Early Holocene and Late-glacial Eruption Frequency Peaks in the Grímsvötn and Bárdarbunga Volcanic Systems Revealed by Tephrochronology.

Eruption frequency is high in Iceland and has been estimated to exceed 20 eruptions per century on average during the Holocene. The majority of the eruptions are mafic ( 90%) and predominantly explosive ( 70%). The dominating explosive volcanism in Iceland allows for the history and behaviour of volcanic provinces to be studied in detail through tephra stratigraphy. For the past 7-8000 years the tephra stratigraphy in Iceland is relatively well known, especially in the southern, northern and central parts of Iceland, being based on numerous soil profiles across the country. To extend and fill in gaps in the eruption history of Icelandic volcanoes the tephrochronology of high-resolution records from soil, lacustrine and marine archives extending beyond 7000 years have been studied. The records investigated here are dominated by basaltic tephra layers and the vast majority originates from the most active Holocene volcanic systems i. e. Grímsvötn, Bárdarbunga and to a lesser extent Katla and Kverkfjöll. There are strong indications that volcanism in Iceland during the Holocene has been periodic at several timescales with apparent periods of 140, 500, and 4-5000 years. A high-resolution lake record from east Iceland (Lake Lögurinn), soil sections from north Iceland (Svartárkot) and marine records from the north Iceland shelf (core MD99 2271) reveal volcanic periodicity even further back in time. The 4-5000 year period is prominent in the tephra records of the Grímsvötn and Bárdarbunga volcanic systems, were eruption frequency peaks are observed between 1-2000, 5-7000, 9-10.000 and 12-14.000 years ago. The early Holocene and Late-glacial peaks have not been observed before and suggest that volcanism in Grímsvötn and Bárdarbunga has been periodic as far back as 15.000 years. The eruption frequency peak between 9-10.000 years ago occurred at the time of a major environmental change associated with deglaciation and rapid isostatic rebound, but the two younger peaks cannot be related to such changes. A more plausible explanation is periodicity in magma supply from the mantle, given the location of the two systems above the centre of the Iceland mantle plume. Work in progress will aim at investigating eruption history and behaviour of the active volcanic systems back to 50.000 years.