Electronic Supplement to
OSL and AMS 14C Dating of the Penultimate Earthquake at the Leigu Trench along the Beichuan Fault, Longmenshan, in the Northeast Margin of the Tibetan Plateau
by Jin Feng Liu, Jie Chen, Jin Hui Yin, Yan Chou Lu, Andrew Murray, Li Chun Chen, Jessica Thompson, and Hui Li Yang
The supplementary data provided in this supplement contain a detailed description of the preparation and the luminescence signal characteristics of the OSL samples. In addition, the uncertainty of OSL dating has been analysed.
OSL Sample Preparation
Samples for OSL dating were processed and measured at the Luminescence Dating Laboratory of the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. The samples were processed under subdued red light (center wavelengths 660 nm) in a laboratory darkroom. Sediment that could have been exposed to light was removed from each end of the steel tube. Next, 30-50g was extracted to measure the field water content and saturation water content of the sample. This portion of the sample was then dried at low temperature (< 60°C), and ground in a mortar until <63 μm for measuring U, Th and K concentrations within the sample. The remainder of the sample was sieved to <300 μm, and treated with 30% H2O2 and 37% HCl to remove organic matter and carbonate, respectively. Since the sedimentary layers identified in the Leigu trench consists mainly of clay or sand-gravel alluvial deposits, most fractions are coarser than 300μm or finer than 90μm. Stokes’ method, based on grain size settling velocities, was used to separate the 4-11μm fraction of the sample. The sample was immersed in 30% fluorosilicic acid for 20-30 days to dissolve any feldspar minerals completely. The OSL IR depletion ratio method described by Duller (2003) and 110 °C thermoluminesce (TL) peak method (Aitken, 1998) were used to test the samples for quartz purity. Finally, the quartz grains were mounted on a 9.7 mm diameter steel discs using acetone.
OSL Measurement Instruments
OSL signal measurements were conducted on a Daybreak 1100 TL/OSL automatic measurement system. Stimulating light sources were blue light (470±5 nm wavelength, maximum luminous intensity 50 mW/cm2) for OSL and infrared ray (880 ± 60 nm wavelength, maximum luminous intensity 80 mW/cm2) for IR. We conducted measurements using 80% of the maximum power for both light sources. The OSL signal was detected by EMI QA9235 type photomultiplier tubes (PMT), with two U-340 filters in front of the PMT. We used the OSL signal derived from the first 1 s integral of the stimulation decay curve less the average value of the last 10 s integral (background) to calculate the equivalent dose (De).
Preheat Plateau, Recycling Ratio, and Recuperation Experiments
To choose an appropriate preheating temperature, we conducted a preheat plateau experiment on the samples 08XY06-1 and 08XY10-2 using the SAR (Murray and Wintle, 2000; 2003) and the SMAR (Sensitivity-corrected Multiple Aliquot Regenerative-dose) (Lu et al., 2007) protocol. The preheating temperature (PH1) of the natural and regenerated doses varied from 160°C to 300°C, with a preheat time of 10 s.
OSL results for the preheat plateau experiment and recuperation and recycling ratio tests are shown in Fig. S1.
Figure S1. Plots of De, recuperation and recycling ratios as a function of preheat temperature for sample 08XY06-1(a,b) and 08XY10-2(c,d).
Dose Recovery Experiments
Dose recovery experiments were also conducted on the sample 08XY06-1 using SAR protocol. The purpose of this is to test whether our SAR protocol is able to accurately measure a dose given in the laboratory before any heating of the sample. The natural luminescence signal was zeroed using a light source, and then each aliquot was given a known laboratory dose (11.6 Gy). The aliquots were then treated in the same manner as those used to determine the De. The ratio of measured to given dose should lie between 0.9-1.1 (Wintle and Murray, 2006). Experimental results are shown in Fig. S2. The dose recovery ratios within the preheating interval of 200-240°C are between 0.9-1.1. This indicates that measurement protocol is suitable and that the luminescence sensitivity correction is effective within this temperature range.
Figure S2. Plot of recovery ratio as a function of preheat temperature for samples 08XY06-1(a) and 08XY10-2(b).
Uncertainty of OSL Dating
The experiments outlined above give us confidence that the equivalent dose, De, has been estimated as accurately as possible, but ultimately the only reliable test for this accuracy is agreement with independent age control. This, and the question of the completeness of bleaching, is discussed in the main text. Our methods for determining the dose rate do not allow us to examine the possibility of disequilibrium in the uranium series, but this is unlikely to be of great significance, because the uranium series only contributes ~30% to the total dose rate. Alpha counting provides a moderately accurate estimate of the present day dose rate from the main contributors in the U-series (as well as the Th-series; Aitken, 1985), so only a very large degree of disequilibrium at deposition could have allowed the dose rate to have changed significantly through time, such that today’s estimate was significantly inaccurate. The dose rates are high, so that uncertainties in the burial depth (and so in the cosmic ray dose rate contribution of typically ~0.15 Gy/ka) do not lead to significant uncertainties in the dose rate. Finally the water content is moderately high (~0.6 of saturation). Typically a 1% increase in water content give rise to a 1% decrease in age. We have assumed a 4% uncertainty in the water content used in calculations, which at 2 standard deviations covers a range from ~0.4 to 0.9 of saturation.
Tables
| Deposit Unit | OSL Sample | Radio Carbon Sample | |
|---|---|---|---|
| C | 08XY05-2, 08XY06-1, 08XY06-2 | Hanging wall | |
| D | 08XY09 | Foot wall | |
| E | 08XY03, 08XY04 | Hanging wall | |
| F | 08XY01, 08XY02, 08XY07 | LG-SY1-3 | Foot wall |
| baked layer | 08XY10-2 | LG-SY1-5, 08XY10-2 | Foot wall |
| Step | Treatment | Observed |
|---|---|---|
| 1 | 6-8 aliquots for nature signal, bleach 5-8 aliquots with SOL2 | |
| 2 | Dn, giving dose, Di(i=1,2,3,4,0(zero),1,4) | |
| 3 | Preheat 220° for 10s | |
| 4 | Blue LED stimulate for 40 s at 125° | LN and Li |
| 5 | Give test dose, Dt | |
| 6 | Preheat 160° for 2s (cutheat) | |
| 7 | Blue LED stimulate for 40s at 125° | TN and Ti |
References
Aitken, M. J. (1998). An introduction to optical dating. Oxford: Oxford University Press
Duller, G. A. T. (2003). Distinguishing quartz and feldspar in single grain luminescence measurements, Radiation Measurements 37, 161-165.
Jacobs, Z., A. G. Wintle and G. A. T. Duller. (2006). Evaluation of SAR procedures for De determination using single aliquots of quartz from two archaeological sites in South Africa, Radiation Measurements 41, no. 5, 520-533.
Lu, Y. C., X. L. Wang and A. G. Wintle. (2007). A new OSL chronology for dust accumulation in the last 130,000 yr for the Chinese Loess Plateau, Quaternary Research 67, 152-160.
Murray, A. S. and A. G. Wintle. (2000). Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol, Radiation Measurements 32 , no. 1, 57-73.
Murray, A. S. and A. G. Wintle. (2003). The single aliquot regenerative dose protocol: potential for improvements in reliability, Radiation Measurements 37, 377-381.
Rees-Jones, J. (1995). Optical dating of young sediments using fine-grain quartz, Ancient TL 13, no. 2, 9-14.
Wintle, A. G. and A. S. Murray. (2006). A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols, Radiation Measurements 41, no, 4, 369-391.