Tephrochronology
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'''Tephrochronology''' is a [[Geochronology|geochronological]] technique that utilises discrete layers of [[tephra]]—volcanic ash from a single eruption—to create a chronological framework in which [[Paleoenviroment|paleoenvironmental]] or [[Archaeology|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 that correlates climactic sequences and events.
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.
Early tephra horizons were identified with the [[Saksunarvatn tephra]] (Icelandic origin, ca 10.2 cal. ka BP), forming a horizon in the late [[Boreal (period)|Pre-Boreal]] of Northern Europe, the Vedde ash (also Icelandic in origin, ca 12.0 cal. ka BP) and the Laacher See tephra (in the Eifel volcanic field, ca 12.9 cal. ka BP). 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: [[Hayes Volcano]] is responsible for a series of six major tephra layers in the Cook Inlet region of Alaska. Tephra horizons provide a synchronous check against which to correlate the palaeoclimatic reconstructions that are obtained from terrestrial records, like fossil pollen studies ([[palynology]]), from [[varve]]s in lake sediments or from marine deposits and [[Ice core|ice-core records]], and to extend the limitations of [[carbon-14 dating]].
A pioneer in the use of tephra layers as marker horizons to establish chronology was [[Sigurdur Thorarinsson]], who began by studying the layers he found in his native Iceland. Since the late 1990s, techniques developed by Chris S. M. Turney ([[QUB]], Belfast) and others for extracting tephra horizons invisible to the naked eye ("cryptotephra")<ref>Turney, D.D. Harkness and J.J. Lowe, (1997). "The use of microtephra horizons to correlate late-glacial lake sediment successions in Scotland." ''Journal of Quaternary Science'' '''12''', pp 525-531.</ref> have revolutionised the application of tephrochronology. This technique relies upon the difference between the specific gravity of the microtephra shards and the host sediment matrix. It has led to the first discovery of the Vedde ash on the mainland of Britain, in Sweden, in [[the Netherlands]], in the Swiss Lake [[Soppensee]] 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).
==Notes==
{{reflist}}
== References ==
*Davies S.M., Wastegård S., Wohlfarth B (2003). "Extending the limits of the Borrobol Tephra to Scandinavia and detection of new early Holocene tephras", ''Quaternary Research'', 59: 345-352.
*Dugmore A., Buckland P.C. (1991). "Tephrochronology and Late Holocene soil erosion in South Iceland", ''Environmental Change in Iceland: Past and Present'' (eds. J.K. Maizels and C. Caseldine). Dordrecht: Kluwer Academic Publishers, pp. 147-159.
*Keenan D.J. (2003). "[http://www.informath.org/pubs/G%5E303a.pdf Volcanic ash retrieved from the GRIP ice core is not from Thera]", ''Geochemistry, Geophysics, Geosystems'', 4, doi: 10.1029/2003GC000608.
*Pearce N.J.G., Eastwood W.J., Westgate J.A., Perkins W.T. (2002). "Trace-element composition of single glass shards in distal Minoan tephra from SW Turkey", ''Journal of the Geological Society, London'', 159: 545–556.
*Þórarinsson S. (1970). "Tephrochronology in medieval Iceland", ''Scientific Methods in Medieval Archaeology'' (ed. R. Berger). Berkeley: University of California Press, pp. 295-328.
*Smith D.G.W., Westgate J.A. (1969). "Electron probe technique. for characterizing pyroclastic deposits", ''Earth and Planetary Science Letters'', 5: 313–319.
== External links ==
* [http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/teph/tech.html USGS tephrochronology technique]
* [http://www.geo.ed.ac.uk/tephra/tephrochron.html Tephra and Tephrochronology, The University of Edinburgh]
* [http://people.bu.edu/marchant/themesTephro2.htm Antarctic Research Group]
* [http://www-odp.tamu.edu/publications/186_SR/107/107_3.htm TEPHROCHRONOLOGY AND HIGH-PRECISION ANALYSIS]
* [http://www.geo.ed.ac.uk/tephra/ TephraBase]
* [http://www.colorado.edu/INSTAAR/AW2004/get_abstr.html?id=67 International Arctic Workshop, 2004. Stefan Wastegård et al., "Towards a tephrochronology framework for the last glacial/interglacial transition in Scandinavia and the Faroe Islands": (Abstract)]
{{Chronology}}
[[Category:Incremental dating]]
[[Category:Geochronology]]
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