasaltic/basaltic trachyandesitic composition except the oldest layer, which is of basanitic composition. An attempt was made to correlate the tephra record of Caveiro bog with the previously investigated Lake Caveiro. A tephra-based correlation between the Caveiro bog and Lake Caveiro is not straightforward and only three tephras in Caveiro bog can possibly be correlated with tephras found in the sediments of Lake Caveiro.
Proximal glass data from the Furnas volcano on São Miguel suggest that distal crypto-tephras found in Ireland may have an origin in the Azores and not on Jan Mayen as previously has been suggested. The similarity of the proximal tephras on São Miguel and distal tephras in Ireland is demonstrated by high similarity coefficients (>0.95) and biplots showing major element composition also support a correlation between Azorean eruptions and distal tephras in Ireland. Thus, trachytic tephras erupted from explosive eruptions on the island of São Miguel may have a potential to contribute to the construction of a European-wide tephrochronology framework.
Trachytic tephras erupted from explosive eruptions on the Azores may be more widely dispersed than previously thought and may provide useful isochrones for correlation of paleoclimate archives in the north-central Atlantic, North Africa and the Iberian regions. The trachybasaltic/basaltic trachyandesitic tephras erupted from cinder cones on Pico Island are probably only useful for a local tephrochronology in the Azores region, and not for a wider Atlantic or European framework.
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