This study presents single-grain optically stimulated luminescence (OSL) results from pottery and sediments from the archaeological site “The burials in Khutag Uul Mountains (Mongolia)”. A global fitting procedure was used on a selection of single grains from pottery,
For pottery, the relative spread in CAM
- single grain luminescence
- ultrafast component
Optically stimulated luminescence (OSL) dating of sediments and thermoluminescence (TL) dating of heated ceramics, terracotta figurines and potteries has proven to be useful in investigating geo-archaeological sites. Especially single-grain quartz optically stimulated luminescence dating has shown to provide reliable ages for samples from a range of archaeological contexts (Guérin et al., 2012; Jacobs et al., 2012; Solongo et al., 2016; Solongo et al., 2014). Single grain luminescence dating is often preferred in order to distinguish grains which were well bleached at time of deposition from those incomplete bleached (Jacobs and Roberts, 2007), grains from all mixed-age deposits (Duller, 2008), and to identify post-depositional mixing of sedimentary units (Arnold and Roberts, 2009). The OSL characteristics of the individual grains have been analysed in view of the various factors, among which the statistical uncertainty due to photon counting and instrumental uncertainties to determine
Our study examines single-grain chronologies for the archaeological site “the burials in Khutag Uul Mountains (Mongolia)”, associated with human activities from the Xiongnu period (3rd century BC – 2nd century AD) and from Turk period (552–745 AD) – using pottery and sedimentary samples from the same archaeological context. Based on the previous findings, e.g. the advantage of luminescence dating at the archaeological site using different dating materials (Solongo et al., 2019), the main aim of the present work includes the study of the luminescence characteristics of single grains from pottery quartz and sedimentary quartz. For single-grain dating, the dominance of the fast component using the fitting procedure will be quantified, and the effect of the ultrafast component on the dose evaluation will be examined and analysed further by identifying the appropriate integration time integral using De-(t) plots.
The Orkhon Valley was the centre of numerous nomadic states; the Xiongnu (3rd century BC – 1st century AD), Turkic (552–745 AD), and Mongol Empires (12th – 14th centuries) succeeded one another in the steppes of Mongolia. Archaeological research in Orkhon Valley has concentrated on numerous steles with runic inscriptions, but also on numerous archaeological burials. Khutag Uul Mountains (47°36’N, 102°47’) is located on the left bank of the Khogshin Orkhon River on the eastern edge of the upper Orkhon valley; it was once a tribute to the people of ancient times. The Khutag Uul Cemetery is the largest in its territory. In 2009, during the fieldwork, a total of 121 burial sites were recorded in the south and additionally 43 burials on the east (Bayar et al., 2010), which immediately became the 2nd Khutag Uul Mountain Cemetery.
The settlement history at this site and the characteristic types of archaeological monuments, especially the sections of burial structures and tombs have been associated with human activities from the Turkic period. However, the graves under study had similar exterior surface structures mounded by an oval or circular stone construction with a diameter of 6–8 m, reflecting a standard tomb of the Xiongnu period. Pottery fragments L-EVA-1201 and associated sedimentary samples L-EVA-1202 and L-EVA-1203, L-EVA1204 from nearby sites ‘MKC 65’ (layer 2) and ‘MKC 4’ (layer 1) were collected taken for single grain measurements (
Sample preparation and luminescence measurements were carried out in the Luminescence laboratory at the MPI for evolutionary anthropology, Leipzig (Germany) and followed the standard procedure reported in (Solongo et al., 2019).
Single grains of quartz were measured using a Risø TL-DA-20 reader (Bøtter-Jensen et al., 2003) fitted with a single grain attachment. A 10 mW Nd:YVO4 solid-state diode-pumped green laser was used for quartz single-grain stimulation, and the signal was detected through Hoya U-340 filters. A 130 mW infrared (IR) diode emitting at 870 ± 40 nm was used for IR depletion test.
Grains were rejected if the resulting OSL data failed to satisfy the criteria similar to those proposed by (Jacobs et al., 2006), namely if: the relative uncertainty on the first test dose signal TN was less than three sigma above its corresponding background or its relative standard error is >20%; the recycling ratio is >10%, and the recuperation is >10% of LN/TN; (C) the LN/TN value exceeded the laboratory-measured dose-response curves; OSL / IRSL depletion ratio differed from unity by more than two sigma and the TL110°C test. To check for the presence of feldspar, an IR depletion-ration test applied to the aliquot was incorporated at the end the SAR sequence (using an IRSL exposure for 100 s at 20°C). The incorporation of TL test to check whether the feldspar component is present involved checking TL whether the range of 150– 220°C is at the background level (e.g. Solongo et al., 2019; Hu and Li, 2019).
For each grain, the sensitivity-corrected regenerated OSL signals (Lx/Tx) were fitted with a single saturating-exponential function in the form:
where I is the luminescence intensity, Imax is the intensity at saturation,
Previously, Roberts and Duller (2004) reported for multi-grain Imax values of 36 to 52 and
Following this first analysis, we calculated the dose estimates obtained when the relative uncertainty on the natural test dose, and the error in
For the sedimentary samples L-EVA1202 and L-EVA1203, the quartz grains exhibit significant grain-to-grain variability in terms of OSL decay rate and inherent brightness. The sensitivity of sedimentary quartz grains has been suggested to be associated with different factors such as the source of origin of the mineral grains (Fitzsimmons, 2011), and their sedimentary/thermal history (Sawakuchi et al., 2011). Altogether 500 grains of L-EVA1202 were measured from which 149 grains (29.8%) were accepted. For representative ‘dim’ grains from the sedimentary quartz L-EVA1202 that passed the acceptance criteria, the natural OSL, test dose OSL and test dose OSL decay curves after IR stimulation are shown in
The dose-response curves of a selection of grains (L-EVA1202,
Similarly, we calculated the dose estimates for precision in
It is worth to note that 51 grains (with signals up to 30,000 cts in the first 0.035 s of stimulation of the natural signal) were registered; however, these bright grains failed the rejection criteria; specifically, the sensitivity-corrected natural signal does not result in a finite dose estimate. In addition, the bright grains failed the TL 110°C test and the IR depletion test, suggesting the presence of a feldspar component. The LM-OSL measurements from a bright grain (
The addition of the ‘fast ratio’ (Durcan and Duller, 2011) and thus removing the grains with low fast ratios as a criterion for selecting single grains for dose estimation was proposed recently (Duller, 2012); however, it might also lead to the removal of a high proportion of signals (Thomsen et al., 2016). The application of “fast ratio” by calculating the ratio between the fast and medium bleaching components for each grain from both datasets was tested here.
The results of the fitting showed up to three components with decay rates on average of 53 ± 1 s–1, 13.4 ± 0.7 s–1 and 1.2 ± 0.2 s –1. Our results are in agreement with the decay rates of 10.3 ± 3.4 s–1 for the fast and 2.1 ± 0.7 s–1 for the medium components reported by (Feathers and Pagonis, 2015), and of 11 ± 4.3 s–1 and 1.9 ± 1.0 s–1 obtained by (Duller, 2012). We assume that the additional component with the decay rate of 56.6 ± 2.4 s–1 obtained in our measurements may correspond to a very rapidly decaying ultrafast OSL (UF component) in quartz samples, reported earlier for multi-grain OSL by (Jain et al., 2008).
This ultrafast component was identified in samples from different areas around the world, and the optical cross-section of the responsible trap under blue light stimulation is about 14 times larger than that of the fast component (Jain et al., 2008). For dating, the authors suggested removing the UF component by IR bleaching above room temperature, removing by high preheat temperature (>200°C) or rejection of the very initial OSL signal. Interestingly, the preheat did not remove the hypothesised UF component; SAR protocol used in our studies employed preheat at 260°C and cut heat at 220°C, and these preheats are assumed to be high enough to remove the UF component (Jain et al., 2008). This implies that this UF is a) different to Jain et al.’s (2008) and b) that is reasonably thermally stable. Further tests are needed, e.g. high-temperature IR sine IR stimulation at room temperature did not impact the UF. The only evidence that this is a component other than the fast component is the fitting results and the proof of an additional component to the previously reported one.
The initial OSL signal integration interval is assumed to include the fast component preferentially, and rejection of the very initial OSL signal might remove the UF. In the following we examined the effect of variation for the integration intervals on
The results are displayed in
The archaeological site at the Khutag Uul Mountains (Mongolia) was investigated using single-grain quartz OSL on pottery and sedimentary samples. Detailed luminescence investigations revealed a presence of an ultrafast component – not detected previously in the single grain measurements – which might lead to erroneous ‘fast ratio’ estimates. As the fitting results of pottery quartz display, fitting was done using ultrafast, fast plus background. It is worth mentioning that the introduction of the third component, namely the medium or slow component did not improve the fit and was equal the background. For the sedimentary quartz grains, fitting was done using a combination of ultrafast and medium with a small fast contribution, whereas the other datasets were fitted using fast and medium components only. Therefore, it was impossible to obtain fast ratio values for the whole dataset.