Sr-Nd isotope evidence for modern aeolian dust sources in mountain glaciers of western China

Abstract

In order to apportion the dust sources of mountain glaciers in western China, the Sr-Nd isotopic compositions of insoluble particles were determined in snow samples collected from 13 sites. The combined plot of ⁸⁷Sr/⁸⁶Sr and ε_Nd(0) demonstrates a distinctive geographic pattern over western China, which can be classified into three regions from north to south. Samples from the Altai mountains show the lowest ⁸⁷Sr/⁸⁶Sr ratio and the highest ε_Nd(0) value, similar to the data of deserts in the north of China such as the Gurbantunggut desert. Samples from the southern Tibetan Plateau (TP) and Himalaya show the highest ⁸⁷Sr/⁸⁶Sr and lowest ε_Nd(0) values, resembling the local and regional dust sources found in the southern TP and Himalaya-India region. Samples from the Tien Shan and northern Tibetan Plateau exhibit intermediate ⁸⁷Sr/⁸⁶Sr and ε_Nd(0) values, similar to the data reported for the northern margin of the TP (NM_TP). However, three sampling sites, JMYZ (Jiemayangzong) located in the Himalaya and ZD (Zadang) and YL (Yulong) located in the southeast TP, presented distinctive Sr-Nd isotopic signatures typical of the NM_TP, suggesting potential long-range and high-altitude dust transport across the TP.

1. Introduction

Glaciers in the mountains of western China are unique recording media of paleoclimate and paleoenvironment in mid-latitudes (e.g. Thompson and others, 2000). Based on particle concentrations in ice cores, researchers can back- calculate the historical aerosol loadings and composition, which reflect environmental change, such as historical dust- storm frequencies (Xu and others, 2007), extreme drought events (Thompson and others, 2000) and anthropogenic impacts (Hong and others, 2009; Kaspari and others, 2011) in the source area. Yet the connection between dust sources and sinks remains uncertain. For example, there is little knowledge of how dust gets lost and is altered during and after transportation. In the past decade, isotopic and geochemical methods have been developed to address these uncertainties. The strontium-neodymium (Sr-Nd) isotopic composition was found to be particularly important as different source materials have unique profiles reflecting their origins and ages, and these profiles undergo little change during weathering, transportation and deposition (Biscaye and others, 1997;Grousset and Biscaye, 2005; Bory and others, 2010).

Arid and semi-arid areas in Central Asia including the Taklimakan, Gobi and Thar deserts make up the second largest dust source on Earth. There has been increasing focus on the dust transportation and deposition from these areas due to the large amount of emitted dust and its significant climatic and environmental impacts (Duce and others, 1980; Huang and others, 2010). For instance, there is growing concern regarding dust emission due to its effect on glacier melting and alteration of biogeochemical cycles (Fujita, 2007); however, available datasets in these areas remain limited to the identification and distinguishing of dust sources. The Sr-Nd isotopic method has only been applied at a few sites in the mountain glaciers of these areas to trace the dust sources (Xu and others, 2009; G. Wu and others, 2010). The goal of this study is to characterize the dust sources and investigate the transport of dust in these areas utilizing a more comprehensive Sr-Nd isotopic databset.

2. Sampling Sites and Methods

Between September 2008 and November 2010, 15 snow pits were collected from 13 mountain glaciers in western China (Fig. 1). Table 1 presents the details of sampling location, collection time and snow-pit depth. At each sampling site, snow samples were collected from the snow pit at a vertical resolution of 5-20cm, following the clean- hands-and-dirty-hands protocol with sampling personnel wearing integral Tyvek^® bodysuits, non-powdered gloves and masks to avoid possible contamination (Fitzgerald, 1999). All sampling sites except Yala were chosen from the accumulation area of glaciers, and most snow pits have accumulated for more than 1 year (Zhang and others, 2012). The Yala samples were collected from the terminus of a glacier due to its harsh condition. Insoluble particles in the snow samples for each research site were obtained by centrifugation (15 000rpm) and heating evaporation. After each centrifugation cycle (30min), the supernatant was discarded using a syringe, and the remaining water (~10 mL) was evaporated in a class 100 laminar flow box.

Fig. 1. Location map of snow samples collected from western China between 2008 and 2010. The distribution of dust sources and general weather patterns in winter (blue) and summer (red) circulation systems are also shown.

Table 1. Summary of the sampling location, time and snow-pit depth at each site in western China

Extracted particles were dissolved in an HF-HClO₄ mixture, and kept at 100-120°C for 7 days. Taking into account the low quantity of the samples, the Sr-Nd separation and purification were carried out following the low-background method. Briefly, rubidium (Rb)-Sr and light rare-earth elements were isolated on quartz columns by conventional ion-exchange chromatography with a 5 mL resin bed of AG 50 W-X12 (200-400 mesh) after sample decomposition. Nd and samarium (Sm) were separated from the other rare-earth elements on quartz columns using 1.7 mL of Teflon powder coated with di (2-ethylhexyl) orthophosphoric acid (HDEHP) as the cation exchange medium. Sr-Nd isotopic data were acquired using a Finnigan MAT-262 mass spectrometer (Thermo Scientific, Waltham, MA, USA). The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios were corrected for mass fractionation relative to ⁸⁶Sr/ ⁸⁸Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively (all errors were kept at 2σ) (Table 2). Meanwhile, five independent runs of standard Ames metal solutions and NBS987 yielded mean values of 0.51244 ± 0.000018 for the ¹⁴³Nd/ ¹⁴⁴Nd ratio and 0.710246 ± 0.000021 for the ⁸⁷Sr/⁸⁶Sr ratio. The maximum values of blank samples were <0.1 ng for both Sr and Nd. For convenience, the ¹⁴³Nd/¹⁴⁴Nd ratios were normalized and denoted as ε_Nd(0) = [(¹⁴³Nd/¹⁴⁴Nd)/ 0.512638-1] × 10⁴.

Table 2. Sr–Nd isotopic composition of snow samples from mountain glaciers of western China. NA: no data available

3. Results and Discussion

3.1. Sr-Nd isotopic composition

The Sr and Nd isotopic compositions show large and structured variations ranging from 0.713 to 0.753 in ⁸⁷Sr/ ⁸⁶Sr, and -4.8 to-16.3 in ε_Nd(0) (Table 2). Overall, the Sr-Nd isotopic compositions reveal a strong geographic trend in western China, with high-ε_Nd(0)-low-⁸⁷Sr/⁸⁶Sr signature in the north and low-ε_Nd(0)-high-⁸⁷Sr/⁸⁶Sr signature in the south (Fig. 2). The MSD sample collected from the Altai mountains shows the highest ε_Nd(0) and lowest ⁸⁷Sr/⁸⁶Sr values of all the samples, similar to values from the north of China and south of Mongolia (Chen and others, 2007). The Sr-Nd isotopic composition at the TS (Tien Shan) site is similar to that of the deserts of the northern margin of the Tibetan Plateau (NM_TP;e.g. Taklimakan and Qaidam deserts) (Chen and others, 2007). Two samples, however, show relatively high ε_Nd(0) values, suggesting possible mixing of dust sources with that from the north of China (e.g. Gurbantunggut and Gobi deserts), likely transported along with the winter monsoons (Sun and others, 2001). The Sr-Nd isotopic compositions in samples from the northern TP, including MS, LHG, DKMD, ZSGR and GL (Table 1), are highly homogeneous, and all fall within the range of NM_TP deserts (Chen and others, 2007) (Fig. 2), suggesting a similar dust source at these sites. Compared to the available dataset for TP, the e_Nd(0) values in ZSGR (-9.97 to -9.6) are significantly different from the TP local dust source (central Qiangtang Terrane) (-20.72 to -14.60) (Zhang and others, 2007; W. Wu and others, 2010), indicating a possible long- range transport of dust from the northern deserts.

Fig. 2. The Nd vs Sr isotopic composition of insoluble particles in snow samples collected from 13 glaciers over western China. Data obtained from potential source areas in the northern boundary of China (NB_China) (Chen and others, 2007), the northern margin of TP (NM_TP) (Chen and others, 2007), the Himalaya (Najman, 2006) and the northwest Indian arid areas (NW_India) (Tripathi and others, 2004) are also shown.

Most Sr-Nd isotopic compositions for samples in the southern TP and the Himalaya (JMYZ, QM, Yala and DML) are close to the values of the Himalaya-India regional dataset (Fig. 2) (Tripathi and others, 2004;Najman, 2006). Note that due to very low insoluble particle concentration in the QM sample, we used the isotopic data from previous studies (Xu and others, 2009). The dust source in QM has been suggested to be a mixture of dust with a minor fraction from the northwest Indian arid region and a major fraction from the local source (from glacial debris and exposed rock walls) (Xu and others, 2009). The Sr-Nd isotopic compositions in Yala, DML and two JMYZ samples also resemble those of QM, indicating the local source in these sites. This phenomenon can probably be explained by the fact that glacier debris can enter the glacier easily, owing to the high ablation rate and high physical-chemical weathering of the mountain (e.g. the DML (Hengduan mountains) site). The Sr- Nd isotopic composition in ZD (northern Nyainqentanglha mountain) mostly ranged near the edge of the NM_TP, and one sample was close to the Himalayan value (Fig. 2), indicating a transitional region of dust source or a mixing of dust deposition. During the sampling period at ZD, dirty layers were very noticeable and accumulated. The mode of size distribution was ~11 mm and the dust deposition rate was much higher than those at GLDD and ZSGR (two sites in the northern TP) (Xu and others, unpublished information), suggesting that the dominant sources of these sites are relatively close. Compared to the available dataset, the ε_Nd(0) values in ZD fall into the range of samples from the Lhasa Terrane (-17.69 to -6.38), where ZD is located (Fig. 1) (Zhang and others, 2007; W. Wu and others, 2010). Owing to the large range of ε_Nd(0) in this dataset, we cannot rule out other sources from the NM_TP or Himalaya-India region which we discuss further, based on the air-mass back trajectories, in Section 3.2.

The largest variation in Sr-Nd isotopic compositions is found in samples from JMYZ and YL, with one JMYZ sample and two YL samples falling into the isotopic range of NM_TP. The uncertainty of ¹⁴³Nd/¹⁴⁴Nd for this one JMYZ sample is 0.000051 (1 ct), and the ε_Nd(0) range is -9.46 to -11.45, still larger than that of the other two data points and close to the results for the NM_TP. The uncertainties of ¹⁴³Nd/¹⁴⁴Nd for these two YL samples are also small (0.000020 and 0.000038). These results suggest the reality for these data. The trajectories analyses (Section 3.2) also indicate that the potential dust source for JMYZ can be the NM_TP. For the YL sampling site, although it is much further away from the NM_TP and the trajectories analysis shows that >85% of the air mass comes from the local and/or nearby regions, the isotopic signatures clearly differ from those of DML, while their location and back-trajectory profiles are almost identical. DML likely had a local dust source. These results indicate that dust may have been carried to these two sites from the northern deserts via long-range transport over the TP. The estimated contribution from these dust sources is presented in Section 3.3.

3.2. Possible dust source and transport route

To facilitate identification of source regions, air-mass back trajectories (for a 3 day period, ending at each sampling site) are computed and classified using GIS-based software (TrajStat; Wang and others, 2009) with storm frequency data during the high dust-storm frequency season, March- June, in 2008 and 2009 (downloaded from the US National Oceanic and Atmospheric Administration (NOAA) gridded meteorological archives, http://www.arl.noaa.gov/ready/hysplit4.html) (Fig. 3). Overall, most clustering trajectories in the sampling sites originated from the west, reflecting the typical westerlies climate of Central Asia, while some clusters originated from the northwest at the northern sites, and others from the southwest at the southern sites.

Fig. 3. Three-day backward air-mass trajectory cluster maps of each sampling site during the high-frequency dust storm season (March–June) between 2008 and 2009. The trajectories are calculated every 6 hours at 500m above each sampling site using the TrajStat software package.

About 60% of the trajectories at MSD originated from the west and northwest, while the rest were short trajectories from the direction of the Gobi desert (Fig. 3), supporting the similarity of Sr-Nd isotopic composition between the MSD samples and samples from the northern boundary of China and south of Mongolia. Over 70% of trajectories at TS traveled along the Taklimakan desert, while about 10% originated from the northern region of TS which covers the Gurbantungut desert. This finding agrees with the speculation, based on the isotopic data, that dust can also travel from northern arid areas. The sampling sites in the northern TP (LHG, ZSGR, DKMD and GL) show similar clustering patterns, with the majority of trajectories coming from the Taklimakan desert and the Tarim basin, which agrees with the relatively homogeneous Sr-Nd isotopic compositions of these sites. These trajectories are also consistent with results from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), which show that dust primarily originated from the Taklimakan desert and the Tarim basin, traveled eastward and covered the northern slope and eastern region of the TP (Liu and others, 2008). Over 90% of the trajectories at MS originated from the west, northwest or southwest, while only 6.5% originated from the Taklimakan desert. However, it is still possible that dust at MS could be mainly from this desert due to its short distance from Taklimakan.

Most trajectories at sites in the Himalaya (JMYZ, QM and Yala) originated from the west and/or the southwest, and partly (~5%) from the north. JMYZ is close to the arid regions of northwest India, and dust can be transported along the Himalaya during the pre-monsoon as seen from aerosol results from the Nepal Himalaya (Carrico and others, 2003). The Sr-Nd isotopic data also suggest that the limited number of trajectories (~4%) originating from the northern desert were able to deliver dust to JMYZ despite the huge elevation difference (~6000m) between the NM_TPand the Himalaya. Satellite data have demonstrated that dust layers may be lifted to altitudes of 11-12 km to the TP in the spring (Liu and others, 2008). In addition, JMYZ is the closest of these three Himalayan sampling sites to the Taklimakan desert. The dust source at QM and Yala should be similar to that of JMYZ, which shows the mixed dust signal because of the similar air-mass trajectories and identical location, but the isotopic signal from the NM_TP at these two sites is likely weakened by the low dust flux or the annual variation due to the different sampling time for these three sites.

Sampling sites in the southeast TP (ZD, DML and YL) also exhibit clustering trajectories from the west and southwest, although with relatively low speed. Meanwhile, relatively large portions of the trajectories (~15%) compare with the Himalayan sites, coming from the north with high speed. At ZD, 44% of trajectories came from the Indian arid regions and then crossed the western TP. This route can not only transport dust from Indian arid areas but also may blow up huge amounts of local dust which can be transported to the mountain areas and the surrounding cities such as Lhasa, as has been shown in published satellite images (Fang and others, 2004). The dust from northern trajectories (~14%) can also travel to ZD via the high-elevation route. From isotopic features, the dust source at ZD seems likely to be a mixture of dust sourced from the Indian arid regions, locally and from the NM_TP. At DML and YL, the clustering trajectories mainly originate from the surrounding regions with low speed except for those from the north. Liu and others (2008) suggested that dust particles traveling from the northern TP may be trapped by spiral airflows caused by the geological structure of the region, causing the northern and southern westerly jets in the southeastern TP to join. This could explain why the Sr-Nd isotopic composition in YL shows a dust signature typical of the northern TP margin. The signals of isotopic composition in DML, which did not show the dust signal of the NM_TP, were likely weakened due to high local dust deposition.

3.3. Case study of northern deserts' dust contribution to the southern sites

To estimate the contribution of dust in the northern deserts to the three southern sites, ZD, JMYZ and YL, we adopted theconventional mixing equation:

where X is the fraction of contribution from the northern desert, (ε_Nd(0))_SAMP is the ε_Nd(0) value at any one of the three sampling sites, and (ε_Nd(0))_{NOR_MAX} (-7.4) and (ε_Nd(0))_HM__AVG (-15.78) are used as the potential end- members, and represent the maximum value of ε_Nd(0) in the northern desert and the average value of ε_Nd(0) in the Himalaya, respectively. The Nd isotopic data were applied to this equation due to the small influence of particle sizes on the isotopic values (Chen and others, 2007). The (ε_Nd(0))_{NOR_MAX} values were taken from a dataset of the NM_TP (Chen and others, 2007). The (εN_d(0))_HM__AVG values were from the Himalayan dataset reported by Tripathi and others (2004).

The results show that 35-56% of dust in ZD, 18-63% of dust in JMYZ and 53-68% of dust in YL were likely transported from the northern dust sources, implying that long-range and high-altitude transport might make an important contribution to dust deposition at these sites. The dust deposition flux in ZD, JMYZ and YL was 1830, 300 and 730 mgcm^-2a^-1 respectively (Xu and others, unpublished information). Thus the contributions from the northern dust source could be around 641-1025 mgcm^-2a^-1 in ZD, 54-190 mgcm^-2a^-1 in JMYZ and 384-495 mgcm^-2a^-1 in YL. These dust deposition values are lower than those of other northern sites (Xu and others, unpublished information). However, this type of transport may have significant environmental impacts on the TP, as aerosols, including dust and anthropogenic particles, can potentially regulate the climate (Lau and others, 2006).

Summary

The Sr-Nd isotopic compositions dataset presented here confirms that the dust source in most mountain glaciers of western China is local and/or regional, while dust in glaciers of the Himalaya and southeast TP can be transported from the northern desert via long-range and high-altitude transport routes, which is consistent with satellite remote detection. These results provide a better understanding of the characterization of dust transport over the mountain areas of western China and are useful when interpreting the dust provenance in ice-core records from these areas.