Tephrochronology

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Tephrochronology is a geochronological technique that utilises discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which palaeoenvironmental or archaeological records can be placed. Such an established event provides a "tephra horizon". Each volcanic event has a unique chemical 'fingerprint' that is identifiable in its fallout.

The main advantages of the technique are that the volcanic ash layers can be relatively easily identified in many sediments and that the tephra layers are deposited relatively instantaneously over a wide spatial area. This means they provide accurate temporal marker layers which can be used to verify or corroborate other dating techniques, linking sequences widely separated by location into a unified chronology.

The problems associated with tephochronology are that its use has been limited to areas of frequent large scale volcanic activity and that tephra chemistry can become altered over time. It also requires accurate geochemical fingerprinting (usually via an electron microprobe) and radiometric dating of proximal tephra deposits.

Major volcanoes which have been used in tephrochronological studies include Vesuvius, Hekla and Santorini. Minor volcanic events may also leave their fingerprint in the geological record: Saksunarvatn tephra form a horizon in the late Pre-Boreal of Northern Europe. Hayes Volcano is responsible for a series of six major tephra layers in the Cook Inlet region of Alaska.

Since the late 1990s, techniques developed by C.S.M. Turney and others for extracting tephra horizons invisible to the naked eye ("cryptotephra") have revolutionised the application of tephrochronology. This technique relies upon the difference between the specific gravity of the tephra shards and the host sediment matrix. It has led to the first discovery of the Vedde Ash on the mainland of Britain, in Sweden and in two sites on the Karelian Isthmus of Baltic Russia. It has also revealed previously undetected ash layers, such as the hitherto unrecorded Borrobol Tephra, dated to ca. 14,400 years BP calibrated (Wastegård 2004).

[edit] References

Dugmore, A. and Buckland, P.C. (1991). "Tephrochronology and Late Holocene soil erosion in South Iceland." in 'Environmental Change in Iceland: Past and Present' (eds. J.K. Maizels and C. Caseldine). Dordrecht: Kluwer Academic Publishers, pp. 147-159.
Þórarinsson, S. (1970). "Tephrochronology in medieval Iceland." in 'Scientific Methods in Medieval Archaeology (ed. R. Berger). Berkeley: University of California Press, pp. 295-328.