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 paleoenvironmental or archaeological records can be placed. Such an established event provides a "tephra horizon". Each volcanic event has a unique chemical "fingerprint" that is believed to be 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.

Tephrochronology requires accurate geochemical fingerprinting (usually via an electron microprobe).[1] An important recent advancement is the use of LA-ICP-MS (i.e. laser ablation ICP-MS) to measure trace-element abundances in individual tephra shards.[2] One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras.[3]

Early tephra horizons were identified with the Saksunarvatn tephra (Icelandic origin, ca 10.2 cal. ka BP), forming a horizon in the late 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 varves in lake sediments or from marine deposits and ice-core records, and to extend the limits 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.[4] 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")[5] 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).

[edit] Notes

  1. ^ Smith & Westgate (1969)
  2. ^ Pearce et al. (2003)
  3. ^ Pollard et al. (2003)
  4. ^ Alloway et al. (1997)
  5. ^ Turney et al. (1997)

[edit] Sources

  • Alloway B.V., Larsen G., Lowe D.J., Shane P.A.R., Westgate J.A. (2007). "Tephrochronology", Encyclopedia of Quaternary Science (editor—Elias S.A.) 2869-2869 (Elsevier).
  • 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) 147-159 (Dordrecht: Kluwer Academic Publishers).
  • Keenan D.J. (2003). "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. doi:10.1029/2003GC000672
  • Pollard A.M., Blockley S.P.E., Ward K.R. (2003). "Chemical alteration of tephra in the depositional environment", Journal of Quaternary Science, 18: 385–394. doi:10.1002/jqs.760
  • Smith D.G.W., Westgate J.A. (1969). "Electron probe technique for characterizing pyroclastic deposits", Earth and Planetary Science Letters, 5: 313–319.
  • Þórarinsson S. (1970). "Tephrochronology in medieval Iceland", Scientific Methods in Medieval Archaeology (ed. R. Berger) 295-328 (Berkeley: University of California Press).
  • Turney C.S.M., Harkness D.D., Lowe J.J. (1997). "The use of microtephra horizons to correlate late-glacial lake sediment successions in Scotland", Journal of Quaternary Science, 12: 525-531.

[edit] External links

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