Environmental records of snow pits in Yuzhufeng Glacier and Xiao Dongkemadi Glacier in the Tibetan Plateau

QuanLian Li , XiaoBo Wu, JianChen Pu, JianQiao He

State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

Environmental records of snow pits in Yuzhufeng Glacier and Xiao Dongkemadi Glacier in the Tibetan Plateau

QuanLian Li*, XiaoBo Wu, JianChen Pu, JianQiao He

State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

The contents and distribution characteristics of ions,n-alkanes, and polycyclic aromatic hydrocarbons in snow pits on the Yuzhufeng (YZF) Glacier and the Xiao Dongkemadi (XDKMD) Glacier are studied. Parameter characteristics and correlation coefficients between ions and two organic compounds are used to explore the possible sources of these chemical compositions. The results indicated that both glaciers are influenced by west wind circulation, but the contents of ions,n-alkanes, and polycyclic aromatic hydrocarbons in the YZF Glacier are higher than in the XDKMD Glacier because of differences in geographical position.The ratios of ∑nC21-/∑nC22+and CPI values (CPI: carbon preference index) indicate that then-alkanes from natural sources in these two glaciers are mainly derived from higher plants, whereas the contribution from lower organisms was small, also,n-alkanes from anthropogenic sources in the YZF Glacier are higher than in the XDKMD Glacier. The ratios of LPAHs/HPAHs and (Fly+Pyr)/(BghiP+INP) indicate that the polycyclic aromatic hydrocarbons in these two glaciers are mainly derived from low temperature combustion of coal and biomass, and, in the XDKMD Glacier, partially from the vehicle exhaust.

Tibetan Plateau; snow pit; ion;n-alkanes; polycyclic aromatic hydrocarbons

1. Introduction

Glaciers result from atmospheric processes and also serve as the semi-permanent reservoirs for a variety of materials. Many chemical substances and microorganisms are stored in the glaciers. They initially deposited on glaciers as snow crystal nuclei and gradually accumulated in the glacier interiors through snow metamorphism and glacier evolution. These substances are thus important indicators of the climate and environmental changes in ancient times. The scientific interpretation of the relationship between these indicators is essential for paleoclimatology research. Earlier studies on Tibetan Plateau snow pits mainly focused on the inorganic components, and reports on organic pollutants are scarce. In 1997, Xieet al. (1999)for the first time studied the biomarkers in the Dasuopu Glacier of Mount Xixiabangma, and 10 years later, Wang and various researchers (Wanget al., 2007; Wanget al.,2008a; Wanget al., 2008b) carried out additional work in the Himalayas. Among the organic contaminants, aliphatic and polycyclic aromatic hydrocarbons (PAHs) have received much attention (Herbertet al., 2005; Maggiet al., 2006;Nemirovskayaet al., 2006).

PAHs are organic compounds with strong carcinogenic,teratogenic, and mutagenic effects (Olivellaet al., 2006).There are two main sources of PAHs (Baeket al., 1991;Simoneitet al., 1999): natural and anthropogenic sources,the anthropogenic predominate (Garbanet al., 2002).n-Alkanes are widespread biomarkers with high-energy carbon-carbon bonds and stable chemical structures, their substance sources are closely related to the prevailing environmental and climatic conditions. Anthropogenic sources ofn-alkanes typically include the combustion of fossil fuels,wood and agricultural debris or leaves, and biogenic sources including epicuticular waxes of vascular plants, direct suspension of pollen, and microorganisms and insects (Simoneitet al., 1997).

As the "third pole" of the Earth, the Tibetan Plateau is affected by the Asian monsoon and the central Asian arid or semi-arid climate, and has substantial impact on both the global climate and the global environment. In this paper, the content characteristics of ions,n-alkanes, and PAHs in snow pits on the Xiao Dongkemadi (XDKMD) Glacier and the Yuzhufeng (YZF) Glacier are researched, and the possible sources of these chemical compositions are explored, which will provide a scientific reference for the interpretation of ice core records.

2. Materials and methods

2.1. General research areas

The Tanggula Mountains are located in high central Asia at the transition zone between the arid/semi-arid continental climate and the humid maritime climate. The XDKMD Glacier (33°4'18"N, 92°14'01"E) is located on the central Tibetan Plateau in the north area of the Tanggula Mountains. It has a continental arid climate with sparse vegetation and a harsh environment. It also suffers from strong dust storms because the land surface is dominated by thermal lows in the summer and the near-ground air rises strongly. The glacier has an area of 16.40 km2and a length of 5.4 km. The maximum glacier altitude is 6,104 m and the end altitude is 5,275 m. The snow line is at 5,600 m and the glacier surface is smooth (Houet al.,1996).

The YZF Glacier (35°39'37"N, 94°14'28"E) is located in the eastern Kunlun Mountains in the northern highlands, and has a cold and arid climate. The glacier has an area of 6.75 km2and a length of 5.7 km, it’s maximum altitude is 6,178 m and the end altitude is 5,080 m. The glacier surface is smooth (Pu , 1994). The locations of the sampling sites are shown in Figure 1.

2.2. Sample collection and pretreatment process

Snow samples were collected from 40-cm-deep snow pits in the YZF Glacier and the XDKMD Glacier on May 18 and 26, 2008, respectively. One set of five snow samples for organic compounds and ion analysis was taken in 8-cm descending intervals at each sampling site. Because the snow pits were relatively shallow (40 cm), we reasoned from the sampling date that these two glaciers may have merely been some spring snow. The pretreatment process of the samples followed the method of Liet al. (2009).

Figure 1 Locations of the sampling sites

2.3. Instrument analysis parameters

Organic compounds were analyzed using a gas chromatograph (GC) with an ion-trap mass spectrometer (MS)(Finnigan Trace GC/PolarisQ). Separation and quantification were achieved on a DB-5 column (30m×0.25mm×0.25μm film thickness). The GC parameters for then-alkanes were as follows: the injector temperature was 280 °C,high-purity helium was used as a carrier gas at a constant flow rate of 1.0 mL/min, and the temperature of the transfer line was 300 °C. Each sample was analyzed under a splitless injection mode, column temperature program: 50°C held for 2 min, ramped up at 6 °C/min to 320 °C. The MS was operated with a 70 eV electron impact (EI) mode with selected ion monitoring (SIM) scan mode. The GC parameters for the PAHs were as follows: the initial temperature (50 °C) was held for 2 min, after which the temperature was raised to 180 °C at 20 °C/min, then further raised to 250 °C at 4 °C/min, the final temperature was raised to 310 °C at 10 °C/min and held for 5 min. The MS was operated in a selected ion monitoring (SIM) scan mode with a 70 eV electron impact (EI) source. For standard reagents, 16 U.S. priority PAHs containing naphthalene (Nap),dihydroacenaphthylene (Any), acenaphthene (Ane), fluorine(Fle), phenanthrene (Phe), anthracene (Ant), fluoranthene(Fla), pyrene (Pyr), benzo[a]anthracene (BaA), chrysene(Chry), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene(BkF), benzo[a]pyrene (BaP), dibenzo[a,h]anthracene (DaA),indeno[1,2,3-cd]pyrene (INP) and benzo[ghi]perylene(BghiP), were used (Supelco Company, USA). Nineteen normal alkanes (nC14-nC32) and these 16 U.S. priority PAHs were adopted as the calibration standards in this study.

Cations were tested on a DX-600 ion-exchange chromatograph using a Dionex Ion Pac CS-12A 2-mm analytical column, a CG-12A guard column, a CSRS-ULTRA 2-mm suppressor, a 25-μL injection loop, a 800-μL sample injecttion tube, and a 25-mm MSA eluent. Anions were tested on an ICS-2500 ion-exchange chromatograph using a Dionex Ion Pac As11-HC 4-mm analytical column, an AG-11 guard column, an ASRS-ULTRA II 4-mm self-regenerating micro-membrane suppressor, a conductivity detector, and automatically generated KOH as the eluent.The detection limit was down to the level of ng/g and the error of the tested data was less than 5%.

3. Results and discussion

3.1. The content characteristics of various components

Table 1 shows the content of ions,n-alkanes and PAHs in the snow pits of the XDKMD Glacier and the YZF Glacier. Ca2+and Mg2+are indicators of atmospheric mineral aerosols, in which Ca2+is a more effective indicator. It is generally believed that a high Ca2+concentration suggests that continental substances are abundant and the glacier area is relatively dry, whereas a low Ca2+concentration suggests that continental substances are few and the glacier area is relatively humid (Shenget al., 1999). It can be seen from Table 1 that the Ca2+concentration was the most predominant among all cations in the snow pits in both glaciers,indicating that they were largely influenced by the continental substances (i.e., the regional dry environment of the Tibetan Plateau). The Ca2+and Mg2+contents in the snow pits of both glaciers are similar, which suggests that both glaciers are substantially influenced by the dust from the continental crust of the arid/semi-arid environment in the vast northern Tibetan Plateau.

Compared with the XDKMD Glacier, the YZF Glacier had 4.33 times more Cl-, 1.53 times more SO42-, and 3.39 times more Na+, which is likely related to salt lake substances from the Qaidam Basin at the north end of the YZF Glacier. There are numerous salt lakes in the Qaidam Basin,and this basin has a dry and windy climate. Its annual evaporation is 2,000-3,000 mm and can reach as high as 3,700 mm, whereas its precipitation is less than 50 mm. This is one of the most highly evaporative regions in the world(Wang, 1990). Evaporation of salt lakes and weathering of salt minerals contribute to the material input of the YZF Glacier, especially Na+, Cl-, and SO42-.

The average content of totaln-alkanes in the snow pits of the YZF Glacier and the XDKMD Glacier were 1,220 ng/L and 720 ng/L, respectively, and the average content of the total 16 priority PAHs were 60.57 ng/L and 20.45 ng/L,respectively. Compared with the XDKMD Glacier, the YZF Glacier had 1.69 times moren-alkanes and 2.64 times more PAHs. Xieet al. (2009) found that culturable bacteria in the snow pits of the YZF Glacier and the XDKMD Glacier were 35.52 CFU/mL and 18.54 CFU/mL, respectively. The glaciers are both influenced by westerly circulation but have very different amounts of ions,n-alkanes, PAHs, and culturable bacteria, which may be resulted from their different geological locations.

Table 1 Content characteristics of chemical compositions of snow pits in the YZF Glacier and XDKMD Glacier

Research by Fanet al. (2008) indicates the climate in the Tibetan Plateau, from the southeast to the northwest, is changing gradually from warm and moist to cold and dry,and vegetation coverage is also decreasing gradually from the southeast to the northwest. The XDKMD Glacier is located on the northern slope of the Tangkula Pass, in the central part of the Tibetan Plateau, where the vegetation is mainly that of a cold steppe landscape. The YZF Glacier is located in the eastern Kunlun Mountains on the northern margin of the Plateau,which has less forest coverage due to higher altitude, the cold and arid climate, and the many gorges in the region. The high mountains in the eastern section of the Kunlun Mountains are surrounded by small quantities of fern, forest and some bushes. The ferns monitored in this areas are all terrestrial plants (i.e., no epiphyte or aquatic plants (Wuet al., 1994)).From the Tanggula Mountains to the eastern Kunlun Mountains, plant species decline sharply and the ecological environment becomes worse. Moreover, because the YZF Glacier is adjacent to the Qaidam Basin, gobi deserts, and the coal mining region to the north (where the action of wind and sand is very strong), substantial terrestrial source materials are contributed to migration and sediment in that glacier,so the content of dust and microorganisms are higher than that of the XDKMD Glacier. Moreover, the distance between the YZF Glacier and the Qinghai-Tibetan Railway was only 10 km, so it is seriously affected by human activities. This is also the reason for the higher content ofn-alkanes and PAHs in the YZF Glacier.

3.2. Correlation analysis among ions, n-alkanes and PAHs

The correlation rate reflects the degree of relationship between factors. Tables 2 and 3 show the correlation coefficients among ions,n-alkanes and PAHs in the snow pits on the YZF Glacier and the XDKMD Glacier, respectively.Ions such as Na+, Cl-and SO42-in the snow pits are obviously correlated with each other, indicating that they have the same origin or pathway of import, which may be the salt from the Qaidam Basin (NaCl, Na2SO4,etc.) (Huo and Yao,2001). In the XDKMD Glacier, Na+, Cl-and SO42-are all strongly correlated with Ca2+, indicating that Na+, Cl-and SO42-are also come from continental aerosols. In contrast,no such correlation is observed for the YZF Glacier, suggesting that Na+, Cl-and SO42-in the YZF Glacier come mainly from salt lake sediments.

The correlation coefficient between totaln-alkanes(T-HCs) and the total polycyclic aromatic hydrocarbons(T-PAHs) in the YZF Glacier is 0.74, indicating that some of then-alkanes in this glacier come from human activities. The NO3-ion has a variety of sources and comes mainly from lightning, stratospheric exchange, sea salt, continental material and human pollution (Lyons and Mayewski, 1983; Shenget al.,1996; Wanget al., 1998). In the YZF Glacier, the NO3-has no correlation with Ca2+but has a significant correlation with T-HCs, and its correlation coefficient with total PAHs is 0.62.It is generally believed that PAHs mainly come from human pollution, since the YZF Glacier is at the northern end of Tibetan Plateau, it can be inferred that the NO3-in the YZF Glacier does not come from sea salt but mainly from human activities and partly from continental sources (Xiaoet al., 2001).

In the XDKMD Glacier, the correlation coefficient between T-HCs and Na+, Cl-and SO42-is greater than 0.64,and T-HCs is also significantly correlated with Ca2+, indicating that then-alkanes are mainly from continental sources.Also, T-PAHs and NO3-are significantly correlated, and neither of them is correlated with Ca2+, Na+, Cl-, SO42-or T-HCs. Therefore, NO3-and T-PAHs in the XDKMD Glacier mainly come from human activities.

Table 2 The correlation coefficients among ions and organic compounds in the YZF Glacier

Table 3 The correlation coefficients among ions and organic compounds in the XDKMD Glacier

3.3. Composition characteristics and sources of n-alkanes and PAHs

In general,n-alkanes from lower plants such as oceanic algae and microbes are dominated by short carbon chains with CmaxatnC15,nC17andnC19, Cmaxin the combustion of fossil fuel is atnC20,nC21. Neither of them have an odd/even preference, and they have CPI values (CPI: carbon preference index) near 1 (Wakeham, 1990; Pavlova and Pazapova, 2003).n-Alkanes from higher plants are dominated by longer chain-length homologous with CmaxatnC27,nC29ornC31,usually with strong odd carbon predominance in thenC23-nC35range, and have CPI values ＞5 (Jaffeet al., 1993).Double peak distribution of then-alkanes are believed to be from a mixed source.

Figure 2 shows the distribution characteristics ofn-alkanes in the snow pits on the YZF Glacier and the XDKMD Glacier. The carbon chain length ofn-alkanes in the XDKMD Glacier showsnC14-nC32, but in the YZF Glacier presentsnC17-nC32. Except for a single sample, these two glaciers show a strong odd carbon number predominance ofnC25ornC27and a single peak distribution, which have features similar to those of epicuticular leaf waxes.Some of these high carbonn-alkanes with odd-even predominance can originate from vegetation in the local area,while others may be derived from long-distance transport through atmospheric circulation (Fanget al., 1999).

Modern organic geochemistry of molecules shows that the ratio of ∑nC21-/∑nC22+ofn-alkanes reflect the relative proportion of lower bacteria and algae to higher plants (Xieet al., 2003). The distribution range of ∑nC21-/∑nC22+ofn-alkanes in the YZF Glacier and the XDKMD Glacier were 0.09-0.32 and 0.1-0.48 respectively. The average ratio of∑nC21-/∑nC22+were 0.19 in the YZF Glacier and 0.22 in the XDKMD Glacier, indicating thatn-alkanes in this study were derived from plant waxes, and lower organisms were minor contributors.

The CPI value ofn-alkanes refer to the ratios of the amounts of odd-carbon numberedn-alkanes to those with even carbon numbers, and this can be used to identify the distribution of recent biogenic matter and anthropogenic materials. CPI is related to its derivation and the temperature experienced by the organic matter, the higher the temperature, the lower the CPI (Simoneitet al., 1991). Table 4 indicates that the CPI values of the YZF Glacier were in the range of 1.25 to 1.73, and the average ratio was 1.47. This narrow distribution implies a greater burden from fossil fuel residues, and minor higher plant contribution. The CPI values ofn-alkane in the XDKMD Glacier were in the range of 1.46 to 4.12 (Simoneit, 1986), this is lower than those of rural Los Angeles in Southern California (3.92 to 13.0), suggesting then-alkanes in XDKMD Glacier are affected by higher plant and fossil-fuel combustion products, and the higher plants are the main contributors.

Figure 2 Distribution characteristic of n-alkanes in snow pits in the YZF Glacier and the XDKMD Glacier

Table 4 Parameter characteristics of snow pits in the YZF Glacier and the XDKMD Glacier

The ratios of low molecular weight PAHs to high molecular weight PAHs (LMW-PAHs/HMW-PAHs, the ratio of two and three rings PAHs to PAHs of more than three rings)might reflect the distinctly different PAHs input sources to specific areas. The lower LMW-PAHs/HMW-PAHs values indicate a relatively high degree of high-temperature pyrogenic contamination, likely from vehicular exhausts. The higher LMW-PAHs/HMW-PAHs ratios demonstrated PAHs from low-temperature combustion and petroleum-derived residues (Maiet al., 2003).

The ratio ranges of LMW-PAHs/HMW-PAHs were 2.95 to 5.64 in the YZF Glacier and 1.0 to 3.91 in the XDKMD Glacier (Table 4). The mean ratio of LMW-PAHs/HMW-PAHs were 4.39 in the YZF Glacier and 2.38 in the XDKMD Glacier. They were lower than in soil samples (3 to 393) and grass samples (4 to 36) obtained from the Himalayan Mountains by Wang and his colleagues, due to the facile adsorption of the PAHs by the lipids of vegetation and organic matters in the soil (Wanget al., 2007).

Different PAHs have different sources. Phenanthrene and fluorene are derived mainly from the low temperature combustion of biomass and coal. Mastoralet al. (1996) have shown that fluorene has the highest concentration among the PAHs emitted under different conditions of coal combustion.Khaliliet al. (1995) also experimentally confirmed that fluorene and other light components are the dominant PAHs within Chicago coke-oven emissions. Simciket al. (1999)also considered the source apportionment of fluorene and anthracene.

Figure 3 shows that phenanthrene was the most abundant component, and dibenzoanthracene was not detected in these two glaciers. Dihydroacenaphthylene was the second most abundant component in the samples from the XDKMD Glacier, whereas fluoranthene was the second most abundant component in the samples from the YZF Glacier. The concentration of fluoranthene from the YZF Glacier was 4.49 ng/L, which was higher than that of the XDKMD Glacier, indicating that the PAHs in the YZF Glacier are derived from low-temperature combustion of coal and biomass, which is relevant to the local coal mining region. Among them, benzo[k]fluoranthene, indeno[1,2,3-c,d]pyrene and benzo[g,h,i]perylene,etc. have been confirmed by many researches as the characteristic markers of motor vehicle exhaust emissions (Grimmeret al.,1983). Fluoranthene and pyrene are mainly derived from the combustion of coal and petroleum (Mascletet al., 2000).

Figure 3 Distribution characteristics of individual PAHs in the YZF Glacier and XDKMD Glacier

None of the five samples from the snow pits in the YZF Glacier contained benzo[ghi]perylene, and only a fat lot samples contained very low levels of indeno[1,2,3-cd]pyrene and benzo[k]fluoranthene. The ratio of (Fly+Pyr)/(BghiP+INP) ranged from 58.94 to 59.86 with an average of 59.40, which suggests that vehicle exhaust has an almost negligible effect on chemical compositions of the YZF Glacier. The contents of benzo[k]fluoranthene, benzo[a]pyrene and indeno[1,2,3-cd]pyrene in the XDKMD Glacier was very low but higher than those in the YZF Glacier. In the XDKMD Glacier, the ratio of (Fly+Pyr)/(BghiP+INP)ranged from 5.95 to 13.01 with an average of 9.64, which indicates that the PAHs in the XDKMD Glacier mainly derived from the low-temperature combustion of biomass and coal but also partly from vehicle exhaust.

4. Conclusions

In this paper, we preliminarily analyze the contents and distribution characteristics of ion,n-alkanes, and PAHs in snow pits on the YZF Glacier and the XDKMD Glacier.The results indicate that both glaciers are influenced by west wind circulation, but the contents of ions,n-alkanes and PAHs in the YZF Glacier are higher than in the XDKMD Glacier, because the YZF Glacier is adjacent to the Qaidam Basin, the Qinghai-Tibet Railway, gobi deserts, and the coal mining region to its north. Generally, the major constituents of then-alkanes are in the higher molecular weight range,such asnC27,nC29ornC31, which have features similar to those of epicuticular leaf waxes. The carbon chain length ofn-alkanes in the XDKMD Glacier showsnC14-nC32, but the YZF Glacier presentsnC17-nC32. Except for a single sample,these two glaciers show a strong odd carbon number predominance abovenC25ornC27and single peak distribution,which also has features similar to those of epicuticular leaf waxes. The ratios of ∑nC21-/∑nC22+and CPI indicate that then-alkanes from natural sources in these two glaciers mainly come from the input of higher plants, whereas the contribution of the lower organisms was small, also,n-alkanes from anthropogenic sources in the YZF Glacier are higher than in the XDKMD Glacier. The ratios of LPAHs/HPAHs and (Fly+Pyr)/(BghiP+INP) indicate that the PAHs in these two glaciers are mainly derived from low-temperature combustion of coal and biomass, and, in the XDKMD Glacier, partially from vehicle exhaust gas.

The authors are very thankful to two reviewers for proposing good suggestions. This project is supported by the Global Change Research Program of China (No.2010CB951404), the National Natural Science Foundation of China (Grant No. 40930526, 40801023, 40871038), and the West Light Foundation of the Chinese Academy of Sciences (Grant No. 290928601).

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10.3724/SP.J.1226.2011.00392

*Correspondence to: Dr. QuanLian Li, Assistant Professor of Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. No. 320, West Donggang Road, Lanzhou, Gansu 730000, China. Tel: +86-931-4967344;Email: liql@lzb.ac.cn

21 April 2011 Accepted: 27 July 2011