AMS radiocarbon dating of late Quaternary glacial landforms, source of the Urumqi River, Tien Shan—a pilot study of 14C dating on inorganic carbon
Introduction
Dating till is important for reconstructing the glacial geological history of a region. However, glacial till is lacking in organic matter for direct 14C dating, except for paleosols developed on till at rare sites. Many researchers have tried to use 14C for dating pedogenic carbonate (e.g. Williams and Poliach, 1969; Pazdur and Pazdur, 1986; Quade et al., 1989; Wang et al., 1994). As the carbonates are not closed systems and may exchange 14C with modern or dead 14C after their formation, many studies failed. However, some data proved to be useful for sediment ages or explanation of paleoenvironments (Swett, 1974; Chen and Polach, 1986; Courty et al., 1994; Geyh and Eitel, 1998; Pustovoytov, 1998). Subglacial precipitation of carbonate coatings was found in carbonate rock areas (Bauer, 1961; Ford et al., 1970; Souchez and Lorrain, 1975; Hallet (1976a), Hallet (1976b); Fairchild and Spiro, 1990; Sharp et al., 1990). Its presence implies that subglacial calcium carbonate coatings could be used for 14C dating. However, dead carbon from carbonate rock will make 14C dates much older than the actual age of moraine and would make the dating of inorganic carbon in glacial moraine impossible. As well, the concentration of carbonate cement and precipitates in till is low in China. When HCl solution is added to till samples, there is no apparent reaction of carbonate with the acid, suggesting that carbonate content is less than 1%. It is therefore difficult to determine the age by mass spectrometry 14C dating of carbonate because of the low carbonate concentrations. Hence, many studies on Quaternary glacial geology in China lacked absolute dating data (e.g. Yi, 1989; Cui et al., 1993). The experimental studies of this paper deal with the radiocarbon dating of subglacial precipitates in a non-carbonate rock area, which though less spectacular are probably more widespread than those on carbonate bedrock, in the source area of the Urumqi River, Tien Shan.
Section snippets
Geological setting at sampling sites
The source area of the Urumqi River is on the northern slope of the main range of the Karauchen Mountains in the central Tien Shan (43°07′N, 86°49′E, Fig. 1). The elevation of the ridge in the region is mainly between 4100 and 4300 m asl, with the highest peak, Tianger Peak, having an attitude of 4486 m asl. The elevation of the modern snowline is about 4000–4100 m asl. Modern glaciers, including cirque glaciers, small-valley glaciers and hanging glaciers, are present at elevations of 3765–3700 m
Sampling
Samples W1 and W2 (W=Wangfeng) were collected from an end moraine in the Upper Wangfeng Moraine Group near the Wangfeng road maintenance station. The end moraine comprises two distinct tills, and is covered by a loess cap (Fig. 2).
The lower till (Fig. 2, Fig. 3) is more consolidated and is finer grained, with a high silt content (Cui, 1981; Wang and Zhang, 1981). The fabrics of clasts (Feng and Qin, 1984) and of fine particles (Yi and Zhijiu, 2001) within these tills are strongly oriented, as a
Morphology and mineral constituents of the coatings
Yi (1992) studied the occurrence and mineral constituents of the coatings on the roche moutonnees in the front of Glacier 1. The coatings, where Samples R1 and R2 were collected, are yellowish white and distributed on the polished top surface of the roche moutonnee. They are about 150–200 μm thick, which is observed in polarizing microscope in thin section. The maximum thickness reaches 200–300 μm. The main mineral component of the precipitate on the roche moutonnee (sample R) is calcite with
Discussion
Some 14C dates for dating pedogenic carbonate proved to be useful indications of sediment ages or the explanation of the paleo-environment in arid or cold regions (e.g. Swett, 1974; Chen and Polach, 1986; Courty et al., 1994; Geyh and Eitel, 1998; Pustovoytov, 1998). The study area of this paper is also in the sub-arid and cold region with average annual temperature and annual precipitation of −5.2°C and 439.3 mm respectively (data from 1958 to 1998 at the Glacier Observatory Station). Secondary
Conclusions
Results of stable isotope carbon and radiocarbon dating, and sedimentary properties of modern till and modern coating on the roche moutonnees, suggest that the source of inorganic carbon in them is synchronous with the moraine formation. The carbon in the fine matrix of till in this non-carbonate area can be used to date the moraine age, as the inorganic carbon in till may be preserved with little or no fractionation. The paper reveals that the age of the Last Glacial Maximum is about 19–23 ka
Acknowledgments
This work was supported by the 100 talents project C2002-43 of the Chinese Academy of Sciences, NSFC Grant 4997108, the LICCRE Grant BX2001-02, and Tianshan Glacier Observatory. Lewis A. Owen helped improve the English. We also acknowledge two anonymous reviewers’ constructive comments.
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