The Saksunarvatn Ash and the G10ka series tephra. Review and current state of knowledge

Abstract

The Saksunarvatn Ash, first found in the Faroe Islands, is a tephra produced by the Grímsvötn volcanic system in Iceland. Since its discovery in the Faroe Islands, dark tephra with a similar stratigraphic position has been described at numerous locations around the North-Atlantic region; including 46 sites in Iceland (soil and lake sediments), 37 marine sediment cores from the North-Atlantic, 23 terrestrial locations in northern Europe (Faroe Islands, Scotland, Orkney, Shetland, Norway and Germany), and 4 sites from the Greenland Ice Sheet. The chemical composition of the tephra found around the North-Atlantic is, in most cases, within the published chemical range of the Saksunarvatn Ash originally found in the Faroe Islands, i.e. tholeiitic basalt with MgO and K2O wt% that places it in the more evolved part of the Grímsvötn chemical field. Published ages of the inferred Saksunarvatn Ash range significantly, dating from 10,625 ± 53 to 9586 ± 315 cal yr BP, although the widespread usage of the ice-core age of ~10,300 yr BP has given the tephra high chronological importance. Based on the reported sites, the tephra covers an area of about 2 million km2. However, in the last decade new studies have shown that the Grímsvötn volcanic system produced several widely distributed tephra layers of very similar chemical composition in the time period from 10,400–9900 yr BP. Hence, the Saksunarvatn Ash appears to be one of multiple early Holocene Grímsvötn tephra layers distributed around the North-Atlantic area. Where such tephras are identified, they therefore reflect a time interval rather than a precise marker as previously anticipated. Although still chemically indistinguishable, these Grímsvötn tephra layers represent a marker horizon around the North-Atlantic region spanning approximately 500 years, and are referred to as the G10ka series tephra. The exact number of eruptions that form the tephra marker horizon remains unknown, but up to seven have been proposed.