MET-post-IR IRSL luminescence dating of cobbles buried in fluvial terraces in the Northern Chinese Tian Shan
Introduction
The responses of fluvial system to climate and/or regional tectonics bear great significance for understanding the landscape evolution and mass redistribution of the Earth's surface (e.g. Molnar et al., 1994; Starkel, 2003; Bridgland and Westaway, 2008). Aggradation, beveling and subsequent incision leads to the formation of river terraces, providing key records of past changes in climate and/or regional tectonics. Assigning accurate chronological frameworks to these terraces is a prerequisite for retrieving such information. The fine sands or silts, which are commonly used for conventional optically stimulated luminescence (OSL) dating, are rare in fluvial terraces in piedmont areas. However, cobbles are ubiquitous. Luminescence dating of buried cobbles has been applied to a variety of sedimentary environments such as glacial deposits (e.g. Rades et al., 2018; Jenkin et al., 2018; Chiverrell et al., 2020), fluvial deposits (e.g. Liu et al., 2018; Bailiff et al., 2021; Ishii et al., 2021), coastal deposits (e.g. Simms et al., 2011; Souza et al., 2019, 2021; Hong et al., 2020) and archaeological sites (e.g. Chapot et al., 2012; Sohbati et al., 2012, 2015; Freiesleben et al., 2015; al Khasawneh et al., 2018, al Khasawneh et al., 2019; Ageby et al., 2021; Gliganic et al., 2021).
Successfully luminescence dating the burial age of cobble is dependent upon how well the signal of cobble surface was bleached before burial. A plateau of age versus depth in luminescence-depth profiles can be used to diagnose the bleaching extents of signals before burial (Fig. 1). However, age-depth (A-D) plateau does not bear information about the effect of anomalous fading. Instead, buried ages obtained by using the fading-correction IRSL50 signal of cobbles were consistent with the independent age (e.g. Sohbati et al., 2015; Jenkin et al., 2018; Ishii et al., 2021; Chiverrell et al., 2020). The IRSL stimulation at elevated temperature following prior IRSL stimulation at low temperature is capable of isolating less fading or even non-fading signals (Thomsen et al., 2008; Buylaert et al., 2009, 2011; Li and Li, 2011, 2012; Thiel et al., 2011). With sequential IRSL stimulations at monotonically increased temperatures in a multi-elevated-temperature post-infrared infrared stimulated luminescence (MET-post-IR IRSL) dating protocol, the emergence of an age-temperature (A-T) plateau suggests the ages in the plateau region may be free of anomalous fading (e.g. Li and Li, 2011, 2012; Fu et al., 2012; Fu and Li, 2013). Moreover, the MET-post-IR IRSL measurements are also used in conjunction with the paired measurements of Jenkin et al. (2018) to pick out “bleached cobbles”. Since the bleaching rates of MET-post-IR IRSL signals decrease with their stimulation temperature (e.g. Li and Li, 2011; Reimann et al., 2015; Luo et al., 2018; Smedley et al., 2021), an A-T plateau is also suggestive of sufficient bleaching for the MET-post-IR IRSL signals of corresponding temperature region (Fig. 1C). Therefore, it is expected that the MET-post-IR IRSL measurement would provide diagnostics for both the resetting before burial and the fading during burial for IRSL signals used for dating.
Another methodological aspect of the cobble luminescence dating is the mineralogical heterogeneity across the cobble samples, especially for the internal potassium beta dose rate (here after termed internal dose) contribution from K-feldspar to the IRSL signals, which originate from the grains themselves (Mejdahl, 1979). Fang et al. (2018) discussed the uncertainty of beta dose rate due to heterogeneity and complex mineralogical structures. μ-XRF measurements can map the element concentration across the rock slices (e.g. Rades et al., 2018; Thomsen et al., 2018; Ishii et al., 2021), which may facilitate a rough but fast assessment of the uncertainty resulted from highly varying grain-sizes of K-feldspars.
In this study, we employed MET-post-IR IRSL single-aliquot regeneration (SAR) protocol to obtain the burial ages of cobbles from high terrace of Manas River in northern Chinese Tian Shan. In addition to the conventional A-D plateau in luminescence-depth profiles, the A-T plateau was employed to evaluate the resetting and fading of MET-post-IR IRSL signals used for equivalent dose (De) determination. In regards to dose rate determination, μ-XRF mapping was used to estimate the grain-sizes distribution of K-feldspar of the rock slices, and the internal dose rate were calculated accordingly. Finally, the ages determined by using the MET-post-IR IRSL signals were compared with the quartz OSL ages of conventional sand-sized sample in the same strata level to evaluate the reliability of cobble sub-surface luminescence dating.
Section snippets
Study area and sampling
The Manas River originates from Tian Shan and flows northward to the Junggar basin, with well-developed terrace sequences (Fig. S1). The deposits of terraces are characterized by cobbles with size between 3 cm and 30 cm, well sorted and rounded, clast-supported. Six light-color granite cobbles and two sand-sized samples from silty sand lens at the top of high terrace deposits were collected. The depositional age of this terrace is 14.5 ± 2.1 ka (mean of five sand-sized samples) by using
Coarse-grained quartz age
A large scatter was observed for coarse-grained quartz De values (Fig. S3), which may be a result of incomplete bleaching at the time of deposition (Arnold and Roberts, 2009; Galbraith and Roberts, 2012). The minimum age model (MAM, Galbraith et al., 1999) with the σb of 0.2 (Galbraith and Roberts 2012) was used for modeling the De of well-bleached grains. The MAM De values of the two samples were 37.1 ± 8.1 Gy (21–20) and 31.2 ± 4.9 Gy (21–22), and result in the depositional ages of 15.7 ± 3.6
Dual test for anomalous fading and bleaching using A-T plateau
The A-T plateau obtained by MET-post-IR IRSL measurement could serve as an indicator of both negligible fading and complete bleaching for the targeted luminescence signals. It would complement conventional age-depth (A-D) plateau in luminescence-depth profile to evaluate resetting of signals (Fig. 1B). For cobble R19-20, an A-T plateau from 170 °C to 225 °C shows that these two signals were free of anomalous fading during burial and completely bleached before burial, which is evidenced by the
Conclusion
The low temperature IRSL signal (IRSL50) has been widely employed for rock surface luminescence dating, although its potential anomalous fading may complicate its routine use. While the elevated temperature post-IR IRSL signals are more difficult to bleach, they are less likely to fade. The age-temperature (A-T) plateau generated by applying a MET post-IR IRSL protocol is proposed to serve as a dual diagnosis for evaluating both the bleaching extent and anomalous fading of the IRSL signals
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by the National Nonprofit Fundamental Research Grant of China, Institute of geology, China Earthquake Administration (grant project no.: IGCEA 1810), the State Key Laboratory of Earthquake Dynamics (project no.: LED 2016A04), and the National Natural Science Foundation of China (project no.: 41911530109 and 42171017). Critical and thorough reviews of the manuscript by an anonymous reviewer significantly improved the current version.
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