5.2.1 Dating

The principal applications of archaeological science techniques in Scottish Mesolithic studies have been in the area of dating.  Radiocarbon (14C) dating has been routinely applied to Scottish Mesolithic sites since the 1960s. The majority of samples analysed prior to the mid-1990s were dated by radiometric methods (gas proportional counting or liquid scintillation counting). Typically, these were bulk samples of wood charcoal, or occasionally of animal bones or marine shells. Where the sample material was limited in quantity error terms were often large. Moreover, charcoal samples were not always identified to species and hence the 14C age measurements carry the possibility of an old wood effect. Similarly, shells have a marine reservoir effect, the magnitude of which is still not well established for the Mesolithic time range. The introduction of the AMS technique in the early 1980s allowed the dating of much smaller samples, thereby opening the possibility of single-entity dating of artefacts and ecofacts. However, error terms associated with dates produced by first generation accelerator mass spectrometers were often large (±80–100 14C yr), although the current generation is capable of routine precision measurements of ±25–40 14C years. With the use of the AMS technique, dating of bone collagen has become much more routine. With the older radiometric methods a large amount of bone was required (equivalent to an entire major limb bone, e.g. humerus). Currently, 0.5–1g of compact bone is normally sufficient for reliable dating. In terms of collagen extraction there is no single preferred technique, with some laboratories favouring a modified Longin method, and others favouring this method combined with ultrafiltration.

Also relevant to understading the changes taking place in the Lateglacial landscape of Scotland is cosmogenic isotope (10Be and 36Cl) dating of the exposure of rock surfaces (e.g. Ballantyne 2010; Everest and Kubik 2006).

The principal non-isotopic dating technique applicable to the Mesolithic time-range is luminescence dating, of which there are various forms (TL, OSL, IRSL, etc.). The advantage of the luminescence technique is that it can be used to date directly certain non-carbon containing materials, e.g. burnt flint. In practice, however, errors associated with single measurements are large; an accuracy of 5–10% of age will be routinely obtained, while for selected samples with appropriate external dosimetry and suitable luminescence properties it may be possible to achieve a precision of 3–5%. With replication, precision may be improved to 2–3%, but with obvious cost implications. Consequently, there have been very few applications of this technique in the Scottish Mesolithic (Mithen et al. 1992; Melton and Nicholson 2004).

Tephra is volcanic ash which can be found within sediments in western Scotland. Such ash will have predominantly originated from volcanic eruptions in Iceland and it is important to Mesolithic archaeology because the specific geochemical characteristics of each tephra horizon can link it to a very specific volcanic event (see Lowe 2011 for a recent review of tephrochronology). As such this provides a means of additional absolute dating for sediments, which enables the verification of radiocarbon dating and contributes towards constraining chronologies for palaeoenvironmental reconstructions. Tephra analysis involves the extraction of tephra from sediments and their characterisation by using a variety of methods including SEM WDX and microprobe analysis. As yet tephra dating has had limited direct application within Palaeolithic and Mesolithic Scotland but the potential is substantial as demonstrated by the discovery of the Hoy tephra on Orkney dating to c. 5500 BP and Lairg A and B tephras in Sutherland dating to c. 6000 BP (Dugmore et al. 1995), and this is an active research area with numerous recent advances (e.g. Davies et al. 2001; Matthews et al. 2011; Pyne-O’Donnell 2007).

Mesolithic Radiocarbon Assessment

Radiocarbon dating has undergone radical change in recent years with the introduction of new techniques such as AMS dating, ultra-filtration pre-treatment of samples, and the application of statistical modelling including, amongst others, Bayesian modelling. One of the results of these new applications is that more precise, and more accurate, chronological control is being afforded to archaeological contexts and this is having a profound effect on understanding of the timing and duration of hunter-gather, and indeed later, activity (see for example the results for the Howick settlement: Waddington 2007).

Of course, the accuracy of the chronological control also depends fundamentally on the calibration procedure that converts 14C ages into calendar age ranges. Accurate conversion is easily accomplished for short-lived, terrestrially derived Mesolithic samples (e.g. round wood charcoal, ungulate bones, etc) using one of the freely available calibration programs such as OxCal or Calib. However, marine samples such as fish bone and shell are more problematic. They derive their carbon from the marine environment which is depleted in 14C relative to the contemporaneous atmosphere/terrestrial biosphere. This is the so-called marine reservoir effect (MRE). If the effect were constant, it would be a relatively simple task to allow for this depletion in the age calculation, however it is well established that the MRE varies both temporally and spatially (e.g. Ascough et al.2004; 2006). The current global average surface water MRE is 400 years and deviations from this value are designated by a ΔR value. Thus, a ΔR value that is negative is indicative of a reduced MRE while a positive value indicates an increased MRE. Therefore, prior knowledge of the ΔR value for a site and period is required in order to achieve the most accurate calibration of marine samples. In the absence of an appropriate ΔR value, a value of zero is typically assumed. Ascough et al. (2007) have derived two ΔR values for the Mesolithic, from Sand and Northton, both of which are positive (64 ± 19 and 79 ± 32, respectively).

The calibration process is further complicated for human remains when there has been consumption of a significant quantity of marine-derived resources. Not only is an appropriate value for ΔR required for accurate calibration but also, it is important to have an accurate estimate of the proportion of marine resources contained in the diet. This estimate is typically made by reference to the δ13C value of the collagen (e.g. Arneborg et al. 1999).

The date list (see the downloadable ScARF datelist) compiled for this assessment consists of a list of raw dates that have not been edited or checked for their accuracy, the usefulness of the sample, or the reliability of the sample and the context from which it came. Therefore, the date list must be used with care as some of the dates would not be considered scientifically acceptable. The calibrations were undertaken in November 2009 using the program Oxcal 4.1 and the calibration curve INTCAL04.

A pie chart displaying the percentage of mesolithic material radiocarbon dated by material showing unidentified charcoal and identified carbonised material to be the most popular material types dated

Percentage of ‘Mesolithic’ radiocarbon measurements from Scotland by material dated

The figure above shows that the about 50% of radiocarbon measurements for the list as it stood in November 2009 at best provide terminus post quems for their contexts and this is probably being generous given that no assessment of the taphonomy of most samples has been undertaken.

A very brief assessment highlights the following:

Unidentified charcoal and wood: can have an unknown age-at-death offset (Bowman 1990); unless identifications are given the dates on these materials only provide tpqs.

Identified carbonised: material – some of this is short-lived, however, a number of samples contain bulked material and may contain material of different ages (Ashmore 1999).

Shell: this needs to be calibrated using the appropriate calibration data (marine offset; see Ascough et al. 2007). This has not been undertaken because it requires some work to identify appropriate regional values from published data.

Animal: unless these are identified as articulated (and this in situ) they only provide tpqs for their context.

Human: these need stable isotope measurements to correct for dietary offsets (marine diet, etc).

Soil: of little value due to uncertainty over precise contextual associations (though interestingly microfossils in sediment are much used by geoscientists to get dates from sediment cores).

Antler: without an indication of the taphonomic relationship to the context they only provide a tpq.

Further Work

The problems identified above do not only relate to the Mesolithic but all periods that use radiocarbon. A proper assessment for all periods as undertaken above would highlight why most dates are at best only tpqs and could be used to produce some generic recommendations on how to approach the use of radiocarbon. This should be undertaken as a minimum before any chronological research questions are formulated.

A study is required to review the date list, removing dates that cannot be demonstrated to be reliable. The resultant dates can then be used for modelling and related purposes to provide a wide variety of useful chronological data that can be used to assist in understanding questions relating to settlement, artefact chronology and environmental change/impacts as well as contributing to the production of historical narrative for the period.

It is also recommended that the current Excel spreadsheet format for the date list is retained, or a variant of it, as this is a simple format that allows for rapid calibration of all dates using the Oxcal program, as well as for easy combining and so forth of dates for statistical modelling. It also means that the date register can be easily updated by non-specialists whilst also not being too onerous for researchers who may produce fairly large numbers of dates for any given project.

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