2. Scientific Dating Methods for the Pleistocene

David R Bridgland

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The relative sequence provided by stratigraphy can be placed on a calendar timescale using various methods of scientific dating. Many of the most important advances come from understanding radioactivity. Detailed explanations of the major techniques follow, including a series of case studies. Further information is provided by Walker (2005), Lowe and Walker (2015) and Rixhon et al. (2017).

The application of scientific dating methods in the Pleistocene is generally limited by the availability of suitable material for dating. Replicate measurements should be obtained and, wherever possible, ages should be obtained from more than one technique. This enables comparison of the results produced by different dating techniques to be assessed and a chronology to be constructed.

Stratigraphy also provides a key method for assessing the reliability of scientific dating. Results should be consistent with the relative dating provided by the stratigraphy. Bayesian chronological modelling can be employed as an explicit methodology for combining these disparate strands of evidence.

Certain questions must be considered before embarking on any dating programme:

  • Applicability: is there something datable within the deposit?
  • Taphonomy: how did the material being dated become incorporated into the deposit?
  • Time range: is there a technique suitable for the expected time-range of the deposit (see Figure 9)?
  • Precision/Accuracy: are the available techniques capable of providing sufficient precision/accuracy to resolve the archaeological problem of interest?
  • Cost/facilities: is there sufficient funding and can the necessary measurements be obtained within the required timescale?

Some general rules should be adopted for scientific dating programmes in the Pleistocene:

  • The application of scientific dating techniques should, wherever possible, be underpinned by a thorough understanding of the site sediments and their stratigraphy.
  • Some types of material or deposit may provide a means of relative dating (e.g. biostratigraphy, pedostratigraphy, morphostratigraphy).
  • Multiple age determinations from a single stratigraphical unit should be compatible.
  • Independent dating techniques from the same stratigraphical unit should give consistent ages.
  • Scientific dates should conform with the stratigraphy (i.e. the oldest dates at the bottom and youngest at the top).
  • Where deposits can be tied into the Marine Oxygen Isotope stages or the Greenland Ice Core record, results should be comparable to these timescales.

2.1 Radiometric methods

These methods make use of radioactive isotopes, which decay at rates predicted by their half-lives. Different isotopes are used for dating different time ranges (Figure 9).

Radiocarbon dating is used in the very Late Pleistocene and through the Holocene because its half-life is 5730 years.

Isotopes with half-lives suited to dating earlier parts of the Pleistocene are more restricted. Argon-Argon (40Ar-39Ar) and Potassium-Argon (40K-40Ar) dating is of considerable precision, but lavas and tephra suitable for these methods are not commonly found in the English Quaternary record.

Uranium-series (including Uranium-Thorium dating) requires that these elements are present and for there to be a closed system. It has mainly been applied to calcareous deposits in caves.

Cosmogenic nuclide dating is based on the reaction between cosmic rays and certain elements in minerals within rocks. The continuous bombardment by cosmic rays is predictable, with certain provisos. It leads to the formation and accumulation of ‘cosmogenic isotopes’ in rock surfaces (Text Box 3). This technique is potentially a powerful tool but requires a good understanding of erosion history and adequate sampling strategies. At present, its use for archaeological applications has been limited. It is expensive and should only be used in collaboration with expert practitioners.

TEXT BOX 3: Cosmogenic Nuclide dating

There are two contrasting approaches to using cosmogenic nuclides for age estimation: exposure dating and burial dating.

Exposure dating measures the time that has elapsed since rock surfaces became exposed to cosmic radiation. It has been used to date past glaciation, for example by dating ice-moulded bedrock and erratic boulders (e.g. Ballantyne 2010). 36Cl, 10Be and 26Al isotopes collectively cover timescales from a few ka to 4 Ma.

The amount of an isotope accumulated in the uppermost few cm of exposed rock is proportional to the length of time elapsed since the initial exposure of the rock surface.

Burial dating is based on the differential decay of at least two nuclides, where at least one of them is a radionuclide. These can indicate the time elapsed since they were sealed from cosmic-ray bombardment (Dunai 2010).

The nuclide pair 26Al/10Be is frequently employed for this method, as both isotopes are readily produced in quartz by the action of cosmic rays at a ratio that is essentially independent of latitude and altitude.

Burial dating using these isotopes depends on the quartz having been exposed to cosmic rays for a period during which they accumulate in the sediment. Burial must then be rapid and at sufficient depth to prevent further cosmogenic nuclide production. As the isotopes decay at differing rates, and the surface concentration ratio is well understood, the ratio of the buried sample can be measured and dated.

2.2 Trapped charge methods

These techniques use signals from charge trapped in the structure of crystalline minerals to calculate the time since the ‘traps’ were emptied by a ‘zeroing’ event, for example exposure to sunlight or heating. Radioactive decay within the environment supplies a stream of charge that will progressively fill these traps at a predictable rate after the ‘zeroing’ event. Once the majority of traps are occupied, the mineral is saturated, which constitutes a limitation to the dating timescale.

  • For luminescence techniques, the trapped charge is measured by the amount of light emitted by charge released from traps.
  • Electron Spin Resonance (ESR) does not evict the charge. Instead, the strength of the signal emitted by the trapped charge is measured.

In both techniques, measurements are made on each sample following laboratory irradiation with calibrated radiation sources. These enable calculation of how much radiation dose the samples were exposed to during burial. This burial dose must be divided by the dose rate (how much dose per thousand years). The dose rate is generated by the level of radioactivity in the sediment from which the dating sample was collected.

Different techniques have different applications and timespans. The timespan is always dependent on the natural level of radioactivity on the site. Low levels of natural radioactivity can allow dating over a longer-timescale (see Luminescence dating):

  • Thermoluminescence (TL) This was the first luminescence dating technique to be used widely. Heat is used to release the trapped charge. It is applicable as a measure of the time that has passed since the heating of burnt flint recovered from hearths.
  • Optically Stimulated Luminescence (OSL) is applied to sediments. Blue or green light is used to release the trapped charge from grains of quartz. When used, it is important to consider whether the grains were fully zeroed by light exposure when deposited, which is why the method works best for dating wind-blown sediments.
  • Infrared-Stimulated Luminescence (IRSL) is also applied to sediments. Infra-red light is used to release the trapped charge in feldspars, not quartz. This technique has the advantage that feldspars normally saturate at higher doses than quartz and thus can date older sediments.
  • Electron Spin Resonance (ESR) measures the trapped charge by looking at the absorption of microwave energy by a mineral as the strength of an applied magnetic field is varied. The dating range is dependent on the type of sample (i.e. tooth enamel or sedimentary quartz grains) and on the concentration of radionuclides in the surrounding environment. Its range is between a few thousand and more than a million years (see Text Box 4).

OSL has been applied successfully to sediments in many areas of Britain, especially in the past two decades (see The Axe Valley at Broom; Marine Aggregate Licence Area 240; Lynford Quarry). Incomplete bleaching of sediments and the unsuitability of the available quartz sand grains, however, may prevent successful dating (for example Pennine quartz in northern England). The IRSL signal from feldspars is less variable from region to region and, although the method is more complex, it may be more suitable in some areas.

ESR has been widely used in France but applications in Britain have been infrequent and yielded inconsistent outcomes (Grün and Schwarcz 2000; Voinchet et al. 2015). This technique should only be employed in collaboration with expert practitioners

TEXT BOX 4: Electron Spin Resonance (ESR)

Electron Spin Resonance (ESR) is a trapped-charge dating technique in the same group as luminescence (Duval 2016; Rixhon et al. 2017). The materials that can be dated include phosphates, carbonates and silicates.

Fossils (especially teeth) and optically bleached quartz grains are the most common applications to Pleistocene deposits in Britain. The main difference from luminescence dating (see Luminescence dating) is that the equivalent dose is obtained using ESR spectroscopy — i.e. the measure of energy stored in traps in the crystal lattice.

For teeth, uptake of Uranium is common, and this makes the dose rate change through time, so it is normally necessary to undertake Uranium-series analyses in parallel with ESR to quantify this effect.

For sedimentary quartz a number of different ESR signals can be used, some of which are reset by light faster than others, and some of which are more stable than others. Quoted errors are typically 15% of the estimated age.

2.3 Other scientific dating methods

There are also non-radiometric methods that have proved to be valuable for dating Pleistocene contexts.

  • Amino-Acid Racemisation dating (AAR) is based on the predictable diagenesis of proteins within biological materials after organisms die, particularly the shells of molluscs. The method has been applied to British Lower to Middle Palaeolithic materials, with emphasis on fluvial localities containing both artefacts and molluscan fauna.
  • Palaeomagnetism is valuable in providing isochrons (age-equivalent stratigraphical horizons). One of the most important is the Matuyama–Brunhes magnetic reversal (when the north and south poles reversed) marking the start of the Middle Pleistocene (780 ka; see Figure 1). This marker can be an important element in dating river terrace sequences, although the reversal itself has yet to be located in Britain.
  • Tephrochronology is a useful means for identifying isochrons across widespread areas, making use of volcanic ash layers (tephras) distributed by wind. It is used when such layers can be correlated with particular eruptions using geochemical analyses. In Britain work has mainly focused on Late Pleistocene isochrons.

2.4 Relative dating methods

There are several approaches which provide relative, rather than calendar, dating. These underpin the understanding of landscape development and stratigraphy, providing a framework into which other dating evidence can be incorporated.

Biostratigraphy is a key relative dating method. This may consist of identifying remains that are only known to have lived at a certain time, or taxa with particular niche requirements, such as temperature, that only occur at specific times in a glacial–interglacial cycle. An important part of the approach is the correlation of sediments in different geographical locations using biostratigraphic characteristics.

Biostratigraphy may be based on a single taxon, on assemblages of taxa, on relative abundances, and on specified morphological features, including evolutionary changes. The last can be the most powerful biostratigraphical technique (see The ’Vole Clock’). Mammalian faunas have proved to be the most effective for dating, as they show greater change during the Quaternary than other biological remains.

This principle is also the basis for ‘archaeostratigraphy’, in which diagnostic archaeological artefacts (for example, types of worked flint) are used to infer the age of the deposits.