Tephrochronology iceland.JPG
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250px, Tephra horizons in south-central Iceland. The thick and light coloured layer at the height of the volcanologist's hands is rhyolitic tephra from Hekla. 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
paleoenvironmental Paleoecology (also spelled palaeoecology) is the study of interactions between organisms and/or interactions between organisms and their environments across geologic timescales. As a discipline, paleoecology interacts with, depends on and informs ...
or
archaeological Archaeology or archeology is the scientific study of human activity through the recovery and analysis of material culture. The archaeological record consists of artifacts, architecture, biofacts or ecofacts, sites, and cultural landscap ...
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 Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
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). An important recent advance is the use of LA-ICP-MS (i.e.
laser ablation Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...
ICP-MS) to measure trace-element abundances in individual tephra shards. One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras. Early tephra horizons were identified with the
Saksunarvatn tephra In the science of tephrochronology, the Saksunarvatn tephra is volcanic ejecta that formed an ash layer that is useful in dating Northern European sediment layers that were laid down during the Boreal period, the warm climate phase that followed th ...
(Icelandic origin, cal. ka BP), forming a horizon in the late
Pre-Boreal The Preboreal is an informal stage of the Holocene epoch. It is preceded by the Tarantian and succeeded by the Boreal. It lasted from 10,300 to 9,000 BP in radiocarbon years or 8350BC to 7050BC in Gregorian calendar years (8th millennium BC). ...
of Northern Europe, the Vedde ash (also Icelandic in origin, c. 12.0 cal. ka BP) and the
Laacher See Laacher See (), also known as Lake Laach or Laach Lake, is a volcanic caldera lake with a diameter of in Rhineland-Palatinate, Germany, about northwest of Koblenz, south of Bonn, and west of Andernach. It is in the Eifel mountain range, and ...
tephra (in the
Eifel The Eifel (; lb, Äifel, ) is a low mountain range in western Germany and eastern Belgium. It occupies parts of southwestern North Rhine-Westphalia, northwestern Rhineland-Palatinate and the southern area of the German-speaking Community of ...
volcanic field, c. 12.9 cal. ka BP). Major volcanoes which have been used in tephrochronological studies include Vesuvius, Hekla and
Santorini Santorini ( el, Σαντορίνη, ), officially Thira (Greek: Θήρα ) and classical Greek Thera (English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast from the Greek mainland. It is the ...
. Minor volcanic events may also leave their fingerprint in the geological record:
Hayes Volcano Hayes Volcano is a stratovolcano in southwestern Matanuska-Susitna Borough, Alaska, 135 km northwest of Anchorage, that was not discovered until 1975. It is responsible for a series of six major tephra layers in the Cook Inlet region of Alas ...
is responsible for a series of six major
tephra layer Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
s 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 Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was dev ...
. 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; now University of Exeter) and others for extracting tephra horizons invisible to the naked eye ("cryptotephra")Turney et al. (1997) 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 __NOTOC__ Soppensee is a lake in Canton of Lucerne, Switzerland. The surface area is . The lake and its surroundings are located in the municipalities of Buttisholz, Menznau and Ruswil Ruswil is a municipality in the district of Sursee in the can ...
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 c. 14.4 cal. ka BP, the microtephra horizons of equivalent geochemistry from southern
Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
, dated at 13,900 Cariaco varve yrs BP and from northwest Scotland, dated at 13.6 cal. ka BP.Ranner et al. (2005)


Notes


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). * * * * * * * * * Þórarinsson S. (1970). "Tephrochronology in medieval Iceland", ''Scientific Methods in Medieval Archaeology'' (ed. R. Berger) 295–328 (Berkeley: University of California Press). *


External links


USGS tephrochronology technique







TephraBase


{{Chronology Geochemistry Geochronology Incremental dating Tephra