The widespread ~10ka Saksunarvatn tephra is not a product single eruption

Tuesday, 16 December 2014: 4:45 PM
Thor Thordarson, University of Iceland, Nordic Volcanological Center, Institute of Earth Sciences, Reykjavik, Iceland
Since its identification as a 45 cm-thick basaltic ash horizon in a sediment core from Lake Saksunarvatn on Streymoy in the Faeroe archipelago the ~10 ka Saksunarvatn ash has been taken to represent one discrete stratigraphic marker horizon in early Holocene sedimentary sequences across the North Atlantic; from Greenland to Western Europe, carrying the inference that it was produced by one continuous eruption. If taken at face value, the volume of this basaltic tephra would be >450 km3, which not only makes it - by far - the largest tephra fall deposit produced by an explosive eruption at an Icelandic volcano but also puts it in the category of ‘super-eruptions’. It also implies that the inferred ‘Saksunarvatn eruption’ would have been about 40 times bigger than the ~3 ka H3 and ~4.2 ka H4 events, two of largest post-glacial silicic explosive eruptions in Iceland. Also, the largest known post-glacial basaltic explosive eruption in Iceland produced tephra volumes of 5-8 km3, or factor of 60 smaller than the anticipated ‘Saksunarvatn’ event. Although the geological record in Iceland contains ample examples of volcanic events with magnitudes in the range of VEI5-7, it does not reveal evidence of events on the super-eruption scale. These question the validity of the inference that the tephra referred to as the Saksunarvatn ash is a product of a single eruption. New studies on lake sediment cores and soil profiles in Iceland have identified several tephra layers in the time interval 10.4 – 9.9 ka that exhibit major element composition identical to that of the Saksunarvatn ash. Thus, over a period of ~500-years the Grímsvötn volcano produced a series of events erupting tephra of composition identical to that of the Saksunarvatn ash and we will present stratigraphic and geochemical evidence to demonstrate that the 10.4 – 9.9 ka interval of the early Holocene sediment archives contains at least six widespread tephra layers of Saksunarvatn composition and that an early Holocene tephra of such composition cannot be assumed to represent a unique correlative time marker horizon.