Research progress on behaviors and environmental effects of mercury in the cryosphere of the Tibetan Plateau:a critical review

ShiWei Sun,ShiChang Kang,QiangGong Zhang,JunMing Guo,XueJun Sun

1.State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research,CAS,Beijing 100101,China

3.CAS Center for Excellence in Tibetan Plateau Earth Sciences,Beijing 100101,China

4.University of Chinese Academy of Sciences,Beijing 100049,China

ABSTRACT The behavior and fates of environmental pollutants within the cryosphere and the associated environmental impacts are of increasing concerns in the context of global warming.The Tibetan Plateau (TP), also known as the "Third Pole", represents one of the most important cryospheric regions in the world.Mercury (Hg) is recognized as a global pollutant.Here,we summarize the current knowledge of Hg concentration levels,pools and spatio-temporal distribution in cryospheric environments (e.g., glacier, permafrost), and its transfer and potential cycle in the TP cryospheric region.Transboundary transport of anthropogenic Hg from the surrounding heavily-polluted regions, such as South and Southeast Asia, provides significant sources of atmospheric Hg depositions onto the TP cryosphere.We concluded that the melting of the cryosphere on the TP represents an increasing source of Hg and brings a risk to the TP environment.In addition,global warming acts as an important catalyst accelerating the release of legacy Hg from the melting cryosphere, adversely impacting ecosystems and biological health.Furthermore,we emphasize on the remaining gaps and proposed issues needed to be addressed in future work, including enhancing our knowledge on some key release pathways and the related environmental effects of Hg in the cryospheric region, integrated observation and consideration of Hg distribution, migration and cycle processes at a key region,and uses of Hg isotopic technical and Hg models to improve the understanding of Hg cycling in the TP cryospheric region.

Keywords:mercury;cryosphere;environmental effects;Tibetan Plateau

1 Introduction

"Cryosphere" refers to the portions of the Earth's surface where frozen water is found, including glaciers(e.g.,ice sheet),frozen soils(i.e.,permafrost and seasonally frozen ground), snow cover, sea ice, river ice and lake ice,etc.(Qin, 2017).The Cryosphere occupies 52%-55% of the global land area, with mountain glaciers and polar ice sheets covering 10% of the land area, frozen ground covering 42%-45% of the land area, and snow cover occupying 1.3%-30.6%(Kanget al., 2020a).Global warming has caused rapid cryosphere changes during the last decades,characterized by glacier retreat, permafrost degradation, and reductions in snow cover extent and duration.This is particularly significant in high mountain areas, where dramatic temperature increases and cryosphere decline have been observed (Hocket al., 2019; Kanget al.,2020b).

The environmental effects triggered by cryospheric changes have become a topic of global concern in recent decades.Cryospheric changes can significantly affect the biogeochemical cycling of trace constituents, such as nutrients (e.g., carbon and trace elements)and pollutants (e.g., heavy metals and persistent organic pollutants (POPs); Bogdalet al., 2009; Legrandet al.,2013;Hoodet al.,2015;Zhanget al.,2017).Of particular concern is that some legacy pollutants preserved in the declining cryosphere could be remobilized and subsequently released into the environment(e.g., atmosphere, oceans, lake and river ecosystems),impacting the ecosystems and biological health adversely (Zhanget al., 2017).For instance, glaciers and permafrost are two key cryospheric components.Studies in the Arctic have emphasized that the export of pollutants from melting glaciers is becoming an important source of pollutants to downstream aquatic ecosystems (Blaiset al., 2001; Bogdalet al., 2010).Additionally,legacy POPs and some volatile substances in glaciers can be re-emitted back into the atmosphere (Dommergueet al., 2003; Maet al., 2011).Permafrost covers approximately 25% of the Northern Hemisphere land area (Schusteret al., 2018).A few studies in the Arctic have suggested that thawing of permafrost potentially serves as an important source of organic carbon/organic carbon-bound trace metals to the environment (e.g., Arctic marine and lake ecosystems; Schusteret al., 2011; Sternet al.,2012).The contribution of pollutants from the melting cryosphere (e.g., glacier retreat and permafrost thawing) to the environment could be similar to, or even higher than the atmospheric depositions previously regarded as the dominant input pathway for ecosystems (Blaiset al., 2001; Klaminderet al., 2008;Maet al.,2011).

Mercury (Hg) is recognized as a global pollutant due to its persistence, long-range transport via the atmosphere, and biomagnification through the food chains(Selin,2009).A significant amount of Hg is released into the atmosphere by anthropogenic activities(e.g., fossil fuel combustion and non-ferrous metal mining; 2,000-2,500 t/a; Streetset al., 2005; Pirroneet al.,2010;Obristet al.,2018),as well as natural processes (e.g., Hg evasions from the earth surface, volcanic and geothermal activities, biomass burning;1,000-5,500 t/a; Lamborget al., 2002; Pirroneet al.,2010; Outridgeet al., 2018).Polar regions have been suggested as the net sink of atmospheric Hg, especially during the AMDEs (Atmospheric Mercury Depletion Events) period in springtime (Schroederet al.,1998;Steffenet al.,2014).Increasing evidence shows that climate warming stimulates Hg release from the cryosphere (Gamberget al., 2015; Zhanget al., 2017,2019; Schusteret al., 2018).For instance, the Northern Hemisphere (NH) permafrost soils are globally important storage pools for Hg (Olsonet al., 2018;Schusteret al.,2018;Limet al.,2020).Schusteret al.(2018) estimated that the NH permafrost soils (0-3 m)contain 1,656 Gg Hg,nearly twice as much as estimated in other soils, oceans, and atmosphere combined(Schusteret al., 2018).Limet al.(2020) estimated a lower, yet still globally significant Hg pool of 597 Gg in the NH permafrost soils (0-3 m; Limet al., 2020).Due to the high affinity of Hg to soil organic matter(SOM), permafrost thawing can accelerate SOM decomposition and facilitate land Hg emissions into the atmosphere (Ciet al., 2016, 2018; Sunet al., 2017a).Other studies also reveal the release of Hg from thawing permafrost to freshwater and marine ecosystems as a result of accelerated thermokarst activity and soil erosion (Gamberget al., 2015; Muet al., 2020).The output flux of Hg from thawing permafrost can be significant, even higher than the atmospheric deposition flux of Hg (Leitch, 2006; Klaminderet al., 2008;Gamberget al., 2015).Unexpected high levels of Hg have been observed in biota of the Arctic and mountain cryospheric regions (Yanget al., 2011; Zhanget al.,2014;Bondet al.,2015).

The Tibetan Plateau (TP; 2,572.4×103km2), also known as the "Third Pole", is geomorphologically the largest and highest mountain region on Earth with an average elevation of ＞4,000 m above sea level(a.s.l.),which provides a natural cold environment for cryosphere development (Qiu, 2008; Yaoet al., 2012).The TP and its surrounding areas are home to the largest aggregate of glaciers outside the polar regions,with a total area of about 98,740 km2and ice volume of 7,481 km3(Zempet al., 2019).Regarded as the"Asian Water Tower", the TP serves as the source region of Asia's major rivers (e.g., the Yangtze River,the Yellow River, Indus and Ganges), and provides water resources for billions of people living downstream.The TP is endowed with the largest permafrost bodies at low- and mid-latitudes (approximately 1.06×106km2),which accounts for 74.5%of the Northern Hemisphere's mountain permafrost (Zouet al.,2017).As a sensitive area to climate change, the TP has warmed at an alarmingly accelerated rate(0.3-0.4°C/decade) since the 1960s, nearly twice as much as the global average.Consequently, the TP cryospheric region has been undergoing dramatic changes, including glacier retreat,permafrost degradation,glacial runoff increase, lake expansion and hydrologic cycle enhancement (Kanget al., 2010; Yaoet al., 2012; Yanget al., 2019).This is undoubtedly causing remobilization and release of pollutants from the melting and thawing cryosphere to the environment.Rivers originated from the"Asian Water Tower"provide fresh water resources and fish productions for Asian populations (Immerzeelet al., 2010).This is of particular concern,because the release of Hg from melting cryosphere (e.g., glaciers and permafrost) to the headwaters can be transported downstream and transformed into methyl-Hg (MeHg), which can bioaccumulate into biota (e.g., fish) and humans, potentially endangering the ecosystems and human health of the downstream regions (Zhanget al., 2017; Muet al., 2020).Thus, information on Hg behaviors and its environmental effects in the TP cryospheric region can not only further our understanding of Hg cycles in the cryosphere, but provides theoretical basis for assessing future potential environmental risks of Hg to the fragile cryospheric ecosystems as well as the environmental effects caused by cryospheric changes.

In recent years, great progress has been made to advance the knowledge of Hg dynamics and its environmental effects in the TP cryospheric region.In this study, we review the current knowledge of concentration levels, pools and spatio-temporal distribution of Hg in the cryospheric environments (e.g., glaciers,permafrost soils,and related water bodies),and potentially important input and release pathways of Hg in the TP cryospheric region (e.g., atmospheric deposition,release from melting glaciers and thawing permafrost soils, and land-air exchange dynamics), as well as their environmental effects.The objective of this study is to obtain a comprehensive understanding of Hg dynamics in the cryosphere and the influence of cryospheric changes on regional Hg cycles as well as its impacts on the fragile TP environments under global warming.

2 Spatio-temporal pattern and sources of Hg in the atmosphere

The knowledge of concentrations, spatio-temporal patterns and sources of atmospheric Hg provides important information on sources of atmospheric Hg depositions into the TP cryosphere.The local anthropogenic Hg emission is thought to be quite limited on the TP due to sparse human population and minimal industrial activities (Streetset al., 2005; Zhanget al.,2015b).However,the TP is adjacent to the two largest contributors of anthropogenic Hg emissions (i.e., China and India; Pacynaet al., 2006; Pirroneet al., 2010).The large-scale atmospheric circulation in the TP region is primarily influenced by the Indian summer monsoon (ISM) in the warm season (June-September)and the westerlies in the cold season (October-May).Studies have indicated that anthropogenic pollutants(e.g., Hg) generated in South and Southeast Asia can be transported to the hinterlands of the TP region via the ISM, and subsequently deposited at high altitudes,resulting in the accumulation of pollutants in glaciers,permafrost soils and vegetation (Yanget al., 2013; Liet al.,2016;Kanget al.,2019).

To our best knowledge, there were previous measurements of atmospheric total gaseous Hg (TGM) at eight sites on the TP (Figure 1).Generally, the spatial distribution of annual-mean TGM concentrations is characterized by the lower values at Nam Co(1.33±0.24 ng/m3) and Beiluhe station (1.36±0.17 ng/m3)on the inland TP, and the higher values at Waliguan station (1.98±0.98 ng/m3), Shangri-La station(2.55±0.73 ng/m3), Mt.Ailao (2.09±0.63 ng/m3) and Mt.Gongga area (3.98±1.62 ng/m3) on the eastern edge of the TP.The annual-mean TGM concentrations in the inland TP(1.3 ng/m3)are the lowest values measured at remote sites of China(1.60-5.07 ng/m3;Fuet al.,2015),and are similar to the lower end of the concentrations measured at remote sites in North America and Europe (1.2-1.9 ng/m3; Jiskraet al., 2018), indicating the pristine atmospheric environment over the inland TP with extremely low regional anthropogenic Hg emissions (Streetset al., 2005).Elevated TGM values on the eastern edge of the TP suggest the influence of regional anthropogenic emissions or longrange transport (Fuet al., 2008, 2012; Zhanget al.,2015a).

Figure 1 Summary of atmospheric TGM concentrations at 8 stations on the Tibetan Plateau.The measurement periods are:Beiluhe(June,September,and December 2014 and May-June 2015);Tanggula(June-October,2016);Nam Co(January 2012 to October 2014);Qomolangma(April-August,2016);Waliguan(September 2007 to September 2008);Mt.Gongga(May 2005 to June 2006);Shangri-La(November 2009 to November 2010);Mt.Ailao(May 2011 to May 2012)(Fu et al.,2008,2012;Zhang et al.,2015a,2016;Ci et al.,2016;Yin et al.,2018;Lin et al.,2019;Sun et al.,2020)

Diverse temporal patterns of TGM are observed in different parts of the TP.At stations on the inland TP with minimal influence of anthropogenic emissions,the concentrations and temporal patterns of TGM are suggested to be more influenced by natural processes(i.e., land-air Hg emissions) compared to anthropogenic emissions (e.g., long-range transport).At the remote sparsely-vegetated sites on the inland TP (i.e.,Nam Co and Beiluhe Stations),TGM concentration in the warm season (1.5 ng/m3) is approximately 1.4 times higher than that in the cold season (1.1 ng/m3),which is consistent with the seasonality of land-air Hg emissions over bare soils (Ciet al., 2016; Yinet al.,2018), suggesting that elevated TGM values in the warm season are mainly caused by enhanced land-air emissions.Growing evidence suggests that transboundary transport of Hg from Southern Asia(e.g.,Indo-Gangetic Plain (IGP)) provides additional sources of Hg, promoting the elevated TGM during the ISM season.The potential source contribution function(PSCF) results at Nam Co station indicate that the IGP is likely a major potential anthropogenic source region of TGM,and ISM serves as the major driver in transboundary transport of Hg to the inland TP(Yinet al., 2018).At a remote densely-vegetated site (Tanggula station;5,100 m a.s.l.),Sunet al.(2020)reported a decreasing trend of TGM concentrations over the alpine meadow with plant growing (emergence (2.32±0.51 ng/m3) ＞active growth period (2.01±0.25 ng/m3) ＞senescence (1.90±0.25 ng/m3) ＞peak vegetation period(1.80±0.26 ng/m3)).This seasonal pattern is consistent with that of land-air emissions, but inconsistent with that of long-range transport,providing direct evidence of the dominant role of land-air Hg exchange in regulating atmospheric TGM at remote sites of the inland TP(Sunet al.,2020).

In contrast, at stations adjacent to anthropogenic source regions, the elevated concentrations and temporal patterns of TGM are mostly controlled by anthropogenic emissions (i.e., regional emissions or long-range transport) compared to natural sources.In Mt.Gongga area (a rural site), TGM concentrations are higher in winter (5.65 ng/m3) than in summer (3.02 ng/m3),which is attributable to enhanced regional anthropogenic emissions in winter (e.g., coal consumptions for household heating; Fuet al., 2008).At remote stations located close to South Asia and Southeast Asia,studies found that TGM concentration is higher in the ISM period than in the non-ISM period (Zhanget al.,2016; Linet al., 2019).For instance, compared to the non-ISM period, TGM concentration during the ISM period increases by 20%and 12%at Qomolangma station and Mt.Ailao station, respectively (Zhanget al.,2016; Linet al., 2019).At Shangri-La station, high TGM episodes (2-6 ng/m3) occasionally occur in the ISM period (Zhanget al., 2015a).The seasonal TGM patterns indicates that the transboundary transport of Hg from South and Southeast Asia plays an important role in regulating the concentrations and temporal patterns of atmospheric Hg at these stations (Zhanget al.,2015a,2016).

In the northern parts of the TP hardly influenced by ISM, the transported Hg from industrial regions in northwestern China could also be an important source of atmospheric Hg.At Waliguan station where the surrounding areas mainly consist of scattered grasslands,the highest monthly-mean TGM concentration is observed in January, which is likely caused by longrange transport of Hg from Northern India.Except January, the monthly-mean TGM concentrations are generally higher in the warm months than those in the cold months,and can be attributed to long-range transported Hg from its adjacent eastern urban and industrial areas in Northwest China (e.g., Xinning and Lanzhou) during the warm season under the influence of the Plateau monsoon(Fuet al.,2012).Enhanced landair emission is likely to be an additional contributor to higher TGM levels in the warm months at Waliguan station.However, the frequently observed high TGM episodes in warm months indicate a more pronounced impact of long-range transport compared to the landair emissions(Fuet al.,2012).

3 Atmospheric Hg deposition onto the TP cryosphere

3.1 Dry deposition

Atmospheric deposition (wet and dry deposition)represents an important source of Hg to terrestrial environments, especially for the remote cryospheric regions with limited human activities.Dry deposition of Hg on the TP remains poorly understood due to the lack of direct and accurate measurements.Two model studies estimated that the dry deposition flux of PHg could be up to 135 μg/(m2·a) in Kathmandu Valley and 35 μg/(m2·a) in Lhasa City (Huanget al., 2016a;Guoet al.,2017).A recent model study estimated that the average deposition flux of Hg over the TP is 3.3 μg/(m2·a),with approximately 76% contributed by dry deposition (2.5 μg/(m2·a); Guet al., 2020), indicating that dry deposition may act as an important process for the atmospheric Hg loadings to the TP.

3.2 Wet deposition

3.2.1 Spatial patterns and potential sources

The monitoring of wet deposition of Hg has been well documented at eleven remote and urban sites on the TP (Figure 2 and Table S1).Except for the forest site (SET station), wet deposition of Hg is primarily in the form of particulate-bound Hg (PHg) over the TP (PHg/THg ratios (PHg%): 59%-93%; Table S1).The major mechanism contributing Hg to precipitation is associated with below-cloud scavenging of atmospheric PHg (Huanget al., 2012c, 2013, 2016a).Dissolved Hg (DHg), however, is the dominant Hg species at the SET station, due to the "forest filter effect" (i.e., the filtering and intercepting effects of forest canopy on atmospheric particulate-bound pollutants; Huanget al., 2015).Generally, precipitation total Hg (THg) concentrations over the TP are closely related to anthropogenic sources and the distance from dust source regions(Huang,2011).

Figure 2 Summary of the concentrations and wet deposition fluxes of THg in precipitation over the Tibetan Plateau and its surrounding area(Fu et al.,2010;Huang,2011;Huang et al.,2012c,2013,2015;Tripathee et al.,2019)

For the TP urban and rural sites, studies revealed that local and regional anthropogenic sources are the largest contributors of precipitation Hg (Huanget al.,2013, 2016a; Guoet al., 2017, 2020).However, the influence of anthropogenic emissions on precipitation Hg is less significant at the TP urban and rural sites,compared to the heavily air-polluted metropolitan areas in eastern China.The volume-weighted mean(VWM) THg concentrations and wet fluxes at the TP rural and urban sites (6-25 ng/L, 8-35 μg/(m2·a), respectively) are generally higher than those measured at remote sites worldwide(2-6 ng/L,＜1-12 μg/(m2·a)),and substantially lower than those of large industrial and urban sites in mainland China (102ng/L, 102μg/(m2·a);Table S1).In Lhasa,the main source of PHg in precipitation is associated with industrial emissions (e.g.,power plants, cement factories) during the non-monsoon season, and local vehicular traffic and religious activities emissions (e.g., religious burning of biomass and incense in and around the temples or households),and partial long-range transport of Hg from Indian subcontinent during the monsoon season (Huanget al.,2013,2016a,2016b).In Kathmandu,the capital city of Nepal, a large proportion of PHg in precipitation is from local anthropogenic sources (e.g., brick kilns, vehicular emissions and small generators).Crustal dust originated from the suspension of local road soil dust,and biomass burning may be additional sources of precipitation Hg (Guoet al., 2017; Tripatheeet al.,2019,2020).

For the TP remote areas, the concentration levels and spatial pattern of precipitation THg are controlled by the distance from dust source regions.The transported PHg associated with natural crustal dust originating from dust storm activities is suggested to be the largest source of Hg deposition in remote regions,especially in the northern TP (Loewenet al., 2007;Huanget al., 2012a, 2020a; Zhanget al., 2012).High atmospheric dust loading and PHg are usually observed in the northern region due to its lower precipitation and closer proximity to major dust source regions in Northwestern China (e.g., the Qaidam Basin and Taklimakan Desert).The atmospheric PHg concentrations in the Taklimakan Desert (86-517 pg/m3)are even comparable to those measured in most of the Chinese metropolitan cities (Huanget al., 2020a).As a result, precipitation THg concentration is relatively higher in the northern region compared to those in the southern region (Figure 2; Huanget al., 2012a;Tripatheeet al., 2019).At Laohugou station on the northern TP, the concentration levels and wet flux of THg(32.9±54.6 ng/L, 12.1 μg/(m2·a)) are even comparable to those reported for urban sites on the TP(10-33 ng/L;8-35 μg/(m2·a)).At remote sites of the southern TP,the concentration levels (＜5 ng/L) and wet fluxes(1-4 μg/(m2·a)) of THg are comparable to those reported for the most remote alpine and polar regions worldwide.

Precipitation Hg studies also emphasize the importance of ISM circulation in transboundary transport of anthropogenic Hg from southern Asia to the inland TP.For some remote regions of the southern TP(e.g.,Mt.Gongga area, SET station and remote stations in central Himalaya), the transported anthropogenic Hg is suggested as an important, even the primary source of precipitation Hg (Huanget al., 2015, 2016b;Tripatheeet al., 2019; Guoet al., 2020).At remote stations of the central Himalayas, THg in precipitation is found to be highly enriched in both monsoon and nonmonsoon seasons with Enrichment Factor values ＞100,suggesting the importance of transported anthropogenic Hg from the urbanized areas of South Asia for Hg enrichment (Tripatheeet al., 2019, 2020).An increasing trend of precipitation THg concentration is observed at the SET station based on the 3-year wet Hg deposition data (2010-2012), synchronous with the recent economic development in South Asia (Huanget al.,2015;Kanget al.,2016).

Data is still scarce for MeHg, the most toxic Hg species and a neurotoxin, in precipitation over the TP.Concentration levels and wet deposition flux of Me-Hg at Nam Co Station are among the lowest reported for the remote alpine and polar regions globally,whereas they are among the highest globally reported values at the SET station (Table S1).Higher MeHg concentrations and wet flux were observed in the Mt.Gongga area(Fuet al.,2010),which is possibly attributed to higher atmospheric Hg loadings available for methylation due to local and regional Hg emissions from industrial activities and coal combustions (Hammerschmidtet al.,2007).

3.2.2 Seasonal pattern

Consistent with the seasonal variations of atmospheric PHg (Huanget al., 2016a; Guoet al., 2017,2020),precipitation THg concentrations are 1.5-5 times higher in the non-ISM season than those in the ISM season.This is attributed to both the dilution effect of increased precipitation in the ISM season,and high local anthropogenic Hg emissions (for urban/rural sites)or increased long-range transported PHg to the TP in non-ISM season (for remote sites; Huanget al.,2012c, 2013, 2015).For instance, in Lhasa city, higher THg concentrations in the non-ISM season are ascribed to high local anthropogenic sources (e.g., cement factories; Huanget al., 2013).At Nam Co station, higher THg concentrations in the non-ISM season are attributed to enhanced transported PHg from the arid dust source regions in central and southern Asia to the hinterland of the TP through westerlies(Huanget al., 2012c).At STE station, a significant amount of Hg is transported to this site with polluted air masses (e.g., biomass burning and fossil fuel combustion) originating from its adjacent, polluted regions in southeastern Asia, resulting in elevated THg concentrations during the non-ISM period (Huanget al., 2015).In contrast, wet THg fluxes are higher in the ISM season than those in the non-ISM season, attributed to more precipitation in the ISM season(Huanget al.,2012c,2013,2015).

4 Hg in glacierized environments

Anthropogenic Hg from the surrounding polluted regions could be deposited onto glaciers over the TP,as evidenced by the atmospheric Hg and precipitation Hg studies mentioned earlier.Hg export from the melting glaciers represents an important source of Hg to the downstream aquatic ecosystems.Once transported to the aquatic ecosystems, inorganic Hg species can be transformed into MeHg,posing a potential threat to human health by biomagnification through the food chains.

4.1 Hg in glaciers

4.1.1 Speciation and concentration levels

Concentration levels, speciation and spatiotemporal distribution of Hg have been well documented in glacier snow on the TP (Figure 3, Tables 1, S2).PHg is the dominant form of Hg in glacier snow and ice,with average PHg% of 55%-90% at various sites(Tables 1, S2).Combined with the wet deposition studies,high PHg%in glacier samples confirm that particulate matter plays an important role in atmospheric Hg transport and deposition over the TP, and PHg is the dominant species influencing the levels and distribution of Hg in glacier snow/ice(Loewenet al.,2007;Zhanget al.,2012).

Figure 3 Summary of THg concentrations in glacial snowpits on the Tibetan Plateau and its surrounding area(Loewen et al.,2007;Zhang et al.,2012;Huang et al.,2012a,2014;Sun et al.,2018a;Paudyal et al.,2017,2019).The values for LHG,GQ,XDKMD,ZD,ER and YL are the averages of several mean THg concentrations in snowpits taken at each individual site.

Generally, mean THg concentrations in snowpits range between 0.9-10.8 ng/L (range: ＜1-50 ng/L;Figure 3 and Table S2), which are at the lower end of those measured in the French Alps(13-130 ng/L;Ferrariet al.,2002),and northern regions of western China adjacent to major dust source regions(e.g.,Urumqi No.1 Glacier (TS, 14.7±8.1 ng/L); Figure 3).These values, however, are higher than those reported for Arctic snow in the absence of AMDEs (＜1-5 ng/L;Ferrariet al., 2005; Steffenet al., 2008).The remarkably high Hg concentrations in snowpits(up to 50 ng/L)generally correspond with the distinct yellowishbrown dust layers (Loewenet al., 2007; Zhanget al.,2012).

With the exception of the summer snow samples of XDKMD and YL and spring snow samples of the ER,mean THg concentrations in surface snow(0-5 cm)at different sites range between 0.7-8.8 ng/L (range:＜1-39.7 ng/L; Table 1), comparable to those measured in snowpits.High THg concentrations in summer aged snow of XDKMD (＞30.6±53.9 ng/L) and YL (37±26 ng/L) have been thought to be caused by the influence of local or long-range transported anthropogenic emissions from South Asia (Paudyalet al., 2017, 2019).However, snowpit Hg concentrations in the two glaciers are low (XDKMD:3.1 ng/L,YL: 1.5 ng/L).It should be noted that these summer snow samples were mainly collected in the glacier ablation zone (where the annual loss of glacier mass exceeds the gain).Thus, we propose that high Hg concentrations in the summer snow samples reflect accumulation of PHg on the glacier surface during the melting season.During the summer melting process, PHg can be left behind in the snow and even forms dirty cones in the ablation zone, resulting in accumulation of PHg in surface snow (Loewenet al., 2007; Zhanget al., 2012).As a result, THg concentrations in surface snow are highest in August in XDKMD with values up to 247 ng/L,when the strongest snowmelt occurs (Paudyalet al., 2017).Such high snow THg concentrations are not observed in snowpits in the accumulation zone of glaciers(where the annual gain of glacier mass exceeds the loss).Also, extremely high THg concentrations (up to 70 ng/L) are observed in the ablation zone of the ER(Sunet al.,2018a).

Hg Data in glacier ice are relatively limited (Table 1).THg concentrations are low with a mean of ＜1 ng/L(range: ＜0.5-9.8 ng/L) in the ice core of the accumulation zone of GQ.THg concentrations are higher in surface ice collected in the ablation zone of LHG(96.9 ng/L) and ER (21.3±30.0 ng/L), attributable to settling of PHg from the upper snow layers onto the glacier surface(Huanget al.,2014;Sunet al.,2018a).

4.1.2 Spatio-temporal pattern

Similar to the precipitation Hg, THg concentrations in glacier snowpits of the accumulation zone are generally higher in the northern region of the TP than in the southern region,and higher in the non-monsoon season than in the monsoon season.This spatiotemporal variation of THg is consistent with that of atmospheric dust loadings, further suggesting that dust storm activities may provide the largest source of Hg deposition on the TP (Loewenet al., 2007; Zhanget al., 2012).The northern region generally has high atmospheric dust loadings and THg concentrations due to the close proximity to the major dust source regions (Huanget al., 2014).In the non-monsoon season,especially winter and spring,dust storm activities are prevalent over the TP, resulting in relatively high dust loadings (Wanget al., 2004).However, in glacial surface snow of the ablation zone, THg shows an opposite seasonal pattern,with higher values in the summer melting season than in the non-monsoon season(Paudyalet al., 2017, 2019), due to enrichment of PHg during the melting season.

For spatial distribution of Hg in an individual glacier,Huanget al.(2012b)reported an increase in THg with altitude in surface "aged snow" (coarse-grained snow)in four high-altitude glaciers(i.e.,MZ,GQ,ZD and ER), and proposed a possible altitude magnification effect of Hg deposition onto alpine snow (i.e.,low temperature at high altitudes may enhance deposition efficiency of atmospheric Hg (e.g., PHg) onto glacier surface; Huanget al., 2012b).This indicates that high alpine glaciers probably act as a convergence zone for atmospheric Hg deposition.However,some studies reported a general decreasing trend, or no clear trend of THg with altitude in surface "aged snow", such as LHG, XDKMD and YL(Huanget al.,2014; Paudyalet al., 2017, 2019; Sunet al., 2018a),which have challenged the "altitude magnification effect" of Hg in glacier snow.There is yet a consensus on the reasons explaining the discrepancies between these findings.We find that an important difference between Huanget al.(2012b) and other studies is that snow samples in Huanget al.(2012b) are collected in the cold season (for GQ, ZD and ER), or in the accumulation zone in the summer season (MZ; Table 1),and thus suffer no or slight melting (i.e., no surface meltwater or meltwater flow were observed on the glacier surface).Whereas snow samples in other studies (LHG, XDKMD and YL) are collected mainly in the ablation zone in the summer melting season.PHg accumulates on the surface at lower elevations due to more intense melting, likely resulting in a negative correlation of THg concentrations with elevation during the summer melting season.This mechanism could mask the "altitude magnification effect" of Hg in glacier snow.

Table 1 Summary of Hg concentrations in ice and surface snow on the Tibetan Plateau(ELA:equilibrium line altitude)Glacier name LHG(the Laohugou No.12 Glacier)MZ(Muztagata Glacier)GQ(Guoqu Glacier)XDKMD(the Xiao Dongkemadi Glacier)ZD(Zhadang Glacier)QY(Qiangyong Glacier)ER(East Rongbuk Glacier)YL(The Baishui No.1 Glacier)Study region Northeastern TP Northwestern TP Central TP Central TP Southern TP Southern TP Southern edge of the TP Southeastern TP Sample type Fresh snow and aged Snow surface ice Coarse-grained snow Coarse-grained snow Ice core Surface snow Coarse-grained snow Fresh snow Surface snow Coarse-grained snow Intensive surface snow coarse-grained snow Fine-grained snow Surface ice Aged snow Depth(cm)0-5 0-5 0-5 0-147 m 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 Date July,2013 July,2010 April,2009 November,2005 May-October,2015 May,2009 August,September,2011 August,2012 October,2010 April,2016 April,2016 April,2016 May-August,2015 ELA(m a.s.l.)4,800 4,800-5,200＞5,300＞5,300 5,620 5,750 5,750 5,600 6,419 6,419 6,419 6,419 4,900 Altitude(m a.s.l.)4,452-5,038 4,400-4,900 5,400-5,800 5,200-5,700 5,750 5,400-5,700 5,500-5,750 5,550-5,800 5,101-5,597 6,300-6,550 6,280 6,300-6,700 6,250-6,400 4,640-4,800 THg(ng/L)mean±SD 5.1±8.8 96.9 8.6±3.1 3.6±1.1 0.8±0.8 30.6±53.9 0.9±0.3 0.7±0.2 8.8±0.5 2.2±0.6 3.9±1.2 19.1±16.5 21.3±30.0 37±26 range＜1-39.7 20.1-306.5＜4-13 2.5-7.5＜0.5-9.8 1.0-246.9＜1-1.5 0.4-1.4 7.6-10.6 1-3 2.6-6.4 9.6-69.8 7.6-90.7 3.1-137.8 PHg--- - - 72.9%- - 71.9%-70.7%±6.6%87.8%±6.0%89.7%±6.0%55%Reference Huang et al.,2014 Huang et al.,2012b Huang et al.,2012b Kang et al.,2016 Paudyal et al.,2017 Huang et al.,2012b Sun et al.,2018a Sun et al.,2016 Huang et al.,2012b Sun et al.,2018a Paudyal et al.,2019

THg concentrations in surface ice show an increasing trend with elevation, due to more intensive glacier melt and greater removal of Hg (especially PHg settled on the ice surface)by meltwater at low altitudes(Huanget al.,2014;Sunet al.,2018a).

4.2 Hg export from glaciers

4.2.1 Post-depositional fate

Previous studies have shown that a fraction of Hg in glacier snow could be re-emitted back into the atmosphere via a sunlight-induced mechanism (Lalondeet al., 2002, 2003; Poulainet al., 2004) and an additional fraction of Hg may be released with meltwater,ultimately affecting the health of aquatic ecosystems(Dommergueet al.,2003;Zhanget al.,2019).Studies at the Arctic (Poulainet al., 2004; Dommergueet al.,2010) and some mid-latitude sites (Lalondeet al.,2002, 2003; Poulainet al., 2007) have shown that Hg deposited onto glacier snow is mainly in the form of reactive gaseous Hg (RGM, mostly Hg(II)).A major fraction of the deposited RGM likely re-emits back into the atmosphere as Hg0via rapid photoreduction of Hg(II), which results in generally ＞40% loss of THg contents in surface snow within 24 h after deposition.

Inconsistent with glacial snow Hg measured at the Arctic and sub-Arctic sites, the deposition of Hg onto glacier snow over the TP is primarily associated with PHg, which is suggested to be more stable and far less reducible than deposited Hg(II) in redox reactions, and thus is less influenced by the photoreduction process(Loewenet al., 2007; Durnford and Dastoor, 2011;Zhanget al., 2012).Evidence for this includes 1) Reactive Hg (RHg) represents mostly ionic (Hg2+) plus dissolved gaseous Hg (Hg0), and Hg fractions weakly bound to inorganic/organic complexes and leachable from the particulate matter (Dalziel, 1995).The measured RHg concentration in wet precipitation (mean:0.5 ng/L at the SET station and Lhasa; Huanget al.,2013, 2015) and glacier snow of the TP is very low(e.g., ＜detection limit of 0.2 ng/L in the fresh snow of ZD; Sunet al., 2018a), suggesting a small fraction of easily-reducible Hg available for photoreduction in glacier snow.2) High THg peaks in snowpits correspond well with high particulate loads, and significant correlations between insoluble particles and THg concentrations are observed in snowpits and ice cores of the TP(Loewenet al., 2007; Zhanget al., 2012; Kanget al.,2016),indicating that PHg associated with dust and particles is well preserved in glacier snow.3) Previous observations emphasized that the deposited Hg in glacier snow of TP has a high potential to be retained in glacier snow (Huanget al., 2012b; Sunet al., 2018a).To be more specific, Huanget al.(2012b) reported an overall 31% loss of THg concentrations in snow during the 5-day monitoring at Nam Co station.The 3-day observation in LHG showed a 28% decrease of THg concentration in glacier snow (Huanget al., 2014).THg concentration in glacier snow of XDKMD shows a 34% decrease in one day.Recently, diurnal THg variations in glacier snow of ZD (fresh snow) and ER (coarsegrained snow) showed 18.9%-34.7% decreases in concentration during a snowmelt day, ＞70% of which contributed by decrease of PHg(Sunet al.,2018a).The relatively small loss of THg in glacier snow of the TP compared to the Arctic/sub-Arctic snow (e.g., 92% loss in snow THg concentrations within 48 hrs; Poulainet al.,2004) indicates that the deposited Hg in the TP mountain glaciers is less influenced by postdepositional processes(e.g.,photoreduction and reemission),and is most likely retained within glacier after deposition (Huanget al.,2012b,2014;Paudyalet al.,2017).

Instead, the deposited Hg in the TP glaciers is mainly released with meltwater and enters the aquatic ecosystem when significant snowmelt occurs (Sunet al., 2018a).Sunet al.(2018a) investigated the diurnal migration process of speciated Hg during snowmelt,and their results showed that concentrations of speciated Hg are quite stable when the temperature is below 0 ℃.A sudden release of DHg from snow is observed with the earliest meltwater fractions at the onset of snowmelt ("ionic pulse"), whereas PHg becomes relatively enriched at the earlier stage of snowmelt and is released later with increasing snowmelt intensity.Furthermore, THg is positively correlated with PHg and crustal major ions(e.g.,Ca2+,Mg2+)during snowmelt, indicating that Hg is mainly transported with particulates.The main pathway of Hg loss during snowmelt is most likely associated with the release of PHg with meltwater, which is greatly influenced by the ablation intensity of snow/ice(Sunet al.,2018a).Snowpit Hg studies revealed that the settling of PHg and percolation of soluble Hg transport Hg from the upper snow layers to the lower strata (e.g.,ice layer), resulting in enrichment and peak values of THg in the dust and ice layers of glacier snowpits(Loewenet al., 2007; Huanget al., 2012a, 2014；Zhanget al., 2012).As ablation intensifies as a result of climate warming, the previously retained Hg (especially PHg) in the TP mountain glaciers will mostly enter aquatic environments through the glacial-fed river ecosystem,likely to impact the human health inhabiting downstream adversely after methylation and biomagnification effect(Durnford and Dastoor,2011).

4.2.2 Downstream Hg export

Mean THg concentrations in supraglacial and glacial-fed stream water range between 0.8-22.8 ng/L at various sites, with the majority fall in the range of 0.8-6.8 ng/L(Table 2),which are comparable to those determined in uncontaminated freshwaters worldwide(＜5 ng/L; Ullrichet al., 2001).However, the mean THg concentrations in glacial-fed river water during the summer melting season are quite high such as in the LHG (22.8 ng/L) and XDKMD (18.6±17.8 ng/L).We suspect that these high THg values result from the contribution of PHg from the basal and bank bedrock erosion under high flow conditions(Sunet al.,2017b).

Table 2 Summary of Hg concentrations in glacial meltwater and glacial-fed river water on the Tibetan Plateau Glacier name LHG XDKMD ZD QY ER YL Study region Northeastern TP Central TP Southern TP Southern TP Southern edge of the TP Southeastern TP Sample type Supraglacial streamwater Proglacial streamwater Glacial-fed riverwater(daily)Glacial-fed riverwater(hourly)Supraglacial streamwater Proglacial riverwater Glacial-fed riverwater(hourly,UPMP)Glacial-fed riverwater(hourly,DMP)Proglacial lakewater Glacial-fed riverwater Supraglacial lakewater Supraglacial streamwater Proglacial lakewater Glacial-fed riverwater Snow meltwater beneath the snowpit Date July,2013 July-August,2015 August,2015 August-September,2011 August-September,2011 August,2011 August,2011 August,2012 April,2016 April,2016 April,2016 April,2016 May,2015 Altitude(m a.s.l.)--5,058-5,263 5,220 5,580 5,545 5,400 4,740 4,770 4,869-4,891 6,278 5,750 5,214 5,151-THg(ng/L)mean±SD1.1 22.8 18.6±17.8 18.9±6.7 2.4±1.0 1.1±0.8 0.8±0.4 1.2±0.3 0.9±0.4 1.1-2.5 6.8±1.0 4.6±0.4 2.2±0.2 1.9±0.4 21.2±7.8 range 0.9-1.2 20.3-25.3 6.6-92.5 7.7-37.3-- - - 0.4-1.8-5.8-7.9 4.2-5.0 1.9-2.4 1.5-2.3 10-36 PHg-- - -87.7%79%86.2%83.6%--83%79%58%41%-Export flux(g/a)-- -747.43-8.76 7.3 157.85------ -Reference Huang et al.,2014 Paudyal et al.,2017 Sun et al.,2017b Sun et al.,2016 Sun et al.,2018a Paudyal et al.,2019

Generally, PHg is also the dominant Hg species in glacial meltwater and glacial-fed river/lake water(PHg%: 58%-88%).Observation in the migration process of speciated Hg during snowmelt shows enhanced release of Hg into meltwater as ablation intensity increases (Sunet al., 2018a).Daily variation of THg concentrations in glacier meltwater is observed to increase with increasing temperature and glacier melt intensity (Paudyalet al., 2019).Moreover, diel THg variations in glacial-fed riverwater are consistent with those of the runoff, indicating strong impact of glacier melt on Hg export and transport (Sunet al., 2016, 2017b; Paudyalet al., 2017).High THg concentrations correspond well with total suspended particle (TSP) contents and elevated discharge in the glacial-fed river water, indicating that downstream transport and export of Hg are closely related to processes influencing TSP and PHg (Paudyalet al., 2017; Sunet al., 2017b).The proposed stimulative effects of massive ablation intensity and high flows enhancing Hg export from the glacierized basin include 1) more glacier Hg is released into meltwater as glacier melt intensifies, and glacial meltwater can flush more particulates from glacier surface (Sunet al., 2016, 2018a); 2) more solutes and Hg-containing particles from the eroded soils,and basal bedrocks/riverbank are released into runoff under high flow condition (Huanget al., 2014; Sunet al.,2017b,2018b);3)disturbance of riverbed sediments by runoff results in resuspension of particulates and release of DHg from the sediment pore waters(Sunet al.,2016).

Limited data on annual Hg export estimations indicate low Hg export yet remarkably high Hg yields from the TP glacierized basin (Paudyalet al.,2017; Sunet al., 2017b).The estimated Hg exports from glacial-fed rivers are approximately 158 g/a in the Qugaqie river basin (Sunet al., 2017b), and 747 g/a in the Dongkemadi river basin in the inland TP(Paudyalet al., 2017).Such Hg export amounts are 1-4 lower in magnitudes compared to those in rivers of the Arctic and sub-Arctic regions (Schusteret al.,2011; Sendergaardet al., 2012; Sondergaardet al.,2015), attributable to relatively small basin area and water discharge for the TP glacierized basins.However, the alpine glacier basins show higher THg yields (Qugaqie river basin: 2.74 μg/(m2·a); Dongkemadi glacier basin: 14.6 μg/(m2·a)) compared to the Arctic and sub-Arctic river basins (0.2-5.2 μg/(m2·a)).This indicates that the mountain glacier basin has high efficiency in producing and transporting Hg,due to its general steep topographic incline facilitating the rock weathering and erosion within the basin, and efficient transport of particulates in the runoff.

4.3 Potential environmental risks of Hg in downstream aquatic ecosystems

The water bodies on the TP are mainly supplied by glacial melt or precipitation, and recent studies have indicated that there is low Hg contamination in the TP lakes and rivers.For instance, THg concentrations in the Yarlung Tsangbo River range between 1.5-5.0 ng/L, which is the background Hg level observed in uncontaminated waters (Zhenget al., 2010).THg concentrations in lake waters range between＜1-40.3 ng/L, with most lakes (36 of 38 lakes) with THg concentrations ＜8 ng/L (Liet al., 2015).MeHg concentrations in the surface waters (i.e.,glacial meltwater, lakes and rivers) and lake sediments on the TP are generally low and within the range of those reported for the global remote areas (Xuet al., 2016; Maet al.,2017;Sunet al.,2018b).This suggests that the environmental conditions on the TP appear to be unfavorable for MeHg production due to low environmental Hg loading,high pH(＞7),low temperature,low organic carbon content, and strong ultraviolet radiation(Yanget al., 2011; Zhanget al., 2014; Maet al.,2017).

Generally, the environmental risks of Hg remain poorly studied in the TP glacierized environments.Some downstream aquatic ecosystems (lakes and wetlands) are suggested to behave as net sinks for Hg from upstream glacial-fed runoff (Sunet al., 2016,2018a, 2018b).Previous investigations show significant decreases in THg concentrations during the downstream transport in glacierized river basins,especially when flowing through lakes and wetlands.This is likely caused by sedimentation of PHg in meltwater runoff as topographic slope reduces and meltwater runoff slows down in the lake and wetland area (Sunet al., 2016, 2017b, 2018a, 2018b).In the Qiangyong Glacier Basin of the southern TP, 56% decrease in THg concentration in meltwater runoff is observed after flowing through two proglacial lakes (Sunet al.,2016).Also, in the Zhadang-Qugaqie Basin (ZQB) of the southern TP, wetland water has a lower THg concentration (1.77±0.72 ng/L) compared to supraglacial meltwater (2.67±1.58 ng/L) and glacial-fed runoff(2.51±1.46 ng/L; Sunet al., 2018b).More importantly,lake and wetland areas serve as active Hg methylation zone and thus aggravate the potential risks of Hg released from melting glaciers.A previous study in ZQB reveals that MeHg concentration and %MeHg are higher in wetland water than those in the supraglacial meltwater and upstream runoff.However, both THg and MeHg levels in the ZQB watershed are low and comparable to the global background levels (Sunet al.,2018b).

Despite low Hg contamination in the TP lakes and rivers, high Hg concentrations have been observed in biota of the TP aquatic ecosystems (Yanget al., 2011;Zhanget al., 2014; Shaoet al., 2016; Liuet al.,2018), indicating high sensitivity of biota to Hg input in alpine aquatic ecosystems.High %MeHg is observed in zooplankton (53.1%-66.4%) and benthic amphipods (61.7%±9.7%) in the alkaline TP lake(i.e., Nam Co lake; Zhanget al., 2014), as compared to those previously reported values in the non-acidified lakes(＜30%;Back and Watras,1995),suggesting the efficient transfer of MeHg from the base of aquatic food web in the Tibetan lake ecosystems (Zhanget al., 2014).Yanget al.(2011) reported high fish Hg concentrations in 8 alpine lakes across the TP compared to other mountain regions(THg:243-2,384 ng/g dry weight (dw), MeHg: 131-1,610 ng/g dw;Yanget al., 2011).Zhanget al.(2014) reported wet weight(wwt)MeHg concentrations of 24.9-1,196 ng/g in the fish of 13 rivers and lakes across the southern TP,with approximately 45% of fish samples exceeding the U.S.Fish and Wildlife Service Criteria(100 ng/g),which is among the highest measurements of Hg concentrations detected in wild fish in China (Zhanget al., 2014).Shaoet al.(2016) reported high THg(11-2,097 ng/g dw)and MeHg levels(14-1,960 ng/g dw) in fish from 4 rivers on the TP (Shaoet al.,2016).Such high MeHg levels in fish can be mainly attributed to long lifespan, slow growth rate, low Me-Hg excretion and efficient transfer of MeHg along the food chain under the cold,oligotrophic alpine environments(Zhanget al.,2014;Shaoet al.,2016).Risk assessment of MeHg in fish indicates that consumption of some fish species in the Tibetan aquatic ecosystems will lead to high human exposure to MeHg(Shaoet al.,2016).

It should be noted that glacier Hg export and risks will be aggravated under global warming.Based on snowpits THg concentrations and glacier volume loss,Zhanget al.(2012) made a rough estimate of 2.5 tons Hg release from glaciers in the past 40 years (Zhanget al., 2012).The current warming trend results in increased glacier ablation not only in the ablation zone but also in the accumulation zone, probably causing an overall enhancement of snow/ice loss and glacier Hg release (Zhanget al., 2015c; Kanget al., 2016).Moreover, increased glacier melt and water discharge have a magnification effect on Hg export flux from the alpine glacierized catchments due to increased bedrock and riverbank erosion.Thus,the environmental risks of Hg in TP aquatic ecosystems will likely increase as the climate warms.

5 Hg in the permafrost region

5.1 Hg in soils

Some recent studies have been conducted to investigate Hg concentrations, distribution, storage and its influencing factors in soils of the TP permafrost region (Sunet al., 2017a; Guet al., 2020; Huanget al.,2020b; Muet al., 2020).Generally, studies reported lower THg concentrations of ＜80 ng/g in soils of the TP permafrost region, as compared to some other northern soils(e.g.,100-200 ng/g in tundra soils;Obristet al., 2017; Olsonet al., 2018).For instance, Ciet al.(2018)reported THg concentrations of 2-25 ng/g in soil profiles (0-40 cm,active layer)across the central TP (Ciet al., 2018).Muet al.(2020) reported THg concentrations of 1-60 ng/g in the active layer(0-50 cm) and 15 permafrost cores (0-18 m) from representative landscapes across the TP permafrost region (i.e., alpine wet meadow, alpine meadow, and alpine steppe; Muet al., 2020).Guet al.(2020) reported comparable THg concentrations of 2.8-80 ng/g(mean:19.8±12.2 ng/g)in the alpine grasslands(0-3 m)based on measurements at 114 sites across the TP(Guet al., 2020).The trend of topsoil Hg concentrations among three representative grasslands is alpine swamp meadow ＞alpine meadow ＞alpine steppe (Guet al.,2020; Muet al., 2020).Generally, over large areas of the TP permafrost region, the spatial pattern of THg concentrations in surface soils is characterized as higher in the south and lower in the north (Guet al.,2020; Huanget al., 2020b).This spatial pattern can be explained by the variations in atmospheric depositions, as a result of difference in distance from elevated anthropogenic Hg emissions in South Asia.For the vertical pattern, THg concentrations decrease with depth followed by relatively low and stable values,which are consistent with the vertical pattern of soil organic carbon (SOC) concentrations.SOC and atmospheric deposition are important factors influencing Hg concentrations, pools and spatial variations in soils of the permafrost region (Schusteret al., 2018;Guet al.,2020;Huanget al.,2020b;Muet al.,2020).Hg inherited from parent rock and Hg eluviation is also important factors influencing soil Hg concentrations, especially for the deeper soil layers (Sunet al.,2017a;Guet al.,2020).

Information on stocks, stability of Hg and their vulnerability to permafrost degradation under climate warming is still limited.THg storage in the 25 m-depth soil is estimated to be 125 Gg in the TP permafrost region, of which 16.58 Gg is stored in the active layer (Muet al., 2020).The estimated THg storage in the upper 3 m soil of the TP permafrost region(21.7 Gg; Muet al., 2020) is approximately 1.3% of that in the Northern Hemisphere permafrost regions(1,656±962 Gg; Schusteret al., 2018).A recent estimation reported higher Hg storage of 86.6±101.2 Gg in alpine grasslands of the TP permafrost region for the top 3 m, with 63.2 Gg stored in the active layer(Guet al.,2020).

Previous studies on the TP have indicated that permafrost thawing triggered by climate warming has a high potential to remobilize and accelerate the release of substantial Hg pools in the permafrost region,which is expected to serve as an important source of Hg to the environment (e.g., atmosphere, rivers and lakes) and the global Hg cycle (Sunet al., 2017a; Ciet al., 2018, 2020; Muet al., 2020).A previous study revealed that compared to the highly stable permafrost zone, topsoil Hg pools (0-60 cm) decrease by approximately 33% in the extremely unstable permafrost zone.Enhanced soil Hg emissions as a result of increased temperature and accelerated SOC decomposition can be an important pathway for Hg loss with permafrost degradation and readmission to the atmospheric Hg cycle (Sunet al., 2017a).Studies also found that permafrost thawing can greatly enhance soil Hg release by driving thermokarst degradation(e.g., thermokarst collapse and thermokarst ponds).Muet al.(2020) reported a 17.6%-30.9% decrease in Hg pools(0-30 cm)in surface soils experiencing thermokarst collapse compared to the non-thermokarst surfaces, due to enhanced soil Hg emissions (increased temperature and exposure to UV light) and soil erosion (removing surface material relatively enriched in Hg; Muet al., 2020).A recent study revealed that the release of Hg from thawing permafrost soils with permafrost melt waters is the dominant source of photoreducible Hg in thermokarst ponds (Ciet al.,2020).

5.2 Land-air Hg dynamics

The knowledge on land-air Hg dynamics in the TP alpine regions is still limited, presenting a gap for the understanding the role of alpine permafrost region in regional and global Hg biogeochemical cycles.Currently,only two flux studies quantified land-air Hg exchanges at Beiluhe station and Tanggula station of the central TP(Ciet al.,2016;Sunet al.,2020).

Measurements of Hg fluxes over bare soils at the Beiluhe station showed that the unvegetated surface represents the net source of atmospheric TGM (2.86 ng/(m2·h) or 25.05 μg/(m2·a); Ciet al.,2016),which is significantly higher than the wet deposition flux at remote stations of the TP(1-12 μg/(m2·a)),within the range of land-air fluxes over background soils (-10 to +10 ng/(m2·h); Schroederet al., 2005;Ericksenet al., 2006).The significantly lower Hg emission fluxes than those over Hg-enriched soils(102-103ng/(m2·h)) indicate the influence of soil Hg content on flux magnitude (Agnanet al., 2016).Hg fluxes show strong seasonality with net emissions in the warm season (1.95-5.16 ng/(m2·h)) and small deposition in winter (-0.62 ng/(m2·h)).Solar radiation(especially UV radiation) and temperature are the two most important environmental variables regulating Hg fluxes (Ciet al., 2016).Moreover, precipitation (rainfall and snowfall) also significantly influences Hg emissions from the dry soils,with a large and immediate increase in Hg emissions during rainfall and snowmelt (Ciet al., 2016).On this basis, vast permafrost regions on the TP are proposed to become increasing sources of TGM due to the stimulative effects of warmer temperatures and more precipitation under global warming (Ciet al.,2016).A similar conclusion was also drawn by Ciet al.(2018), which investigated temperature sensitivity of soil Hg(0)in the unvegetated active layer (0-40 cm) at Beiluhe station.Their results revealed that soil Hg(0) concentrations are relatively low and stable under the frozen condition, and increase exponentially with temperature under the unfrozen condition.Furthermore, temperature increase and permafrost thawing are projected to result in up to 54.9% increase in surface soil Hg(0) concentrations by 2100, which will stimulate soil Hg(0) emissions from the climate-sensitive TP permafrost region (Ciet al.,2018).

Thermokarst lakes and ponds commonly exist in the ice-rich permafrost landscapes and play an important role by degassing terrestrial Hg.A recent study in the Beilu river basin found that permafrost melt water is the dominant source of photoreducible Hg in thermokarst ponds (81.2%-91.2%), and thus controls Hg(0) emissions from the thermokarst pond water(30.8-31.6 μg/(m2·a); Ciet al., 2020).This indicates that thermokarst ponds serve as an active converter of photoreducible Hg to Hg(0), and drives the release of permafrost "legacy Hg" into the atmosphere and modern-day Hg cycle.Thermokarst pond Hg(0) emissions are estimated to increase at a rate of 3% per decade in the study region due to permafrost thawing and thermokarst pond expansion under climate warming (Ciet al.,2020).

Alpine grasslands (1,201×103km2) constitute the dominant ecosystem over the TP cryospheric region,covering approximately 80% of the total area of the TP permafrost region (Wanget al., 2016).Sunet al.(2020) proposed a mechanism that climate warming inhibits Hg emissions over the vast TP permafrost region by stimulating vegetation greening.Flux measurements over the alpine meadow at Tanggula station showed that the TP grassland serves as a small source of atmospheric TGM during vegetation period(0.3 ng/(m2·h); Sunet al., 2020).A logarithmic decrease in Hg fluxes is observed with the plant growing,indicating that the presence of alpine grassland inhibits land-air Hg emissions by light shading and plant Hg uptake.Thus,determination of future change in land-air Hg fluxes over the vast TP permafrost region remains a challenge due to the opposite effects of vegetation greening and rising temperature on landair emissions under global warming.Long-term annual ecosystem-scale flux data are required to further assess the impacts of climate warming on Hg fluxes.

6 Knowledge gaps and proposed issues

The existing body of evidence indicates that warming is driving Hg release from the melting and thawing cryosphere and poses a risk to the environment.However,there are still some emerging scientific issues that needed to be addressed to get a further understanding of the influence of cryospheric change on Hg cycling and its potential environmental effects in the fragile TP environments,including:

1) Currently, studies are conducted in different regions, and provide episodic information on Hg behaviors in different components of the TP cryospheric regions.The integrated observation and consideration of Hg dynamics (e.g., Hg pools, input/output processes) and its environmental impacts at some key specific regions are needed to obtain a comprehensive recognition of Hg cycling in the cryospheric region(such as the Nam Co basin).Modeling is a powerful tool to simulate important environmental Hg-cycling processes,such as atmospheric transport and deposition(Gustinet al., 2015), Hg export in river (Schusteret al.,2011), Hg release from thawing permafrost (Schaeferet al., 2020), and land-air exchange (Khanet al.,2019).However, no such work has been done in the TP region.It will be helpful and necessary to develop Hg models to improve the understanding of Hg cycling in the TP cryospheric region.

2)There is a lack of MeHg dataset in the environmental media of the TP(e.g.,glaciers,water and soils),which greatly limits the assessment of environmental risks of Hg in the cryospheric region.Currently, little is known about the Hg methylation mechanism, bioavailability, transmissibility to trophic organisms along the food chain and current status of Hg bioaccumulation in organisms (especially for the terrestrial food chains).Further studies on MeHg levels and factors controlling Hg methylation in the TP cryospheric environment are therefore urgently needed.

3) Only a few studies have reported an overall loss of Hg pools in the active layer with permafrost degradation (Sunet al., 2017a; Muet al., 2020).There is a lack of knowledge on speciation (e.g., distribution between inorganic and organic Hg), mobility(e.g., partitioning of Hg between pore water and solid phase) and release pathways of Hg in permafrost soils, in particular, the "legacy Hg" in permafrost directly affected by permafrost thawing and little is known about its release pathways.This has resulted in large uncertainties on how and to what extent climate warming and permafrost thawing will drive the release of Hg from permafrost soils.

4) Land-air Hg dynamics represent an important Hg input or output pathways.A recent study has shown that alpine grasslands have a significant influence on land-air Hg fluxes and atmospheric Hg through light shading and direct plant Hg uptake (Sunet al., 2020).There is large spatial heterogeneity in vegetation features (e.g., vegetation coverage, biomass/productivity) over the TP, thus, more flux monitoring over the TP grasslands representing various vegetation features is needed to better quantify fluxes over the Tibetan grasslands.Moreover, the driven atmospheric Hg deposition by plant uptake represents an input pathway to the terrestrial ecosystems.However,there is still no detailed assessment of the role of plant Hg uptake in land-air Hg dynamics (e.g., Hg accumulation in plant tissues versus measured fluxes,and potential impacts of phytovolatilization), and its contribution to soil Hg pools in the TP cryospheric region.Further monitoring of the following subjects,such as plant Hg concentrations, biomass, and foliarair Hg exchanges for grassland species during the plant growing season, is essential for a more detailed assessment.

5) Stable Hg isotopes are useful tracers to understand the sources and biogeochemical processes of Hg in the environment.In recent years, there have been some Hg isotope studies exploring the source of Hg in the aquatic ecosystems on the TP(Xuet al.,2016;Liuet al., 2018), and deposition and accumulation of Hg in forest ecosystems on the eastern TP (Wanget al.,2017, 2020; Liuet al., 2019).Hg isotope techniques should be used to explore more processes and pathways influencing the release, transport, transformation,and bioaccumulation of Hg in the TP cryospheric region.

Supporting Information:

Research progress on behaviors and environmental effects of mercury in the cryosphere of the Tibetan Plateau: a critical review.

Table S1 Summary of concentrations and wet deposition fluxes of THg in precipitation over the Tibetan Plateau and other typical regions Sites Muztag Station,Northwestern TP Laohugou Station,Northeastern TP Nam Co Station,Southern TP SET Station,Southeastern TP Two sites in central Himalaya Mt.Gongga,Southeastern TP Dhunche,central Himalaya Yulong Station,Southeastern TP Lhasa,Capital of Tibet Kathmandu,Nepal Mt.Changbai,northeastern China Mt.Leigong,southwestern China Pengjiayu,Taiwan,China Three sites in southwestern China Beijing,China Changchun,China Kodiak,USA Experimental Lakes Area,Canada Churchill,Canada North America MDN(＞100 sites)Korea Durham,USA New York,USA Eastern Ohio,USA Toronto,Canada Seoul,Korea Site type Remote Remote Remote Remote Remote Rural Rural Urban Urban Urban Alpine Alpine Remote Rural to Suburban Urban Urban Sub-Arctic Boreal Sub-Arctic Remote to industrial Rural Rural Rural Urban Urban Urban Period July-October,2010 July-October,2010 2009-2011 2010-2012 2011-2012 2005-2007 2011-2012 August-October,2010 2010 2011-2012 2005-2006 2008-2009 2009 2005-2006 1994-1995 1999-2000 2008 1992-1994 2007 2008 2006-2008 2007,2008 2003-2005 2003,2004 2005-2008 2006,2007 Annual precipitation(mm)200 369 364.9 978-1,818-921 359-630 1,533 1,438 1,120-1,230 647 567 2,500 730 332-1,062 114-160 110--1,235-1,645 Volume-weighted mean concentration(ng/L)THg--4.8±5.94.0-14.36.7-24.8 18.3 13.34.0 8.85 12.9-32.3 224 3542.14.06.2 2.1-18.78.8 8-8.15.5 13.5-14 22.0 10.1-16.3 MeHg--0.03 0.11-0.16-----0.040-----0.052--------Mean THg(ng/L)10.3±11.5 32.9±54.6 6.1±6.9 3.4±1.6 6.5-7.1-8.0±8.3 11.4±5.8 32.6±34.9 19.8±18.3----------------PHg 69.5%77.1%71.2%43.6%63%-80%-60%92.6%77%±12%59%----------------Wet deposition flux(μg/(m2·a))THg2.1 12.1 1.753.9-26.1 15.9 10.58.2 34.98.46.1 10.18 16.8-29.0115 152.45.22.9 0.54 1.9-25.09.4 8.4-12.35.9 13.5-19.7 18.60 16.8-20.2 MeHg--0.01 0.11-0.30-----0.06-----0.04--------Reference Huang,2011 Huang,2011 Huang et al.,2012b Huang et al.,2015 Tripathee et al.,2019 Fu et al.,2010a Tripathee et al.,2019 Huang,2011 Huang et al.,2013 Tripathee et al.,2019 Wan et al.,2009 Fu et al.,2010b Sheu and Lin,2013 Wang et al.,2009 Liu,1997 Fang et al.,2004 MDN,2010 Louis et al.,1995 Sanei et al.,2010 MDN,2010 Ahn et al.,2011 Lombard et al.,2011 Lai et al.,2007 Keeler et al.,2006 Zhang et al.,2012b Seo et al.,2012

Table S2 Summary of Hg concentrations in glacial snowpits on the Tibetan Plateau Site the Laohugou No.12 Glacier Muztag Glacier Guoqu Glacier Xiao Dongkemadi Glacier Zhadang Glacier Demula Glacier East Rongbuk Glacier Baishui No.1 Glacier Study region Northeastern TP Northwestern TP Central TP Central TP Southern TP Southern TP Southern edge of the TP Southeastern TP Mountain range Qilian Kunlun Tanggula Tanggula Nyainqêntanglha Kangri Garpo in Eastern Himalaya Middle Himalayas Hengduan Sample type snowpit 2 snowpits snowpit 2 snowpits snowpit 2 snowpits snowpit snowpit snowpit 9 snowpits snowpit snowpit snowpit snowpit snowpit 3 snowpits Depth(cm)0-130 0-40 0-150 0-90 0-70 0-45 0-110,0-40 0-200 0-210 0-45 0-180 0-150 0-115 0-140 0-295 90,110,160 Date October,2008 July,2013 July,2010 October,November,2005 April,2009 June,July,2015 June,October,2006 September,2008 May,2009 August,September,2011 September,2008 April,2005 May,2009 April,2016 May,2009 May,2015 Altitude(m a.s.l.)5,026 5,040 5,725 5,750;5,820 5,765 5,678 5,800 5,758 5,797 5,800 5,404 6,536 6,525 6,460 4,747 4,700 THg(ng/L)mean±SD 10.8±4 8.8,10.6 3.2±0.9 3.7±2.4 0.9±0.8 1.9,4.3 7.0±8.8,7.1±6.9 8.1±9.2 5.5±6.2 2-6.9 4.9±3.5 1.7±0.8 1.1±1.3 2.8±5.0 3.5±2.2 1.25-1.65 range 4.9-19.9＜1-50 1.2-4.3 1.2-8.3＜0.3-2 0.47-10.05 2.3-43.2 0.8-38.2 0.3-22.2＜1-20.8 0.4-11 0.5-3 0.3-6.5 1.5-21.1 1-7.5 0.01-3.8 PHg- - - - -64.3%,81.0%- - - 76.6%- - -78.3%±10.3%-55%Reference Zhang et al.,2012a Huang et al.,2014 Zhang et al.,2012a Loewen et al.,2007 Zhang et al.,2012a Paudyal et al.,2017 Loewen et al.,2007 Zhang et al.,2012a Zhang et al.,2012a Huang et al.,2012a Zhang et al.,2012a Loewen et al.,2007 Zhang et al.,2012a Sun et al.,2018 Zhang et al.,2012a Paudyal et al.,2019