Tephrochronology

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rhyolitic tephra from Hekla
.
Geologist explaining the importance of tephrochronology to students on field in Iceland.

Tephrochronology is a

paleoenvironmental or 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. It is a variant of the basic geological technique of stratigraphy
.

The main advantages of the technique are 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 climatic sequences and events. This results in "age-equivalent dating".[1]

Effective tephrochronology requires accurate geochemical fingerprinting (usually via an

ICP-MS) to measure trace-element abundances in individual tephra shards.[3] One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras.[4] Some tephra horizons and the use of zircon directed techniques are more useful than others in linking layers over wide areas and determining eruption details.[5] For example the often very explosive nature of rhyolytic eruptions will cause wider distribution, the higher potassium content of rhyolite allows more accurate time determinations, and the location of a deposit will influence its potential for chemical alteration after being laid down.[5] Zircon techniques applied to tephra and other samples from the same eruption, may allow magma sources, magma residence times and the geochemical conditions of the magma formation to be better understood with dating of more than just the eruption itself, but also when the magma first evolved separately, or incorporated other rocks.[5]

History of speciality

The term tephrochronology appears to have been used by Sigurdur Thórarinsson as early as 1944.[6] A key point in the establishment of this scientific field of study with what evolved to be a unique geoscientific method was in 1961 after a proposal supported by him led by Japanese researchers including Professor Kunio Kobayashi resulted in the establishment of an international scientific group. Much work had preceded this, but was limited by the techniques available at the time in geology. This had resulted in tephra formations not being linked and inaccurate timings that could not be related to events say with worldwide traces.

What would now be known as cryptotephra studies occurred in sea floor samples in the 1940s but Christer Persson in Scandinavia, was the first to publish articles in this field in the 1960s.

stalagmites as well as contemporary eruption deposits.[6]

Early tephra horizons were identified with the

carbon-14 dating
.

A pioneer in the use of tephra layers as

Sigurdur Thorarinsson, who began by studying the layers he found in his native Iceland.[7] 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")[8] 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 c. 14.4 cal. ka BP,[8] the microtephra horizons of equivalent geochemistry from southern Sweden, dated at 13,900 Cariaco varve yrs BP[9] and from northwest Scotland, dated at 13.6 cal. ka BP.[10]

Since 2010 Bayesian age modelling built around ever-improving 14C-calibration curves and other age-related data,such as zircon double dating continues to better define tephrochronology.[6]

References

  1. ISBN 9789400763036
    .
  2. doi:10.1016/S0012-821X(68)80058-5
    .
  3. S2CID 129240868
    .
  4. S2CID 140624059
    .
  5. ^
    doi:10.1029/2020GC009255
    .: Sections:1 Introduction, 2 Geologic Setting and Background 
  6. ^
    hdl:10289/15024
    .
  7. ^ Alloway et al. (2007)
  8. ^
    doi:10.1002/(SICI)1099-1417(199711/12)12:6<525::AID-JQS347>3.0.CO;2-M
    .
  9. doi:10.1016/j.quascirev.2003.11.006
    .
  10. S2CID 126677732
    .

Sources

External links

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