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Module 8 - How accurate is a luminescence age?

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11 min read

There are many factors that contribute to the uncertainty of a luminescence age. When considering uncertainty, it is important to distinguish between precision and accuracy.

The precision of a dating technique refers to the reproducibility of dating results. In other words, how much do the results vary with repeated measurements under identical conditions? Extremely precise age estimates can be made on very bright minerals with optical properties suited to our measurement procedures from homogeneous sediments with well defined dose rates.

The main sources of uncertainty of luminescence ages are, of course, derived from the uncertainties associated with the two main variables of interest: the equivalent dose (De) and the dose rate (Dr). These are summarized in Table 1.

Table 1. Sources of uncertainty in luminescence ages (Aitken, 1998; Mahan et al., 2022).

Measured variable

Type of error

Sources of uncertainty

Equivalent dose (De)

Instrument-based sources of error

  • temperature variation during heating

  • power stability of stimulation source

  • movement of discs in reader

  • loss of sediment between measurements

  • single-grain laser positioning

Measurement protocol error

  • imperfect sensitivity correction

  • photon counting error

  • different radiation conditions between the reader and in nature

  • undesirable optical properties in the sample

Statistical modeling

  • dose response curve fitting error

  • uncertainty in depositional environmental conditions (bleaching, heterogeneity of dose rate, etc.)

Dose rate (Dr)

Environmental sources

  • uncertainty in sample water content during its burial history

  • beta microdosimetry effects

  • uncertainty in radioisotopic equilibrium conditions during sample burial history

  • changes in burial depth during sample burial history

  • uncertainty with respect to internal radioactivity of mineral grains

Methodological sources

The accuracy of a technique refers to how close the results are to the true age of the deposit. This can be harder to ascertain as independent age control may not be available. The accuracy of luminescence ages are tested, often using comparisons with radiocarbon ages. These comparisons are not always ideal, as radiocarbon ages provide the time of death of an organism, while luminescence ages provide the time of last exposure to light or heat; two different events (Schiffer, 1986). However, reproducible ages are consistent with other age controls when applied to suitable settings (Figs 1-3).

Figure 1. C-14 and luminescence ages from the Nuβloch loess section in South West Germany are plotted according to the depth of sampling. Luminescence ages date polymineral silt. The location of major soils is indicated and the dashed line plots a sedimentation rate of 1.3 mm per year. An enlargement of the upper 14 m is given in the inset. From Lang et al. (2003).

Figure 2. Comparison between luminescence ages obtained from quartz (A) and feldspar (B) samples and independent age control. Samples are obtained from a range of depositional environments. Data for quartz are from Rittenour (2008), modified and updated from Murray and Olley (2002). “OSL” refers to optically stimulated luminescence. Data for feldspar are from Buylaert et al. (2011; 2012); Kars et al. (2012); Li and Li (2011; 2012); Lowick et al. (2012); Ramos et al. (2012); Roskosch et al. (2012); Schatz et al. (2012); Stevens et al. (2011); Thiel et al. (2011; 2012); Vasiliniuc et al. (2012).

Figure 3. A) Comparison of feldspar ages to independent age control. Samples are from fluvial, alluvial and lake shoreline deposits in active tectonic settings in California, Mexico, Tibet and Mongolia. From Rhodes (2015). B) Comparisons of young luminescence ages obtained from quartz and independent age control. The samples consist of aeolian and waterlain deposits from coastal and inland areas. Data are from Madsen and Murray (2009).