Permafrost degradation enhances the risk of mercury release on Qinghai-Tibetan Plateau

Journal Pre-proofs Permafrost degradation enhances the risk of mercury release on Qinghai-Tibetan Plateau Cuicui Mu, Paul.F. Schuster, Benjamin.W. Abbott, Shichang Kang, Junming Guo, Shiwei Sun, Qingbai Wu, Tingjun Zhang PII: DOI: Reference:

S0048-9697(19)35119-8 https://doi.org/10.1016/j.scitotenv.2019.135127 STOTEN 135127

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Science of the Total Environment

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11 June 2019 19 September 2019 21 October 2019

Please cite this article as: C. Mu, Paul.F. Schuster, Benjamin.W. Abbott, S. Kang, J. Guo, S. Sun, Q. Wu, T. Zhang, Permafrost degradation enhances the risk of mercury release on Qinghai-Tibetan Plateau, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv.2019.135127

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Permafrost degradation enhances the risk of mercury release on Qinghai-Tibetan Plateau Cuicui Mu1, 2, 3*, Paul. F. Schuster4, Benjamin. W. Abbott5, Shichang Kang2, 6, Junming Guo2, Shiwei Sun2, Qingbai Wu3, Tingjun Zhang1, 7*

1

Key Laboratory of Western China's Environmental Systems (Ministry of Education), College of

Earth and Environmental Sciences, Lanzhou University, Lanzhou, Gansu 730000, China 2

State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and

Resource, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China 3

State Key Laboratory of frozen soil engineering, Northwest Institute of Eco-Environment and

Resource, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China 4

U.S. Geological Survey, National Research Program, Boulder, CO, USA

5 Department

6 CAS

7

of Plant and Wildlife Sciences, Brigham Young University, Provo, Utah, USA

Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100875, China

University Corporation for Polar Research, Beijing 100875, China

Corresponding to C.C. Mu, [email protected]; T.J. Zhang, [email protected] Key words: mercury, permafrost, thermokarst, climate warming, soil organic carbon, Qinghai Tibetan Plateau

Abstract Permafrost on the Qinghai-Tibetan Plateau (QTP) has been degrading in the past decades. While the degradation may mobilize previously protected material from the

permafrost profile, little is known about the stocks and stability of mercury (Hg) in the QTP permafrost. Here we measured total soil Hg in 265 samples from 15 permafrost cores ranging from 3 to 18 m depth, and 45 active layer (AL) soil samples from different land cover types on the QTP. Approximately 21.7 Gg of Hg was stored in surficial permafrost (0-3 m), with 16.58 Gg of Hg was stored in the active layer. Results from six permafrost collapse areas showed that much of the thawed Hg is mobile, with decreases in total Hg mass of 17.6-30.9% for the AL (top 30 cm) in comparison with non-thermokarst surfaces. We conclude that the QTP permafrost region has a large mercury pool, and the stored mercury is sensitive to permafrost degradation.

Key words: mercury, permafrost, thermokarst, climate warming, soil organic carbon, Qinghai Tibetan Plateau

1. Introduction Environmental mercury (Hg) attracts worldwide attention due to its toxicity, ability to bioaccumulation, and threat to human’s health (Wang and Wong, 2003). Soil is a major component of Hg pools and the mobilization of Hg from soils can affect human and ecology health (Driscoll et al., 2013; Obrist et al., 2018). Atmospherically deposited Hg can bind with organic carbon and accumulate in permafrost soils (Turetsky et al., 2006). Recently, the Hg pools in arctic permafrost regions became a major public concern due to the potential release of huge Hg pools from the frozen

soils and the atmospheric elemental Hg can be uptaken by tundra (Obrist et al., 2017). Under global warming scenarios, widespread permafrost degradation has been projected (Slater and Lawrence, 2013) and models predict 30 to 90% of permafrost in the northern hemisphere will disappear by 2100 (Schuur et al., 2015). Because Hg is strongly associated with organic carbon (Olson et al., 2018; Schuster et al., 2018), permafrost degradation may increase the potential release of Hg from thawing permafrost soils. The permafrost regions on the Qinghai-Tibetan Plateau (QTP), covering an area of 1.06×106 km2 (Zou et al., 2017), store approximately 160 Pg organic carbon in the top 25 m (Mu et al., 2015; Mu et al., 2016a), suggesting that this area also potentially stores significant masses of Hg. The effects of soil erosion on the Hg release on the QTP in the future have been evaluated (Liu et al., 2018), while the Hg pools in these regions may also be affected by permafrost degradation since permafrost degradation can lead to the release of soil Hg (Schuster et al., 2018). QTP permafrost is sensitive to climate warming and is undergoing rapid thawing and degradation (Qin et al., 2018; Wu et al., 2015; Wu and Zhang, 2010). Permafrost degradation has been widely recorded on the QTP, including thaw slumps, thermo-erosion gullies, and active layer detachment slides (Niu et al., 2012). The thermokarst formation stimulates horizontal and vertical soil carbon loss (Abbott and Jones, 2015; Mu et al., 2016b), which may thus further accelerate Hg release from affected soils. Estimating the Hg storage and understanding potential Hg loss on the QTP are of great importance to human health because this area is the headwater for many large

Asian rivers which provide water for more than 1.35 billion people (Yang et al., 2014). Since Hg can accumulate in fish tissues (Eaglessmith et al., 2018; Lavoie et al., 2013), release of these Hg pools to the headwaters can potentially affect the downstream people because these rivers provide many fish production for Asian people. However, largely is unknown about the stocks of permafrost Hg on the QTP and their vulnerability to permafrost degradation. In this study, we obtained 265 samples from 15 permafrost cores ranging in the depth from 6 to 18 m, and 45 samples from the active layer (seasonally-thawed surface soil) across the QTP. We analyzed these samples to calculate the total mass of Hg in permafrost regions of the QTP. In addition, to examine the effects of permafrost degradation on the Hg pools, we also selected typical thermokarst landforms to analyze the loss of Hg. This study can provide insights for the assessment of the potential ecological and environmental risk of Hg in the QTP permafrost regions with climate warming.

2. Materials and Methods 2.1 Field sampling The sampling sites were on three representative land cover types of the permafrost regions on the QTP: alpine wet meadow, alpine meadow, and alpine steppe. During 2009 to 2013, 15 permafrost cores varied from 6 m to 18.5 m were drilled by machine drilling rigs (Table S1, Fig. S1). Each core was broken into 30-40 cm sections, which were wrapped, labeled (Fig. S2), and stored in a portable freezer. In addition, we also collected 45 surface samples (Fig. 1) at a depth of 50 cm using a soil auger with a

diameter of 2.5 cm. These 45 soil samples, together with the surface soil samples (17 samples) from the permafrost cores were used to examine the distribution of soil Hg content in the surface soil layer among different land cover types. The 45 soil samples were not included in the Hg pool calculation because of lacking soil bulk density data. We selected six typical thermokarst landscapes (Fig. 2; Tables S2 and S3) to represent permafrost collapse, ground subsidence, thermal erosion and thaw slump (Jorgenson and Osterkamp, 2005) on the QTP. Permafrost collapse observed at the Eboling (EBO) site, and ground subsidence was observed at the eastern Bei Luhe (EBLH) site. Thermal erosion processes were observed at the Fenghuo Shan (FSH) and Qu Malai (QML) sites. Thaw slump was observed at the Bei Luhe (BLH) and northern Bei Luhe (NBLH) sites, where active layer detachment slides and retrogressive thaw slump occurred (Fig. 2). Although the thermokarst area showed great heterogeneities in surface soils and some of the surface layers were removed by erosion, the thermokarst chronosequence can be classified as three grades (Abbott and Jones, 2015; Mu et al., 2017): grade 1, 2 and 3 (Fig. 2). Grade 1 represents the part that has not been affected by thermokarst; grade 2 represents the collapsing part; and grade 3 represents the subsided or exposed part. In the thermokarst areas, we observed the grade 2 and grade 3 areas according to the microfeatures including vegetation cover, erosion state and slumping features (Fig. 2). The surface soil samples at a depth interval of 10 cm for the 0-30 cm layers were collected by a soil auger at the three stages of thermokarst. The samplings at each stage were performed at three transects. At each site, three samples were

collected and then mixed. All samples were transported to laboratory and stored at -20℃ until processing and analysis. Before measurement, all Hg subsamples were placed in a preprocessed, Hg-free ziplock bags. To scale patch-level measurements to the feature level, the perimeters of thermokarst features were measured by ground-based surveying using a GPS, and then the areas were calculated.

2.2 Soil processing and analysis All frozen cores were cut in half lengthways (Fig. S3). One half of the core was used for the physical and chemical characteristics analysis (Schuster et al., 2018). The other half was cut in half lengthways again to obtain two parallel samples, which were cut into approximately 10-cm slices for Hg content measurement. The samples for soils Hg measurement were selected according to the soil horizons, i.e., soil color, texture. Soil samples were freeze dried, manually ground with a quartz mortar, and hand-sieved through a 100-mesh sieve per inch to remove large debris before chemical analysis. For each soil variables and Hg content, three replicates were analyzed using subsamples from the slice. Soil organic carbon of homogenized samples was analyzed by dry combustion on a Vario EL elemental analyzer (Elementra, Hanau, Germany) and was pretreated by HCl to remove carbonate (Wu et al., 2017). Bulk density and water content were determined by measuring the volume of a section of frozen core, and then drying the segment at 105℃ and determining its mass. The pH of the soil suspension (1:5 soil:

water ratio) were measured using an acidity meter (PHS-3C). The total soil mercury samples were measured by using a Direct Mercury Analyzer (Hydra-IIC, Leeman Lab Hydra, Teledyne Leeman Laboratories, Hudson, NH), following the US EPA Method 7473 (USEPA, 2000). The instrument detection limits for THg ranged from 0.19 to 0.29 ng/g for a 0.2 g sample mass. Detection limits were at least five to ten times less than the smallest measure THg value (Schuster et al., 2018). Analytical accuracy and precision for the measurements were determined by analysis of Chinese geochemical standard reference material GSS-9 and Tort-2 Lobster Hepatopancreas (NRCC, Ontario, Canada) (Guo et al., 2017) with a precision of 5%.

2.3 Carbon dating We selected the northern QTP sites for accelerator mass spectrometry (AMS) radiocarbon (14C) dating radiocarbon dating to determine net rates of SOC and Hg accumulation (Table S1). Soils at seven depths (EBOA) were selected to investigate the vertical distribution of THg contents at different depths. We also selected samples from deep permafrost cores (ranged from 5 to 11.5 m) at other 7 sites in this area to examine the relationship between the THg and age (Table S1). The samples pretreatment followed the procedure of the Key Laboratory of Western China’s Environmental Systems, Ministry of Education, Lanzhou University (Dong, 2014). The 14C dating analysis was conducted at the Laboratory of Quaternary Geology and Archaeological Chronology of Peking University, Beijing. The data were calibrated using the program CALIB v 7.02 and the IntCal13 curve (Reimer et al., 2013).

2.4 Calculation of mercury pools We used the total Hg data and the bulk densities to calculate the Hg pools on the QTP. The Hg pool data were calculated as: n 3

Hg pool   THg  r i  (1  g i )  H i i 1

Where the Hgpool unit is Gg, THg is the soil total Hg contents (ng Hg g soil-1), r is the bulk density, g is the gravel content (g cm-3), H is the area of land cover or the Quaternary stratigraphies (×106 km2). For the upper 3 m soils, i defines the three representative land cover types (alpine wet meadow, alpine meadow, and alpine steppe). For the 3-25 m soils, i defines the three Quaternary stratigraphies (Quaternary, Triassic, and Permian). The losses of THg pool for the upper 30 cm soils in the thermokarst-affected areas were calculated as:  L  1  

 S G 2  AG 3  S G 3   100 ( AG 2  AG 3)  S G1 

A

G2

Where the L is loss rates of soil Hg pool (%), AG2, AG3 were the area of Grade 2 and Grade 3 (m2), SG1, SG2 and SG3 were total Hg stocks for the upper 30 cm soils (g m-2).

2.5 Statistical analysis All the analyses including bulk density, SOC, soil moisture and Hg contents were conducted in triplicate using subsamples. The statistical analysis was performed using R.3.3.3 for windows. The linear model was used to descript the relationship between

soil Hg and other physiochemical characteristics. The uncertainty analysis was conducted using the Percentile Method, and we use an interquartile confidence interval to find the range of Hg pools for the different soil layers.

3. Results 3.1 Contents and stocks of soil mercury High Hg content was associated with high soil organic carbon (SOC) content for the top 50 cm of soils (Fig. S4). Total Hg content was 6.48 ng Hg g soil-1 (median) in alpine steppe, 18.11 ng Hg g soil-1 in alpine meadow, and 37.45 ng Hg g soil-1 in alpine wet meadow (Fig. S5). The ratio of Hg to SOC (RHgC) varied considerably among land cover types, with median values of 1.07 µg Hg g C-1 for alpine steppe, 0.86 µg Hg g C-1 for alpine meadow, and 0.36 µg Hg g C-1 for alpine wet meadow (Fig. S6). Total Hg content in permafrost cores (deeper than 3 m) ranged from 1.31 to 58.67 ng Hg g soil-1, with a median value of 6.14 ng Hg g soil-1. The total Hg contents were higher in cores under alpine wet meadow, and lower in cores under alpine steppe (Fig. S7). In cores of alpine wet meadow (PT9, EBO-A and EBO-B), the SOC has a significantly strong positive relationship with Hg content (Pearson’s R = 0.81, p < 0.01). In soil cores under alpine steppe and alpine meadow, the SOC also significantly positively correlated with Hg (Pearson’s R = 0.38, p < 0.01, Fig. S8). The SOC and total Hg mass for alpine wet meadow at all the depths were markedly higher than those of other land covers (Fig. S9), and the SOC and total Hg contents under alpine wet meadow decreased with soil depth. The median values of RHgC for

alpine wet meadow, alpine meadow and alpine steppe were 1.00, 0.87 and 1.68 µg Hg g C-1, respectively. The variation in SOC content (Coefficient of variation for SOC was 1.51) was higher than that in Hg content (Coefficient of variation for SOC was 0.87), and thus the much lower soil organic carbon contents in AS soils lead to the higher RHgC. For all samples, the median value of RHgC was 0.85 µg Hg g C-1, and the peaks of RHgC probability distribution appeared at 0.6 and 0.9 µg Hg g C-1 (Fig. S10). The Hg stored at depths of 0-1 m, 1-2 m, 2-3 m and 3-25 m in the QTP permafrost region are shown in Table 1. Based on the land cover areas (0.050×106 km2 for the alpine wet meadow, 0.584×106 km2 for the alpine meadow, and 0.850×106 km2 for the alpine steppe), a total of 21.65 Gg (interquartile: 18.43-24.24) Hg is stored in the upper 3 m depth soils. Based on the Quaternary stratigraphies data (0.194×106 km2 for Quaternary, 0.238×106 km2 for Triassic and 0.135×106 km2 for Permian) and the assumption that the Hg contents in the 15 deep permafrost cores were representative of the 3-25 m soils, the total permafrost soil Hg mass at the 3-25 m soil was 103.52 Gg (interquartile: 84.52-114.45).

3.2 Accumulation of soil mercury Soil total Hg decreases with age for an exponential curve fit to 14 samples with carbon dating

(Fig. S11). The youngest sample was 849 years before present (YBP) from Ebo-A at 0.6 m depth and the oldest sample was 27,760 YBP from PT9 at 6.7 m depth, indicating that all samples predate the industrial era. From the 7 samples collected at Ebo-A site, the 14C age-depth profiles showed that each 8 cm soil represents 100 years

(Fig. S12).

3.3 Relationships among Hg and soil variables The linear regressions using SOC, water content, and bulk density showed that SOC and TN explained 49.6% and 57.9% of the total variance in Hg (adjusted R2), with smaller coefficients for water content, bulk density, and C:N ratio (Fig. S13). The Hg content was negatively correlated with bulk density, pH, and C/N ratios, but was positively correlated with soil water content (Fig. S14).

3.4 Effect of thermokarst on soil mercury In the thermokarst landscapes, soil Hg content ranged from 5.64 to 39.19 ng Hg g soil-1, SOC content ranged from 0.63 to 6.97%, and total nitrogen ranged from 0.032 to 1.24%. Soils affected by thermokarst formation had substantially lower Hg, SOC, and TN contents for all the sampling areas (Fig. 3). At the EBO site, the Hg, SOC, and TN contents at the Grade 2 were lower than the Grade 1 and 3. While at the other sites, although is no consistent trend for the Hg, SOC, and TN contents in the subsoils, the Hg, SOC, and TN contents at the Grade 3 was lower than Grade 2 and Grade 1 in the topsoils. We calculated the changes of Hg, SOC, and TN storage in the thermokarst-affected area (stage 2 and 3) compared with unaffected (stage 1) sites. Hg stocks in the upper 30 cm were decreased by 24.9% (17.6 to 30.9%), SOC stocks were decreased by 17.5% (11.4 to 22.2%), and TN stocks were decreased by 22.7% (8.5-30.9%), based on the thermokarst areas and bulk density (Fig. 4).

4. Discussion A previous study found that the mean soil Hg content was 21 ng Hg g soil-1 in the non-permafrost regions on the southern QTP, where there is a larger human population and greater anthropogenic loading of Hg compared to our study areas (Zhang and Zhu, 1994). Soil Hg contents from this study were much lower than those of many soils across the world. The background average value of soil Hg in China is 40 ng Hg g soil-1 (Zhang and Zhu, 1994). In the Alaskan interior and north slope, the average soil Hg (ng Hg g soil-1, dry weight) for 13 cores up to 1m in length was 47 ng Hg g soil-1, ranging from 19 to 208 ng Hg g soil-1 (Schuster et al., 2018). The median value of RHgC in the circum-Arctic regions was 1.6 (Schuster et al., 2018), indicating more Hg were bound to the organic matter in comparison with this study. Among many factors, the differences of the soil Hg in these regions can largely be explained by the variability in atmospheric deposition of Hg (Yu et al., 2014). The Arctic regions have been recognized as an important global sink for atmospheric Hg (Lindeberg et al., 2006; Schroeder et al., 1998), and Hg can be potentially be transported from Asia and Europe and deposited in Arctic soils (Durnford et al., 2010). On QTP, the Hg deposition flux (0.74 to 2.97 μg m-2 y-1) is relatively lower than

the global natural Hg deposition rates (2 to 5 μg m-2 y-1) (Kang et al., 2016;

Swain et al., 1992). In addition, the lower soil Hg may be also related to exposure to strong ultraviolet light and high temperature. It is because solar radiation can promote the photo-reduction of Hg(II) to form Hg(0), higher soil temperature can favor the

Hg(0) production, and solar radiation and high soil temperature can reduce the apparent activation energy of Hg(0) desorption and increase Hg(0) emission from surface soils (Ci et al., 2016; Lalonde et al., 2016; Gustin et al., 2002). The QTP accounts for about 8% of the permafrost in northern hemisphere (Zhang et al., 2008), while the Hg pools for 0-3 m soils was only about 1.3% of that in circumarctic regions (1,656 ± 962 Gg) (Schuster et al., 2018). Although 1.3% is a small part of the global permafrost Hg pools, 21.65 Gg in a globally important headwater region is still a large Hg pool concerning human health. Since the active layer on the QTP is large about 2 m (Qin et al., 2017), the Hg pools in the active layers on the QTP can be calculated as 16.58 Gg (Interquartile: 14.55-18.48 Gg). In addition, our study reported the QTP permafrost Hg pools in the deep soils. Although there are large uncertainties in the permafrost Hg pools due to the limited data, our study highlighted the importance of the deep permafrost Hg pools on the QTP. It was reported that soil Hg emission has a strongly positive relationship with temperature in the QTP permafrost (Ci et al., 2018). Soil temperature and moisture can control CO2 respiration rates (Natali et al., 2014; Wickland et al., 2006), which were linearly correlated with soil Hg emissions (Obrist et al., 2010). Our results showed significantly negative relationships with pH, bulk density, but positive relationship with soil water contents. These findings can be explained as that the lower pH, bulk density, and higher soil moisture contents benefit the accumulation of soil organic matter and Hg (Olson et al., 2018; Schuster et al., 2018). As a result, permafrost degradation, which usually leads to lower soil water content, higher bulk

density, and higher pH, will also increase the soil organic matter decomposition and Hg emissions. Since soil organic carbon stored in cold regions is more vulnerable to climate change due to thermal adaptation of microbes (Karhu et al., 2014), it can be concluded that permafrost degradation on the QTP will stimulate soil Hg emissions. Because QTP permafrost soil temperatures are close to the freezing point, climate warming will likely result in a large amount of carbon decomposition (Mu et al., 2016a), possibly increasing Hg release over the study area. The decrease in soil Hg stocks in thermokarst-affected could be explained by several processes (Fig. 5). First, increased temperature and O2 availability during thermokarst formation can stimulate organic matter decomposition (Mu et al., 2016b) and increase methylmercury (MeHg) production (Yang et al., 2016), potentially leading to gaseous Hg emission (Obrist et al., 2010) or accumulation in food webs. Second, thermokarst increases exposure to UV light, facilitating photo-degradation of organic matter and photoreduction of deposited Hg2+ (Ferrari et al., 2004; Lalonde et al., 2016). Third, thermokarst increases soil erosion (Abbott and Jones, 2015), removing surface material that is relatively enriched in Hg. Together, these factors could accelerate Hg transport to atmosphere and then reenter the global cycles during permafrost degradation where Hg could accumulate in aquatic food webs. The lowest Hg contents observed at the Grade 2 of EBO site was attributable to the lowest soil water content during collapsing (Mu et al., 2016b). The distribution of Hg with depth in the topsoils suggest that permafrost collapse can significantly decreased soil Hg content. No consistent trend for soil Hg in the subsoils could be explained by the

infiltration of dissolved organic matter, the vertical distribution of particulate matter, and overlapping of the overlying soil and vegetation (Mu et al., 2016b). Once the lost mercury was converted into methylmercury and accumulated in the food web, it can potentially threaten human health. Although the QTP permafrost contains much less Hg on an areal basis compared with circumarctic regions, our results suggest that thermokarst development (Lara et al., 2016) can greatly enhance soil Hg release. Because the QTP is the headwater source of many of Asia’s largest rivers, including the Yangtze, Yellow, Mekong, and Brahmaputra, understanding Hg dynamics in this region is vital to the drinking water safety of billions of people (Yang et al., 2014). The most effective strategy to reduce potential Hg release is to reduce global greenhouse gas emissions (Abbott et al., 2016) . Additionally, on regional and local scales, limiting excessive grazing (Yang et al., 2004) and mitigating construction practices may slow permafrost degradation (Wu et al., 2016). It was reported that the mean emission rate of Hg from surface soils in the permafrost regions was 2.86 ng m-2 y-1, the surface soils were the net emission source of Hg (Ci et al., 2016). With climate warming, the Hg emission can be accelerated by permafrost degradation, thus regular monitoring of surface waters (rivers and lakes) for Hg content should be performed in the rivers and lakes of the QTP permafrost region.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant

No. 41871050, 41630754), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA20100313; XDA20100103), the Open Foundations of the State Key Laboratory of Cryospheric Science (Grant No. SKLCS-OP-2018-05), and the Open Foundations of the State Key Laboratory of Frozen Soil Engineering (Grant No. SKLFSE201705).

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2527-2542.

Table 1 Median values (with interquartile confidence) of mercury pools for different soil layers under three land cover types (data from deep soil cores, AWM, alpine wet meadow; AM, alpine meadow; AS, alpine steppe; the 0-2 m layer can be considered as the active layer because the active layer thickness on the QTP is about 2 m (Qin et al., 2017))

Land cover

AWM

AM

AS

Total

1.02

3.07

5.94

10.03

(0.86-1.44)

(3.04-3.14)

(5.62-6.44)

(9.52-11.01)

1.58

3.49

1.48

6.55

(0.61-1.60)

(3.16-4.09)

(1.26-1.78)

(5.03-7.47)

0.76

2.59

1.73

5.07

(0.43-1.06)

(2.03-2.65)

(1.42-2.06)

(3.88-5.76)

3.35

9.15

9.15

21.65

(1.91-4.10)

(8.23-9.87)

(8.30-10.27)

(18.43-24.24)

Quaternary

Triassic

Permian

Total (3-25m)

36.58

21.68

45.26

103.52

(25.39-39.86)

(21.05-23.09)

(38.08-51.50)

(84.52-114.45)

0-1 m

1-2 m

2-3 m

0-3 m

Geological stratigraphy

3-25 m

125.17 0-25 m (102.95-138.69)

FIGURES Figure 1. Locations of permafrost coring sites (purple flags), surface soil sampling sites (black dots) and thermokarst study area (red dots) with the land cover types in the permafrost regions of Qinghai Tibetan Plateau. The white area shows the non-permafrost areas.

Figure 2. Typical thermokarst features and sampling sites in QTP permafrost regions with vegetation types of alpine wet meadow, alpine meadow and steppe. The areas within cyan solid lines indicate grade 3 and areas outside these lines are either grade 2 (collapsing) or grade 1 (unaffected).

Figure 3. Distribution of soil Hg, SOC, TN in the upper 30 cm layers at the different stages on

the six areas with thermokarst features.

Figure 4. Loss of total Hg, SOC, and TN stocks for the upper 30 cm soil layer at the six thermokarst areas.

Figure 5. A conceptual framework of permafrost degradation effects on soil Hg.

Figure 1. Locations of permafrost coring sites (purple flags), surface soil sampling sites (black dots) and thermokarst study area (red dots) with the land cover types in the permafrost regions of Qinghai Tibetan Plateau. The white area shows the non-permafrost areas.

Figure 2. Typical thermokarst features and sampling sites in QTP permafrost regions with vegetation types of alpine wet meadow, alpine meadow and steppe. The areas within cyan solid lines indicate grade 3 and areas outside these lines are either grade 2 (collapsing) or grade 1 (unaffected).

Figure 3 Distribution of soil Hg, SOC, TN in the upper 30 cm layers at the different stages on the six areas with thermokarst features

Figure 4. Loss of total Hg, SOC, and TN stocks for the upper 30 cm soil layer at the six thermokarst areas

Figure 5. A conceptual framework of permafrost degradation effects on soil Hg

Highlights 

The permafrost regions on the Tibetan Plateau store about 21.65 Gg (interquartile: 18.43-24.24) Hg in 0-3m soils.



The active layers on the Tibetan Plateau store 16.58 Gg (Interquartile: 14.55-18.48 Gg) Hg.



The mercury in the Tibetan Plateau is sensitive to permafrost degradation.