science

1.7 Tephrochronology

Tephrochronology is a dating technique based on the identification and correlation of deposits of volcanic ejecta, (volcanic ash or pumice, all known as tephra) (Thórarinsson 1944, 1981). In practice in Scotland this means either atmospheric fallout of particles <200 microns in size, or cobble-grade piece of pumice dispersed over the ocean by wind and waves; virtually all Scottish tephra deposits originate in Iceland (Dugmore 1991; Dugmore et al. 1995b). Dating does not take place on the tephra itself, but on the eruption that produced it. As a result it is the correlation of a tephra to its source eruption enables the dating of that event to be applied to tephra wherever it is found (Dugmore and Newton 2009). Dating may be derived from a number of sources such as written records, ice core dates, sediment accumulation rates and radiocarbon. Correlation of tephra deposits generally relies on accurate grain specific chemical analysis of major and minor elements.

Firmly identified tephra deposits have the potential to define an isochron, or horizon of equal age. This may be very precisely defined even if the absolute age of the tephra is unknown. The best isochrons in Scotland are formed by in situ deposits from atmospheric fallout that were originally deposited within a matter of few hours or days (Dugmore et al. 1995a). Pumice in Scotland is rarely found in situ in natural contexts partly due to sea level changes, but it does occur in a range of archaeological contexts that are not contemporaneous with the original, pumice-forming eruptions. Since the first discovery and identification of Icelandic volcanic ash in microscopic amounts (cryptotephras) in Scotland (Dugmore 1989), tephrochronology (Lowe 2010) has become a standard palaeoenvironmental tool in Scotland. Tephra layers have been identified in Scotland’s peat deposits and lake sediments dating from the late glacial until the eruption of Hekla in 1947 (Dugmore et al. 1995b; Turney et al. 1997; Lowe et al. 2008; Housely et al. 2010). Recent events, such as the 2010 eruption of Eyjafjallajökull, have highlighted the importance of understanding the distribution of volcanic ash deposits, but the tephra preserved in Scotland’s peat and lake deposits contributes directly to science-based archaeology. The presence of unambiguously identified and dated tephra layers can provide a crucial test of other chronological methods (e.g. Dugmore et al. 1995 a and b), as well as providing otherwise unobtainable precise dating of palaeoenvironmental and proxy climate records (e.g. Langdon and Barker 2004). This allows precise correlations to be made between high resolution palaeoenvironmental records within Scotland, across Greenland, the North Atlantic and north-west Europe.

Scotland also has excellent facilities to geochemically analyse volcanic ash. The Natural Environmental Research Council Tephra Analytical Unit (http://www.geos.ed.ac.uk/facilities/tephra/) (electron and ion microprobes) is based in the School of GeoSciences at the University of Edinburgh (School of GeoSciences 2010).

It is through the definition of isochrons and the precise correlation of environmental records and chronology that tephra allows, that the greatest potential contribution to Scottish archaeology by this means can be expected. While the presence of more than a dozen tephras can add spot dates of great utility to long-term records (such as 1510 AD), it is the use of precisely-defined isochrons that lie across most of Scotland that holds the greatest future potential. Spatial patterns and their changes through time are crucially important to our understanding of the past, particular during periods of rapid cultural or environmental change and tephrochronology can enable correlations to be made to within a year. This can add a spatial dimension of great utility to other very precise records such as dendrochronology.

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1.4 Luminescence dating in archaeology

Luminescence dating utilises energy deposited in mineral lattices by naturally occurring ionising radiation to record information encoding chronology, depositional process information, and thermal history records in ceramics, lithics, and sedimentary materials. The information is stored through charge trapping processes in populations of point defects in common minerals, and can be reset by heating (for ceramics and heated lithic materials) and/or exposure to light (for sediments and exposed rock surfaces). Luminescence dating quantifies the radiation exposure experienced by target minerals (usually quartzes or feldspars) from the sample as an “equivalent dose”, measured in Grays (Gy), and representing the mean radiation dose which would reproduce the observed natural signal levels of the sample as prepared in the laboratory. The “dose rate”, measured in mGy/a, is determined by combining field and laboratory analyses of the levels of naturally occurring radionuclides and cosmic radiation with an appropriate microdosimetric model for the mineral phase in question. The luminescence age is estimated from the quotient of “stored dose” over “dose rate”.

The age range of luminescence dating extends from modern samples (<10 years) to 105-106 years, thus covering all periods of known human occupation of Scotland, and much of the Palaeolithic elsewhere. Precision of dating varies from sample to sample, and from context to context, depending on individual sample characteristics (mineralogy, luminescence sensitivity, stability and homogeneity of the radiation environment, and the quality of initial zeroing). Typically precisions of ± 2-10% of age can be achieved with 5% dating precision representing a reasonable target for general purposes. A well calibrated laboratory can produce accuracy at the lower end of the precision scale. For high quality work it is important that the environmental gamma dose rates are recorded in-situ at time of excavation, which is most readily facilitated by involving the dating laboratory in fieldwork.

The key importance of luminescence dating within Scottish Archaeology lies in the nature of the events represented by the various dating materials. In this respect, and in extending the range of dating materials and questions available, there have significant developments in recent years, and more can be anticipated.

For heated materials both thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) can be applied. TL analysis has the advantage that it can also reveal thermal history information – enabling the thermal exposures of early ceramics, and heated stones to be estimated as a by product of dating. This has provided evidence for fuel poverty in prehistoric island communities in Scotland, and also in a contemporary setting has been used to assist civil engineers with assessing fire damage of modern concrete structures (notably the Storebaelt and Channel Tunnel fires). In addition to ceramics the application of TL/OSL dating to hearthstones provides a direct indication of abandonment, since the last heating of a hearth relates directly to the end of an occupation phase in a domestic structure. This has been applied to prehistoric settlements in Orkney, where there is evidence of abandonment of marginal settlements at times of environmental stress, and to Iron Age hut circles in the Scottish Borders, where abandonment coincides with the Roman occupation of the region. Other fire damaged structures, including spectacularly vitrified forts, can be dated by TL, as can burnt stone mounds which remain an abundant and enigmatic resource within the landscape.

In the sedimentary field there have also been important developments. The recognition in the early 1980’s that luminescence signals could be zeroed by light exposure has led to the development of photostimulated luminescence (PSL) or Optically Stimulated Luminescence (OSL) methods and associated instrumentation which have dramatically enhanced the potential chronometers for studying archaeological sites and landscapes. A wide range of aeolian, fluvial, alluvial and colluvial materials have been studied worldwide for mainly quaternary research purposes. Archaeological applications are also increasingly prominent in the literature. The ability to date clean wind-blown sand layers in archaeological landscapes and sequences provide important opportunities to examine human-environment interactions, and in particular the impact of past storminess on early communities. This has external links to the North Atlantic climate system, and also to the development of greater understanding of the environmental factors which accompany changes in settlement pattern, and population movements. Within sites the recognition that some constructional activities can also reset the luminescence age system for underlying sedimentary substrates has profound implications for dating incised features and certain built structures. Within Scotland this has been used to date, for example the re-setting of the Hilton of Cadboll stone, and other monuments in Orkney including the ditch-fills of the Ring of Brodgar within the World Heritage Site in Orkney. On the international scene, researchers based in Scotland have used these methods to provide the first dates for ancient canals in the early rice cultivating empires of SE Asia, and to understand the dynamics of incised features within Neolithic sites in the Mediterranean and Southern Italy. Importance aspects of OSL dating of sediments within complex sites relate to establishing whether individual samples have been effectively zeroed at time of final enclosure within the site formation processes, and whether they have been subject to post-depositional disturbances resulting in mixed-age materials. Two significant developments have greatly assisted the task of resolving such complexity. One is the development of rapid OSL profiling methods, originally using series of small samples analysed in the laboratory to map sedimentary stratigraphy, and more recently with the development, at SUERC, of field portable instrumentation which can be used to detect inversions, age-discontinuities, and redepositional sequences during excavation and sampling. The other is the development of small-aliquot and single-grain OSL methods which can generate dose-distributional information using automated equipment in the laboratory, and thus provide a means of monitoring and accounting for mixed-age and partially zeroed materials within sediment samples.

Finally there has been recent recognition of the potential for surface bleaching of luminescence signals in rock surfaces and worked stone objects to generate dating opportunities for these classes of materials. Further methodological research is required to refine physical models for these processes and to develop robust means of extracting dating information from them. But these approaches may hold new opportunities for dating lithic monuments such as standing stones, chambered tombs or other built monuments which can currently only be placed into their cultural and chronological contexts using indirect means. They may also provide a means of enhancing understanding of the duration of use of portable stone objects (including handaxes) prior to deposition within archaeological sites and landscapes.

Scotland is well placed to contribute to high quality research, development and application of luminescence dating, having the combination of a good science base with well established and developed expertise in this area, and a wide range of monuments, landscapes and materials covering the last 8000 years to which these methods could be applied. Critical to such work is developing and sustaining effective collaborative working arrangements between archaeological units and the research laboratories, since provision for luminescence dating and its associated sampling needs to be embedded into excavation planning and operation from the outset to achieve effective results.

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