Towards a Holocene tephrochronology for the Azores

Abstract: The Azores is situated in the North Atlantic Ocean and is one of the most active volcanicregions in the Northern Hemisphere. The volcanic history of the islands is fairly wellknown and several explosive trachytic eruptions have been reported but the geochemicalcompositions of the glass component of the tephra as well as the dispersal oftephras to distal areas are less well known. The purpose of this study is twofold: (1) topresent major element geochemistry of the glass component from several historic aswell as prehistoric tephras, including the trachybasaltic Capelinhos AD 1957‐58eruption 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 ofSão Miguel; (2) to present a refined tephrostratigraphy for the island of Pico. Analyses ofmajor element geochemistry suggest that tephras from the three active stratovolcanoeson 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 radiocarbondated core with eight tephra layers extending back to c. 7000 BP. All tephras are oftrachybasaltic/basaltic trachyandesitic composition except the oldest layer, which is ofbasanitic composition. An attempt was made to correlate the tephra record of Caveirobog with the previously investigated Lake Caveiro. A tephra‐based correlation betweenthe Caveiro bog and Lake Caveiro is not straightforward and only three tephras inCaveiro bog can possibly be correlated with tephras found in the sediments of LakeCaveiro.Proximal glass data from the Furnas volcano on São Miguel suggest that distal cryptotephrasfound in Ireland may have an origin in the Azores and not on Jan Mayen aspreviously has been suggested. The similarity of the proximal tephras on São Miguel anddistal tephras in Ireland is demonstrated by high similarity coefficients (>0.95) andbiplots showing major element composition also support a correlation between Azoreaneruptions and distal tephras in Ireland. Thus, trachytic tephras erupted from explosiveeruptions on the island of São Miguel may have a potential to contribute to theconstruction of a European‐wide tephrochronology framework.Trachytic tephras erupted from explosive eruptions on the Azores may be more widelydispersed than previously thought and may provide useful isochrones for correlation ofpaleoclimate archives in the north‐central Atlantic, North Africa and the Iberian regions.The trachybasaltic/basaltic trachyandesitic tephras erupted from cinder cones on PicoIsland are probably only useful for a local tephrochronology in the Azores region, andnot for a wider Atlantic or European framework.