Distribution and isotopic compositions of n-alkanes and n-alkenes in the cryoconites from the Tibetan Plateau glaciers

Quanlian Li; Shichang Kang; Shijin Wang; Ninglian Wang; Xiaobo Wu; Wasim Sajjad; Huan Yang; Yao Li; Jingquan Wu

doi:10.1017/jog.2024.23

Distribution and isotopic compositions of n-alkanes and n-alkenes in the cryoconites from the Tibetan Plateau glaciers

Published online by Cambridge University Press: 14 March 2024

Xiaobo Wu ,

Huan Yang ,

Yao Li and

Jingquan Wu

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Quanlian Li*: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China Yulong Snow Mountain Station of Cryosphere and Sustainable Development, State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
Shichang Kang: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China
Shijin Wang: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China Yulong Snow Mountain Station of Cryosphere and Sustainable Development, State Key Laboratory of Cryospheric Sciences, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
Ninglian Wang: Affiliation:
Shaanxi Key Laboratory of Earth Surface System and Environmental Carrying Capacity, Xi'an 710127, China Institute of Earth Surface System and Hazards, Northwest University, Xi'an 710127, China College of Urban and Environmental Sciences, Northwest University, Xi'an 710127, China
Xiaobo Wu: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China
Wasim Sajjad: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China
Huan Yang: Affiliation:
Hubei Key Laboratory of Critical Zone Evolution, School of Geography and Information Engineering, China University of Geosciences, Wuhan 430074, China
Yao Li: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China
Jingquan Wu: Affiliation:
Northwest Institute of Eco-Environment and Resources, State Key Laboratory of Frozen Soil Engineering, SKLCS Lanzhou, Gansu, CN, CAS, Lanzhou 730000, China
*: Corresponding author: Quanlian Li; Email: liql@lzb.ac.cn

Article contents

Abstract
Introduction
Materials and methods
Results and discussion
Conclusions
References

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Abstract

In the current study, cryoconite samples were collected from six glaciers on the Tibetan Plateau to analyze n-alkanes and n-alkenes. The findings revealed that the concentrations of n-alkanes and n-alkenes varied from 40.1 to 496.1 μg g−1 and from 4.6 to 13.8 μg g−1, respectively. The carbon preference index of the long-chain n-alkanes ranged from 3.3 to 8.4, and the average chain length ranged from 28.7 to 29.3. Moreover, the δ13C of the n-alkanes in cryoconites were within the range of C3 plants, demonstrating that the n-alkanes in cryoconites were only derived from vascular plants. However, the δDmean were more negative than that of C3 plants, which could be caused by dry and humid conditions of glaciers. Unlike n-alkanes, n-alkenes ranged from C17:1 to C30:1 and showed a weak even-over-odd carbon number preference in the Dongkemadi, Yuzhufeng, Laohugou and Tianshan glacier, but a weak odd carbon preference in the Qiyi glacier. The n-alkenes in the YL Snow Mountains showed an obvious odd-over-even carbon number predominance from C17:1 to C22:1 with Cmax at C19:1, and the even-over-odd carbon number preference from C23:1 to C30:1 with Cmax at C28:1. This demonstrated that the n-alkenes of cryoconites may be mainly derived from in situ production in glaciers.

Keywords

cryoconite glacier n-alkane n-alkene Tibetan Plateau

Information

Type: Article
Information: Journal of Glaciology , Volume 70 , 2024 , e38

DOI: https://doi.org/10.1017/jog.2024.23 [Opens in a new window]
Creative Commons: This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives licence (http://creativecommons.org/licenses/by-nc-nd/4.0), which permits non-commercial re-use, distribution, and reproduction in any medium, provided that no alterations are made and the original article is properly cited. The written permission of Cambridge University Press must be obtained prior to any commercial use and/or adaptation of the article.
Copyright: Copyright © The Author(s), 2024. Published by Cambridge University Press on behalf of International Glaciological Society

1. Introduction

Cryoconites are dark-colored, spherical granules on the glacier surface (Langford and others, Reference Langford, Irvine-Fynn and Edwards2014; Cook and others, Reference Cook, Edwards and Takeuchi2016; Takeuchi and others, Reference Takeuchi, Sakaki and Uetake2018). Cryoconite granules are often composed of filamentous cyanobacteria entangled with minerals and organic particles (Takeuchi and others, Reference Takeuchi, Nagatsuka and Uetake2014). Due to their darker tone, cryoconites can absorb more solar radiation than the surrounding ice, which promote the melting of the ice beneath the granules and form cryoconite holes (Chandler and others, Reference Chandler, Alcock and Wadham2015; Tedesco and others, Reference Tedesco, Doherty and Fettweis2016). Cryoconite holes are regarded as unique glacier ecosystems, in which liquid water is available on the glacier surface during the ablation season. Therefore, cryoconite holes could act as biologically active habitats for a diverse community of microorganisms, including algae, cyanobacteria, bacteria, archaea, viruses, rotifers, tardigrades and insects (Bagshaw and others, Reference Bagshaw, Tranter and Wadham2016; Zhou and others, Reference Zhou, Zhou and Hu2019; Rozwalak and others, Reference Rozwalak, Podkowa and Buda2022). Generally, algae and cyanobacteria usually grow at the bottom of cryoconite holes, and they produce organic matter through photosynthesis, which supports the proliferation of heterotrophs (Takeuchi and others, Reference Takeuchi, Sakaki and Uetake2018).

The organic matter accumulated on cryoconites usually has both autochthonous and allochthonous sources (Hood and others, Reference Hood, Fellman and Spencer2009; Singer and others, Reference Singer, Fasching and Wilhelm2012; Stibal and others, Reference Stibal, Šabacká and Žárský2012). The autochthonous organic matter in cryoconites is produced in situ via heterotrophic or autotrophic microbial activities (Anesio and others, Reference Anesio, Hodson and Fritz2009; Smith and others, Reference Smith, Foster and McKnight2017), whereas allochthonous organic matter (such as levoglucosan) may be derived from biomass burning (Li and others, Reference Li, Wang and Barbante2019). However, the particles deposited on glaciers as well as the autochthonous and allochthonous organic matter are darker, so they reduce the albedo of the glacier surface and accelerate glacier melting (Takeuchi and others, Reference Takeuchi, Kohshima and Seko2001; Takeuchi, Reference Takeuchi2002; Rozwalak and others, Reference Rozwalak, Podkowa and Buda2022). Until now, there have been only sporadic reports on organic matters in cryoconites. Xu and others (Reference Xu, Simpson and Eyles2010) found normal alkanes in cryoconites from Athabasca Glacier of Mount Rocky, Canada were derived from mossy and vascular plant origin. They also detected higher concentrations of phospholipid fatty acid, indicating that the glacier surface was dominated by Gram-positive and Gram-negative bacteria, as well as cyanobacteria. Pautler and others (Reference Pautler, Dubnick and Sharp2013) also found that Antarctic cryoconites contained microbial proteins, peptides and phospholipid fatty acids. The above results suggested these organic matters in cryoconites from Arctic and Antarctic were derived from the combined contributions of both higher plants and in situ microorganism's activities.

Normal alkanes (n-alkanes), which are stable, long-lived and non-polar-saturated hydrocarbon molecules that originate from epicuticular waxes of vascular higher plants (Eglinton and Hamilton, Reference Eglinton and Hamilton1967; Bush and McInerney, Reference Bush and McInerney2013; Diefendorf and Freimuth, Reference Diefendorf and Freimuth2017). Short-chain n-alkanes in the range of C₁₄–C₂₂ are produced mainly by bacteria, algae and microbial organisms (Han and Calvin, Reference Han and Calvin1969; Grimalt and Albaiges, Reference Grimalt and Albaigés1987). Mid-chain n-alkanes (C₂₃–C₂₅) have been mainly detected in aquatic macrophytes (Ficken and others, Reference Ficken, Li and Swain2000). n-Alkanes are strongly recalcitrant due to their hydrophobic nature and excellent chemical stability during transportation, deposition and burial (Schwark and others, Reference Schwark, Zink and Lechterbeck2002). Moreover, the stable carbon isotope compositions (δ¹³C) of n-alkanes in different matrixes have been widely employed to understand their source regions because they are sensitive to the plant types (C₃ or C₄) they are derived from (Chikaraishi and Naraoka, Reference Chikaraishi and Naraoka2003; Schefuß and others, Reference Schefuβ, Schouten and Schneider2005; Hockun and others, Reference Hockun, Mollenhauer and Ho2016). In addition, the hydrogen isotope ratios (δD) of higher plant waxes, primarily reflect the δD of the precipitation during photosynthesis (Sachse and others, Reference Sachse, Radke and Gleixner2006, Reference Sachse, Billault and Bowen2012). Hence, the combination of distribution and dual-isotope ratios (δ¹³C and δD) of n-alkanes provides a powerful tool for accessing their source regions (Diefendorf and Freimuth, Reference Diefendorf and Freimuth2017; Zhang and others, Reference Zhang, Fu and Yu2023). Compared with the n-alkanes, the n-alkenes are geologically unstable due to the sensitivity of the double bond to oxidation. The long-chain n-alkanes and n-alkenes have been detected previously in Antarctic soils (Matsumoto and others, Reference Matsumoto, Akiyama and Watanuki1990), peat (Xie and others, Reference Xie, Nott and Avsejs2004) and sediments (Zhang and others, Reference Zhang, Su and Liu2015; Pu and others, Reference Pu, Wang and Meyers2017). However, investigations for these compounds in the glacier are still a fairly novel field. Therefore, to fill this information gap, the cryoconite samples were collected from six glaciers on the Tibetan Plateau (TP) to probe the molecular distribution characteristics of n-alkanes and n-alkenes in cryoconites and discussed their potential sources based on δ¹³C and δD. For the first time, this paper provides an account of the unusual distribution of n-alkenes in cryoconites from the TP and discusses their possible origins.

2. Materials and methods

2.1. Study site

During July and August 2014, cryoconite samples were collected from six glaciers which are presented in Figure 1 and the sampling information is provided in Table 1. The Ürümqi Tianshan (TS) Glacier No.1 is located on the eastern side of the Tien Shan (Fig. 1). The Laohugou (LHG) Glacier No. 12 and the Qiyi (QY) glacier are located on the northern slope of the Qilian Mountains in the northeastern TP. The Yuzhufeng (YZF) glacier is located on the eastern Kunlun Mountains in the northern TP. These four glaciers are affected by the westerlies throughout the year. The Dongkemadi (DKMD) glacier is located on the northern slope of the Tanggula Mountains in the central TP, which is the transitional region of the South Asian monsoon and westerly winds. The Baishui Glacier No. 1 (YL Snow Mountains) is located in the southeastern TP, which has a typical Indian monsoon climate.

Figure 1.

Sampling sites of cryoconites on the TP glaciers.

Table 1.

Information about the cryoconite samples collected from different glaciers on the Tibetan Plateau

2.2. Sample collection

The cryoconite samples were collected from the ablation areas of each glacier using a sterile stainless-steel scoop. All samples were sealed and labeled immediately and stored in a freezer at −20°C until further analysis.

2.3. Extraction and measurement

The cryoconite samples were lyophilized at −70°C and ground into powder. Cryoconite powder (20 g) was extracted using a Soxhlet extractor with dichloromethane and methanol (9:1 v/v) for 72 h. The aliphatic hydrocarbon fractions were loaded onto precleaned silica gel columns and were eluted with hexane. The aliphatic hydrocarbons fractions were determined by a gas chromatography–mass spectrometry (GC–MS) using an Agilent 7890A gas chromatograph coupled with an Agilent 5975C mass spectrometer. The chromatographic column was a DB-1 MS capillary column (30 m × 0.25 mm × 0.25 μm). The GC oven temperature was programmed from 70 to 320°C in steps of 4°C min⁻¹ and held for 30 min. Samples were injected in splitless mode (1 μL) and high purity helium (99.999%) was used as the carrier gas at a flow rate of 1.0 mL min⁻¹. The ion source temperature was 250°C, and ionization energy was 70 eV. The full scanning mode was used to select the characteristic ions (m/z 85) for the detection of n-alkanes. The identification and quantification of n-alkanes were achieved by comparing peak areas with external n-alkane standards (mixture of C₂₁, C₂₅, C₂₇, C₂₉, C₃₁ and C₃₃) of known concentration.

Compound-specific carbon and hydrogen isotopes were measured using an HP 6890 GC, interfaced with a Finnigan MAT Delta plus XL isotope ratio mass spectrometry instrument by a high temperature pyrolysis reactor. In all analyses, an Agilent HP-1 MS (30 m × 0.32 mm i.d., 0.25 μm film thickness) capillary column was used, and helium was used as the carrier gas. δ¹³C and δD values are reported in per mil (‰) relative to Vienna Pee Dee Belemnite and Vienna Standard Mean Ocean Water, respectively. Each sample was independently measured thrice. Isotopic measurements were calibrated by coinjected 4–6 n-alkane standards. Detailed experimental procedure of compound-specific carbon and hydrogen isotope can be found in Bi and others (Reference Bi, Sheng and Liu2005). Achieved precision, expressed as the average std dev., was 4‰ for the standard mixtures.

2.4. n-Alkane indices

The carbon preference index (CPI) and the average chain length (ACL) were used as n-alkane indices. The CPI measures the odd-over-even carbon number predominance of the n-alkane, which is used to characterize the sources of n-alkane (Eglinton and Hamilton, Reference Eglinton and Hamilton1967). The ACL of n-alkanes from the higher plant is predominantly a function of vegetation type (Li and others, Reference Li, Li and Tian2013). The CPI and ACL were calculated as follows:

(1)$$\eqalign{& {\rm CP}{\rm I}_{n \hbox{-}{\rm alkanes}} = \cr & \quad \displaystyle{1 \over 2} \times \left[{\displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{25}-{\rm C}_{33}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{24}-{\rm C}_{32}} ) }} + \displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{25}-{\rm C}_{33}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{26}-{\rm C}_{34}} ) }}} \right]}$$

(2)$$\eqalign{& {\rm AC}{\rm L}_{n \hbox{-}{\rm alkanes}} = \cr & \quad \displaystyle{{27 \times {\rm C}_{27} + 29 \times {\rm C}_{29} + 31 \times {\rm C}_{31} + 33 \times {\rm C}_{33} + 35 \times {\rm C}_{35}} \over {{\rm C}_{27} + {\rm C}_{29} + {\rm C}_{31} + {\rm C}_{33} + {\rm C}_{35}}}\;}$$

(3)$$\eqalign{& {\rm CP}{\rm I}_{18\colon 1-24\colon 1} = \cr & \quad \displaystyle{1 \over 2} \times \left[{\displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{19\colon 1}-{\rm C}_{23\colon 1}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{18\colon 1}-{\rm C}_{22\colon 1}} ) }} + \displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{19\colon 1}-{\rm C}_{23\colon 1}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{20\colon 1}-{\rm C}_{24\colon 1}} ) }}} \right]\;}$$

(4)$$\eqalign{& {\rm CP}{\rm I}_{23\colon 1-30\colon 1} = \cr & \quad \displaystyle{1 \over 2} \times\! \left[{\displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{23\colon 1}-{\rm C}_{29\colon 1}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{22\colon 1}-{\rm C}_{28\colon 1}} ) }} + \displaystyle{{\mathop \sum \nolimits_{{\rm odd}} ( {{\rm C}_{23\colon 1}-{\rm C}_{29\colon 1}} ) } \over {\mathop \sum \nolimits_{{\rm even}} ( {{\rm C}_{24\colon 1}-{\rm C}_{30\colon 1}} ) }}} \right]\;}}$$

Odd-over-even predominance (OEP), similar to CPI, is another n-alkane ratio proxy for the predominance of odd-over-even. The OEP of n-alkanes and n-alkenes were calculated as follows:

(5)$${\rm OE}{\rm P}_{n \hbox{-}{\rm alkanes}} = \displaystyle{{{\rm C}_{27} + {\rm C}_{29} + {\rm C}_{31}} \over {{\rm C}_{26} + {\rm C}_{28} + {\rm C}_{30}}}$$

(6)$${\rm OE}{\rm P}_{17\colon 1-22\colon 1} = \displaystyle{{{\rm C}_{17\colon 1} + {\rm C}_{19\colon 1} + {\rm C}_{21\colon 1}} \over {{\rm C}_{18\colon 1} + {\rm C}_{20\colon 1} + {\rm C}_{22\colon 1}}}\;$$

(7)$${\rm OE}{\rm P}_{23\colon 1-30\colon 1} = \displaystyle{{{\rm C}_{23\colon 1} + {\rm C}_{25\colon 1} + {\rm C}_{27\colon 1} + {\rm C}_{29\colon 1}} \over {{\rm C}_{24\colon 1} + {\rm C}_{26\colon 1} + {\rm C}_{28\colon 1} + {\rm C}_{30\colon 1}}}\;$$

3. Results and discussion

3.1. Concentrations of the aliphatic hydrocarbons in cryoconites

The concentrations of the n-alkanes and n-alkenes in cryoconite samples of different glaciers varied from 36.5 to 496.1 μg g⁻¹ and from 4.8 to 17.0 μg g⁻¹, respectively (Fig. 2). The average concentration of n-alkanes was highest in the YL Snow Mountains (496.1 ± 68.0 μg g⁻¹) and the lowest in the DKMD Glacier (36.5 ± 3.9 μg g⁻¹). The average concentrations of n-alkanes were 118.9 ± 5.7 μg g⁻¹ in the TS Glacier, 76.0 ± 2.7 μg g⁻¹ in the LHG Glacier, 70.2 ± 8.2 μg g⁻¹ in the QY Glacier, and 49.8 ± 6.4 μg g⁻¹ in the YZF Glacier. Similarly, the highest total concentration of n-alkenes was measured in the YL Snow Mountains (17.0 ± 5.0 μg g⁻¹), while the lowest concentration was measured in the YZF Glacier (4.8 ± 2.1 μg g⁻¹) rather than in the DKMD Glacier (7.2 ± 0.7 μg g⁻¹). Overall, n-alkanes are 4.6–29.3 times larger than the corresponding carbon number of n-alkenes in six glaciers. Moreover, the concentrations of n-alkane in the cryoconites were much higher than those previously reported in Antarctic soil (0.013–2.2 μg g⁻¹ from n-C₁₄ to n-C₃₅) (Matsumoto and others, Reference Matsumoto, Akiyama and Watanuki1990), but lower than those in the cryoconites reported in Western Canada (290–2990 μg g⁻¹ from n-C₁₈ to n-C₃₃) (Pautler and others, Reference Pautler, Dubnick and Sharp2013) and other Antarctic soil (800–13 400 μg g⁻¹ from n-C₁₈ to n-C₃₃) (Xu and others, Reference Xu, Simpson and Eyles2010).

Figure 2.

Concentration of n-alkanes and n-alkenes in cryoconites.

3.2. Distribution characteristics of n-alkanes

Vascular plants typically show a strong odd-over-even carbon number predominant distribution, with a maximum abundance at n-C₂₇, n-C₂₉ or n-C₃₁ (Eglinton and Hamilton, Reference Eglinton and Hamilton1967; Collister and others, Reference Collister, Rieley and Stern1994). The CPI values of n-alkanes from higher plants are generally >5 (Eglinton and Hamilton, Reference Eglinton and Hamilton1967), whereas the n-alkanes from lower organisms, such as bacteria and algae, as well as fossil fuels, have low CPI values close to 1 (El Nemr and others, Reference El Nemr, Moneer and Ragab2016). The average relative abundance of the n-alkanes and n-alkenes in the cryoconites of different glaciers is shown in Figure 3. The cryoconite samples in the DKMD, YZF, QY, LHG and TS glaciers contain a suite of n-alkanes ranging from n-C₁₄ to n-C₃₅, with C _max values at n-C₂₉ or n-C₃₁. This finding is comparable with the distribution of n-alkanes in 154 lacustrine surface sediments on the TP (Xia and others, Reference Xia, Xu and Mügler2008). The n-alkanes in the YL Snow Mountains show the wider distribution ranged from n-C₁₃ to n-C₃₇ compared with the other five glaciers. However, the n-alkanes of n-C₁₈, n-C₁₉ and n-C₂₀ in the DKMD, YZF, QY, LHG and TS glaciers and n-C₁₉ in the YL Snow Mountains are below detection limit. The GC–MS figure is provided in Supplementary information 1.

Figure 3.

Average relative abundances of the n-alkanes and n-alkenes in the cryoconites.

As shown in Table 2, the average CPI values of the n-C₂₅ to n-C₃₃ n-alkanes in the cryoconites varied from 3.3 in the YL Snow Mountains to 8.4 in the QY Glacier (Table 2), similar to the Japan Sea sediments (Yamada and Ishiwatari, Reference Yamada and Ishiwatari1999), showing an obvious odd-over-even carbon number predominance, which indicates that the n-alkanes in cryoconites were derived from higher plants. The CPI value is the lowest in the YL Snow Mountains, because the higher CPI values usually correlate with cold and dry environments, whereas the smaller CPI values usually correlate with warm and humid environments (Ankit and others, Reference Ankit, Mishra and Kumar2017). In addition, CPI values were also related to local vegetation type (Bai and others, Reference Bai, Azamdzhon and Wang2019). The OEP value of n-alkanes in the polluted substrates is in the range of 1.0–1.2 (Snedaker and others, Reference Snedaker, Glynn and Rumbold1995), so OEP can determine whether the sedimentary area is polluted by petroleum hydrocarbons (Snedaker and others, Reference Snedaker, Glynn and Rumbold1995). The OEP of cryoconites ranged from 2.7 to 8.6, indicating that cryoconites on the TP glaciers were not polluted by petroleum and its derivatives. However, previous studies showed that unpolluted environments had stable ACL values (Sikes and others, Reference Sikes, Uhle and Nodder1993). The ACL of >27 represents an input of terrigenous higher plants (Ankit and others, Reference Ankit, Mishra and Kumar2017). In these six glaciers, the ACL of the cryoconite sample varied from 28.8 to 29.3, it was basically stable at ~29, indicating that the main source of n-alkanes in cryoconites was terrestrial higher plants.

Table 2.

Molecular distribution of the n-alkanes and n-alkenes in the cryoconites

“a” indicated the ΣC₂₁⁻/ΣC₂₂⁺ of n-alkanes, “b” indicated the ΣC₂₁⁻/ΣC₂₂⁺ of n-alkenes.

For n-alkanes, the n-C₂₇ and n-C₂₉ are diagnostic of woody plants while the n-C₃₁ is diagnostic of herbaceous plants (Meyers, Reference Meyers2003). Therefore, the n-C₂₇/n-C₃₁ ratio of n-alkanes is usually used to evaluate the relative contribution of herbaceous and woody plants (Meyers, Reference Meyers2003; Bush and McInerney, Reference Bush and McInerney2013). The ratio of n-C₂₇/n-C₃₁ < 1 indicates an input increase of herbaceous plants, and the ratio of n-C₂₇/n-C₃₁ > 1 indicates an input increase of woody plants. The ratio of n-C₂₇/n-C₃₁ in cryoconites ranged from 0.50 and 0.89, which indicates n-alkanes in TP glaciers could be mainly derived from herbaceous plants. The ΣC₂₁⁻/ΣC₂₂⁺ ratio of n-alkanes reflect the relative abundance changes of lower bacterial algae organisms and higher plants (Meyers, Reference Meyers2003). When ΣC₂₁⁻/ΣC₂₂⁺ < 1, it indicates that the soils are in the early development stage and greatly influenced by the input of terrigenous higher plants. The ΣC₂₁⁻/ΣC₂₂⁺ of n-alkanes is between 0.09 and 0.16, indicating that n-alkanes in cryoconites mainly come from terrigenous higher plants, and cryoconite has not been formed in soil.

3.3. Distributions and source of n-alkenes

The n-alkenes ranged from C_17:1 to C_30:1 with a C _max value of n-C_19:1 and were detected in all cryoconite samples (Fig. 3). The relatively n-alkene abundance of C_17:1−C_20:1 in DKMD, YZF, QY, LHG, and TS glaciers were higher than that of C_21:1−C_30:1. Moreover, n-alkenes with C_17:1−C_30:1 were also detected in lower concentration than the corresponding n-alkanes. The n-alkenes in the YL Snow Mountains showed a bimodal distribution ranging from C_17:1 to C_33:1. The main peak was C_19:1 and the second peak was C_28:1, which was in line with the occurrence of the n-alkenes in lake sediments (Pu and others, Reference Pu, Wang and Meyers2017) and Antarctic soils (Matsumoto and others, Reference Matsumoto, Akiyama and Watanuki1990). The CPI_17:1–23:1 and OEP_17:1–22:1 of n-alkenes were 4.40 and 6.32, respectively. Meanwhile, the CPI_23:1–30:1 and OEP_23:1–30:1 of n-alkenes were 0.39 and 0.37 in the YL Snow Mountains, respectively. This demonstrated an obvious odd-over-even carbon number predominance from C_17:1 to C_22:1, and an even-over-odd carbon number preference from C_23:1 to C_30:1 in the YL Snow Mountains. The CPI_17:1–23:1 and OEP_17:1–22:1 of n-alkenes in the DKMD, YZF, LHG and TS glaciers ranged from 0.89 to 1.35 and from 0.76 to 0.98, respectively. The CPI_23:1–30:1 and OEP_23:1–30:1 of n-alkenes in the DKMD, YZF, LHG and TS glaciers ranged from 0.56 to 0.96 and from 0.65 to 0.91, respectively. This showed a weak even-over-odd carbon number preference from C_17:1 to C_30:1 in these four glaciers. In contrast, the CPI_17:1–23:1 and OEP_17:1–22:1 of n-alkenes in the QY Glacier were 1.87 and 1.03, respectively. The CPI_23:1–30:1 and OEP_23:1–30:1 of n-alkenes in the QY Glacier were 1.15 and 1.12, respectively. This showed a weak odd-over-even carbon number predominance from C_17:1 to C_30:1 in the QY Glacier. The ΣC₂₁⁻/ΣC₂₂⁺ of n-alkenes were 0.69 in the YL Snow Mountains, and ranged from 5.56 to 23.40 in the other five glaciers, demonstrating that n-alkenes in the YL Snow Mountains has a different source from the other five glaciers.

The n-alkenes coexisted with n-alkanes in the cryoconite, but it was inferred that n-alkene has a different source from n-alkane due to the unusual distribution characteristic, different carbon number ranges and C _max between them. Previous studies indicated that n-alkenes could originate from rare cases, e.g. the epicuticular waxes of higher plants (Grimalt and Albaigés, Reference Grimalt and Albaigés1990; Pu and others, Reference Pu, Cao and Jia2018), algae, fungi and cyanobacteria organisms living in aquatic environment (Patterson, Reference Patterson1967; Gelpi and others, Reference Gelpi, Oró and Schneider1968, Reference Gelpi, Schneider and Mann1970; Matsumoto and others, Reference Matsumoto, Akiyama and Watanuki1990), the reduction of diagenesis of monounsaturated fatty acids for the main source of n-alkene (Ekpo and others, Reference Ekpo, Oyo-Ita and Wehner2005) and the microbial transformation of the corresponding n-alkanes or direct inputs from organisms (Jaffé and others, Reference Jaffé, Mead and Hernandez2001). The relatively n-alkene abundance of C_17:1–C_20:1 in the DKMD, YZF, QY, LHG and TS glaciers was higher than that of C_21:1–C_30:1, and the n-alkanes of C₁₈, C₁₉ and C₂₀ were below detection limit. This indicated that C_17:1, C_18:1, C_19:1 and C_20:1 n-alkenes in the DKMD, YZF, QY, LHG and TS glaciers could be derived from the microbial transformation of the corresponding n-alkanes. However, n-alkenes were not detected in aerosols and the surface soil around the TP glacier (Matsumoto and others, Reference Matsumoto, Akiyama and Watanuki1990; Bai and others, Reference Bai, Tian and Fang2014), which combined with the special distribution in the cryoconites determines that the n-alkenes in the cryoconites are from microorganisms.

Cryoconite holes are covered by ice lids and snow accumulation on the glacier, which can keep them isolated from the atmosphere. Therefore, wind-borne materials cannot be deposited in cryoconite holes in winter (Foreman and others, Reference Foreman, Sattler and Mikucki2007). During the summer melting season, as the snow and ice on the glacier slowly melt, more and more cryoconite holes are opened to the atmosphere, and more microbes enter or in situ production occurs in the cryoconite holes (Musilova and others, Reference Musilova, Tranter and Wadham2017). According to the previous reports (Takeuchi and others, Reference Takeuchi, Kohshima and Seko2001), a large amount of snow algae is contained in the cryoconite. Singer and others (Reference Singer, Fasching and Wilhelm2012) identified a prominent population of unsaturated aliphatic lipids and peptides in glaciers, which supports the in situ production of these compounds by microorganisms. Antony and others (Reference Antony, Grannas and Willoughby2014) reported that the aliphatic molecules (double bond equivalents per carbon atom) were most likely produced from microbial and algal biomass, constituting ~37–52% of the total formulas assigned to each glacial ice sample. Moreover, a large number of cyanobacteria have been identified on glacial surfaces on the TP (Takeuchi and Li, Reference Takeuchi and Li2008; Liu and others, Reference Liu, Yao and Jiao2009; Feng and others, Reference Feng, Xu and Kang2016). Therefore, we conclude that n-alkene identified in the cryoconites from the TP glacier may have been mainly produced in situ by various microbes. More knowledge is needed about the biosynthetic origins of long-chain n-alkenes and factors deciding their accumulation in cryoconites.

3.4. Carbon stable isotope compositions of n-alkanes

Figure 4 illustrates the δ¹³C values of the n-alkanes (n-C₂₇, n-C₂₉, n-C₃₁) in the cryoconite samples of six glaciers. Generally, the δ¹³C values of the n-C₂₉ and n-C₃₁ alkanes in most glaciers were more negative than those of the n-C₂₇ alkanes. The difference was likely due to the involvement of different biosynthetic pathways of these long-chain n-alkanes (Collister and others, Reference Collister, Rieley and Stern1994). In this study, the δ¹³C values of the n-C₂₉ and n-C₃₁ n-alkanes were more negative in the YL Snow Mountains than those of the other five glaciers. The reason may be that YL Snow Mountains are located in the southeastern TP and are a part of the southern Hengduan Mountains, which have a warm, humid climate and surrounded by dense vegetation. In contrast, cold and dry climate highly influence the other five glaciers, because the compound-specific carbon isotopic values of the long-chain n-alkanes are sensitive to regional moisture (Wiltshire and others, Reference Wiltshire, Waine and Grabowski2023). A previous study indicated that ¹³C enrichment in plants occurs under drought stress because leaf stomata are closed to prevent transpiration. Therefore, the ratio of intercellular to the ambient CO₂ concentration decreases, resulting in increased carbon isotope fractionation (Diefendorf and Freimuth, Reference Diefendorf and Freimuth2017). DKMD and YZF glaciers in the central TP are surrounded by Taklamakan Desert and the Qaidam Basin with sparse vegetation coverage and arid climate. In summer, the evaporation of the soil water and leaf surface water is stronger in the central TP than in the YL Snow Mountains and QY, LHG and TS glaciers. Therefore, the largest δ¹³C values of the n-C₂₇, n-C₂₉ and n-C₃₁ n-alkanes ranged from −32.0 to −31.4‰ in the YZF Glacier and from −33.5 to −31.7‰ in the DKMD Glacier. The TS, QY and LHG glaciers are all located in the northern edge of the TP, which has a lower relative humidity than the southeastern TP. The δ¹³C values of the n-C₂₇, n-C₂₉ and n-C₃₁ n-alkanes detected in the northern edge of TP were at intermediate levels, ranging from −33.7 to −32.9‰ in the TS Glacier, from −33.3 to −30.8‰ in the QY Glacier and from −34.5 to −31.9‰ in the LHG Glacier. Similar n-alkane distributions have been documented in the snow pits from the TP glacier (Xie and others, Reference Xie, Yao and Kang2000; Li and others, Reference Li, Wang and Wu2009), fresh snow in Sapporo, northern Japan (Yamamoto and others, Reference Yamamoto, Kawamura and Seki2011) and fresh snow in Hokkaido, Japan (Sankelo and others, Reference Sankelo, Kawamura and Seki2013).

Figure 4.

Box plots of the mean δ¹³C values of the n-alkanes in all six glaciers.

The δ¹³C of n-alkanes can provide information on the source of organic materials (Aichner and others, Reference Aichner, Herzschuh and Wilkes2010a). For example, plants with different photosynthetic pathways have distinctive δ¹³C values: C₃ plants produce long-chain n-alkanes with δ¹³C values between −32 and −39‰, while C₄ plants have δ¹³C values between −18 and −25‰ (Collister and others, Reference Collister, Rieley and Stern1994). A quantitative estimate of the relative contribution of C₃ plants, compared to that of C₄ plants, was conducted using the δ¹³C of the n-alkanes and a binary mixing model. The isotopic composition and relative abundance of n-C₂₇, n-C₂₉, and n-C₃₁ alkanes in each cryoconite sample were used to calculate their weighted mean δ¹³C values (δ¹³C_mean) (Eqn (8)). According to a previous study, the endmember values for wax n-alkanes should be −36‰ for C₃ plants and −21‰ for C₄ plants (Collister and others, Reference Collister, Rieley and Stern1994). In Eqn (8), C_n is the concentration (units). The relative contribution of C₃ plants (f) was calculated using Eqn (9). The hydrogen isotope compositions and relative abundances of the n-C₂₇, n-C₂₉ and n-C₃₁ alkanes in each cryoconite sample were used to calculate their weighted mean δD values (δD_mean) (Eqn (10)):

(8)$${\rm \delta }^{13}{\rm C}_{{\rm mean}} = \displaystyle{{{\rm \delta }^{13}{\rm C}_{27} \times {\rm C}_{27} + {\rm \delta }^{13}{\rm C}_{29} \times {\rm C}_{29} + {\rm \delta }^{13}{\rm C}_{31} \times {\rm C}_{31}} \over {{\rm C}_{27} + {\rm C}_{29} + {\rm C}_{31}}}\;$$

(9)$${\rm \delta }^{13}{\rm C}_{{\rm mean}} = f\;( {-36\perthousand } ) + ( {1-f} ) \;( {-21\perthousand } ) \;$$

(10)$${\rm \delta }D_{{\rm mean}} = \displaystyle{{{\rm D}_{27} \times {\rm C}_{27} + {\rm D}_{29} \times {\rm C}_{29} + {\rm D}_{31} \times {\rm C}_{31}} \over {{\rm C}_{27} + {\rm C}_{29} + {\rm C}_{31}}}$$

As shown in Table 3, the weighted mean δ¹³C values (δ¹³C_mean) of the n-C₂₇, n-C₂₉ and n-C₃₁ n-alkanes in all cryoconite samples ranged from −31.7 to −34.8‰. The δ¹³C_mean values were −34.8‰ ± 1.4 in the YL Snow Mountains, −32.9‰ ± 1.3 in the DKMD Glacier, −31.7‰ ± 0.3 in the YZF Glacier, −32.2‰ ± 1.3 in the QY Glacier, −33.7‰ ± 1.4 in the LHG Glacier and −33.2 ± 0.5‰ in the TS Glacier. The application of the two endmember mixing models revealed that C₃ plants contributed 71.3–92.2% of the total n-alkanes in the cryoconites from all glaciers (Table 3), indicating that C₃ plants were the dominant source of the n-alkanes in the cryoconites on the TP glaciers.

Table 3.

δD and δ¹³C values of individual n-alkanes in cryoconites

^a Abundance weighted mean δ¹³C value (δ¹³C_mean‰) of the n-C₂₇, n-C₂₉ and n-C₃₁ n-alkanes.

^b Abundance weighted mean δD values (δD_mean‰) of the n-C₂₇, n-C₂₉ and n-C₃₁ n-alkanes.

3.5. Hydrogen stable isotope compositions of n-alkanes

In all cryoconites, the δD values of the predominant odd carbon number of n-alkanes could be measured, but that of the even carbon number of n-alkanes could not be measured due to their low abundance. The measured δD values of the odd carbon number of n-alkanes (n-C₂₇, n-C₂₉ and n-C₃₁) and weighted mean δD values (δD_mean‰) of the n-C₂₇, n-C₂₉ and n-C₃₁ are listed in Table 3. Previous studies indicated that the δD_wax value of woody, shrub and herbaceous plants decreased sequentially (Hou and others, Reference Hou, D'Andrea and MacDonald2007). In this study, the C _max value was n-C₃₁ in the cryoconite of the DKMD and YZF glaciers, which was consistent with the vegetation coverage around the DKMD and YZF glaciers, indicating that the terrestrial n-alkanes in the cryoconite of these two glaciers mainly came from herbaceous plants around the glaciers. Therefore, the δD_mean of the DKMD Glacier was the most negative (−203.5 ± 9.1) among the six glaciers. However, the YZF Glacier is located near the Qaidam Basin with cold and dry climate, so the δD_mean value of the YZF Glacier was relatively more positive (−174 ± 6.1) than the DKMD Glacier. In the cryoconites of the other four glaciers, including YL Snow Mountains and QY, LHG and TS glaciers, the C _max was n-C₂₉ in these four glaciers, which mainly came from woody plants and shrubs. The vegetation and precipitation of YL Snow Mountains was more abundant than that of the other five glaciers. As a result, the δD_mean value of YL Snow Mountains was the only second negative (−183.5 ± 7.2) to the DKMD Glacier. The δD_mean values from DKMD and YL Snow Mountains were more negative than that from the other four glaciers, consistent with the local river and water vapor isotopic characteristics (Li and Garzioner, Reference Li and Garzione2017). The TS, QY and LHG glaciers all belong to cold and dry climate, but the air temperature and precipitation in the TS Glacier were higher than those in the QY and LHG glaciers. However, the woody plants, shrubs and vegetation in the TS Glacier were more abundant than in the QY and LHG glaciers. Therefore, the δD_mean value of TS Glacier (−178.1 ± 13.6) was more negative than that of QY Glacier (−170.7 ± 6.3) and LHG Glacier (−162.7 ± 6.8).

The plot of δ¹³C versus δD of six glaciers is presented in Figure 5. The weighted mean δ¹³C values of the n-C₂₇, n-C₂₉ and n-C₃₁ alkanes ranged from −33.3 to −35.0‰ in all the cryoconite samples, which were within the range of C₃ leaf wax (−30 to −37.9‰) (Bi and others, Reference Bi, Sheng and Liu2005). Moreover, the δ¹³C_mean values of the n-alkanes in the cryoconites were consistent with the snow in Hokkaido (−28.2 to −34.4‰) (Sankelo and others, Reference Sankelo, Kawamura and Seki2013) and lake sediment on the TP (−20.3 to −35.8‰) (Aichner and others, Reference Aichner, Herzschuh and Wilkes2010a). The weighted mean δD values of the n-C₂₇, n-C₂₉ and n-C₃₁ alkanes ranged from −153.0 to −212.3‰ in the cryoconites from the TP glaciers, which were within the range of the δD values of the n-alkane in snow from Hokkaido (−169.9 to −223.1‰) (Sankelo and others, Reference Sankelo, Kawamura and Seki2013), lake sediments on the TP (−158.0 to −237‰) (Aichner and others, Reference Aichner, Herzschuh and Wilkes2010b), and soil on the TP (−142.3 to −277.1‰) (Luo and others, Reference Luo, Peng and Gleixner2011; Bai and others, Reference Bai, Tian and Fang2014). However, these δD_mean values were more negative than those of C₃ plant leaf wax (−95.9 to −209.1‰) (Bi and others, Reference Bi, Sheng and Liu2005), which could be potentially caused by glacial environment and the local climate due to low temperatures.

Figure 5.

Comparison of the average δ¹³C and δD values of the n-alkanes in all six glaciers.

4. Conclusions

This study provides valuable information about the distribution and sources of n-alkanes and n-alkenes in the cryoconite samples of six glaciers on the TP. The n-alkanes ranged from n-C₁₄ to n-C₃₅, with a C _max value at n-C₂₉ or n-C₃₁ in the DKMD, YZF, QY, LHG and TS glaciers, but showed the wider distribution ranging from n-C₁₃ to n-C₃₇ in the YL Snow Mountains. The CPI is higher in the DKMD, YZF, QY, LHG and TS glaciers under cold and dry environments and the lowest in the YL Snow Mountains due to the higher environmental air temperature and precipitation. The ratio of n-C₂₇/n-C₃₁ ranged from 0.50 and 0.89, which indicates n-alkanes in TP glacier were mainly derived from herbaceous plants. The δ¹³C and δD values of the n-alkanes demonstrated that the isotopic signature of n-alkanes in cryoconites correlated with that derived only from vascular plants. The n-alkenes may have been mainly produced in situ by various microbes in the cryoconite due to their special distribution characteristics. The molecular distribution of the n-alkanes and n-alkenes in the cryoconites revealed that both allochthonous and autochthonous materials were essential contributors to the organic matters in the cryoconites on the TP glaciers.

Supplementary material

The supplementary material for this article can be found at https://doi.org/10.1017/jog.2024.23.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (grant Nos. 41971090 and 42371155) and Major Science and Technology Project of Gansu Province (No. 22ZD6FA005). We gratefully thank the fieldwork staff for their hard and excellent glacier sampling work. The authors are grateful to the anonymous reviewers and editor for their valuable comments which have greatly improved this work.

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