Quaternary glaciations and glacial landform evolution in the Tailan River valley, Tianshan Range, China
Introduction
Glaciers are highly sensitive to climate change and have been responsible for shaping some of the most spectacular landscapes throughout the world (Bennett and Glasser, 2009, Benn and Evans, 2010, Ehlers et al., 2011). These spectacular landforms are direct imprints of past glacial fluctuations and represent important archives of past climatic and environmental information. Studies of glacial landforms can define the timing, extent, properties, and types of ancient glaciers, and can improve our understanding of temporal and spatial shifts of past glaciers (Shi et al., 2006, Shi et al., 2011, Ehlers et al., 2011), which can help us understand paleoenvironments. Glaciers are often highly erosive (Hallet et al., 1996, Brocklehurst and Whipple, 2006, Ehlers et al., 2006), and the denudation caused by glaciers has the potential to influence climate by increasing chemical weathering and, ultimately, lowering atmospheric concentrations of CO2 (Raymo and Ruddiman, 1992). In addition, glaciers enhance rates of valley incision during glaciation and the resulting isostatic rebound can cause high mountain crests to rise, which in turn has an influence on climate (Molnar and England, 1990). Therefore, in tectonically active regions, studies of past glaciation provide important information for understanding the history of uplift.
Dating glacial landforms is a fundamental component of studying landform evolution and also of paleoenvironmental reconstruction. In the past several decades, electron spin resonance (ESR) dating, which utilizes radiation-defects in quartz to determine the ages of sediments, has been increasingly applied to studies of Quaternary geology (Hennig and Grün, 1983, Grün, 1989, Ikeya, 1993, Rink, 1997). Dating of unconsolidated sediment using ESR was first proposed by Yokoyama et al. (1985). Tanaka et al. (1985) used germanium (Ge) centers to date sun-bleached sediment for the first time, and obtained results that were consistent with the geological setting. Grün (1991) proposed that Ge centers should be used in future ESR dating studies, and Schwarcz (1994) suggested that glacial till could be dated using ESR. Since then, the potential value of ESR for dating Quaternary glacial deposits has been explored by several scholars (e.g., Yi et al., 2002a, Yi et al., 2002b, Zhou et al., 2002, Zhou et al., 2006, Zhao et al., 2006, Zhao et al., 2010, Xu and Zhou, 2009, Wang et al., 2011) and comparisons have been made between ESR and other independent dating methods, including optically stimulated luminescence (OSL) and cosmogenic radionuclides (CRN) (Xu et al., 2010, Zhou et al., 2010, Zhao et al., 2012). Additional studies have demonstrated the value of ESR in augmenting conventional mapping and relative dating studies in advancing research on Quaternary glaciation in China (Shi et al., 2006, Shi et al., 2011).
The Tianshan range is a tectonically active mountain system in Central Asia that formed as a result of the collision of the Indian and Eurasian continental plates. The Tumur Peak area of the Tianshan range includes more than forty peaks above 6000 m asl, with Tumur Peak being the highest (7435 m asl) and the largest center of modern glaciation in the range. During Quaternary glacial–interglacial cycles this area was extensively and repeatedly glaciated, and moraines and associated fluvioglacial landforms from multiple glaciations are present in the valleys, basins and the piedmonts of the range. Studies of these landforms can provide insight into temporal and spatial variations of glaciers and contribute to paleoenvironmental reconstructions and understanding the tectonic uplift history of the Tumur Peak area, and the Tianshan range.
Quaternary glacial landforms, including moraines in the Tailan River valley on the southern slope of Tumur Peak have been studied extensively since the 1940s (Huang, 1944, Feidaoluoweiqi and Yan, 1959, Feidaoluoweiqi and Yan, 1960, Liu et al., 1962, Shi et al., 1984, Su et al., 1985b). Unfortunately, no geochronological framework for these landforms has been established yet, and this restricts our understanding of the change of past glaciers in this drainage and also of regional paleoenvironmental variations. Thus over the past decade we have used remote-sensing techniques, field mapping, and ESR dating technique to constrain the timing, style and nature of Quaternary glaciations in the Tailan River valley.
Section snippets
The Tailan River valley
The Tailan River originates on the southern slope of Tumur Peak and flows southwards into the Tarim Basin (Fig. 1). The drainage area above the Tailan Hydrological Observation Station is about 1324 km2 and the length of the river is about 80 km. The river has an annual discharge of about 7.5 × 108 m3, with more than 60% being glacier meltwater. This runoff provides a vital water resource for the Zhamutai oases in the eastern part of Wensu County, Aksu District.
The heights of the mountain ridges
Quaternary glacial landforms
Glaciers in the Tailan River valley have been experiencing extensive melting over the past several decades and most of the glaciers are retreating and have clear glacial karst features on their surfaces. Glacial landforms deposited during the recent retreat are poorly preserved at the terminus of the Qiongtelian Glacier. However, five older moraine complexes occur in locations ranging from close to the terminus of the glacier to the piedmont beyond the end of the main valley (Fig. 1).
The first
Methods and results
To constrain the chronology of glaciation in the Tailan River valley, samples for ESR dating were collected from natural or human-made sections from the second, third and fourth sets of moraines (Fig. 1). The samples were kept in opaque plastic bags to ensure that they were not exposed directly to sunlight. Grinding, collision and heating were also avoided during transportation. The samples were pretreated in the OSL chronology laboratory at the Cold and Arid Regions Environmental and
Resetting of the Ge centers in glacial quartz grains
Previous studies have confirmed that Ge centers in quartz grains are sensitive to sunlight, UV-light and grinding (Tanaka et al., 1985, Buhay et al., 1988, Jin et al., 1991, Ye et al., 1993, Ye et al., 1998, Walther and Zilles, 1994, Rink, 1997), and these mechanisms can reset Ge centers effectively. Given the structure and movement of alpine glaciers (Shi et al., 1989) and processes of lateral moraine formation by ice-marginal dumping of debris (Small, 1983), it is reasonable to assume that
Conclusions
Geomorphological mapping, absolute age dating using ESR, and field investigations of the sedimentology and lithology of glacial and fluvioglacial deposits in the Tailan River valley provide new insight into the record of glaciation in the Tumur Peak area, and also contribute to a better understanding of patterns of paleoglaciation along the Tianshan range in central Asia. The end moraines that are present at the terminuses of the modern glaciers in the Tailin River valley were deposited during
Acknowledgements
We thank Deng Xiaofeng, Liu Jingfeng, Han Haidong and Zhao Sandong for their fieldwork assistance. Fan Yuxin for helping prepare the samples in the OSL chronology laboratory at CAREERI, CAS, Lanzhou. Diao Shaobo for helping date the samples in the Open Laboratory of Marine Sedimentology, Qingdao. We thank Feng Zhaodong (guest editor) and two anonymous reviewers for their constructive suggestions and comments. This work was supported by the Program of the Ministry of Science and Technology of
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