High resolution OSL and post-IR IRSL dating of the last interglacial–glacial cycle at the Sanbahuo loess site (northeastern China)
Introduction
The loess record of northern China is considered to be semi-continuous and is one of the most widely studied terrestrial sediments in world (Liu, 1985, An, 2000, Guo et al., 2002, Lu et al., 2010). These loess-palaeosol sequences contain detailed archives of palaeoenvironmental changes and are highly sensitive to climatic changes, especially to shifts in the Asian summer and winter monsoon and/or Northern Hemisphere westerly circulation (Liu and Ding, 1998). Chronological control of loess sequences provides insights on the timing of past glaciation and dust transportation conditions (e.g. Lai, 2010, Stevens et al., 2008). However, the majority of studies have been carried out on the thickest and most complete loess deposits in the Central Loess Plateau and the northeastern Tibetan Plateau (Liu and Ding, 1998, Guo et al., 2002, Stevens et al., 2008, Lu et al., 2010). There are also extensive loess deposits in the downwind areas of the Horqin and Otindag dunefields in northeastern China; these deposits form a valuable archive to study the history of aridity and palaeoclimatic change in this region (Fig. 1). Until recently, only a limited number of studies have been undertaken concerning the loess stratigraphy and chronology (Zeng et al., 2011, Yi et al., 2012). A detailed numerical chronological framework for this region would contribute to a better correlation of individual sections and provide a timeframe for the regional climatic reconstruction in northeastern China.
Optically stimulated luminescence (OSL) dating, using the single aliquot regeneration (SAR) protocol (Wintle and Murray, 2006) and its derivatives is one of the most robust dating methods for establishing absolute chronologies for loess deposits worldwide (Roberts, 2008). Over the past decade, a considerable amount of research has been undertaken on luminescence dating of Chinese loess–paleosol sequences, mainly based on quartz OSL (Stevens et al., 2006, Stevens et al., 2008, Buylaert et al., 2007, Buylaert et al., 2008, Roberts, 2008, Lai, 2010, Li and Li, 2011). Unfortunately, the quartz OSL signal usually saturates at relatively low doses (∼200 Gy) - for loess deposits with a typical dose rate 3–4 Gy/ka, this restricts the use of quartz OSL to the last 50–70 ka (Buylaert et al., 2007, Roberts, 2008). The infrared stimulated luminescence (IRSL) signal from K-feldspar saturates at much higher doses than does quartz OSL, and offers an alternative approach. However, feldspar IRSL is known to often suffer from athermal (anomalous) fading (Wintle, 1973, Spooner, 1994, Huntley and Lamothe, 2001); this leads to an underestimation of the equivalent dose (De) and so the luminescence age of the sample. Recently, it has been suggested that IRSL signals measured at elevated temperatures after an infrared (IR) stimulation are more stable than the standard IRSL signal (Thomsen et al., 2008, Jain and Ankjærgaard, 2011); this has led to the development of so-called post-IR IRSL protocols (Buylaert et al., 2009, Buylaert et al., 2012, Thiel et al., 2011). Since then, several single-aliquot-based pIRIR dating protocols for feldspar have been developed and have been shown to give accurate ages both for very young (<10 ka, Reimann and Tsukamoto, 2012, Fu and Li, 2013) and old (>100 ka, Buylaert et al., 2012, Li and Li, 2011, Li and Li, 2012a, Li and Li, 2012b) samples. Post-IR IRSL dating thus has the potential to extend the loess chronostratigraphy beyond the limits of quartz OSL in northeastern China.
In this work, both standard quartz SAR OSL and K-feldspar post-IR infrared (IR) stimulated luminescence (post-IR IRSL; pIRIR290) methods are employed to date the Sanbahuo (SBH) loess site in northeastern China. First, the quartz SAR OSL characteristics are documented. Then we investigate the luminescence characteristics of the elevated temperature IRSL signals in a SAR post-IR IRSL protocol. The quartz OSL and pIRIR290 ages are compared with each other and with the loess-palaeosol stratigraphy. Finally, the luminescence ages are used to derive sedimentation rates for the upper part of the SBH loess section.
Section snippets
Site description and sampling
The SBH section (42°18′16″N, 118°41′10″E, 677 m.a.s.l) is situated in the vicinity of the city of Chifeng, in southeastern Inner Mongolia (Fig. 1). The whole section is a 60 m thick series of loess intercalated by palaeosols, and is made up of a natural exposure of the upper 48 m (Fig. S1) and a 12 m deep exploratory well. The Matuyama–Brunhes palaeomagnetic boundary and the Jaramillo sub-chron have been identified in the section. Extrapolation based on sedimentation rate between B/M and M/J
Sample preparation and analytical facilities
Under subdued red light conditions, the outer material of each tube was scraped away and used for dose rate and water content measurement (see Section 3.2). The non-light exposed material from the middle part of the tube was pretreated in a routine manner, i.e. with hydrochloric acid (10%) and hydrogen peroxide (30%) to remove carbonates and organic matter, respectively. Pure coarse-grained (63–90 μm) quartz (no significant IRSL signals) was obtained after a 40 min hydrofluoric (40%) acid etch
Quartz OSL characteristics
The quartz De values were determined using the single aliquot regeneration (SAR) protocol (Murray and Wintle, 2003) (Table S1a). Typical dose response curves and OSL decay curves (inset) are shown for the upper (138128) and lower (138175) samples in Fig. 3a, b, respectively. The blue-light stimulated OSL signals decrease very quickly during the first second of stimulation, indicating that the signal is dominated by the fast component (Singarayer and Bailey, 2003).
Appropriate preheat conditions
Quartz and feldspar age comparison
Fig. 6 shows the quartz OSL ages plotted as a function of the feldspar pIRIR290 ages. The quartz and feldspar ages are in good agreement back to ∼44 ka, beyond this the quartz OSL ages systematically underestimate the pIRIR290 ages. The S1 soil provides pedostratigraphic age control at this section; it formed during MIS 5 (130–75 ka). The quartz OSL ages of the samples taken in the unambiguous part of the S1 palaeosol (9.20–9.70 m) range from 62 ± 4 ka at the top of S1 to >74 ka at the bottom
Discussion
Our observations are consistent with other data from the central and northwestern Loess Plateau (Sun et al., 2010, Sun et al., 2012) which indicate higher sedimentation rates during cold periods, and lower sedimentation rates during warm periods, both in response to changes in the strength of the Asian monsoons. It is widely suggested that changes in surface processes and paleoclimate in the dunefield/deserts and other dust source regions are the main influences on loess accumulation rates in
Conclusions
Quartz OSL and feldspar pIRIR290 ages are in good agreement back to ∼44 ka; in older samples quartz OSL increasingly underestimates. In contrast, feldspar pIRIR290 ages are in satisfactory agreement with the expected age of the last interglacial soil (S1).
Following the last interglacial, there appears to have been a period at SBH of very rapid loess deposition (>0.4 m/ka) during MIS 4, followed by a lower sedimentation rate of 0.10 ± 0.01 m/ka during MIS 3 (c.f. Sun et al., 2010, Sun et al.,
Acknowledgements
This research is supported by the National Natural Science Foundation of China (41371203, 41472138). We are indebted to Xu Zhiwei, Zhang Wenchao, Qiu Zhimin and Zhuo Haixin for assistance during field work and to Yang Chuanbin for support in the laboratory. Jan-Pieter Buylaert thanks the Danish Council for Independent Research | Natural Sciences (FNU) for financial support (Steno grant no. 11–104566).
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