The development of the single-aliquot regenerative-dose (SAR) protocol [1
] for optically stimulated luminescence (OSL) dating of quartz has revolutionized the luminescence dating method by giving rise to high precision equivalent dose estimates. Loess-paleosol sequences are important archives of the climatic changes that took place during the Pleistocene, but their significance can only be fully understood once a reliable and absolute chronology is available. Due to its quartz rich and windblown nature, loess is generally considered an ideal material for the application of OSL. However, although more precise ages can be obtained by SAR-OSL, the validation of the accuracy of these OSL ages by independent age control is hindered by the lack of methods which can directly date the depositional time of the sediments. In this context, the identification of the paleosol associated with marine isotope stage (MIS) 5 is known to yield valuable time control as the identification of this paleosol provides a minimum age threshold for the sediments underlying it, which should be no younger than ~130 ka. However, it is well known that the results of luminescence dating methods applied on quartz underestimate the expected ages for samples collected below this soil. For example, an age of 106 ± 16 ka (equivalent dose of 310 ± 9 Gy) was obtained for quartz grains of 4–11 μm from one sample taken immediately below the S1
paleosol (associated with MIS 5) at Mircea Vodă loess paleosol site, Romania, while an increasing degree of age underestimation with depth was observed for samples taken from below S1
paleosols at the same location [2
]. The same trend in age underestimates was reported in China. At Luochuan, Buylaert et al. [3
] obtained an age on coarse (63–90 µm) quartz of 81± 7 ka (De
= 229 ± 16 Gy) for the loess beneath the S1
paleosol while Lai [4
] reported lower ages than expected for samples older than 70 ka on 45–63 µm quartz. In the case of another site on the Chinese Loess Plateau (Zhongjiacai), Buylaert et al. [5
] obtained for a sample taken from the lower part of the last interglacial paleosol an age of 69.8 ± 3.8 ka (De
= 216 ± 6 Gy) on coarse (63–90 µm) quartz.
Another important issue which was raised relates to the choice of the quartz grain size. The use of coarse grains (so called inclusion dating) or fine grains (4–11 µm) has been proposed five decades ago for thermoluminescence dating of pottery, based on the different penetration powers of nuclear radiations in minerals by Fleming [6
] and Zimmerman [7
], respectively. However, in later geological applications in what regards OSL dating technique, the choice between these protocols was dictated by the dominant grain size within the investigated sedimentary unit. Consequently, it is common practice to use only one grain size fraction. A series of investigations carried out by our group during the last decade on quartz of different grain sizes extracted from loess yielded intriguing and concerning results. While ages obtained on fine (4–11 μm) and coarse (>63 μm) quartz samples were in good agreement up until ~40 ka, after this age the optical ages obtained on coarse (63–90 μm) quartz were reported to be systematically higher than those on fine (4–11 μm) quartz [8
In the light of these findings, we are applying here the single aliquot regeneration dating protocol on quartz of different grain sizes for revisiting the chronology of Mircea-Vodă loess paleosol sequence in Romania. This is the site where we have reported for the first time various problems when investigating different quartz grain sizes [8
] and we have subsequently applied alternative luminescence dating protocols on feldspars [14
]. Additionally, there is a limited practice of performing interlaboratory comparison exercises in the field of luminescence dating. This represents an interesting endeavor since this new investigation takes place a decade later and in different laboratories (Ghent, Belgium [2
] and Cluj-Napoca, Romania—current paper), using different samples from the same site. We are testing the robustness of the protocol by performing intrinsic rigor tests and we are discussing the accuracy of the obtained ages, also in the light of the results of the previous studies.
4. Ages and Discussion
Previous luminescence dating studies on Mircea Vodă site (Figure 8
a) revealed an age discrepancy between the two quartz fractions investigated that is still not yet understood. The ages obtained ranged from 8.7 ± 1.3 to 159 ± 24 ka for fine silt-sized (4–11 μm) quartz and from 16 ± 2 ka to 230 ± 31 ka for fine sand-sized (63–90 μm) quartz, the difference varying between 20% to 70% [2
]. Despite the fact that both datasets were consistent with the stratigraphic position of the samples, the fine (4–11 μm) quartz ages for the three samples taken from the L2
loess units were interpreted as underestimates. The post-IR IR225
signal was considered more reliable than the previously obtained quartz ages for the L2
units. These ages are presented alongside the quartz ages in Figure 8
Due to the importance of Mircea Vodă loess-paleosol master section, the current study aimed to obtain a more detailed chronological framework. In this regard 13 samples have been collected from the Holocene soil (S0
) and L1
loess unit and doublet samples have been collected directly beneath the S1
units, respectively. The luminescence ages were found to be mostly in agreement with their stratigraphically corresponding results reported by Timar-Gabor et al. [8
] and Vasiliniuc et al. [14
]. The overall uncertainties associated with the new OSL ages are dominated by the systematic uncertainties caused by the time-averaged water content, a-value and beta attenuation factors. The overall contribution from random sources of uncertainties scatters around 2.5% for the fine (4–11 μm) quartz (generally 10 aliquots per sample) and ranges from 4.2% to 11.8% for the coarse (63–90 μm) quartz. It has been proposed that the source for such a large spread could be attributed to reduced number of coarse quartz grains on the disk compared to fine aliquots, thus reducing the variability in the luminescence properties. Other studies have also reported similar spread in coarse quartz data [23
The top part of the profile that encompasses the Pleistocene/Holocene transition (L1
unit) is characterized by increasing ages with profile depth, ranging from 5.3 ± 0.5 ka to 22.8 ± 2.3 ka for fine (4–11 μm) quartz and from 5.3 ± 0.7 ka to 35.9 ± 3.2 ka for coarse (63–90 μm) quartz (Figure 8
b). In the Holocene soil, for ages up to about 11 ka OSL ages obtained for coarse and fine quartz agree, as expected and previously reported for such young samples [27
]. The older samples taken from the L1
transition and the L1
loess unit yielded ages that no longer agree within uncertainties. The fine (4–11 μm) ages continue to be younger than coarse (63–90 μm) ages. As previously reported by Timar-Gabor et al. [8
] on Mircea Vodă, Timar-Gabor et al. [9
] on Mostiștea, Constantin et al. [12
] on Costinești, Timar-Gabor et al. [10
] on Orlovat in Serbia as well as by Timar-Gabor et al. [11
] in China, the equivalent doses are higher for coarse (63–90 μm) quartz, which is unexpected considering the annual dose rate.
The age discrepancy between the two quartz fractions appears to begin sooner than previously reported. For Romanian, Serbian and Chinese loess samples, SAR-OSL ages divergence arose beyond ~40 ka (>~100 Gy) [11
]. Only for samples collected from Lunca section (southern Wallachian Plain, see Figure 1
) such a difference occurred starting with samples as young as <30 ka (~80 Gy) [39
For the samples starting with L2 downwards, the results obtained on doublet samples on each quartz fractions were found in agreement within uncertainties. Consequently, weighted ages have been calculated for each grain size fraction on the doublet samples.
The doublet samples collected from the L2
loess unit yielded weighted ages on the two quartz fractions of 111 ± 11 ka for fine (4–11 μm) quartz and 130 ± 11 ka for coarse (63–90 μm) quartz. Fine quartz ages underestimate the expected age, while the coarse quartz age is in broad agreement with the expected age. These results are consistent with those reported by Timar-Gabor et al. [8
] and Vasiliniuc et al. [14
The weighted average ages recovered from the L3 samples were that of 177 ± 18 ka for 4–11 μm quartz and 204 ± 18 ka for 63–90 μm quartz. The fine quartz age clearly underestimates the expected age, the offset being of 37%, while for the coarse quartz age the underestimation is of 19%. In the case of coarse quartz it was observed that the signal was close to laboratory saturation levels, with an average of 80%, while in the case of fine quartz, the natural signals were interpolated significantly below the saturation level of the laboratory dose response curve.
In what regards the samples from L4
loess units, the ages on fine (4–11 μm) quartz are 180 ± 15 ka and 198 ± 21 ka and the ages for coarse (63–90 μm) quartz are 230 ± 20 ka and 184 ± 19 ka, respectively. The two set of ages underestimate severely the expected values from stratigraphical boundaries considerations [50
]. The coarse(63–90 μm) quartz signals were found to be close to laboratory saturation for both L4
samples (85% and 77%, respectively), with the closeness to laboratory saturation being more pronounced when a larger test dose was used. On the other hand, the same statement cannot be made for fine (4–11 μm) quartz (e.g., for 2MVL5 sample the natural signals were at 60% of the laboratory saturation level).
The reasons behind the age discrepancies between fine (4–11 μm) quartz and coarse (63–90 μm), especially in the case of the Mircea Vodă site have been proposed, discussed and investigated in previous studies. Microdosimetry could be a reason, but one should bear in mind the fact that such an age discrepancy could only arise from a dose rate difference of approximately 1 Gy/ka [8
]. The purity of the quartz extracts was checked by comparing the natural and regenerated signals with the calibration quartz, by performing the IR depletion test [69
] and by checking the 110 °C TL peak recorder during preheats. The results dismiss the possibility of a feldspathic component which would end in different age results between the two fractions. This also concurs with previous time resolved OSL (TR-OSL) results reported by Timar-Gabor et al. [28
]. In what regards partial bleaching, residual doses of the order of at least tens or even hundreds of Gy would be needed in order to cause such an age offset for all samples investigated. This is not to be expected in the case of quartz.