bubble emissions from thermokarst lakes in the qinghai–xizang plateau

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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 article info Article history: Available online 25 December 2013 abstract It is important to understand the role of QinghaieXizang Plateau thermokarst lakes in the global atmo- spheric methane (CH 4 ) and carbon dioxide (CO 2 ) 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 (N 2 ), oxygen (O 2 ) and carbon dioxide (CO 2 ) were the predominant constituents of bubbles, but argon (Ar) and methane (CH 4 ) were also present. The N 2 was primarily atmospheric in origin, although in part likely originated in sediment organic matter. Isotopic analysis of CO 2 and CH 4 suggested that CO 2 in the bubbles was a mixture of CO 2 from decomposed lacustrine carbonate and oxidized organic mass, except for CO 2 from organic mass oxidized in Bucha Lake. CH 4 in bubbles primarily originated from thermogenic sources and old sediment organic matter. Ó 2013 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction 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 uxes 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 CH 4 and CO 2 from permafrost, and geological strata un- derlying permafrost, in Siberia and Alaska (Walter et al., 2006, 2007, 2008). Extrapolation of CH 4 uxes 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). The concentration of atmospheric CH 4 is the highest over 65 e 70 N(Fung et al., 1991; IPCC, 2001), and has risen 58% during recent decades (Dlugokencky et al., 1998), demonstrating a new feedback with climate warming (Walter et al., 2006). The global circum- uence 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 CH 4 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 CO 2 and CH 4 concentration and its stable isotope between QXP thermokarst lakes and Siberian thaw lakes were compared. 2. Methods 2.1. Study lakes Thermokarst lakes are widespread in permafrost regions such as the QXP, where many are currently releasing gases via bubbling * 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). Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint 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 Quaternary International 321 (2014) 65e70

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Page 1: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

lable at ScienceDirect

Quaternary International 321 (2014) 65e70

Contents lists avai

Quaternary International

journal homepage: www.elsevier .com/locate/quaint

Bubble emissions from thermokarst lakes in the QinghaieXizangPlateau

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, ChinabBeiluhe Observation Station of Frozen Soil Environment and Engineering, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Science, Lanzhou, Gansu 730000, Chinac 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

Article history:Available online 25 December 2013

* Corresponding author. State Key Laboratory of Fand Arid Regions Environmental and EngineeringAcademy of Science, Donggang West Road 320#, Lan

E-mail addresses: [email protected], wu_qingbai_10

1040-6182/$ e see front matter � 2013 Elsevier Ltd ahttp://dx.doi.org/10.1016/j.quaint.2013.11.028

a b s t r a c t

It is important to understand the role of QinghaieXizang Plateau thermokarst lakes in the global atmo-spheric methane (CH4) and carbon dioxide (CO2) budget. This study investigated the gas components andisotopic characteristics of bubble gas collected from six thermokarst lakes. Themajor gas component of thebubbles varied greatly among lakes and bubble sources. Nitrogen (N2), oxygen (O2) and carbon dioxide(CO2) were the predominant constituents of bubbles, but argon (Ar) andmethane (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 amixture of CO2 from decomposedlacustrine 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.

� 2013 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction

Climate change has resulted inwidespread permafrost warmingand 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 inground surface subsidence and thermokarst (van Everdingen,2005), partly or completely destroying the existing landscape andecosystem. Thermokarst alters the energy fluxes as well as thewater and carbon balance between land and the atmosphere(Osterkamp et al., 2000; Hinzman et al., 2005). A recent studyindicated that distribution ebullition seep from thermokarst lakesreleases CH4 and CO2 from permafrost, and geological strata un-derlying permafrost, in Siberia and Alaska (Walter et al., 2006, 2007,2008). Extrapolation of CH4 fluxes from thermokarst lakes in NorthSiberia has resulted in estimates of methane emissions fromnorthern wetlands increasing by 10e63% from present conditions(Walter et al., 2006).

rozen Soil Engineering, ColdResearch Institute, Chinese

zhou, Gansu 730000, [email protected] (Q. Wu).

nd INQUA. All rights reserved.

The concentration of atmospheric CH4 is the highest over 65�e70�N (Fung et al., 1991; IPCC, 2001), and has risen 58% during recentdecades (Dlugokencky et al., 1998), demonstrating a new feedbackwith climate warming (Walter et al., 2006). The global circum-fluence model predicted that the greatest climate warming wouldoccur at high-altitude zones during the 21st century (IPCC, 2001,2007). Furthermore, permafrost degradation will lead toincreased CH4 emissions in northern lakes and wetlands, therebyincreasing 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 alarge amount of gas being released from thermokarst lakes inQinghaieXizang Plateau (QXP). Therefore, in this study, the gasesbubbling from thermokarst lakes were collected and analyzed forconcentration and isotopic characteristics. Moreover, the differencein CO2 and CH4 concentration and its stable isotope between QXPthermokarst lakes and Siberian thaw lakes were compared.

2. Methods

2.1. Study lakes

Thermokarst lakes are widespread in permafrost regions such asthe QXP, where many are currently releasing gases via bubbling

Page 2: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

Q. Wu et al. / Quaternary International 321 (2014) 65e7066

(Fig. 1) for the first time in recorded history. During the summer of2012, six thermokarst lakes showed unusually high ebullition seepalong the QinghaieXizang Highway (QXH) (Fig. 1), and intermittentand continuous ebullition seeps could be seen in the surface ofthermokarst lakes. Table 1 provides a detailed description of theselakes. During winter 2012, we further investigated ebullition seepsfrom thermokarst lakes from the Kunlun Mountains to the TuotuoRiver along the QXH. Gas bubbles were present within the lake iceof almost all the thermokarst lakes (Fig. 2). As shown in Fig. 2, thetwo types of bubble clusters described by Walter et al. (2006) werepresent, hotspots (a) and kotenoks (b, c, d). Here, we investigatedgas collected from thermokarst lakes during summer 2012.

Table 1Geographical information of thermokarst lakes

Name Locations Latitude (�N) Longitude (�E) Area of lake (m2) PT (m)

LDG Liangdaogou 34.58 92.75 450 20e40BL Bucha Lake 34.05 92.68 100 10e30BLH Beiluhe 34.83 92.94 750 30e60CL Chumaer River 35.38 93.48 180 20-40TTH1 Tuotuo River 34.2 92.56 320 10e30TTH2 Tuotuo River 34.28 92.18 220 10e30

PTdpermafrost thickness.Table 2Gas concentrations in bubbles from thermokarst lakes (%)

Location Name N2 O2 Ar CO2 CH4 N2/Ar O2/Ar

Liangdaogou LDG-1 67.10 17.21 0.82 14.87 0.004 81.83 20.99LDG-2 58.36 15.89 0.72 25.02 0.001 80.72 21.98LDG-3 37.83 10.59 0.48 51.10 0.003 78.32 21.93LDG-4 28.05 9.06 0.50 62.38 0.0023 56.56 18.27

Bucha Lake BL-1 87.96 8.41 1.32 2.15 0.131 66.54 6.36BL-2 85.21 11.53 1.25 1.72 0.281 68.17 9.22BL-3 82.90 14.97 1.13 0.99 0.002 72.85 13.15

Beiluhe BLH-1 78.19 20.41 0.93 0.38 0.091 83.72 21.85BLH-2 85.40 2.58 0.31 9.55 1.9621 276.08 8.352

Chumaer High-plain CL 87.38 11.32 1.05 0.23 0.0 83.26 10.78Tuotuo River TH1-1 78.08 20.75 0.94 0.22 0.003 82.45 21.91

TH1-2 18.15 3.91 0.23 77.62 0.062 78.26 17.0TH2 84.92 11.31 0.99 2.77 0.0 85.96 85.96

Atmosphere Atm 78.08 20.94 0.93 83.60 22.42Dissolved aira DA 16.07 12.75 0.42 38.01 30.14

a Value at 0 �C under gasewater saturated conditions.

2.2. Collection of gas

We collected bubbles that could be seen by eye on the lakesurface of six thermokarst lakes. Gas samples were collected usingNalgene hand-operated vacuum pumps connected to a glass bottlewith a sealed valve. First, we inverted a small glass funnel that wasconnected to the glass bottle onto the gas seep, after which we ranhand-operated vacuum pumps via a sebific duct. During collection,air was repeatedly removed from the glass bottle and sebific ductusing hand-operated vacuum pumps. Finally, gas was slowlyintroduced into the glass bottle. After an adequate amount of gaswas obtained, we turned off the sealed valve and brought the gasbottles to the laboratory for analysis. Overall, 13 gas samples werecollected from six thermokarst lakes along the QXH.

2.3. Gas concentration and isotope analysis

Gas samples were analyzed using an MAT-271 mass spectrom-eter. The carbon isotopic composition of CH4 and CO2 weremeasured using a GCeCeMS (Delta Plus XL mass spectrometer)with a d13C value (PDB) precision of �0.5&. All gas analysis wasconducted in the Laboratory of Lanzhou Center for Oil and Gas

Fig. 1. Gas release from thermokarst lakes in the QinghaieXizang Plateau, a, Buch

Resources, Institute of Geology and Geophysics, Chinese Academyof Science.

3. Results

3.1. Gas concentration in bubbles from thermokarst lakes

Table 2 shows the concentrations of various gases in bubblesfrom thermokarst lakes in the study area. N2, O2 and CO2 were thedominant gases, with concentrations of 37.83e87.96% with anaverage of 66% being observed for N2, 8.41e20.75% with an averageof 12.2% being observed for O2, and 0.22e77.62% with an average of20.73% being observed for CO2 (Fig. 3), respectively. Ar and CH4were also present, but at lower levels. Ar concentrations rangedfrom 0.23 to 1.32% with an average of 0.8% and CH4 from 0.0001 to1.96% with an average of 0.21% (Fig. 3). Bubbles from BL had arelatively lower ratio of N2/Ar and O2/Ar, ranging from 66.54 to72.85 and 6.36 to 13.15, respectively. However, the ratios of N2/Arand O2/Ar for the other five thermokarst lakes were similar to thoseof the atmosphere (83.602 and 22.428, respectively) (Table 2).

CH4 concentrations differed by orders of magnitude betweenthawing lakes in North Siberia and the Arctic, where CH4 accountedfor 73e99% of the ebullition gas by volume (Walter et al., 2008;2010). As shown in Table 2 and Fig. 3, CO2 and CH4 concentra-tions changed greatly in different locations of the thermokarstlakes. The CO2 and CH4 concentrations for the continuous ebullition

a Lake (BL), b, Beiluhe (BLH), c, Liangdaogou (LDG), and d, Tuotuohe1 (TH1).

Page 3: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

Fig. 2. Photographs of bubbles trapped in lake ice.

Q. Wu et al. / Quaternary International 321 (2014) 65e70 67

seeps (9.55% and 1.96%, respectively for BLH, and 77.62% and0.062%, respectively for TH1) were greater than those of theintermittent ebullition seeps (0.38% and 0.09%, respectively, forBLH, 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 toanalyze isotope compositions, only part CH4 and CO2 isotopecompositions of bubbles from these thermokarst lakes weremeasured. Fig. 4 shows the results of CH4 and CO2 isotope com-positions of bubbles from thermokarst lakes of QXP. The d13CPDBisotope 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 fromintermittent ebullition seeps were lighter than those from contin-uous ebullition seeps.

4. Discussion

4.1. Characteristics and relevance of N2, O2, CO2, Ar and CH4 inbubbles

4.1.1. Change in N2/Ar ratio and N2 sourceN2 is a common gas, but its content varies greatly owing to

differences in geological and geochemical backgrounds. Accord-ingly, investigation of the geochemical behavior of N2 will facilitateunderstanding of the source and evolution of N2 in bubbles fromthermokarst 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 temperaturemetamorphism. Many studies have shown that N2 from differentsources has different geochemical characteristics in rocks, soils, andwater.

N2 and Ar in atmosphere are gases with relatively stablechemical properties, representing about 78.084% and 0.934% ofatmosphere volume, respectively, and N2/Ar equals 84.4. N2 and Arare dissolving gases at 0 �C and N2/Ar, is about 38.012 when gas andwater are saturated. The ratio of N2/Ar in the atmosphere variesfrom 38.0 to 84.4, and therefore, this value is used to identify theorigin of N2. N2 is considered to originate from the atmospherewhen 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 Lakeranged from 28.05% to 67.1% with an average of 47.84%. That ofBeiluhe Lake ranged from 78.19% to 85.4%, with an average of81.80%; and that of Tuotuo Lake ranged from 18.15% to 84.92% withan average of 60.38%. The average ratio of N2/Ar is about 69.19 forBucha Lake, 74.36 for Liangdaogou Lake, 179.9 for Beiluhe Lake, and82.44 for Tuotuo Lake, respectively, which is greater than the N2/Arratio of dissolved air (38.012), but less than that of air (83.602).Accordingly, the N2 in the sampled bubbles had both atmosphericand non-atmospheric origins, indicating that they may be influ-enced by sediment organic matter at the bottom of the lakes.

4.1.2. O2 and N2 distribution in bubbles from thermokarst lakesBubbles from thermokarst lakes contain O2 at levels lower than

those of the atmosphere (20.948%). In bubbles from the thermo-karst lakes, an inverse relationship between N2 and O2

Page 4: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

Fig. 3. CO2 and CH4 concentration of bubbles from thermokarst lakes.

Q. Wu et al. / Quaternary International 321 (2014) 65e7068

concentration occurred at the break point of atmospheric N2 and O2concentration (Fig. 5a). Specifically, the N2 concentration in bubblesincreased as the O2 concentration increased (Fig. 5a) in Liangdao-gou Lake, while the opposite was true in Bucha Lake (Fig. 5a),indicating differences in the N2 and O2 sources. Bubbles in BuchaLake contained N2 of atmospheric and non-atmospheric origin(Fig. 5b), while those in Liangdaogou Lake primarily consisted ofatmospheric N2 (Fig. 5b). Bubbles of anaerobic lacustrine sedimentcontain free O2 that has diffused into the bubbles, which may be aresult of photosynthesis of sediments surface beneath the lakebase. Aerobic processes are related to methane oxidation (Walteret al., 2007).

4.1 3. CO2 and CH4 in bubbles from thermokarst lakeThe concentration of CO2 in bubbles varied greatly (0.22%e

62.38%) among thermokarst lakes and different locations withinindividual lakes. Bubbles in the LDG contained the highest CO2levels (14.87%e62.38%, average ¼ 38.34%), while those in BuchaLake contained the lowest concentrations (0.98%e2.15%,average ¼ 1.62%). There was also a large difference in CO2 levels inbubbles from point sources and hotspots in BLH and TH1, withlevels of 0.38% and 9.55% observed for the former and 0.22% and77.62% for the latter, respectively. As shown in Table 2, CH4 waspresent in bubbles from Bucha Lake (0.131%, 0.281%) and BeiluheLake (up to 1.96%), while the other 10 gas samples had lower levels.

Fig. 4. CO2 (a) and CH4 (b) isotope composition of bubbles from thermokarst lakes.

There was a large difference in CH4 concentrations in bubblescollected from intermittent and continuous ebullition seeps in BLHand TH1, with values of 0.091% and 1.96% being observed in theformer 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 lowlevels of CO2, showing a positive correlation (Fig. 6a), while anegative correlation was observed for Liangdaogou Lake (LDG) andTuotuo1 Lake (TH1) (Fig. 6a). N2 and CH4 in bubbles showed un-certain relationships (Fig. 4b). However, N2 and CH4 in bubbles fromBucha Lake (BL) had an approximately linear relationship (Fig. 6b),which was unique among the investigated lakes.

4.2. Isotopes of CO2 in bubbles from thermokarst lake

Variations in d13CCO2are closely related to the carbon source.

The d13CCO2value of decomposed lacustrine carbonate is �5.0&,

while that of atmospheric carbon is about �7.0&, and d13CCO2from

organic mass oxidation is usually lower. Generally, CO2 could be aproduct of organic mass oxidationwhen d13CCO2

is less than�7.0&.The d13CCO2

in bubbles from thermokarst lakes ranges from�11.1&to �26.2& in Shuchi Lake of Siberia, and �9.6& to�19.6& in TubeDispenser Lake of Siberia, indicating that it is the product of organicmass oxidation (Walter et al., 2008).

The d13CCO2in bubbles collected in the present study ranged

from�4.2& to�15.9&, and could be divided into two areas (A andB in Fig. 7). The variation of d13CCO2

in area A was large, rangingfrom �8.4& to �26.2&. However, the variation in CO2 concentra-tion was small (0.2%e2.15%). In contrast, the variation of d13CCO2

inarea B of Fig. 7 was small, ranging from more than �5.0& to lessthan�7.0&. However, the variation in CO2 concentrationwas large,ranging from 0.22% to 77.62%. For area A, variation in the CO2concentration in bubbles was small, while d13CCO2

was larger thanthat of the atmosphere, implying that the majority of CO2 presenthad been released by organic mass oxidation. Remarkably, althoughthe d13CCO2

of area B varied from �4.2& to �7.7&, the CO2 con-centration in bubbles (except for those from Tuotuo1 Lake) washigh (12.21%e77.62%). It is difficult to explain this change based ontrace CO2 from the atmosphere. Accordingly, these findings indicatethat CO2 in the bubbles collected from area B are mixed products ofdominant lacustrine carbonate decomposition and oxidation oforganic matter.

4.3. Methane production pathways in bubbles

The d13CCH4d values could only be determined for six gas sam-

ples owing to the CH4 concentrations. The d13CCH4values ranged

from �28.3& to �17.99&, indicating that the methane had ther-mogenic origins (d13CCH4

varies from �50& to �35& or heavier;C1/C2þ < 100) (Schoell, 1980, 1988; Whiticar et al., 1986; Oremland,1988; Whiticar, 1994, 1999). However, d13CCH4

in bubblesapproached that of d13CCH4

for CH4 produced by acetate fermen-tation of sediment with an extremely insufficient supply of freshorganic material (�27&) (Nakagawa et al., 2002). This may reflectold organic material, isotope composition and the extent of thermaldevelopment beneath the current sediments in the thermokarstlakes. The possibility that CH4 was produced by acetate fermenta-tion in an extreme environment is small.

Fig. 8 shows the relationship between d13CCH4and d13CCO2

inbubbles from BL, BLH and TH1 and a comparisonwith data for lakesin Siberia (Nakagawa et al., 2002; Walter et al., 2008). CH4 inbubbles from thermokarst lakes in QXP differed from that in ShuchiLake and Tube Dispenser Lake in Siberia. Specifically, the d13CCH4

ofbubbles in lakes in Siberia indicated an origin of microbial methane(Nakagawa et al., 2002;Walter et al., 2008), whereas that of bubbles

Page 5: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

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.

Fig. 7. d13CCO2in bubbles from thermokarst Lakes in the QinghaieTibet Plateau.

d13CCO2in Lakes of the Shuchi and Tube areas in Siberia (Walter et al., 2008).

Fig. 8. d13CCH4and d13CCO2

in bubbles from thermokarst lakes observed in the presentstudy and in lakes in the Shuchi and Tube areas of Siberia (Walter et al., 2008).

Q. Wu et al. / Quaternary International 321 (2014) 65e70 69

from thermokarst lakes in the QXP was characteristic of thermo-genic methane, indicating that the methane originated from oldsediment organic materials. The relationships among gas compo-nents in bubbles also support this conjecture (Figs. 5 and 7).Moreover, a negative correlation of N2 and O2 and trend of heavierd13CCH4

and lighter d13CCO2indicate geochemical characteristics

related to the process of O2 depletion and oxidation of CH4.

5. Conclusions

Thermokarst lakes, which are widespread in permafrost regionsof the QXP, are experiencing a large amount of gas release via un-usually distributed ebullition seeps. Gas concentrations and CO2and CH4 isotopes were analyzed to identify the components andsources 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. Theconcentration of CO2 and CH4 varied greatly among thermokarstlakes and bubble source, with CO2 levels of 0.22%e77.62% and CH4concentrations of 0.001e1.96% being observed. These valuesdiffered by orders of magnitude from those observed for thawinglakes 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 intermittentseeps were lighter than those from continuous ebullition seeps.

CO2 in bubbles likely primarily originated from decomposedlacustrine carbonate as well as oxidized organic material. d13CCH4 inbubbles from thermokarst Lakes in QXP showed an isotope char-acteristic of thermogenic methane, indicating that the methaneoriginates from old sediment organic materials.

Acknowledgments

This study was supported by the Key Program of the ChineseAcademy of Sciences (Grant No. KZCX2-XB3-03), the Global Change

Page 6: Bubble emissions from thermokarst lakes in the Qinghai–Xizang Plateau

Q. Wu et al. / Quaternary International 321 (2014) 65e7070

Research Program of China (Grant No. 2010CB951402), and theNational Natural Science Foundation of China (Grant No. 41121061).

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