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Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau
Quaternary International 321 (2014) 65e70
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Bubble emissions from thermokarst lakes in the QinghaieXizang Plateau Qingbai Wu a, b, *, Peng Zhang a, Guanli Jiang a, Yuzhong Yang a, Yousheng Deng a, Xianbin Wang c a State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Lanzhou, Gansu 730000, China b Beiluhe Observation Station of Frozen Soil Environment and Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Lanzhou, Gansu 730000, China c Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Science, Lanzhou, Gansu 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 25 December 2013
It is important to understand the role of QinghaieXizang Plateau thermokarst lakes in the global atmospheric methane (CH4) and carbon dioxide (CO2) budget. This study investigated the gas components and isotopic characteristics of bubble gas collected from six thermokarst lakes. The major gas component of the bubbles varied greatly among lakes and bubble sources. Nitrogen (N2), oxygen (O2) and carbon dioxide (CO2) were the predominant constituents of bubbles, but argon (Ar) and methane (CH4) were also present. The N2 was primarily atmospheric in origin, although in part likely originated in sediment organic matter. Isotopic analysis of CO2 and CH4 suggested that CO2 in the bubbles was a mixture of CO2 from decomposed lacustrine carbonate and oxidized organic mass, except for CO2 from organic mass oxidized in Bucha Lake. CH4 in bubbles primarily originated from thermogenic sources and old sediment organic matter. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
The concentration of atmospheric CH4 is the highest over 65 e 70 N (Fung et al., 1991; IPCC, 2001), and has risen 58% during recent
Climate change has resulted in widespread permafrost warming and degradation during the last few decades (Smith et al., 2005; Cheng and Wu, 2007; IPCC, 2007; Wu and Zhang, 2008; Romanovsky et al., 2010). Thawing of permafrost can result in ground surface subsidence and thermokarst (van Everdingen, 2005), partly or completely destroying the existing landscape and ecosystem. Thermokarst alters the energy fluxes as well as the water and carbon balance between land and the atmosphere (Osterkamp et al., 2000; Hinzman et al., 2005). A recent study indicated that distribution ebullition seep from thermokarst lakes releases CH4 and CO2 from permafrost, and geological strata underlying permafrost, in Siberia and Alaska (Walter et al., 2006, 2007, 2008). Extrapolation of CH4 fluxes from thermokarst lakes in North Siberia has resulted in estimates of methane emissions from northern wetlands increasing by 10e63% from present conditions (Walter et al., 2006).
decades (Dlugokencky et al., 1998), demonstrating a new feedback with climate warming (Walter et al., 2006). The global circumfluence model predicted that the greatest climate warming would occur at high-altitude zones during the 21st century (IPCC, 2001, 2007). Furthermore, permafrost degradation will lead to increased CH4 emissions in northern lakes and wetlands, thereby increasing the risk of permafrost degradation (Sazonova et al., 2004; Lawrence and Slater, 2005; Schuur and Abbott, 2011). We recently found that distribution ebullition seep leads to a large amount of gas being released from thermokarst lakes in QinghaieXizang Plateau (QXP). Therefore, in this study, the gases bubbling from thermokarst lakes were collected and analyzed for concentration and isotopic characteristics. Moreover, the difference in CO2 and CH4 concentration and its stable isotope between QXP thermokarst lakes and Siberian thaw lakes were compared. 2. Methods
* Corresponding author. State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Donggang West Road 320#, Lanzhou, Gansu 730000, China. E-mail addresses: [email protected], [email protected] (Q. Wu). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.11.028
2.1. Study lakes Thermokarst lakes are widespread in permafrost regions such as the QXP, where many are currently releasing gases via bubbling
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(Fig. 1) for the first time in recorded history. During the summer of 2012, six thermokarst lakes showed unusually high ebullition seep along the QinghaieXizang Highway (QXH) (Fig. 1), and intermittent and continuous ebullition seeps could be seen in the surface of thermokarst lakes. Table 1 provides a detailed description of these lakes. During winter 2012, we further investigated ebullition seeps from thermokarst lakes from the Kunlun Mountains to the Tuotuo River along the QXH. Gas bubbles were present within the lake ice of almost all the thermokarst lakes (Fig. 2). As shown in Fig. 2, the two types of bubble clusters described by Walter et al. (2006) were present, hotspots (a) and kotenoks (b, c, d). Here, we investigated gas collected from thermokarst lakes during summer 2012.
Table 1 Geographical information of thermokarst lakes Name Locations
Latitude ( N) Longitude ( E) Area of lake (m2) PT (m)
LDG BL BLH CL TTH1 TTH2
34.58 34.05 34.83 35.38 34.2 34.28
Liangdaogou Bucha Lake Beiluhe Chumaer River Tuotuo River Tuotuo River
92.75 92.68 92.94 93.48 92.56 92.18
450 100 750 180 320 220
20e40 10e30 30e60 20-40 10e30 10e30
PTdpermafrost thickness.
Resources, Institute of Geology and Geophysics, Chinese Academy of Science. 3. Results 3.1. Gas concentration in bubbles from thermokarst lakes Table 2 shows the concentrations of various gases in bubbles from thermokarst lakes in the study area. N2, O2 and CO2 were the dominant gases, with concentrations of 37.83e87.96% with an average of 66% being observed for N2, 8.41e20.75% with an average of 12.2% being observed for O2, and 0.22e77.62% with an average of 20.73% being observed for CO2 (Fig. 3), respectively. Ar and CH4 were also present, but at lower levels. Ar concentrations ranged from 0.23 to 1.32% with an average of 0.8% and CH4 from 0.0001 to 1.96% with an average of 0.21% (Fig. 3). Bubbles from BL had a relatively lower ratio of N2/Ar and O2/Ar, ranging from 66.54 to 72.85 and 6.36 to 13.15, respectively. However, the ratios of N2/Ar and O2/Ar for the other five thermokarst lakes were similar to those of the atmosphere (83.602 and 22.428, respectively) (Table 2).
Table 2 Gas concentrations in bubbles from thermokarst lakes (%) Location
2.2. Collection of gas We collected bubbles that could be seen by eye on the lake surface of six thermokarst lakes. Gas samples were collected using Nalgene hand-operated vacuum pumps connected to a glass bottle with a sealed valve. First, we inverted a small glass funnel that was connected to the glass bottle onto the gas seep, after which we ran hand-operated vacuum pumps via a sebific duct. During collection, air was repeatedly removed from the glass bottle and sebific duct using hand-operated vacuum pumps. Finally, gas was slowly introduced into the glass bottle. After an adequate amount of gas was obtained, we turned off the sealed valve and brought the gas bottles to the laboratory for analysis. Overall, 13 gas samples were collected from six thermokarst lakes along the QXH.
Liangdaogou
Name N2
LDG-1 LDG-2 LDG-3 LDG-4 Bucha Lake BL-1 BL-2 BL-3 Beiluhe BLH-1 BLH-2 Chumaer High-plain CL Tuotuo River TH1-1 TH1-2 TH2 Atmosphere Atm Dissolved aira DA a
67.10 58.36 37.83 28.05 87.96 85.21 82.90 78.19 85.40 87.38 78.08 18.15 84.92 78.08 16.07
O2
Ar
CO2
CH4
17.21 15.89 10.59 9.06 8.41 11.53 14.97 20.41 2.58 11.32 20.75 3.91 11.31 20.94 12.75
0.82 0.72 0.48 0.50 1.32 1.25 1.13 0.93 0.31 1.05 0.94 0.23 0.99 0.93 0.42
14.87 25.02 51.10 62.38 2.15 1.72 0.99 0.38 9.55 0.23 0.22 77.62 2.77
0.004 81.83 20.99 0.001 80.72 21.98 0.003 78.32 21.93 0.0023 56.56 18.27 0.131 66.54 6.36 0.281 68.17 9.22 0.002 72.85 13.15 0.091 83.72 21.85 1.9621 276.08 8.352 0.0 83.26 10.78 0.003 82.45 21.91 0.062 78.26 17.0 0.0 85.96 85.96 83.60 22.42 38.01 30.14
N2/Ar
O2/Ar
Value at 0 C under gasewater saturated conditions.
2.3. Gas concentration and isotope analysis Gas samples were analyzed using an MAT-271 mass spectrometer. The carbon isotopic composition of CH4 and CO2 were measured using a GCeCeMS (Delta Plus XL mass spectrometer) with a d13C value (PDB) precision of 0.5&. All gas analysis was conducted in the Laboratory of Lanzhou Center for Oil and Gas
CH4 concentrations differed by orders of magnitude between thawing lakes in North Siberia and the Arctic, where CH4 accounted for 73e99% of the ebullition gas by volume (Walter et al., 2008; 2010). As shown in Table 2 and Fig. 3, CO2 and CH4 concentrations changed greatly in different locations of the thermokarst lakes. The CO2 and CH4 concentrations for the continuous ebullition
Fig. 1. Gas release from thermokarst lakes in the QinghaieXizang Plateau, a, Bucha Lake (BL), b, Beiluhe (BLH), c, Liangdaogou (LDG), and d, Tuotuohe1 (TH1).
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Fig. 2. Photographs of bubbles trapped in lake ice.
seeps (9.55% and 1.96%, respectively for BLH, and 77.62% and 0.062%, respectively for TH1) were greater than those of the intermittent ebullition seeps (0.38% and 0.09%, respectively, for BLH, and 0.22% and 0.0028%, respectively, for TH1). 3.2. CH4 and CO2 isotopes in bubbles from thermokarst lakes Because gas concentration of CO2 and CH4 is not sufficient to analyze isotope compositions, only part CH4 and CO2 isotope compositions of bubbles from these thermokarst lakes were measured. Fig. 4 shows the results of CH4 and CO2 isotope compositions of bubbles from thermokarst lakes of QXP. The d13CPDB isotope of CO2 varied from 4.2 to 15.9&, but was less than 10& at Bucha Lake (Fig. 4), while the d13CPDB isotope of CH4 varied from 24.6 to 28.3& (Fig. 4). The d13CPDB values of CO2 and CH4 from intermittent ebullition seeps were lighter than those from continuous ebullition seeps. 4. Discussion 4.1. Characteristics and relevance of N2, O2, CO2, Ar and CH4 in bubbles 4.1.1. Change in N2/Ar ratio and N2 source N2 is a common gas, but its content varies greatly owing to differences in geological and geochemical backgrounds. Accordingly, investigation of the geochemical behavior of N2 will facilitate understanding of the source and evolution of N2 in bubbles from thermokarst lakes. N2 can originate from a variety of sources, including (1) the atmosphere, (2) microbial denitrification, (3) ammonification of immature sedimentary organic material, (4)
thermal evolution, thermal cracking, and thermal metamorphism, and (5) inorganic nitrogen fixation under ultrahigh temperature metamorphism. Many studies have shown that N2 from different sources has different geochemical characteristics in rocks, soils, and water. N2 and Ar in atmosphere are gases with relatively stable chemical properties, representing about 78.084% and 0.934% of atmosphere volume, respectively, and N2/Ar equals 84.4. N2 and Ar are dissolving gases at 0 C and N2/Ar, is about 38.012 when gas and water are saturated. The ratio of N2/Ar in the atmosphere varies from 38.0 to 84.4, and therefore, this value is used to identify the origin of N2. N2 is considered to originate from the atmosphere when N2/Ar < 38 and to have non-atmospheric origins when N2/ Ar >> 38. The N2 concentration of Bucha Lake ranged from 82.9% to 87.96% with an average of 85.36% (Table 2). That of Liangdaogou Lake ranged from 28.05% to 67.1% with an average of 47.84%. That of Beiluhe Lake ranged from 78.19% to 85.4%, with an average of 81.80%; and that of Tuotuo Lake ranged from 18.15% to 84.92% with an average of 60.38%. The average ratio of N2/Ar is about 69.19 for Bucha Lake, 74.36 for Liangdaogou Lake, 179.9 for Beiluhe Lake, and 82.44 for Tuotuo Lake, respectively, which is greater than the N2/Ar ratio of dissolved air (38.012), but less than that of air (83.602). Accordingly, the N2 in the sampled bubbles had both atmospheric and non-atmospheric origins, indicating that they may be influenced by sediment organic matter at the bottom of the lakes. 4.1.2. O2 and N2 distribution in bubbles from thermokarst lakes Bubbles from thermokarst lakes contain O2 at levels lower than those of the atmosphere (20.948%). In bubbles from the thermokarst lakes, an inverse relationship between N2 and O2
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There was a large difference in CH4 concentrations in bubbles collected from intermittent and continuous ebullition seeps in BLH and TH1, with values of 0.091% and 1.96% being observed in the former and about 0.002% and 0.062% in the latter, respectively. Bubbles in Bucha Lake (BL), Chumaerhe Lake (CL), Beiluhe Lake (BLH), and Tuotuo2 Lake (TH2) contained high levels of N2 and low levels of CO2, showing a positive correlation (Fig. 6a), while a negative correlation was observed for Liangdaogou Lake (LDG) and Tuotuo1 Lake (TH1) (Fig. 6a). N2 and CH4 in bubbles showed uncertain relationships (Fig. 4b). However, N2 and CH4 in bubbles from Bucha Lake (BL) had an approximately linear relationship (Fig. 6b), which was unique among the investigated lakes. Fig. 3. CO2 and CH4 concentration of bubbles from thermokarst lakes.
4.2. Isotopes of CO2 in bubbles from thermokarst lake 13
concentration occurred at the break point of atmospheric N2 and O2 concentration (Fig. 5a). Specifically, the N2 concentration in bubbles increased as the O2 concentration increased (Fig. 5a) in Liangdaogou Lake, while the opposite was true in Bucha Lake (Fig. 5a), indicating differences in the N2 and O2 sources. Bubbles in Bucha Lake contained N2 of atmospheric and non-atmospheric origin (Fig. 5b), while those in Liangdaogou Lake primarily consisted of atmospheric N2 (Fig. 5b). Bubbles of anaerobic lacustrine sediment contain free O2 that has diffused into the bubbles, which may be a result of photosynthesis of sediments surface beneath the lake base. Aerobic processes are related to methane oxidation (Walter et al., 2007). 4.1 3. CO2 and CH4 in bubbles from thermokarst lake The concentration of CO2 in bubbles varied greatly (0.22%e 62.38%) among thermokarst lakes and different locations within individual lakes. Bubbles in the LDG contained the highest CO2 levels (14.87%e62.38%, average ¼ 38.34%), while those in Bucha Lake contained the lowest concentrations (0.98%e2.15%, average ¼ 1.62%). There was also a large difference in CO2 levels in bubbles from point sources and hotspots in BLH and TH1, with levels of 0.38% and 9.55% observed for the former and 0.22% and 77.62% for the latter, respectively. As shown in Table 2, CH4 was present in bubbles from Bucha Lake (0.131%, 0.281%) and Beiluhe Lake (up to 1.96%), while the other 10 gas samples had lower levels.
Variations in d CCO2 are closely related to the carbon source. 13 The d CCO2 value of decomposed lacustrine carbonate is 5.0&, 13 while that of atmospheric carbon is about 7.0&, and d CCO2 from organic mass oxidation is usually lower. Generally, CO2 could be a 13 product of organic mass oxidation when d CCO2 is less than 7.0&. 13 The d CCO2 in bubbles from thermokarst lakes ranges from 11.1& to 26.2& in Shuchi Lake of Siberia, and 9.6& to 19.6& in Tube Dispenser Lake of Siberia, indicating that it is the product of organic mass oxidation (Walter et al., 2008). 13 The d CCO2 in bubbles collected in the present study ranged from 4.2& to 15.9&, and could be divided into two areas (A and 13 B in Fig. 7). The variation of d CCO2 in area A was large, ranging from 8.4& to 26.2&. However, the variation in CO2 concentra13 tion was small (0.2%e2.15%). In contrast, the variation of d CCO2 in area B of Fig. 7 was small, ranging from more than 5.0& to less than 7.0&. However, the variation in CO2 concentration was large, ranging from 0.22% to 77.62%. For area A, variation in the CO2 13 concentration in bubbles was small, while d CCO2 was larger than that of the atmosphere, implying that the majority of CO2 present had been released by organic mass oxidation. Remarkably, although 13 the d CCO2 of area B varied from 4.2& to 7.7&, the CO2 concentration in bubbles (except for those from Tuotuo1 Lake) was high (12.21%e77.62%). It is difficult to explain this change based on trace CO2 from the atmosphere. Accordingly, these findings indicate that CO2 in the bubbles collected from area B are mixed products of dominant lacustrine carbonate decomposition and oxidation of organic matter. 4.3. Methane production pathways in bubbles 13
Fig. 4. CO2 (a) and CH4 (b) isotope composition of bubbles from thermokarst lakes.
The d CCH4 d values could only be determined for six gas sam13 ples owing to the CH4 concentrations. The d CCH4 values ranged from 28.3& to 17.99&, indicating that the methane had ther13 mogenic origins (d CCH4 varies from 50& to 35& or heavier; C1/C2þ < 100) (Schoell, 1980, 1988; Whiticar et al., 1986; Oremland, 13 1988; Whiticar, 1994, 1999). However, d CCH4 in bubbles 13 approached that of d CCH4 for CH4 produced by acetate fermentation of sediment with an extremely insufficient supply of fresh organic material (27&) (Nakagawa et al., 2002). This may reflect old organic material, isotope composition and the extent of thermal development beneath the current sediments in the thermokarst lakes. The possibility that CH4 was produced by acetate fermentation in an extreme environment is small. 13 13 Fig. 8 shows the relationship between d CCH4 and d CCO2 in bubbles from BL, BLH and TH1 and a comparison with data for lakes in Siberia (Nakagawa et al., 2002; Walter et al., 2008). CH4 in bubbles from thermokarst lakes in QXP differed from that in Shuchi 13 Lake and Tube Dispenser Lake in Siberia. Specifically, the d CCH4 of bubbles in lakes in Siberia indicated an origin of microbial methane (Nakagawa et al., 2002; Walter et al., 2008), whereas that of bubbles
Q. Wu et al. / Quaternary International 321 (2014) 65e70
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Fig. 5. N2 and O2 concentrations (a) and relationship between the ratio of N2/Ar and O2/Ar (b) of bubbles from thermokarst lakes.
Fig. 6. N2eCO2 concentrations (a) and N2eCH4 concentrations (b) in bubbles from thermokarst lakes.
from thermokarst lakes in the QXP was characteristic of thermogenic methane, indicating that the methane originated from old sediment organic materials. The relationships among gas components in bubbles also support this conjecture (Figs. 5 and 7). Moreover, a negative correlation of N2 and O2 and trend of heavier d13 CCH4 and lighter d13 CCO2 indicate geochemical characteristics related to the process of O2 depletion and oxidation of CH4. 5. Conclusions
13
Fig. 7. d CCO2 in bubbles from thermokarst Lakes in the QinghaieTibet Plateau. d13 CCO2 in Lakes of the Shuchi and Tube areas in Siberia (Walter et al., 2008).
Thermokarst lakes, which are widespread in permafrost regions of the QXP, are experiencing a large amount of gas release via unusually distributed ebullition seeps. Gas concentrations and CO2 and CH4 isotopes were analyzed to identify the components and sources of gas being emitted from these lakes. N2, O2, and CO2 were the predominant constituents of bubbles, while argon (Ar) and methane (CH4) were less concentrated. The concentration of CO2 and CH4 varied greatly among thermokarst lakes and bubble source, with CO2 levels of 0.22%e77.62% and CH4 concentrations of 0.001e1.96% being observed. These values differed by orders of magnitude from those observed for thawing lakes in north Siberia and the Arctic. The d13CPDB of CO2 varied from 4.2 to 15.9&, while that of CH4 ranged from 24.6 to 28.3&. Moreover, the d13CPDB values of CO2 and CH4 from intermittent seeps were lighter than those from continuous ebullition seeps. CO2 in bubbles likely primarily originated from decomposed lacustrine carbonate as well as oxidized organic material. d13CCH4 in bubbles from thermokarst Lakes in QXP showed an isotope characteristic of thermogenic methane, indicating that the methane originates from old sediment organic materials. Acknowledgments
13
13
Fig. 8. d CCH4 and d CCO2 in bubbles from thermokarst lakes observed in the present study and in lakes in the Shuchi and Tube areas of Siberia (Walter et al., 2008).
This study was supported by the Key Program of the Chinese Academy of Sciences (Grant No. KZCX2-XB3-03), the Global Change
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Research Program of China (Grant No. 2010CB951402), and the National Natural Science Foundation of China (Grant No. 41121061).
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Bubble emissions from thermokarst lakes in the QinghaieXizang Plateau Qingbai Wu a, b, *, Peng Zhang a, Guanli Jiang a, Yuzhong Yang a, Yousheng Deng a, Xianbin Wang c a State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Lanzhou, Gansu 730000, China b Beiluhe Observation Station of Frozen Soil Environment and Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Lanzhou, Gansu 730000, China c Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Science, Lanzhou, Gansu 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 25 December 2013
It is important to understand the role of QinghaieXizang Plateau thermokarst lakes in the global atmospheric methane (CH4) and carbon dioxide (CO2) budget. This study investigated the gas components and isotopic characteristics of bubble gas collected from six thermokarst lakes. The major gas component of the bubbles varied greatly among lakes and bubble sources. Nitrogen (N2), oxygen (O2) and carbon dioxide (CO2) were the predominant constituents of bubbles, but argon (Ar) and methane (CH4) were also present. The N2 was primarily atmospheric in origin, although in part likely originated in sediment organic matter. Isotopic analysis of CO2 and CH4 suggested that CO2 in the bubbles was a mixture of CO2 from decomposed lacustrine carbonate and oxidized organic mass, except for CO2 from organic mass oxidized in Bucha Lake. CH4 in bubbles primarily originated from thermogenic sources and old sediment organic matter. Ó 2013 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
The concentration of atmospheric CH4 is the highest over 65 e 70 N (Fung et al., 1991; IPCC, 2001), and has risen 58% during recent
Climate change has resulted in widespread permafrost warming and degradation during the last few decades (Smith et al., 2005; Cheng and Wu, 2007; IPCC, 2007; Wu and Zhang, 2008; Romanovsky et al., 2010). Thawing of permafrost can result in ground surface subsidence and thermokarst (van Everdingen, 2005), partly or completely destroying the existing landscape and ecosystem. Thermokarst alters the energy fluxes as well as the water and carbon balance between land and the atmosphere (Osterkamp et al., 2000; Hinzman et al., 2005). A recent study indicated that distribution ebullition seep from thermokarst lakes releases CH4 and CO2 from permafrost, and geological strata underlying permafrost, in Siberia and Alaska (Walter et al., 2006, 2007, 2008). Extrapolation of CH4 fluxes from thermokarst lakes in North Siberia has resulted in estimates of methane emissions from northern wetlands increasing by 10e63% from present conditions (Walter et al., 2006).
decades (Dlugokencky et al., 1998), demonstrating a new feedback with climate warming (Walter et al., 2006). The global circumfluence model predicted that the greatest climate warming would occur at high-altitude zones during the 21st century (IPCC, 2001, 2007). Furthermore, permafrost degradation will lead to increased CH4 emissions in northern lakes and wetlands, thereby increasing the risk of permafrost degradation (Sazonova et al., 2004; Lawrence and Slater, 2005; Schuur and Abbott, 2011). We recently found that distribution ebullition seep leads to a large amount of gas being released from thermokarst lakes in QinghaieXizang Plateau (QXP). Therefore, in this study, the gases bubbling from thermokarst lakes were collected and analyzed for concentration and isotopic characteristics. Moreover, the difference in CO2 and CH4 concentration and its stable isotope between QXP thermokarst lakes and Siberian thaw lakes were compared. 2. Methods
* Corresponding author. State Key Laboratory of Frozen Soil Engineering, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Science, Donggang West Road 320#, Lanzhou, Gansu 730000, China. E-mail addresses: [email protected], [email protected] (Q. Wu). 1040-6182/$ e see front matter Ó 2013 Elsevier Ltd and INQUA. All rights reserved. http://dx.doi.org/10.1016/j.quaint.2013.11.028
2.1. Study lakes Thermokarst lakes are widespread in permafrost regions such as the QXP, where many are currently releasing gases via bubbling
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(Fig. 1) for the first time in recorded history. During the summer of 2012, six thermokarst lakes showed unusually high ebullition seep along the QinghaieXizang Highway (QXH) (Fig. 1), and intermittent and continuous ebullition seeps could be seen in the surface of thermokarst lakes. Table 1 provides a detailed description of these lakes. During winter 2012, we further investigated ebullition seeps from thermokarst lakes from the Kunlun Mountains to the Tuotuo River along the QXH. Gas bubbles were present within the lake ice of almost all the thermokarst lakes (Fig. 2). As shown in Fig. 2, the two types of bubble clusters described by Walter et al. (2006) were present, hotspots (a) and kotenoks (b, c, d). Here, we investigated gas collected from thermokarst lakes during summer 2012.
Table 1 Geographical information of thermokarst lakes Name Locations
Latitude ( N) Longitude ( E) Area of lake (m2) PT (m)
LDG BL BLH CL TTH1 TTH2
34.58 34.05 34.83 35.38 34.2 34.28
Liangdaogou Bucha Lake Beiluhe Chumaer River Tuotuo River Tuotuo River
92.75 92.68 92.94 93.48 92.56 92.18
450 100 750 180 320 220
20e40 10e30 30e60 20-40 10e30 10e30
PTdpermafrost thickness.
Resources, Institute of Geology and Geophysics, Chinese Academy of Science. 3. Results 3.1. Gas concentration in bubbles from thermokarst lakes Table 2 shows the concentrations of various gases in bubbles from thermokarst lakes in the study area. N2, O2 and CO2 were the dominant gases, with concentrations of 37.83e87.96% with an average of 66% being observed for N2, 8.41e20.75% with an average of 12.2% being observed for O2, and 0.22e77.62% with an average of 20.73% being observed for CO2 (Fig. 3), respectively. Ar and CH4 were also present, but at lower levels. Ar concentrations ranged from 0.23 to 1.32% with an average of 0.8% and CH4 from 0.0001 to 1.96% with an average of 0.21% (Fig. 3). Bubbles from BL had a relatively lower ratio of N2/Ar and O2/Ar, ranging from 66.54 to 72.85 and 6.36 to 13.15, respectively. However, the ratios of N2/Ar and O2/Ar for the other five thermokarst lakes were similar to those of the atmosphere (83.602 and 22.428, respectively) (Table 2).
Table 2 Gas concentrations in bubbles from thermokarst lakes (%) Location
2.2. Collection of gas We collected bubbles that could be seen by eye on the lake surface of six thermokarst lakes. Gas samples were collected using Nalgene hand-operated vacuum pumps connected to a glass bottle with a sealed valve. First, we inverted a small glass funnel that was connected to the glass bottle onto the gas seep, after which we ran hand-operated vacuum pumps via a sebific duct. During collection, air was repeatedly removed from the glass bottle and sebific duct using hand-operated vacuum pumps. Finally, gas was slowly introduced into the glass bottle. After an adequate amount of gas was obtained, we turned off the sealed valve and brought the gas bottles to the laboratory for analysis. Overall, 13 gas samples were collected from six thermokarst lakes along the QXH.
Liangdaogou
Name N2
LDG-1 LDG-2 LDG-3 LDG-4 Bucha Lake BL-1 BL-2 BL-3 Beiluhe BLH-1 BLH-2 Chumaer High-plain CL Tuotuo River TH1-1 TH1-2 TH2 Atmosphere Atm Dissolved aira DA a
67.10 58.36 37.83 28.05 87.96 85.21 82.90 78.19 85.40 87.38 78.08 18.15 84.92 78.08 16.07
O2
Ar
CO2
CH4
17.21 15.89 10.59 9.06 8.41 11.53 14.97 20.41 2.58 11.32 20.75 3.91 11.31 20.94 12.75
0.82 0.72 0.48 0.50 1.32 1.25 1.13 0.93 0.31 1.05 0.94 0.23 0.99 0.93 0.42
14.87 25.02 51.10 62.38 2.15 1.72 0.99 0.38 9.55 0.23 0.22 77.62 2.77
0.004 81.83 20.99 0.001 80.72 21.98 0.003 78.32 21.93 0.0023 56.56 18.27 0.131 66.54 6.36 0.281 68.17 9.22 0.002 72.85 13.15 0.091 83.72 21.85 1.9621 276.08 8.352 0.0 83.26 10.78 0.003 82.45 21.91 0.062 78.26 17.0 0.0 85.96 85.96 83.60 22.42 38.01 30.14
N2/Ar
O2/Ar
Value at 0 C under gasewater saturated conditions.
2.3. Gas concentration and isotope analysis Gas samples were analyzed using an MAT-271 mass spectrometer. The carbon isotopic composition of CH4 and CO2 were measured using a GCeCeMS (Delta Plus XL mass spectrometer) with a d13C value (PDB) precision of 0.5&. All gas analysis was conducted in the Laboratory of Lanzhou Center for Oil and Gas
CH4 concentrations differed by orders of magnitude between thawing lakes in North Siberia and the Arctic, where CH4 accounted for 73e99% of the ebullition gas by volume (Walter et al., 2008; 2010). As shown in Table 2 and Fig. 3, CO2 and CH4 concentrations changed greatly in different locations of the thermokarst lakes. The CO2 and CH4 concentrations for the continuous ebullition
Fig. 1. Gas release from thermokarst lakes in the QinghaieXizang Plateau, a, Bucha Lake (BL), b, Beiluhe (BLH), c, Liangdaogou (LDG), and d, Tuotuohe1 (TH1).
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67
Fig. 2. Photographs of bubbles trapped in lake ice.
seeps (9.55% and 1.96%, respectively for BLH, and 77.62% and 0.062%, respectively for TH1) were greater than those of the intermittent ebullition seeps (0.38% and 0.09%, respectively, for BLH, and 0.22% and 0.0028%, respectively, for TH1). 3.2. CH4 and CO2 isotopes in bubbles from thermokarst lakes Because gas concentration of CO2 and CH4 is not sufficient to analyze isotope compositions, only part CH4 and CO2 isotope compositions of bubbles from these thermokarst lakes were measured. Fig. 4 shows the results of CH4 and CO2 isotope compositions of bubbles from thermokarst lakes of QXP. The d13CPDB isotope of CO2 varied from 4.2 to 15.9&, but was less than 10& at Bucha Lake (Fig. 4), while the d13CPDB isotope of CH4 varied from 24.6 to 28.3& (Fig. 4). The d13CPDB values of CO2 and CH4 from intermittent ebullition seeps were lighter than those from continuous ebullition seeps. 4. Discussion 4.1. Characteristics and relevance of N2, O2, CO2, Ar and CH4 in bubbles 4.1.1. Change in N2/Ar ratio and N2 source N2 is a common gas, but its content varies greatly owing to differences in geological and geochemical backgrounds. Accordingly, investigation of the geochemical behavior of N2 will facilitate understanding of the source and evolution of N2 in bubbles from thermokarst lakes. N2 can originate from a variety of sources, including (1) the atmosphere, (2) microbial denitrification, (3) ammonification of immature sedimentary organic material, (4)
thermal evolution, thermal cracking, and thermal metamorphism, and (5) inorganic nitrogen fixation under ultrahigh temperature metamorphism. Many studies have shown that N2 from different sources has different geochemical characteristics in rocks, soils, and water. N2 and Ar in atmosphere are gases with relatively stable chemical properties, representing about 78.084% and 0.934% of atmosphere volume, respectively, and N2/Ar equals 84.4. N2 and Ar are dissolving gases at 0 C and N2/Ar, is about 38.012 when gas and water are saturated. The ratio of N2/Ar in the atmosphere varies from 38.0 to 84.4, and therefore, this value is used to identify the origin of N2. N2 is considered to originate from the atmosphere when N2/Ar < 38 and to have non-atmospheric origins when N2/ Ar >> 38. The N2 concentration of Bucha Lake ranged from 82.9% to 87.96% with an average of 85.36% (Table 2). That of Liangdaogou Lake ranged from 28.05% to 67.1% with an average of 47.84%. That of Beiluhe Lake ranged from 78.19% to 85.4%, with an average of 81.80%; and that of Tuotuo Lake ranged from 18.15% to 84.92% with an average of 60.38%. The average ratio of N2/Ar is about 69.19 for Bucha Lake, 74.36 for Liangdaogou Lake, 179.9 for Beiluhe Lake, and 82.44 for Tuotuo Lake, respectively, which is greater than the N2/Ar ratio of dissolved air (38.012), but less than that of air (83.602). Accordingly, the N2 in the sampled bubbles had both atmospheric and non-atmospheric origins, indicating that they may be influenced by sediment organic matter at the bottom of the lakes. 4.1.2. O2 and N2 distribution in bubbles from thermokarst lakes Bubbles from thermokarst lakes contain O2 at levels lower than those of the atmosphere (20.948%). In bubbles from the thermokarst lakes, an inverse relationship between N2 and O2
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There was a large difference in CH4 concentrations in bubbles collected from intermittent and continuous ebullition seeps in BLH and TH1, with values of 0.091% and 1.96% being observed in the former and about 0.002% and 0.062% in the latter, respectively. Bubbles in Bucha Lake (BL), Chumaerhe Lake (CL), Beiluhe Lake (BLH), and Tuotuo2 Lake (TH2) contained high levels of N2 and low levels of CO2, showing a positive correlation (Fig. 6a), while a negative correlation was observed for Liangdaogou Lake (LDG) and Tuotuo1 Lake (TH1) (Fig. 6a). N2 and CH4 in bubbles showed uncertain relationships (Fig. 4b). However, N2 and CH4 in bubbles from Bucha Lake (BL) had an approximately linear relationship (Fig. 6b), which was unique among the investigated lakes. Fig. 3. CO2 and CH4 concentration of bubbles from thermokarst lakes.
4.2. Isotopes of CO2 in bubbles from thermokarst lake 13
concentration occurred at the break point of atmospheric N2 and O2 concentration (Fig. 5a). Specifically, the N2 concentration in bubbles increased as the O2 concentration increased (Fig. 5a) in Liangdaogou Lake, while the opposite was true in Bucha Lake (Fig. 5a), indicating differences in the N2 and O2 sources. Bubbles in Bucha Lake contained N2 of atmospheric and non-atmospheric origin (Fig. 5b), while those in Liangdaogou Lake primarily consisted of atmospheric N2 (Fig. 5b). Bubbles of anaerobic lacustrine sediment contain free O2 that has diffused into the bubbles, which may be a result of photosynthesis of sediments surface beneath the lake base. Aerobic processes are related to methane oxidation (Walter et al., 2007). 4.1 3. CO2 and CH4 in bubbles from thermokarst lake The concentration of CO2 in bubbles varied greatly (0.22%e 62.38%) among thermokarst lakes and different locations within individual lakes. Bubbles in the LDG contained the highest CO2 levels (14.87%e62.38%, average ¼ 38.34%), while those in Bucha Lake contained the lowest concentrations (0.98%e2.15%, average ¼ 1.62%). There was also a large difference in CO2 levels in bubbles from point sources and hotspots in BLH and TH1, with levels of 0.38% and 9.55% observed for the former and 0.22% and 77.62% for the latter, respectively. As shown in Table 2, CH4 was present in bubbles from Bucha Lake (0.131%, 0.281%) and Beiluhe Lake (up to 1.96%), while the other 10 gas samples had lower levels.
Variations in d CCO2 are closely related to the carbon source. 13 The d CCO2 value of decomposed lacustrine carbonate is 5.0&, 13 while that of atmospheric carbon is about 7.0&, and d CCO2 from organic mass oxidation is usually lower. Generally, CO2 could be a 13 product of organic mass oxidation when d CCO2 is less than 7.0&. 13 The d CCO2 in bubbles from thermokarst lakes ranges from 11.1& to 26.2& in Shuchi Lake of Siberia, and 9.6& to 19.6& in Tube Dispenser Lake of Siberia, indicating that it is the product of organic mass oxidation (Walter et al., 2008). 13 The d CCO2 in bubbles collected in the present study ranged from 4.2& to 15.9&, and could be divided into two areas (A and 13 B in Fig. 7). The variation of d CCO2 in area A was large, ranging from 8.4& to 26.2&. However, the variation in CO2 concentra13 tion was small (0.2%e2.15%). In contrast, the variation of d CCO2 in area B of Fig. 7 was small, ranging from more than 5.0& to less than 7.0&. However, the variation in CO2 concentration was large, ranging from 0.22% to 77.62%. For area A, variation in the CO2 13 concentration in bubbles was small, while d CCO2 was larger than that of the atmosphere, implying that the majority of CO2 present had been released by organic mass oxidation. Remarkably, although 13 the d CCO2 of area B varied from 4.2& to 7.7&, the CO2 concentration in bubbles (except for those from Tuotuo1 Lake) was high (12.21%e77.62%). It is difficult to explain this change based on trace CO2 from the atmosphere. Accordingly, these findings indicate that CO2 in the bubbles collected from area B are mixed products of dominant lacustrine carbonate decomposition and oxidation of organic matter. 4.3. Methane production pathways in bubbles 13
Fig. 4. CO2 (a) and CH4 (b) isotope composition of bubbles from thermokarst lakes.
The d CCH4 d values could only be determined for six gas sam13 ples owing to the CH4 concentrations. The d CCH4 values ranged from 28.3& to 17.99&, indicating that the methane had ther13 mogenic origins (d CCH4 varies from 50& to 35& or heavier; C1/C2þ < 100) (Schoell, 1980, 1988; Whiticar et al., 1986; Oremland, 13 1988; Whiticar, 1994, 1999). However, d CCH4 in bubbles 13 approached that of d CCH4 for CH4 produced by acetate fermentation of sediment with an extremely insufficient supply of fresh organic material (27&) (Nakagawa et al., 2002). This may reflect old organic material, isotope composition and the extent of thermal development beneath the current sediments in the thermokarst lakes. The possibility that CH4 was produced by acetate fermentation in an extreme environment is small. 13 13 Fig. 8 shows the relationship between d CCH4 and d CCO2 in bubbles from BL, BLH and TH1 and a comparison with data for lakes in Siberia (Nakagawa et al., 2002; Walter et al., 2008). CH4 in bubbles from thermokarst lakes in QXP differed from that in Shuchi 13 Lake and Tube Dispenser Lake in Siberia. Specifically, the d CCH4 of bubbles in lakes in Siberia indicated an origin of microbial methane (Nakagawa et al., 2002; Walter et al., 2008), whereas that of bubbles
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Fig. 5. N2 and O2 concentrations (a) and relationship between the ratio of N2/Ar and O2/Ar (b) of bubbles from thermokarst lakes.
Fig. 6. N2eCO2 concentrations (a) and N2eCH4 concentrations (b) in bubbles from thermokarst lakes.
from thermokarst lakes in the QXP was characteristic of thermogenic methane, indicating that the methane originated from old sediment organic materials. The relationships among gas components in bubbles also support this conjecture (Figs. 5 and 7). Moreover, a negative correlation of N2 and O2 and trend of heavier d13 CCH4 and lighter d13 CCO2 indicate geochemical characteristics related to the process of O2 depletion and oxidation of CH4. 5. Conclusions
13
Fig. 7. d CCO2 in bubbles from thermokarst Lakes in the QinghaieTibet Plateau. d13 CCO2 in Lakes of the Shuchi and Tube areas in Siberia (Walter et al., 2008).
Thermokarst lakes, which are widespread in permafrost regions of the QXP, are experiencing a large amount of gas release via unusually distributed ebullition seeps. Gas concentrations and CO2 and CH4 isotopes were analyzed to identify the components and sources of gas being emitted from these lakes. N2, O2, and CO2 were the predominant constituents of bubbles, while argon (Ar) and methane (CH4) were less concentrated. The concentration of CO2 and CH4 varied greatly among thermokarst lakes and bubble source, with CO2 levels of 0.22%e77.62% and CH4 concentrations of 0.001e1.96% being observed. These values differed by orders of magnitude from those observed for thawing lakes in north Siberia and the Arctic. The d13CPDB of CO2 varied from 4.2 to 15.9&, while that of CH4 ranged from 24.6 to 28.3&. Moreover, the d13CPDB values of CO2 and CH4 from intermittent seeps were lighter than those from continuous ebullition seeps. CO2 in bubbles likely primarily originated from decomposed lacustrine carbonate as well as oxidized organic material. d13CCH4 in bubbles from thermokarst Lakes in QXP showed an isotope characteristic of thermogenic methane, indicating that the methane originates from old sediment organic materials. Acknowledgments
13
13
Fig. 8. d CCH4 and d CCO2 in bubbles from thermokarst lakes observed in the present study and in lakes in the Shuchi and Tube areas of Siberia (Walter et al., 2008).
This study was supported by the Key Program of the Chinese Academy of Sciences (Grant No. KZCX2-XB3-03), the Global Change
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Research Program of China (Grant No. 2010CB951402), and the National Natural Science Foundation of China (Grant No. 41121061).
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