Synopsis

Basaltic tephra horizons dominate the Icelandic tephrochronological framework during the last glacial-interglacial transition (LGIT; 15–10 ka cal. bp), providing excellent potential for high precision correlation of palaeoclimate data-sets as recorded in ice, marine and terrestrial sequences of the North Atlantic region. Although significant advances have been made in identifying and extending the known provenance of rhyolitic tephras (in the form of microtephra horizons), the detection of basaltic layers has proved problematic. The bimodal Vedde Ash is an important chronological marker horizon that lies firmly within the GS-1/Younger Dryas chronozone (ca. 10.3 ka 14C bp; 12.0 ka GRIP ice-core yrs) and provides an excellent opportunity to develop methodologies for extracting both rhyolitic and basaltic ash. The recent identification of the rhyolitic component across Europe using flotation techniques has significantly extended the known provenance of this eruption event. However, there is no routine methodology for the identification of basaltic microtephra horizons and the separation of this component of the Vedde Ash has therefore been limited to visible horizons in Iceland, western Norway and the Isle of Skye. Here, we use magnetic separation techniques to identify for the first time basaltic Vedde Ash as a microtephra horizon in minerogenic sequences from two sites in western Scotland.

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