Sr and Nd isotope ratios and REE abundances of moraines in the mountain areas surrounding the Taklimakan Desert, NW China.

Abstract

The first systematic data of Sr and Nd isotopic ratios, REE abundances, major element and mineral compositions are reported for the Chinese moraines from the western Kunlun Mts. and southern and northern flanks of the Tianshan Mts. and soils from the Tibet Plateau. This study was conducted in order to characterize the isotopic and geochemical features of these moraines and soils, which are considered as possible sources of the Taklimakan Desert sands. The moraines and the desert sands consist of quartz and feldspar as dominant minerals, but the former have higher clay and mica and lower carbonate mineral abundances than the latter. In spite of the wide variation, average values of most major elements of the moraines are generally similar to those of the Taklimakan Desert sands, except for CaO depletion and K 2 O enrichment in the moraines. Sr and Nd isotopic ratios are also highly variable for moraines, particularly from the Tianshan northern flank, while those for the desert sands show restricted values. In the Sr-Nd isotopic diagram, the Taklimakan Desert sands are plotted in the narrow field overlapped by both the moraine matrices from the western Kunlun Mts. and from the Tianshan southern flank. In addition, it is noted that average value of Nd isotopic ratios for the moraines is clearly higher than those for the desert sands. Two Tibetan soils of this study show wide variation in mineral composition and major element compositions (SiO 2 = 75% and CaO =1.5% for one sample and SiO 2 = 44% and CaO =31.8% for the other). However, Sr isotopic ratios of both soils are similar to those of the desert sands and their Nd isotopic data are slightly smaller than the desert sands. These results suggest moraine matrices from both the western Kunlun Mts. and the Tianshan southern flank and the Tibetan soils as possible sources for the Taklimakan Desert sands.

The environmental implications for dust in high-alpine snow and ice cores in Asian mountains

“…If this sorting did exist and was responsible for the compositional discrepancy between snow dust (cryoconite) and moraine, the REE abundance of moraine should be less than the cryoconite, since REE abundance increases in the fine fraction of moraine (Chang et al, 2000). However, the snow dust and cryoconite samples display a lower REE abundance than the moraine both in Qiyi and NCG3 (Fig.…”

“…Additionally, a total of 25 samples collected from February to December 2008 were used to analyze snow dust composition. The composition of moraine and cryoconite (silt pellet) samples of UG1 were obtained from previous studies (Chang et al, 2000;Li et al, 2011).…”

“…REE abundance and consequently element ratios are dependent of grain size (Chang et al, 2000). Snow dust particles are finer that those of cryoconite, while cryoconite particles are finer than moraine and surface soil, resulting in some uncertainties in compositional comparison and tracing.…”

“…Therefore, the REE distribution pattern itself seems not effective enough for UG1 dust tracing. The composition of local moraine has a wide range (Chang et al, 2000), but can be distinguished from snow dust and cryoconite by the lower L/H REE, lower δEu, and lower La N /Sm N ratios (Fig. 2).…”

“…3. The composition of new Muztagata aerosol and ice core dust, and their comparison with surface soil (this study) and moraine (Chang et al, 2000). Fig.…”