250px, Tephra horizons in south-central . The thick and light coloured layer at the height of the volcanologist">Iceland. The thick and light coloured layer at the height of the volcanologist's hands is rhyolitic tephra from
Hekla
Hekla (), or Hecla, is a stratovolcano in the south of Iceland with a height of . Hekla is one of Iceland's most active volcanoes; over 20 eruptions have occurred in and around the volcano since 874. During the Middle Ages, the Icelandic Nor ...
.
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
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
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 electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. It ...
). An important recent advance is the use of LA-ICP-MS (i.e.
laser ablation ICP-MS
Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It is ...
) 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 (Icelandic origin,
cal. ka BP), forming a horizon in the late
Pre-Boreal 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 i ...
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
Mount Vesuvius ( ; it, Vesuvio ; nap, 'O Vesuvio , also or ; la, Vesuvius , also , or ) is a somma- stratovolcano located on the Gulf of Naples in Campania, Italy, about east of Naples and a short distance from the shore. It is one of ...
,
Hekla
Hekla (), or Hecla, is a stratovolcano in the south of Iceland with a height of . Hekla is one of Iceland's most active volcanoes; over 20 eruptions have occurred in and around the volcano since 874. During the Middle Ages, the Icelandic Nor ...
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 A ...
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
Palynology is the "study of dust" (from grc-gre, παλύνω, palynō, "strew, sprinkle" and '' -logy'') or of "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, from water, or from deposit ...
), from
varve
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (va ...
s 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 horizon
Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no do ...
s 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
The University of Exeter is a public university , public research university in Exeter, Devon, England, United Kingdom. Its predecessor institutions, St Luke's College, Exeter School of Science, Exeter School of Art, and the Camborne School of Min ...
) 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
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, in the Swiss Lake
Soppensee and in two sites on the
Karelian Isthmus
The Karelian Isthmus (russian: Карельский перешеек, Karelsky peresheyek; fi, Karjalankannas; sv, Karelska näset) is the approximately stretch of land, situated between the Gulf of Finland and Lake Ladoga in northwestern ...
of Baltic Russia.
It has also revealed previously undetected ash layers, such as the Borrobol Tephra first discovered in northern
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
, 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).
*
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*
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 techniqueTephraBase
{{Chronology
Geochemistry
Geochronology
Incremental dating
Tephra