Was the 12.1 ka Icelandic Vedde Ash one of a kind?

Abstract

The Vedde Ash is the most important volcanic event marker layer for the correlation of Late Quaternary palaeoenvironmental archives in Europe and the North Atlantic. First defined from its type site localities near Ålesund, Western Norway, the Vedde Ash has now been traced across much of northern and central Europe, into northwest Russia, within North Atlantic marine sediments and into the Greenland ice cores. The Vedde Ash is thought to derive from an eruption of the Katla volcano in Iceland that occurred midway through the Younger Dryas/Greenland Stadial 1 (GS-1), ∼12.1ka BP. Visible and cryptotephra deposits of the Vedde Ash have been found in numerous stratified sites with robust chronologies, which allow its age to be constrained and its dispersal to be mapped. The eruption must have been highly explosive, however few proximal outcrops have been confirmed and this crucial ash layer remains almost exclusively distally-described and characterised using major element glass compositions. The widespread distribution, stratigraphic associations and consistent major element glass chemistry have led the Quaternary tephrochronological community to see the Vedde Ash as a robust and unique chronological marker layer for the Last Glacial to Interglacial Transition (∼10–18ka BP). Here we present new glass analyses of the Vedde Ash from multiple sites around the dispersal area, using a full suite of compositional analysis, including for the first time, single-grain trace element data. These data demonstrate the strong compositional coherence of Vedde Ash deposits. However, comparison with major, minor, and trace element compositional data from several other distally-described Icelandic tephras reveals that both before and after the Younger Dryas chronozone, there were eruptions that generated widespread tephra layers that have comparable glass shard compositions to the Vedde Ash. This implies that these numerous events not only hail from the same volcanic system, but that the melts share similar crystallisation trends and mixing patterns prior to eruption. It seems therefore that composition alone is insufficient for the correlation of some widespread tephra layers: good stratigraphic information and/or robust dating control are also essential.