This essay will consider both the inherent strengths and weaknesses of Radiocarbon dating and Dendrochronology, and also the ways in which these techniques can be applied inappropriately. As might be expected, each of the techniques has limitations and conditions under which it can be applied; it is when the technique is applied to conditions outside these limitations, perhaps for reasons of interpretative determinism, that the integrity of the technique is undermined.
The analysis of each technique is focussed on the following factors:
- Applicability: what range of subjects and materials can the technique address?
- Accuracy: what resemblance does the achieved determination bear to the actual calendar date of the subject?
- Interpretative outcomes: besides chronological considerations, does the technique add other information to the interpretation? A strength of a technique might lie in its ability to provide additional insights into environmental conditions, but a weakness of a technique might be found in the tenuous link between the dating subject and the context in which it is used to date
Additional to these factors could be a multitude of other considerations not strictly properties of the technique itself. For example, the processes by which the technique is performed and the associated skills and knowledge required to produce accurate determinations will necessarily impact the availability of the technique, but availability (and the associated monetary cost) is not an intrinsic property of the technique itself. In order to evaluate the technique itself, an idealised situation will be considered, whereby it is assumed that an archaeologist would have equal and otherwise unbiased access to a range of dating techniques, and it remains only to choose the one most appropriate to the situation at hand.
Most of the 14C in the atmosphere originates in the action of cosmic rays on Nitrogen in the upper atmosphere. This unstable isotope of Carbon then enters the food chain, and in doing so, forms part of all organic matter (Bayliss et al. 2004). Broadly speaking, anything that was once alive can therefore theoretically have measured the levels of radiocarbon it now contains. It is also possible to obtain radiocarbon determinations from inorganic materials if the process of producing the finished state includes the incorporation of carbon; examples of where this might be possible is the application of lime mortar as carbon dioxide is absorbed by the surface when the mortar hardens (Bowman 1990:13).
In reality, there are difficulties associated with the processing and measurement of certain materials, which reduces the applicability of this technique. For example, bone samples without enough remaining collagen had to be disregarded in a radiocarbon dating exercise targeting the Cotswold-Severn Long Barrows (Smith & Brickley 2006).
Owing to the plateaux in the calibration curve (see Figure 1 below), samples with true dates on these plateaux cannot produce dates with any precision, and may return such wide ranges that the technique may not be the best approach to dating material from that time period.
Figure 1:Radiocarbon versus calendar ages for the period 9000-11000 14C years BP. Source Walker 2005 Figure 2.5
It used to be the case, before mass spectrometry was invented, that a limitation of the applicability of radiocarbon dating was due to the large sample sizes required in order to obtain a statistically-valid count of the beta decay. With Accelerometer Mass Spectrometry, the ratios of the various isotopes of Carbon are measured directly and the amount of 14C calculated from the ratios, rather than relying on detecting the decay of the radionuclide. For the high-precision radiometric laboratories (which use the decay-detection technique) a large sample and a long period of time to perform the count is required (Walker 2005: Section 2), limiting the usefulness to situations where these are not constraints.
Whilst some quite precise dates are being produced by radiometric laboratories in Seattle, Groningen, and Belfast, with standard errors of around 20 years (Walker 2005:Section 2.3.2), the accuracy of the dates so obtained is dependent on the quality, purity and processing of the sample material. Hodgins et al. (2001) showed that the pre-treatment regime for sub-fossil insect remains had an impact on the resulting radiocarbon dates obtained from the samples (Figure 2 below).
Figure 2: Changes in chitin chemistry during pre-treatment for 14C dating. Source: Hodgins et al. 2001 Table 3
Errors can obviously also creep in due to contamination, whether in the laboratory, or at the point at which the sample is taken. This is especially a problem with material that has been stored sub-optimally, handled with unclean hands, or treated with organic chemicals for conservation purposes (Pohl et al. 2009). Unlike dendrochronology, which relies on multiple correspondences to provide a solid chronology (as discussed later), radiocarbon dating has no inbuilt self-test mechanism, so errors of this sort are hard to detect and hard to quantify.
Radiocarbon dating is predicated on the assumption that the level of 14C in the sample at the time it entered the archaeological record is identical to the concentration of 14C in the atmosphere at the time and that these levels of both biosphere and atmosphere are consistent over the entire globe. It is now known that this is not the case, and that there are localised reservoir effects which need to be compensated for in the calibration process. These include the upwelling of 14C-depleted waters from the ocean depths (Barrett et al. 2000), problems of 14C –deficient CO2 emission from volcanoes adjusting the local uptake of 14C (Wiener 2012), and freshwater reservoir effects (Fernandes & Bergemann 2012).
It is known, too, that isotopic fractionation occurs; the lighter molecules of CO2 are taken up in preference to the radioactive and heavier isotopes, so a lower percentage of 14C enters the biosphere in this way than is present in the atmosphere. The reverse is true for the radioactive carbon ratios in the ocean, where 14C is taken up in preference to the lighter isotopes (Walker 2005: Section 2.4.2).
Inaccuracies derived from these two sources cannot be effectively dealt with by multiple readings, as in the case of inaccuracies introduced by incorrect measurement, and so must be estimated and compensated for (Ramsey 2009). This essentially means that the original estimated figure of the amount of 14C in the sample is then subjected to further estimations of factors that might affect the accuracy of that original determination. As time progresses it is hoped that these estimations may become more sophisticated, but it is perhaps fair to say that Radiocarbon dating is not so much measurement as an exercise in statistical analysis.
Parker Pearson (2013: 129-132) gave an example of a radiocarbon determination being (incorrectly) disregarded as it did not fit with the interpretation of the construction sequence of Stonehenge. With a re-assessment of the interpretation of the context in which the sampled material was found, the dates have been attributed to their proper feature and the chronology is internally consistent.
One of the problems with the radiocarbon dating of ecofacts, or of small artefacts found within soil, is that of bioturbation. Parker Pearson (2013: 305-307) gives the example of the huge range of dates obtained by Darvill and Wainwright in 2008 when trying to produce radiocarbon dates for Stonehenge; the date determinations indicated that the sarsen circle dated to AD 1670-1960 as a result of disturbances within the soil by both people and animals. The lesson here is that although a sample may be retrieved from a given context, there is every cause to question whether that was in fact the context from which it originated.
The question of residuality, that is, how long artefacts have been in existence before they enter the archaeological record, is also a factor that can affect the accuracy of radiocarbon dates when applied to a given context. For example, as Hamilton (2011) argued, there is no way of knowing whether the artefact that has been subjected to radiocarbon dating is an heirloom that has been curated for a given period of time, or whether the deposit itself has been reworked in some way as to render the date invalid when applied to anything other than the sample itself. This is not necessarily a weakness of radiocarbon dating, more a pitfall of the application of the technique. An interesting example of this was the evidence for Bronze Age mummies at Cladh Hallan (Parker Pearson et al. 2005), and many other examples from British Prehistory (Booth 2008). The implications of this for the accepted chronologies built on radiocarbon-dating of skeletal material, and the understanding of burial practice in prehistory, are profound (Smith 2013).
A further pitfall can be seen in the less precise determinations and the resulting wide date ranges being used to construct chronologies. Nishitani (2012 :195), in a consideration of the chronology of the pottery from Danebury, points out that even with a difference of 200-300 years between individual samples, at 95.4% confidence level, the calibrated date ranges at least partially overlap, meaning that the detailed chronology constructed by Cunliffe on the basis of radiocarbon dates may be problematic.
An interesting example of the interpretative use of radiocarbon dating in conjunction with dendrochronologically-pinpointed dating was the suggestion that an increase in the amount of 14C in wood dating from AD 774-775 can be attributed to an increase in cosmic rays. This was suggested to be potentially due to increased solar activity or a nearby supernova (Jull 2013). Whether this suggested change in cosmic rays would be perceptible in the archaeological record in terms of an impact on human lives, is debatable, however.
Dendrochronology relies upon strong correlations in patterns of rings to match back to a master chronology. The more rings there are the more likely it is that a several sequences of rings will cross-match to the master chronology and provide a correspondence strong enough to be considered a date. What this means is that the technique requires long-lived species, as at least 100 rings are ideally needed and this limits the applicability in Europe to long-lived Oaks (Baillie 1995:12).
The distinctness of rings in a sample is another consideration. Some trees are more sensitive than other to environmental disturbance and may miss rings or produce multiple rings in the same year, making it more difficult to find a matching pattern as that environmental disturbance may have only affected the trees in the locality of the sample, whereas the master chronology will not reflect this disturbance. This means that the optimal species for dendrochronology are those that are both long-lived and relatively insensitive. However, this lack of sensitivity may cause the tree to not produce distinctive patterns: it is the sensitivity to conditions that causes the variability in ring sizes and greater sensitivity will cause greater variation between extremes and therefore more distinct patterns (Aitken 1990:38).
In order to determine the date of felling, the sapwood, or at least the boundary between the sapwood and the heartwood must be present in the sample as it is these that mark the terminus of the tree’s growth (Baillie 1995:23). If these outer rings are not present then although a match may be made to the master chronology based on the pattern of rings, it will only be possible to say that the tree was alive during these years. Which may be precision enough for most purposes, so the technique may still be applicable.
Dendrochronology can be destructive if a sample is extracted from the wood for analysis. This can limit the applicability of the technique to those artefacts where this is permitted. However, it is possible to lift the pattern of rings using modelling clay, avoiding the need for destructive testing (Baillie 1995:18), but presumably potentially leaving a residue.
These considerations, coupled with the biodegradability of the material itself, limit the opportunities for this technique to be applied. Therefore the main weakness of this technique is its limited applicability (Baillie 1995:13).
The main strength of dendrochronology is its ability to produce absolute dates, sometimes even to the exact season that the wood was felled (Baillie 1995:17) if the final ring is present, providing amazing precision. The problem with this is that there is an expectation that if a wood sample is present, then it can produce these absolute dates, but as discussed above, there is a specific set of circumstances that constrain this, and often the sample is undateable. Samples that are missing the sapwood can still produce dates, but as there is a need to estimate the number of sapwood rings that should be present, the accuracy of the technique is diminished as certainty is replaced by estimation (Baillie 1995:23).
Whilst the potential for achieving precision is high for a ring-complete sample, the accuracy of the technique is dependent firstly on a correct match being determined, and secondly that the chronology it is being matched to is itself accurate. The correct match can be assisted with statistical software that can calculate the co-efficient of correlation for a given matching pattern (Baillie 1995:17) and therefore provide a quantitative assessment of how good the match is, although Baillie went on to argue that this computer-based matching is best thought of as guidance for the experienced dendrochronologist and not the authoritative definition of a match. Nevertheless, a high degree of certainty that the sample is a match for that section of the master chronology can be obtained by insisting on high degrees of correlation and multiple matching points for a given sample.
Figure 3: Completely Integrated Correlations between Independent Chronologies. The Long Sections of English Chronology from Croston Moss, Lancashire Match at Exactly the same Date Against both the Irish and German Master Chronologies. With Highly Significant Correlations. Source: Baillie 1995 Figure 1.3
The accuracy of the master chronologies themselves, for it is these that ultimately returns the value for the calendar date, is the determining factor that will return an accurate date or not. Dendrochronologists use a series of crosslinks between chronologies (as shown in Figure 3 above) to provide reinforcement of the certainty that the chronology is correct.
Dendrochronology can allow the pinpointing in time of the felling of a single tree and the identification of the use of the wood of that tree across a given site. Dendrochronology can also give an insight into climatic conditions during the lifetime of the tree, as evidenced by dimensions and distinctness of the growth rings. This has allowed Dendrochronologists to provide evidence for the timing of such events as the eruption in prehistory of the Santorini volcano (Baillie & Munro 1988).
Dendrochronological dating shares with Radiocarbon dating some of the interpretative issues when the dates are used to provide a chronology or a narrative of a given event. The dates are for the end of the life of the tree, not of the artefact or context that is being interpreted. It has been shown, for example that some of the timbers in the Sweet Track were up to 400 years old when used (Baillie 1995: Chapter 4). This question of residuality must be considered when constructing site chronologies, as the difference between seasoned and green timbers may make a considerable difference if one is looking to determine sub-decade-level chronologies.
Baillie also explained that the problem identified with only partial sets of rings can be compounded when the planks of wood are riven and end up in the same structure, as different parts will essentially contain different snapshots of the tree’s life, representing different sections of time.
Figure 4: Where Tangential Splitting of Riven Oak has been Employed to Produce Building Timbers, the Danger is that Different Elements Within a Structure will be of Intrinsically Different Dates. This is a Particular Danger if the Structure is to be Dated by Radiocarbon. Source: Baillie 1995 Figure 4.5
A major strength of both of these techniques is the ability to give a chronological hook on which to hang an interpretation, and the large window of time that the techniques are applicable across (see Figure 5).
Figure 5: The effective dating ranges of the different techniques discussed in this book. Source: Walker 2005 Fig 1.6
Whilst Dendrochronology promises, and can deliver, precision and accuracy, the technique is weakened by the limited applicability. Radiocarbon has a much wider suitability (and potential for misuse) but the inherent uncertainties of both the process and the determinations mean that accuracy and precision are lacking. However, if one subscribes to the worldview that the course of events seldom alters as the result of a single event, and that a ‘smearing’ of time is not necessarily a hindrance to interpretation, then Radiocarbon offers the greater potential to answer the questions of chronology when considered at the century rather than season scale. However, the sheer number of caveats and adjustments that are required to be applied to the Radiocarbon determinations should be ever-present in the mind whilst using the resulting dates. Essentially, both techniques have their merits, and when used judiciously, and preferably in combinations, useful outcomes may be achieved. (2844 words including headings.)
Aitken, M.J., 1990. Science-Based Dating in Archaeology (Longman Archaeology Series) 1 edition., Routledge.
Baillie, M. & Munro, M., 1988. Irish tree rings, Santorini and volcanic dust veils. Nature, 332, pp.344–346. Available at: http://www.nature.com/nature/journal/v332/n6162/abs/332344a0.html [Accessed December 31, 2013].
Baillie, M.G.L., 1995. A Slice Through Time: Dendrochronology and Precision Dating, Routledge.
Barrett, J.H., Beukens, R.P. & Brothwell, D.R., 2000. Radiocarbon dating and marine reservoir correction of Viking Age Christian burials from Orkney. Antiquity, 74, pp.537–543.
Bayliss, A., McCormac, G. & Plicht, H. Van Der, 2004. An illustrated guide to measuring radiocarbon from archaeological samples. Physics Education, 39(2), pp.137–144.
Booth, T., 2008. An Assessment of the evidence for the widespread practice of mummification at prehistoric British sites with reference to the criteria established at Cladh Hallan, South Uist. Unpublished Masters Dissertation. University of Sheffield. Available at: http://www.academia.edu/1087827/An_Assessment_of_the_Evidence_for_the_Widespread_Practice_of_Mummification_at_Prehistoric_British_Sites.
Bowman, S., 1990. Radiocarbon Dating, London: British Museum Publications Ltd.
Fernandes, R. & Bergemann, S., 2012. Mussels with Meat: Bivalve Tissue-Shell Radiocarbon Age Differences and Archaeological Implications. Radiocarbon, 54(3), pp.953–965.
Hamilton, W.D., 2011. The use of radiocarbon and Bayesian modelling to (re)write later Iron Age settlement histories in east-central Britain. Unpub’d PhD Thesis. University of Leicester. Available at: http://hdl.handle.net/2381/9066.
Hodgins, G.W.L.G. et al., 2001. Protocol development for purification and characterization of sub-fossil insect chitin for stable isotopic analysis and radiocarbon dating. Radiocarbon, 43(2), pp.199–208. Available at: https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/3955 [Accessed September 1, 2013].
Jull, A.J.T., 2013. Some interesting and exotic applications of carbon-14 dating by accelerator mass spectrometry. Journal of Physics: Conference Series, 436.
Nishitani, A., 2012. Typological Classification and the Chronology of Iron Age pottery in central-southern Britain. Unpublished PhD Thesis. University of Durham.
Parker Pearson, M. et al., 2005. Evidence for mummification in Bronze Age Britain. Antiquity, 79(305), pp.529–546. Available at: http://antiquity.ac.uk/Ant/079/0529/ant0790529.pdf [Accessed January 18, 2011].
Parker Pearson, M., 2013. Stonehenge: A New Understanding, New York: The Experiment LLC.
Pohl, C.M. et al., 2009. The Effect of Cyclododecane on Carbon-14 Dating of Archaeological Materials. Journal of the American Institute for Conservation, 48(3), pp.223–233. Available at: http://openurl.ingenta.com/content/xref?genre=article&issn=0197-1360&volume=48&issue=3&spage=223.
Ramsey, C., 2009. Dealing with outliers and offsets in radiocarbon dating. Radiocarbon, 51(3), pp.1023–1045. Available at: https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/3561.
Smith, M., 2013. Context isn’t quite everything: Interpreting complex prehistoric mortuary rituals at Cranborne Chase, Dorset, England. In TAG-On-Sea 2013.
Smith, M. & Brickley, M., 2006. THE DATE AND SEQUENCE OF USE OF NEOLITHIC FUNERARY MONUMENTS: NEW AMS DATING EVIDENCE FROM THE COTSWOLD-SEVERN REGION. Oxford Journal of Archaeology, 25(4), pp.335–355. Available at: http://onlinelibrary.wiley.com/doi/10.1111/j.1468-0092.2006.00265.x/full [Accessed January 1, 2012].
Walker, M., 2005. Quaternary Dating Methods: An Introduction Kindle., Chichester: John Wiley & Sons, Ltd.
Wiener, M., 2012. Problems in the Measurement, Calibration, Analysis, and Communication of Radiocarbon Dates (with Special Reference to the Prehistory of the Aegean World). Radiocarbon, 54(3), pp.423–434. Available at: https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/16231 [Accessed January 5, 2014].