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Luminescence dating techniques are most commonly applied to silt or sand grains, but in recent years, significant advances have been made in dating rock surfaces (Habermann et al. 2000; Greilich et al., 2005; Vafiadou et al., 2007; Liritzis, 2011; Simms et al., 2011; Sohbati et al., 2012; Freiesleben et al., 2015; Simkins et al., 2016; Gliganic et al., 2021; Freiesleben et al., 2023).
Why date rock surfaces?
Luminescence ages from rocks are important for sites that lack adequate sand/silt for traditional luminescence dating techniques, as well as sites that are contaminated by mobile fine grain materials that severely post-date (underestimate) the true age of the landform or artefact. Applications of rock surface dating include refining the chronology of sea level change (Simms et al., 2011), the advance and retreat of glaciers (Rhades et al., 2018; Jenkins et al., 2018), ancient floods (Smith et al., 2023) as well as the construction of monuments (Liritzis, 2010; Feathers et al., 2019; 2022) and rock art (Chapot et al., 2012; Liritzis et al., 2019; Moayed et al., 2022).
A particularly exciting application of rock surface dating is the direct dating of stone tools or lithic artefacts (Gliganic et al., 2019; 2021) as this approach provides us with an opportunity to date many archaeological sites that have, until recently, been deemed undateable. Many archaeological sites in arid or semi-arid lands in Australia and elsewhere consist of artefact scatters situated in or on wind-swept, remobilized sediments, containing little or no organic material (Fig. 1). For these sites, traditional dating methods, such as radiocarbon dating or luminescence dating of sediment, are not effective in determining the time the artefact was created or used.
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Figure 1. A) Archaeologist standing over a scatter of lithic artefacts along the banks of the Middle Son River, Madhya Pradesh, India. B) Close-up of lithic flakes found on the ground surface. Photo: Christina Neudorf.
Rock surface burial dating vs exposure dating
There are two ways one can date the surface of a rock using luminescence: that is by i) burial dating, or ii) exposure dating. Like traditional luminescence dating techniques for sediments, rock surface burial dating determines how long the rock surface has been buried or shielded from light. This approach follows a similar procedure to that of sediments that entails measurement of an equivalent dose (De) and environmental dose rate (Dr). But in this case, De and Dr are calculated or modeled at a series of increasing depths into the rock (see explanation below).
Rock surface exposure dating does not tell us how long a rock has been buried, but rather how long it has been exposed to light. This approach also requires measurement of luminescence with depth into the rock, however this data is then compared to (or calibrated by if you will) similar measurements obtained from a rock surface that has been sun-exposed for a known duration of time. This so-called “calibration sample” may be created artificially, simply by cutting a rock away from its bedrock and exposing it for a year or more (e.g., Gliganic et al., 2019) (Fig. 2). Alternatively, it may be derived from natural exposed rocks of known age (e.g., Lehmann et al., 2018).
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Figure 2. A) A boulder at the lithic quarry site of Su-re in southern Tibet. Dashed lines show location where the boulder was broken to expose a new calibration surface. B) The exposed calibration surface, which was sampled 20 months after creation to calculate the bleaching rate. From Gliganic et al. (2019).
How do we date rock surfaces?
Rock surface dating studies have most commonly targeted cobble or boulder sized clasts, which are sampled under dark or dim red light conditions. Sampling may be done at night and/or with a light-safe tarp or tent to block any ambient sun, moon or artificial light that may reset the luminescence signal. Once in the lab, ~10 mm diameter cores are extracted from the rock and are sliced into sub-millimeter slices. The luminescence signal of these slices is then measured in a luminescence reader and plotted as a function of their depth into the rock surface. This produces a luminescence-depth profile that traces the luminescence signal intensity from the rock surface to depth (Fig. 3). This luminescence-depth profile records the depth of light penetration and helps us calculate the time of past burial periods (Fig. 3).
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Figure 3. The standard approach to rock surface dating. A) A core is extracted from the rock surface. The core is sliced into sub-millimeter thick slices. The luminescence signal from each slice is measured to generate a luminescence-depth profile. From Gliganic et al. (2024). B) Luminescence-depth profiles measured for rock surfaces exposed to light for a range of durations. From Gliganic et al. (2024). C) Luminescence-depth profiles that are expected after i) light exposure of the rock surface (blue line), ii) re-burial a rock surface after an exposure event (red line), and iii) after no light exposure (grey dashed line). From Smith et al. (2023).
Figure 3 (B) illustrates how the luminescence-depth profile changes through time during light exposure. You can see that the S-shaped curve migrates further into the rock with light-exposure time. Theoretical luminescence-depth profiles that would be measured after: i) light exposure of the rock surface, ii) re-burial a rock surface after an exposure event, and iii) after no light exposure or burial of the rock for a very long time are shown in Figure 3 (C). For a cobble that had sufficient sun exposure prior to burial, the time of the most recent exposure event can be calculated from the near-surface plateau of the luminescence-depth profile (red line).
Obtaining ages from rock luminescence-depth profiles requires an estimate of the environmental dose rate at the rock surface, as well as at depth (i.e., a “dose rate-with-depth” profile). Dose rate models, therefore, take into account measured dose rates from the rock and surrounding sediments, and use established beta and gamma attenuation factors to calculate how dose rates will change with depth into the rock (e.g., Jenkins et al., 2018; Riedesel et al., 2020) (Fig. 4). Figure 4 shows the modeled dose rate and calculated age-depth model for a volcanic gravel sized rock from a pluvial lake beach ridge in Nevada, USA (Neudorf et al., unpublished data). The shape of this profile suggests that this rock did not receive adequate sunlight prior to burial to fully deplete the luminescence signal at its surface.
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Figure 4. A) The modeled dose rate with depth into a volcanic rock from a pluvial lake beach ridge in Nevada, USA. The contributions from both the rock and the surrounding sediment are plotted. The alpha contribution from the sediment is deemed negligible due to the short travel distances of alpha particles. From Neudorf et al. (unpublished data). B) The calculated age-depth profile from a volcanic rock from the same site. Dashed lines denote rock slice thickness. Black dots are ages from multi-grain aliquots obtained from each slice, X symbols are rejected aliquots, and red hollow circles are CAM weighted mean ages for each slice. Yellow shading highlights slices used to calculate the age of the plateau. From Neudorf et al. (unpublished data).
Reconstructing rock and artefact transport histories
An amazing consequence of measuring luminescence-depth profiles is our ability to see more than one burial event (Fig. 5). Some rocks show more than one plateau that can provide dates for multiple burial events during the rock’s transport history. For rocks transported in the natural environment, this information can help us understand their mode of transport, and the number of transport events (e.g., Rades et al., 2018) (Fig. 6A). At archaeological sites, such information can help us understand how many times a rock has been moved or re-positioned by people, how many times a building or wall has been re-built, and how artefacts came to their final resting place (e.g., Gliganic et al., 2021) (Fig. 6B).
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Figure 5. A) Modelled normalized luminescence-depth profiles for multiple sequential events of burial and daylight exposure. No trap filling has been considered during daylight exposure, and the dose rate is assumed constant with depth. The sequence of events are: i) burial for a long time sufficient for saturation L0(x), ii) daylight exposure L1(x), iii) burial L2(x), iv) daylight exposure L3(x), v) burial L4(x) (Freiesleben et al., 2015). Depth has been normalized using a light attenuation factor, μ.
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Figure 6. A) A boulder obtained from a terminal moraine (glacial deposits) from Malta Valley, southern Austria. B) At least two exposure/burial events can be observed in the luminescence-depth profile measured from the boulder. Modified from Rades et al. (2018). C) A quartzite artefact collected from an artefact scatter in Tibet. D) The calculated age-depth profile calculated for the artefact in ‘C’. Age plateaus are shown as solid lines (error shown as dashed lines). Note that this profile shows two sequential burial events (at 5.18 ± 0.37 and 2.38 ± 0.37 ka) separated by a brief exposure event of unknown duration. Modified from Gliganic et al. (2021).
Gliganic et al. (2021) illustrates how a luminescence-depth profile may change as a lithic artefact is created and remobilized in the landscape (Fig. 7). You can follow how the luminescence-depth profile will change on two opposing surfaces of the artefact (green and red) as the artefact is removed from the rock face then later discarded and partially buried in the ground.
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Figure 7. Conceptual model showing the dating approach highlighting a common pathway of a lithic artefact in the landscape and the corresponding changes to the luminescence signal on the object surface of interest. (A) Artefact still in its original bedrock context before exposure by quarrying and/or knapping. (B) Artefact use and/or discard by humans leading to exposure and bleaching of the luminescence signal on all surfaces. (C) Artefact settling, embedding, and semiburial in the soil leading to luminescence signal buildup (red and green dotted lines indicate two opposing artefact surfaces). Note that exposure of a subaerially exposed artefact surface before knapping (i.e., prior exposure, while still “in situ” in the quarry setting) such as in (A) will contribute to the bleaching of the artefact surfaces that continues in (B). From Gliganic et al. (2021).