Tephra stratigraphy and geochemistry from three Icelandic lake cores: a new method for determining source volcano of tepra layers

Abstract

At present there is no consistent method for the identification of source volcanoes for a tephra layer found in a stratigraphy. This has led to several studies misidentifying source volcanoes. Geochemical analysis of the tephra layer can identify the source volcano. A methodology for identification of most Icelandic volcanoes is presented here following from the work of Jóhannsdóttir (2007). It has been constructed using an existing major element geochemical dataset from Icelandic tephra layers of known origin. In particular, it improves the difficult task of distinguishing between Hekla and Katla basalts. It is hoped it is simple and quick enough to use to encourage other workers to use it, but to still retain sufficient accuracy that a statistics based computer program could not produce. It was found that Cr, Ni, Sr, F, Cl and S are of no additional help in identifying the source volcano of the tephra layer. In order to aid production of this method and to evaluate its success, three lacustrine cores from Iceland are analysed. 135 primary Holocene tephra layers were identified in the combined stratigraphy, which includes 42 layers from Hestvàtn, 49 layers from Vestra Gìsholtsvàtnand 69 layers from Hvítarvàtn. 106 of these tephra layers were analysed for major element geochemistry. Together with the work of Jóhannsdóttir (2007), this provides a unique tephrochronological record for Central South Iceland, because it constitutes a continuous chronology spanning the last 12kyrs at the resolution of decades to centuries. These results enabled the examination of geochemistry, soil accumulation rate, identification of historic layers, frequency of tephra fall and associated hazard risks and preservation of tephra within lakes. 47.5% of the tephra layers from this study are basaltic, and another 41% contain a basaltic component, highlighting the importance of basaltic tephra layers in the tephrostratigraphy. Few tephra layers from the Western Volcanic Zone are present in the record despite its proximity to the study area. This emphasises the importance of ice caps on volcanoes for producing sufficiently explosive basaltic eruptions to generate widespread tephra fallout. Although not highly accurate the Sediment Accumulation Rate method gives a reasonable age for eruptions that have not been dated by another method. Frequency of tephra fall is highly variable in the area, it averages 8.4layers/500yrs. Katla is the most hazardous volcano for south-western Iceland, even though only 5% of its eruptions have produced tephra that reaches this location.