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Tephra and Tephrochronology

Iceland and north-west Europe

Tephra is a term used to describe all of the solid material produced from a volcano during an eruption (Thorarinsson, 1944). The fine fraction of this material can travel great differences. Tephra from the 1259 AD eruption of El Chichon, Mexico, for example, has been found in both the Greenland and Antarctic ice caps (Palais et al., 1992). Small tephra shards (2-3 µm) from the eruption of Pinatubo circled the globe several times after the 1991 eruption. The interest in the study of tephra layers has proceeded on two fronts: firstly, there is interest of volcanic impact on climate and the environment and secondly, as a chronological tool. Climatological and palaeoenvironmental research has involved studies on the possible major impact of volcanic eruptions on climate, from the possible intensification of ice ages (Ramaswamy, 1992) to localised or short-term climatic change (Baillie and Munro, 1988). The large 1991 eruption of Pinatubo, for example, produced a large eruption column that had a small, but noticeable effect on the Earth's climate (Koyaguchi and Tokuno, 1993). The use of tephra layers as a chronological tool (tephrochronology) was originally developed in Iceland (Thorainsson, 1944) and has since been applied to other volcanically active areas such as Alaska, New Zealand and Mexico. This technique allows isochronous marker horizons, formed by tephra layers, to be mapped across inter-continental scale distances. These can form a dating framework against which other dating techniques can be checked and validated.

Until the 1960’s, tephrochronological studies in north-west Europe were restricted to Iceland, the only country with active volcanoes in the region. This work was pioneered by Sigurdur Thórarinsson, who produced several seminal works (e.g. Thórarinsson 1967). During the 1960’s, research carried out in mainland Scandinavia and the Faroe Islands produced evidence of the presence of Icelandic tephra layers thousands of kilometres from their sources (Persson, 1971). This period also saw the development of marine sediment studies, partly associated with oil exploration, and it became apparent that these contained a long record of Icelandic volcanism (Ruddiman and Glover 1972). Developments in geochemical analysis has enabled tephra layers to be identified independently of other dating methods, e.g. radiocarbon dating. Once a tephra has been geochemically identified, it can be used as a time marker horizon across continental or inter-continental distances over a wide range of depositional environments . This has led to the discovery and identification of Icelandic tephra layers across a large area, covering the period from the Late-Glacial to the present. Discoveries have been made in Norway (Mangerud et al., 1984), the Faroes (Mangerud et al., 1986; Dugmore and Newton, 1998; Wastegard et al; 2001, 2002), Scotland (Dugmore, 1989; Dugmore et al, 1992; Dugmore et al, 1995; Langdon and Barber, 2001), Northern Ireland (Hall and Pilcher et al, 2002; Pilcher and Hall 1992; Pilcher et al., 1995, 1996), Germany (Merkt et al., 1993; van den Bogaard et al, 1994, 2002; van den Bogaard and Schmincke, 2002), Sweden (Boygle, 1998; 2004; Wastegard, 1998; Bergman et al., 2004), Russia (Wastegard et al, 2000) and Greenland ice cores (e.g. Gronvold et al., 1995). Note: that this is not a comprehensive list of all the papers from these regions/countries. More references can be found by searching Tephrabase.

Recently, several papers have been published which have attempted to summarize and collect details on the techniques involved in tephrochronology and the information on the tephra layers. These include Haflidason et al (2000), Turney et al (2004), Dugmore et al (2004), Hunt and Hill (1993, 2001).

The volcanically active areas of Iceland can be divided into 4 volcanic zones. These are the Snæfellsness Zone, the Reykjanes-Langjokull Zone, the Northern Zone and the Eastern Zone. Öræfajökull, south-east Iceland, however does not belong to any of these zones (see location map of Iceland). Within these zones are found volcanic systems (see geological map). Jakobsson (1979) defines a volcanic system as a "spatial grouping of eruption sites in a certain period of time, with particular characteristics of tectonics, petrography and geochemistry". Volcanic systems generally start off as fissure swarms (e.g. Veidivötn), producing mainly basaltic rocks. In time more evolved rocks are produced and activity often becomes concentrated in one area. A caldera, central volcano and high temperature thermal field eventually follows (e.g. Katla Volcanic System). Systems generally have a life of between 300,000 and 500,000 years, but central volcanoes may reach an age of over 2 million years. As each volcanic system is partly defined on the geochemical property of the products it has produced, this helps in identifying the source volcanoes of tephra layers found thousands of kilometres from Iceland. So far all of this work has involved major element analysis of glass shards and this is the data stored on this database. More details on volcanic system found in Iceland are available. Many volcanic systems in Iceland are covered by thick ice-caps, which have a major impact on eruptions.

Central Mexico

Intense volcanic activity in the Transmexican Volcanic Belt (TMVB), which stretches across central México, has occurred since the Oligocene (see map of Mexico). Volcanic activity continues through to the present day (e.g. Popocatépetl since 1994) representing a major potential hazard. Tephra falls from eruptions over the historical period (since 1521 AD) are reported to have effected large areas, some several hundred kilometres from the source volcano, e.g. Volcán de Colima (Martin del Pozzo et al., 1995). Volcanic activity in central Mexico is associated with the subduction of the Cocos and Rivera Plates beneath the North American plate. Although the volcanic activity in central Mexico forms an arcuate distribution, it is not parallel to the subduction, especially the part of the TMVB which lies about 19° north. The reasons for this are not entirely clear.

Present day volcanic activity can be roughly devided between large stratovolcanoes and smaller scale volcanoes such as cinder cones, maars, sheild volcanoes and lava flows. From a tephrochronological point of view tephra produced from stratovolcanoes and cinder cones are the most important features. The map of Mexico shows the location of the main stratovolcanoes in central Mexico and the areas with monogenetic cinder cone activity. The major stratovolcanoes in central Mexico include Nevado de Colima (4,240 metres), Volcán de Colima (3,820 metres), Nevado de Toluca (4,575 metres), Iztaccihuatl (5230 metres), Popocatépetl (5,465 metres), La Malinche and Pico de Orizabo (5,610 metres). The Michoacán-Guanajuato Volcanic Field (MGVF) is dominated by extremely dense volcanic activity in the form of cinder cones and shield volcanoes, with over 900 cinder cones found in an area of 40,000 km² (Hasenaka and Carmichael, 1985; see map). The two youngest cinder cones are Jorullo (AD 1759-1774) and Parícutin (AD 1943-1952). The Chichinautzin monogenetic field (see map) is another area of dense monogenetic activity with about 50 cinder cones produced in the last 50,000 years.

References

Baillie, M.L., and Munro, M.A.R. (1988) Irish tree rings; Santorini and volcanic dust veils. Nature, 332, 344-346.

Bennett, K.D., Boreham, S., Sharp, M.J. and Switsur, V.R. (1992) Holocene history of environment, vegetation and human settlement on Catta Ness, Lunnasting, Shetland. Journal of Ecology, 80, 241-273.

Bergman, J., Wastegard, S., Hammarlund, D., Wohlifarth, B. and Roberts, S.J. (2004) Holocene tephra horizons at Klocka Bog, west-central Sweden: aspects of reproducibility in subarctic peat deposits. Journal of Quaternary Science, 19, 241-249.

van den Bogaard, C. and Schmincke, H.U. (2002) Linking the North Atlantic to central Europe: a high-resolution Holocene tephrochronological record from northern Germany. Journal of Quaternary Science, 17, 3-20.

van den Bogaard, C., Dorfler, W., Sandgren, P. and Schmincke, H-U. (1994) Correlating the Holocene records: Icelandic Tephra found in Schleswig-Holstein (Northern Germany). Naturwissenschaften, 81, 554-556.

van den Bogaard, C., Dorfler, W., Glos, R., Nadeau, M.J., Grootes, P.M. and Erlenkeuser, H. (2002) Two tephra layers bracketing late Holocene paleoecological changes in northern Germany. Quaternary Research, 57, 314-324.

Boygle, J. (1998) A little goes a long way: discovery of a new mid-Holocene tephra in Sweden. Boreas, 27, 195-199.

Boygle, J. (2004) Towards a Holocene tephrochronology for Sweden: geochemistry and correlation with the North Atlantic tephra stratigraphy. Journal of Quaternary Science, 19, 103-109.

Dugmore, A.J. (1989) Icelandic volcanic ash in Scotland. Scottish Geographical Magazine, 105(3), 168-172.

Dugmore, A.J. (1991) Tephrochronology and UK archaeology. IN: Proceedings of the Archaeological Sciences Conference 1989. (Budd P., Chapman B., Jackson C., Jonaway R., Ottaway B. Eds.), Oxbow Books: Oxford, 242-250.

Dugmore, A.J. and Newton, A.J. (1997) Holocene tephra layers in the Faroe Islands. Frodskaparrit, 45, 141-154.

Dugmore, A.J., Larsen, G., Newton A.J. and Sugden D.E. (1992) Geochemical stability of fine-grained silicic Holocene tephras in Iceland and Scotland. Journal of Quaternary Science, 7, 173-183.

Dugmore, A.J., Larsen, G. and Newton, A.J. (1995) Seven tephra isochrones in Scotland. The Holocene, 5(3), 257-266.

Dugmore, A.J., Larsen, L. and Newton, A.J. (2004) Tephrochronology and its application to Late Quaternary Environmental Reconstruction, with Special Reference to the North Atlantic Islands. In: Tools for Constructing Chronologies (Eds Buck, C.E. and Millard, A.R.), pp 173-188, Springer-Verlag, London.

Edwards K.J., Buckland P.C., Blackford J.J., Dugmore A.J. and Sadler J.P. (1994) The impact of tephra: proximal and distal studies of Icelandic eruptions. Muncher Geographische Auhandlungen. B12, 108-126.

Gronvold, K., Oskarsson, 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 based sediments. Earth and Planetary Science Letters, 54, 238-246.

Haflidason, H., Eiriksson, J. and Kreveld, S.V. (2000) The tephrochronology of Iceland and the North Atlantic region during the Middle and Late Quaternary: a review. Journal of Quaternary Science, 15(1), 3-22.

Hall, V.A. and Pilcher, J.R. (2002) Late-Quaternary Icelandic tephras in Ireland and Great Britain: detection, characterization and usefulness. Holocene, 12, 223-230.

Hunt J. and Hill P.G. (1993) Tephra geochemistry: a discussion of some persistent analytical problems. The Holocene, 3(3), 271-278.

Hunt, J.B. and Hill, P.G. (2001) Tephrological implications of beam size-sample-size effects in electron microprobe analysis of glass shards. Journal of Quaternary Science, 16(2), 105-117.

Jakobsson S.V. (1979) Petrology of recent basalts of the Eastern Volcanic Zone, Iceland. Acta Naturalia Islandica, 26, 103 p.

Koyaguchi, T., and Tokuno, M. (1993) Origin of the giant eruption cloud of Pinatubo, June 15, 1991. Journal of Volcanology and Geothermal Research, 55(1-2), 85-96.

Langdon, P.G. and Barber, K.E. (2001) New Holocene tephras and a proxy climate record from a blanket mire in northern Skye, Scotland. Journal of Quaternary Science, 16, 753-759.

Mangerud J., Furnes H. and Johansen J. (1986) A 9,000 year old ash bed on the Faroe Islands. Quaternary Research, 26, 262-265.

Mangerud J., Lie, S.E., Furnes H., Kristiansen I.L. and Lomo L. (1984) A Younger Dryas ash bed in western Norway and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic. Quaternary Research, 21, 85-104.

Merkt J., Muller H., Knabe W., Muller P. and Weiser T. (1993) The early Holocene Saksunarvatn Tephra found in lake sediments in N.W. Germany. Boreas, 22, 93-100.

Palais, J.M., Germani, M.S., and Zielinski, G.A. (1992) Inter-hemispheric transport of volcanic ash from a 1259 AD volcanic eruption to the Greenland and Antarctic Icesheets. Geophysical Research Letters, 19, 801-804.

Persson C. (1971) Tephrochronological investigations of peat deposits in Scandinavia and on the Faroe Islands. Sveriges Geologiska Undersokning, 65, 3-34.

Pilcher J.R. and Hall V.A. (1992) Towards a tephrochronology for the Holocene for the north of Ireland. The Holocene, 2(3), 255-260.

Pilcher J.R., Hall V.A., and McCormac F.G. (1995) Dates of Holocene Icelandic volcanic eruptions from tephra layers in Irish peats. The Holocene, 5(1), 103-110.

Pilcher, J.R., Hall, V.A. and McCormac, F.G. (1996) An outline tephrochronology for the Holocene of the north of Ireland. Journal of Quaternary Science, 11(6), 485-494.

Ramaswamy, V. (1992) Explosive start to the last glaciation. Nature, 359, 14-14.

Turney, C.S.M., Lowe, J.J., Davies, S.M., Hall, V., Lowe, D.J., Wastegard, S., Hoek, W.Z. and Alloway, B. (2004) Tephrochronology of Last Termination Sequences in Europe: a protocol for improved analytical precision and robust correlation procedures (a joint SCOTAV-INTIMATE proposal). Journal of Quaternary Science, 19, 111-120.

Wastegård, S., Björck, S., Possnert, G. and Wohlfarth, B. (1998) Evidence of the occurance of Vedde Ash in Sweden: radiocarbon age estimates. Journal of Quaternary Science, 13(3), 271-274.

Wastegård, S., Wohlfarth, B., Subetto, D.A. and Sapelko, T.V. (2000) Extending the known distribution of the Younger Dryas Vedde Ash into northwestern Russia. Journal of Quaternary Science, 15(6): 581-586.

Wastegard, S., Bjorck, S., Grauert, M. and Hannon, G.E. (2001) The Mjauvotn tephra and other Holocene tephra horizons from the Faroe Islands: a link between the Icelandic source region, the Nordic Seas, and the European continent. The Holocene, 11(1), 101-109.

Wastegard, S. (2002) Early to middle Holocene silicic tephra horizons from the Katla volcanic system, Iceland: new results from the Faroe Islands. Journal of Quaternary Science, 17, 723-730.