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Abstract
Stötter, J. and Wastl, M. (1996). High-resolution Holocene tephrochronology in Northern Iceland. 26th International Arctic Workshop, Arctic and Alpine Environments, Past and Present, Program with Abstracts, 1996, Institute of Arctic and Alpine Research, University of Colorado at Boulder, Boulder, 144-145.


Tephrochronology is an effective tool for dating and chronostratigraphic correlation of different depositional environments (terrestrial, lacustrine, marine, glacial) in Iceland and in the surrounding ocean and land areas. As the deposition of volcanic ash layers is essentially instantaneous on a geological timescale, such layers provide distinct and widespread isochronous stratigraphic markers.
For the younger part of the Holocene the tephrochronological record for Northern Iceland at present comprises eight distinct tephra layers/series of tephra layers of known age. This sequence covers the period from c. 5850 BP.

tephra layer source age
Katla-1918/Askja-1875 (?) Katla (S Iceland)/Askja (SE Iceland) (?) 1918 AD/1875 AD (?)
a-1477 Kverkfjöll (SE Iceland) 1477 AD
Hekla-1104 Hekla (S Iceland) 1104 AD
Landnám (3 layers) Vatnaöldur/Torfajökull (S Iceland) c. 870 AD - c. 930 AD
Hekla-3 Hekla (S Iceland) c. 2800 BP
Hekla-4 Hekla (S Iceland) c. 4000 BP
N (4 layers) ? c. 5000 BP (?)
Hekla-5 Hekla (S Iceland) c. 5850 BP

Table: State of the art of Holocene tephrochronology in Northern Iceland

Recent findings in sections in Northern Iceland where more than 50 tephra layers can be distinguished for the period since the Late Glacial/Early Holocene transition demonstrate the unique potential for establishing a high resolution tephrochronology covering the whole Holocene.
Due to the location of Northern Iceland outside the volcanically active zones only volcanic events connected with tephra fallout covering a rather large area are likely to be seen in the sections. As a consequence the tephra layers known to have been distributed over Northern Iceland during the Holocene have been identified in terrestrial or marine sequences in North-West Europe and the adjacent ocean areas. Recently a number of ash layers found in Northern Iceland have been correlated to the Greenland GRIP ice core record (Grönvold et al., 1995).
Apart from providing a sequence of chronohorizons serving as time control for the records of Holocene environmental and climatic conditions in Northern Iceland, the tephrochronological record is of direct consequence for the reconstruction of environmental and climatic history. The ash layers mark volcanic events connected with the deposition of tephra in Northern Iceland. These volcanic activities caused rapid and - compared to climatically controlled changes - drastic changes in the environment (see e.g. the reports on the consequences of the Laki eruption of 1783 in Gunnlaugsson et al., 1984). The deposition of tephra has both a physical (according to the thickness of the ash layer) and chemical (release of potentially toxic concentrations of carbon dioxide, fluorine, chlorine and compounds of sulphur) impact on the ecosystem. The destructive effect of the deposition of massive tephra layers on the vegetation cover in Northern Iceland is documented by a sharp drop in the rate of organic accumulation in soil profiles. These can be correlated with periods of increased instability of slopes (Stötter, 1991). For the reconstruction of past climate change on the basis of proxy records of environmental conditions it is thus imperative to distinguish endogenic from exogenic environmental changes (Stötter, 1994).

References
Grönvold, K., Óskarsson, N., Johnsen, S. J., Clausen, H. B., Hammer, C. U., Bond, G. and Bard, E. (1995). Ash layers from Iceland in the Greenland GRIP ice core correlated with oceanic and land sediments. Earth and Planetary Science Letters, 135, 149-155.
Gunnlaugsson, G. A., Guðbergsson, G. M., Þórarinsson, S., Rafnsson, S. and Einarsson, T. (eds.) (1984). Skaftáreldar 1783-1784. Reykjavík.
Stötter, J. (1991). Geomorphologische und landschaftsgeschichtliche Untersuchungen im Svarfaðardalur-Skíðadalur, Tröllaskagi, N-Island. Münchener Geographische Abhandlungen, B9, 166 pp.
Stötter, J. (1994). Changing the Holocene record - a call for international interdisciplinary co-operation. In: Stötter, J. and Wilhelm, F. (eds.), Environmental Change in Iceland, Münchener Geographische Abhandlungen, B12, 257-273.
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