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'''Tephrochronology''' is a [[Geochronology|geochronological]] technique that uses 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". The premise of the technique is that each volcanic event produces ash with a unique chemical "fingerprint" that allows the deposit to be identified across the area affected by fallout. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker.
The main advantages of the technique are that the [[volcanic ash]] layers can be relatively easily identified in many sediments and that the [[tephra layer]]s 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 climatic sequences and events.
Tephrochronology requires accurate geochemical fingerprinting (usually via an [[electron microprobe]]).<ref>Smith & Westgate (1969)</ref> An important recent advance is the use of LA-ICP-MS (i.e. [[laser ablation]] [[ICP-MS]]) to measure trace-element abundances in individual tephra shards.<ref>Pearce et al. (2002)</ref> One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras.<ref>Pollard et al. (2003)</ref>
Early tephra horizons were identified with the [[Saksunarvatn tephra]] (Icelandic origin, [[
A pioneer in the use of tephra layers as [[marker horizon]]s to establish chronology was [[Sigurdur Thorarinsson]], who began by studying the layers he found in his native Iceland.<ref>Alloway et al. (2007)</ref> Since the late 1990s, techniques developed by Chris S. M. Turney ([[QUB]], Belfast; now [[University of Exeter]]) and others for extracting tephra horizons invisible to the naked eye ("cryptotephra")<ref name="Turneyet">Turney et al. (1997)</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 Borrobol Tephra first discovered in northern [[Scotland]], dated to