The usable doses measurement of equivalent dose impacts significantly on the ESR dating method accuracy and reliability. The optical bleaching characteristics of the ESR signals in sedimentary quartz is the key to acquire accurate equivalent dose. Pioneering studies demonstrated that the Al centre and multiple Ti centres can be bleached to a steady residual condition or to zero. Hence, both of the two centres are suitable for ESR dosimetry (Yokoyama
Recently, the
The factors affecting the
In this study, we collected one bedrock sample, one current fluvial sediment sample and one Holocene sediment sample to investigate the optical bleaching characteristics of quartz
The study area lies on the southeast margin of the Tibetan Plateau which resulted from the collision of the India and Eurasia continental plates. The sampling sites are located on the middle reaches of the Dadu River between 3500 ~ 5000 m above sea level (Yang
Fig. 1
Sketch map showing study area, sampling sites as well as geomorphological and geological setting of sampling sites. a) Study area located on eastern Tibet Plateau and its geomorphological map; b) geological setting and sampling site of Mian’ning River and Diao’tengzi River. Due to that the Mian’ning River is too small to be marked on the map, we represent both of the two sampling sites as one spot in Fig. 1b). The bedrock age data came from China Intrusive Rock Isotope Age Figure by K-Ar dating method.

List of sample information used in this study. All samples were collected from the southeast margin of the Tibet Plateau, which is located in the junction between the first and second ladder of China terrain. The quartz extracted from bedrock (MN05) was sampled 10 mm beneath its surface; DTZG03 was sampled at 0.7 m beneath the surface deposits; MN06 was collected from the modern floodplain. MN05 and MN06 were collected from the same river drainage with unitary lithology of granite.
No. | Sample name | Quartz origin | Sample type | Latitude (°N) | Longitude (°E) |
---|---|---|---|---|---|
1 | MN05 | granitic quartz | bedrock | 28°31’30” | 102°03’42” |
2 | MN06 | granitic quartz | current fluvial sediment | 28°31’30” | 102°03’42” |
3 | DTZG03 | granitic quartz | Holocene sediment | 29°16’01” | 102°18’26” |
The whole block bedrock sample (MN05) was firstly grinded with an iron rod and iron mortar, and then fractioned to100–200 μm by hand sieving in order to eliminate the grain-size effect and to retain the bedrock’s original information. The size fraction of 100–200 μm of the two sediment sample (MN06 and DTZG03) was separated by wet sieving. After that, three samples were treated with 30% H2O2 for one day for organic material removal then with 40% HCl for another day to remove carbonates. They were then washed and cleaned with distilled water. Magnetic minerals were removed by magnetic selection. Sodium polytungstate was used to separate the minerals of the density ranging between 2.57 and 2.73 g/cm3, then they were etched in 40% HF for 40 min to remove feldspars and the part of grains affected by alpha particles. Subsequently HCl was used again to remove the fluorides created by the HF etching. Finally, all samples were rinsed in distilled water to neutralize the acid and dried at 40°C to obtain pure quartz grains.
All pure quartz samples acquired from above methods started optical bleaching without addition of any artificial irradiation. Each sample was deposited in a clear plastic dish with 10 cm diameter. The quartz mineral particles were evenly dispersed on dishes as a monolayer. The dishes were placed into a 43 cm × 43 cm × 60 cm SOL 2/500S simulator made by Hönle, Germany. Its operating parameters are as follows: the intensity of light is 120,000 lux and 910 W/m2; light spectrum (UVA+VIS+IR) between 320 nm to 3.000 nm and its illumination intensity was 6.5 times higher than outdoor nature light (more detailed information on www.hoenle.de). All dishes were placed 30 cm below the light, and the temperature was kept lower than 30°C. The samples were exposed to the illuminator continuously up to 900 h. In order to make a more clearly and high-resolution bleaching pattern, each sample has been divided into 22 aliquots for the bleaching experiment. The details of the bleaching duration of each measured point are listed in
Each measured point and its bleaching duration.
Measured point No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|---|
Bleaching duration (h) | 0 | 0.5 | 1.5 | 2 | 4 | 8 | 16 | 32 | 84 |
Measured point No. | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
Bleaching duration (h) | 102 | 134 | 151 | 184 | 202 | 231 | 260 | 300 | 350 |
Measured point No. | 19 | 20 | 21 | 22 | |||||
Bleaching duration (h) | 400 | 500 | 700 | 900 |
After 900 h exposed to artificial sunlight, another 13 aliquots of each sample were prepared for irradiation experiments. The added gamma dose are 100, 200, 400, 800, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000, 7000 Gy, respectively, by gamma ray irradiation. The irradiation experiment was carried out in the 60Co laboratory of Peking University, with a dose rate of 42.25 Gy/min.
The ESR signal of the
Fig. 2
Typical E1’ centre ESR spectrum of quartz in MN05 sample.

The three samples were bleached up to 900 h, and the ESR intensities of the
Fig. 3
Bleaching curve of the E1’ centre of quartz extracted from three storing origins.

MN05 collected from bedrock, being exposed to light, exhibits a rapid growth within the first 120 h, then a rapid decrease (between 120 h and 300 h) and then a slower decrease after 300 h. However, MN06 collected from the same bedrock drainage does not show the same bleaching curve of MN05. In fact,
Fig. 4
The E1’ signal intensity change level (%) with different light exposure time. The increase level of the E1’ centre is the ratio between increase value of its natural intensity. All sample nature E1’ centre ESR signal intensity are of numerical normalized as 1 (100%). After 400 h, the E1’ centre of three samples show a relatively steady increasing level with a range of 235 ~ 270%, about 2.5 times of its nature.

In addition, the statistical data are normalized according to the original natural intensity to efficiently address the numerical increased level of the three samples with different light bleaching time, as shown in
sample (MN05) is also higher than that of relevant sediment sample (MN06), which received more sunlight exposure. This result is inconsistent with the fact that sediment sample should show a higher signal intensity with more sunlight exposure than that of bedrock sample. So far, we could not provide a reasonable explanation for the phenomena. This may be caused by variable sunlight optical sensitivity that needs to be certified by future research. Moreover, after 400 h, the numerical gap narrows with longer time exposition to sunlight, and the
Based on our results, the quantitative change of the quartz ESR intensity of the
On the other hand, the basis of paramagnetic centres for ESR dating are that they are well reduced, their values can be brought to zero or brought to a steady residual level before deposition, and after that the grains will receive natural radiation. In case of the Al centre, the signal intensity increases with dose from this “bleached residual” level, therefore, dating is possible when this starting level is estimated. This study has shown that there was also such a “starting” level of the
Fig. 5
The E1’ centre signal intensity changes with dose irregularly.

Toyoda and Hattori (2000) proposed that the heat-treated
The basic provenance tracing idea of the
where A and B is respectively the signal intensity of the two tributary before their junction and C is the one after junction. Based on this study observation, take a modern river sediment sample of the river, and examine the bleaching pattern. If the signal intensity increases with bleaching, the sediment should not have much sunlight exposure. While if they decrease, it indicates that they have quite well been bleached. Then, we can bleach the quartz by the artificial simulative sunlight to the “steady level”, which is a better indicator than “easily changed” indicator. This is because that compared to “easily changed” indicator, the “steady” indicator will keep the most original bedrock information with its resistance to weathering, denudation, erosion, transport and deposition processes. Thus, the “steady level” signal intensity of the
This study reports that the natural signal intensities of various quartz
future work needs to be done to perfect the potential use of the nature signal intensities of quartz
The response of the
Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Each measured point and its bleaching duration.
Measured point No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|---|
Bleaching duration (h) | 0 | 0.5 | 1.5 | 2 | 4 | 8 | 16 | 32 | 84 |
Measured point No. | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
Bleaching duration (h) | 102 | 134 | 151 | 184 | 202 | 231 | 260 | 300 | 350 |
Measured point No. | 19 | 20 | 21 | 22 | |||||
Bleaching duration (h) | 400 | 500 | 700 | 900 |
List of sample information used in this study. All samples were collected from the southeast margin of the Tibet Plateau, which is located in the junction between the first and second ladder of China terrain. The quartz extracted from bedrock (MN05) was sampled 10 mm beneath its surface; DTZG03 was sampled at 0.7 m beneath the surface deposits; MN06 was collected from the modern floodplain. MN05 and MN06 were collected from the same river drainage with unitary lithology of granite.
No. | Sample name | Quartz origin | Sample type | Latitude (°N) | Longitude (°E) |
---|---|---|---|---|---|
1 | MN05 | granitic quartz | bedrock | 28°31’30” | 102°03’42” |
2 | MN06 | granitic quartz | current fluvial sediment | 28°31’30” | 102°03’42” |
3 | DTZG03 | granitic quartz | Holocene sediment | 29°16’01” | 102°18’26” |