1 Introduction

Investigations of trace elements (TEs) in remote areas have obtained information related to past climatic changes (Gabrielli et al., 2005 ; Gaspari et al., 2006 ) and the impact of human activities on the global atmosphere since the industrial revolution (Candelone et al., 1995 ; Hong et al., 2012 ), especially from the late 20th century (Boutron et al., 1991 ; Barbante et al., 2004 ; Schwikowski et al., 2004 ; Gabrieli et al., 2011 ; Eichler et al., 2012 , 2014).

The Qinghai-Tibetan Plateau (QTP) has commonly been regarded as the "third pole," in addition to those found in the Antarctic and Arctic regions (Yao et al., 2012a ). This third-pole environment (TPE) is very important because its water resources and ecosystem unite both on the interior and exterior of the plateau. Various proxies have been used to evaluate the state of the environment and climate, such as the oxygen isotope, dust, black carbon, and trace elements. The snow and ice of the wide-range-distributed high-mountain glaciers have preserved information related to the local, regional, and hemispheric past climate and environment (Thompson et al., 1997 ; Wu et al., 2010 ; Yao et al., 2012b , 2013; Li et al., 2016 ).

Studies focusing on the TEs in the snow and ice of the QTP have also obtained information related to the impact of dust and types of anthropogenic pollution sources, such as fossil fuel combustion, mining, and smelting activities (Li et al., 2006 ; Lee et al., 2008 ; Duan et al., 2009 ; Kaspari et al., 2009 ; Liu et al., 2011a ,b; Huang et al., 2013 ; Dong et al., 2015 , 2016a,b, 2017; Zhang et al., 2016 ). However, these studies were based on various acid-leaching methods. The acid leaching in these investigations was generally conducted for several hours to around 1 week before taking the final measurements of the TEs. Koffman et al. (2014) showed that the relative TE concentrations measured in snow and ice depend not only on the acidification time but also on the acid strength. The acid strength and contact time affect the dust dissolution and consequently result in differential dissolutions of the TEs. In addition, elemental dissolution in different mineral dusts is also time- and mineral-dependent (Rhodes et al., 2011 ). Therefore, the results obtained through different acid-leaching approaches are unlikely to be directly comparable.

Uglietti et al. (2014) compared the acid-leaching test with the full digestion of TEs in ice-core samples from four sites of the QTP. The mass fractions of the TEs in four ice-core samples were addressed in the study. Uglietti et al. (2014) found that the mass fluxes of these TEs from high-altitude glaciers will be significantly underestimated even if the ice-core samples undergo acidification for over 1 month. However, this study focused only on the TEs from ice-core samples from four sites on the QTP. Glaciers extensively distributed across the QTP are subject to influences from various dust sources with different mineralogy. Further study on the TEs in the current snow from other sites at the QTP based on simple acidification and total digestion will provide more information about the mass fraction of trace elements.

This study will characterize TEs, REEs, Y, and Th by comparing their mass fractions, REE normalized distribution patterns, and provenance-tracing proxies related to REEs and Y (such as La/Yb, Nd/Er, Y/La, and Y/Tb) in recent snow samples collected from the QTP, based on traditional acidification and total digestion. The aim is to examine the influence of different sample-preparation methods on the evaluation of the fluxes of the TEs, REEs, Y, and Th; and on the provenance tracing of the dust based on the REEs in the snow of the glaciers studied on the QTP. The results of this study can serve as a reference for evaluating the fluxes of the TEs, REEs, Y, and Th, the provenance tracing of dust and contribution from human activities in the atmosphere of the glaciers studied.

2 Materials and methods 2.1 Sampling

Snow-pit samples from the glaciers MeiKuang (MK) and Xiaodongkemadi (XDKMD) and surface-snow samples from the glaciers Qiumianleike (QMLK) and Gurenkekou (GRHK) were collected between late April and late May in 2013 (Figure 1, Table S-1). All samples collected were kept frozen and then transferred to the cold room of the State Key Laboratory of Cryospheric Science (SKLCS), Northwest Institute of Eco-Environment and Resources (NIEER), Chinese Academy of Sciences (CAS), China, until preparation for analysis.

2.2 Preparation and determination

The acid-cleaning procedure, sample preparation, and determination of dust content were performed at the SKLCS, where all samples for TEs, REEs, Y, and Th determinations were added indium solution to reach 1 ng/mL as internal standard element. All lab ware, including the low density polyethylene (LDPE) sample bottles and polytetrafluoroethylene (PTFE) bombs for digestion, were pre-cleaned through successive procedures that included four acid baths; these cleaning procedures have been described elsewhere (Li et al., 2006 ; Liu et al., 2011b ).

The dust content was determined by an optical particle sizer (AccuSizerTM 780, USA) with a precision of 5%. For the concentrations of TEs, REEs, Y, and Th in the acid-leachable fractions (ALF), the sample-preparation method was as follows. Five mL of the melted snow subsample was acidified with Merck "Ultrapur" 60% HNO₃ (Darmstadt, Germany) until the HNO₃ content reached 1% (v/v). One week after the addition of the acid, the samples were preserved for about 3 weeks in a freezer after and then transferred at room temperature to Italy over about 1 week; the samples rested for 1 more week before the measurements were performed.

For the concentrations of the TEs, REEs, Y, and Th in the digested samples, double-distilled hydrofluoric acid (HF) from Merck 40% HF after being purified using a sub-boiling acid-distillation apparatus (BSB-939-IR, Berghof, Germany) was used, together with the Merck "Ultrapur" 60% HNO₃. A 30-mL melted-snow subsample was transferred into a PTFE beaker, weighed, and then evaporated on a temperature-controlled hot plate to ensure that the sample volume reached about 2 mL. This sample was then transferred into a 10-mL PTFE bomb, and the "Ultrapur" HNO₃ and purified HF were added; the sample was then sealed in a stainless steel pot. The digestion method used in this study was referenced from Qi et al. (2000) and He et al. (2002) .

The standard preparation and measurement of the TEs, REEs, Y, and Th with an Element 2 inductively coupled plasma-sector field mass spectrometer (ICP-SFMS, Thermo Finnigan, Bremen, Germany) were carried out at the Institute for the Dynamics of Environmental Processes, National Research Council (IDPA-CNR) in Venice, Italy. Details about the cleaning conditions, ultrapure water, instrument proxies, and method of determination can be found in Gabrieli et al. (2011) and Planchon et al. (2001) . A standard reference material, TMRAIN-95 (from the National Water Research Institute, Canada), which is made from rain water, was used to validate the quality of the ICP-SFMS analyses. The recoveries for most elements were from 80% to 113%. One exception is the recovery of Al was 187% of that of the certified values (Table S-2). There were no recovery data for the REEs because there are no certified values for REEs in TMRAIN-95.

The effectiveness of the digestion method for the total concentrations of TEs was validated based on a soil-standard reference material GBW07423 (GSS-9, made by the Institute of Geophysical and Geochemical Exploration, Langfang, China). The results showed that the ratios of the certified values of the TEs in the standard reference material, of which the weight was less than 1 mg to 24.2 mg, were in the range of 0.80 (Pb) to 1.21 (Ti) excepting of Al of which the ratio is higher than 1.3 but less than 1.5 (Figure S-1). These data suggest that the accuracy of the digested method is reliable for the determination of the TEs. Although the REEs in the SRM soil were not measured during the determination, the aforementioned accuracy of the determined TEs suggests that the determination of the REEs was reasonably reliable.

The procedural blanks of the TEs, REEs, Y, and Th associated with the evaporation and digestion procedures were evaluated with Milli-Q 18.2 MΩ·cm ultrapure water by treating them using the same procedure as that of the samples. The lowest concentrations of TEs were found in the XDKMD sample. A comparison of the samples with the lowest TE concentrations in the XDKMD sample with their average procedural blanks indicated that the concentrations of the TEs were generally about five times higher than the corresponding blanks. The concentrations of Co in a few samples from XDKMD were around two times higher than the blanks, but this was not expected to influence the interpretation of its significance in environmental records.

The concentrations of most REEs in most samples were at least five times higher than their blanks, except for three samples from XDKMD, for which the concentrations of Tb, Tm, and Lu slightly lower than the blanks. The concentrations of Tm and Lu in the surface samples from GRHK were around 1 to 5 times the blank values. However, these results will not have a major influence on the interpretation of the REE distribution and proxies for provenance tracing.

3 Results and discussion 3.1 Mass fractions of TEs

The mass fraction of TEs was defined as the ratio of the TE concentrations of the acid-leached samples to the digested samples. Our statistical data indicated that the mass fractions varied between TEs, samples, and sites (Figure 2). On average, TEs with mass fractions less than 0.5 were Al (0.24 to 0.34), Mo (0.06 to 0.16), Ti (0.21 to 0.28), Rb (0.32 to 0.41), and V (0.35 to 0.44) respectively. The other TEs had mass fractions more than 0.60, across the four sites. Obviously, the lower mass fractions of Al, Rb, and Ti, which generally were regarded as the references for crust or soil, would lead to relatively high enrichment factor (EF) values of other TEs such as Pb if the EF calculations were based on the three elements (e.g., EF(Pb) = [Pb/Al]_snow/[Pb/Al]_soil or crust). Figure S-2 illustrates the EF values of the trace elements along the depths in the MK snow pit, based on the two different sample-preparation methods. The EF values of trace elements such as Ba, Cu, Mn, Pb, Sb, Sr, and Tl are higher in a few samples that were treated with the traditional acidification method versus those samples that underwent digestion. The resultant higher EF values will lead to a greater contribution of the trace elements from other sources, especially human activities, than the dust source. In addition, the lower mass fractions of these will lead to an underestimation of their fluxes and dust if any of the elements are used as the dust reference.

The average mass fractions of Fe for most samples are 93% (QMLK), 68% (MK), 77% (XDKMD), and 62% (GRHK) which were comparable with the data for Holocene ice from the Antarctic, in which the mass fraction was 70% (Gaspari et al., 2006 ), but higher than that of the Last Glacier Maximum and the present snow data reported by Grotti et al. (2011) , in which the mass fractions of Fe were lower than 20%. The difference between our Fe data and modern Antarctic snow may be the result of the different acidification methods. For our samples, the HNO₃ content was 1% (v/v), whereas the content of HNO₃ in the Antarctic snow samples reported by Grotti et al. (2011) was 0.5%. Moreover, the acidification time before determination was longer in our study than that for the Antarctic samples; the time after acidification was not specified in that study but was presumably shorter than ours because their samples were acidified and refrozen until analysis. In addition, it was found that even after 3 months, Fe was still actively leaching into the solution (Edwards et al., 2006 ; Koffman et al., 2014 ). Our early data show that the Fe concentrations in the snow samples from the Yuzhufeng glacier, located near the MK glacier, increased by 3.1 and 3.5 times after 5 months, relative to the first determination 1 week after acidification (Li et al., 2011 ). Moreover, the difference between the higher mass fractions of Fe in our study and the lower mass fractions in the Antarctic snow may be due to the difference in the mineral dust present in our snow samples and the Antarctic samples. Rhodes et al. (2011) found that the elemental dissolution from four different crushed-rock standards acidified at 1% (v/v) nitric acid was both time- and mineral-dependent. The difference between the mass fractions of Fe of our samples and the Antarctic samples suggests that a comparative investigation for both the ALF with the TD of the TEs is important for sites with considerably different mineral dusts. This approach could be used to help extract information regarding the composition of the mineral dust in the area studied.

3.2 Mass fractions of REEs

All REEs mentioned hereafter were normalized using the data from post-Archean Australian shale (PAAS, Taylor and McLennan, 1985), excluding the Y and Th data, which were not normalized.

The evaluation of the mass fractions of the REEs in the samples from the different glaciers with varying dust content is important when using REEs as dust tracers (Gabrielli et al., 2010 ). The REEs were differentiated into three groups: light REEs (La–Nd, LREE), middle REEs (Sm–Ho, MREE), and heavy REEs (Er–Lu, HREE). On average, the mass fractions of the LREEs, MREEs, and HREEs range from 0.34 to 0.75 and are quite different from each other (Figure 3, Table 1). The Mean values of mass fractions of the MREEs range from 0.46 to 0.75, higher than those of the LREEs (0.45 to 0.57) and HREEs (0.34 to 0.52). The lower mass fractions of the REEs in our study indicate that concentrations of the REEs in the acid-leachable fractions in the snow samples will also result in an underestimated dust flux if using the REEs in the acid-leachable fractions to assess the dust flux. The mass fractions of the REEs in our sites studied are also different from those in the Antarctic ice, as reported by Gabrielli et al. (2010) , who found that the LREEs had a higher mass fraction than the MREEs and HREEs.

3.3 REE normalized distribution patterns

The higher mass fractions of the MREEs, compared with the LREEs and HREEs, in the acid-leached samples of the studied sites led to different distribution patterns.

Figure 4 illustrates the difference in the REE distribution patterns in the acid-leachable fractions from those in the digested samples of the sites studied, together with four selected potential source areas (PSAs) of dust: the Taklimakan Desert, the Qaidam Basin, and the surface soil of the Tibetan Plateau and the Indian Thar Desert.

The REE distribution patterns in the acid-leachable fractions of the samples from the four sites studied were different not only from those of the digested samples but also from the four PSAs. For example, the REEs in the digested samples of the studied sites displayed similar normalized distribution patterns, with a positive slope between the LREEs and MREEs ([La/Gd]_PAAS) ranging from 0.72 to 0.78. This result is comparable to the data for the four PSAs, ranging from 0.69~0.85, and for the negative slope between the MREEs and HREEs ([Gd/Yb]_PAAS), ranging from 1.24 to 1.58. These values are also comparable with the data in the range of 1.15~1.40 for the three PSAs but lower than the value for the Indian Thar Desert (1.70). In contrast, the REE patterns in the acid-leachable fractions display more positive LREE/MREE slope values ([La/Gd]_PAAS values of 0.56~0.78) and more negative MREE/HREE values ([Gd/Yb]_PAAS values of 2.15~2.37) than the digested REE distribution patterns and the four PSAs. The REE distribution patterns in the digested samples can be used to trace the four selected dust sources, as well as other potential dust sources. In contrast, the REE distribution patterns in the acid-leachable fractions will result in unreliable results in tracing the dust sources in the snow because the REE patterns are clearly different from any of the PSAs.

3.4 Characteristics of REE ratios for tracing dust provenance

A combination of the ratios of the REEs and those of Y or Th to the REEs allows for a greater discrimination between these sources. Figure 5 shows the plots of the REE ratios, including the ratios of Y to the REEs for both kinds of samples from the MK and XDKMD snow pits, compared with the four potential areas. One common feature that can be observed from Figure 5 is that the acid-leachable fractions of the samples from MK and XDKMD are located in different areas from those of the digested samples and the four PSAs, which indicates that the dust sources vary based on the method used, so different results will be produced because they will be based on differing REE ratios of the acid-leachable fractions. In contrast, plots of Y/La vs. Tb/Er, Y/La vs. Y/Er, Y/Er vs. La/Th, and La/Yb vs. Y/Tb clearly show that most digested samples are located in the same area as those of the three PSAs mentioned above (apart from the Indian Thar Desert), indicating that the aforementioned three PSAs contributed dust to the studied sites. However, one sample of XDKMD in the plots of Tb/Er vs. Y/La, Y/Er vs. Y/La, and La/Yb vs. Y/Tb falls into the plot area of the Indian Thar Desert (Figure 5), suggesting that dust from remote regions, such as the Indian Thar Desert, may be transported to the XDKMD glaciers.

The plot of Eu/Eu* and Gd/Yb roughly differentiate the acid-leachable fractions, the digested samples, the three Tibetan PSAs (e.g., Tibetan surface soil, Taklimakan Desert, and Qaidam Basin), and the Indian Thar Desert into three areas (Figure 5). The digested samples are located in areas close to the Taklimakan Desert, Qaidam Basin, and Tibetan soil, while the acid-leachable fractions and the Indian Thar Desert soil are located at two areas different from those of the digested samples, which clearly shows that different results will be obtained based on the REEs in the acid-leachable fractions, as well as the REEs in the digested samples.

4 Conclusions

This study compared the mass fractions of the TEs and REEs, distribution patterns of REE, and some proxies related to the REEs and Y, such as La/Yb, Nd/Er, Y/La, and Y/Tb, based on two sample-preparation methods (total digestion and traditional acid-leachable fraction) for the snow-pit and surface-snow samples from four glaciers on the QTP. The following conclusions were obtained from the above analysis and discussion:

1) The mass fractions of the TEs (such as Mo, Ti, Al, V, Rb, REE, Y, and Th in the ALFs) accounted for less than 50%, while those of other TEs (such as Cu, Pb, Mn, and Sr) accounted for more than 90% (on average) in the TD samples. The data of the TEs, REEs, Y, and Th from the acid-leached samples will lead to an overestimated contribution from other sources (especially human activities) and underestimated fluxes of the TEs, REEs, and dust.

2) The higher mass fractions of the TEs, such as Pb in the snow samples of the sites studied, suggest that the samples for tracing pollution sources using Pb isotopes need to be prepared only by traditional acidification. There is no need for full digestion.

3) The sample-preparation methods also influence the REE distribution patterns and proxies used in dust provenance tracing. The REE distribution patterns and proxies in the ALF resulted in unreliable information for tracing the sources of the dust present in the snow. In contrast, the REE distribution patterns and proxies in the TD samples could be used more reliably to trace the potential dust source areas.

Acknowledgments:

This research was supported by grants provided by the National Natural Science Foundation of China (Grant Nos. 41276194, 40771046, and 40601021). The authors also would like to thank members of the spring 2013 fieldwork team on the Tibetan Plateau for all their hard work. We also appreciate two anonymous referees very much for their good reviews and suggestions for improving the paper.