Sources, Fates, and Geochemical Cycling of Mercury in Geothermal Fields: Insights From Mercury Isotopes

Geothermal fields emit remarkable amounts of mercury (Hg) to the environment. To address the source, fate and geochemical cycling of Hg in geothermal fields, we investigated Hg concentrations and isotopic compositions of hot spring water and fumarole gases from Rehai and Dagejia in SW‐China. Elevated Hg concentrations in fumarole gases (10.0–167 ng m−3) and hot spring water (3.44–84.5 ng L−1) were observed, suggesting that both geothermal fields are of environmental concern. The variation in Δ199Hg (−0.06 to 0.23‰) and Δ200Hg (−0.09 to 0.19‰) in hot spring water supports Hg likely originating from endogenous volcanic degassing and/or rainwater. Negative and nearly zero Δ199Hg in fumarole gases (−0.32 to 0.03‰) supports volcanic degassing and background atmosphere origin. The ranges of δ202Hg in fumarole gases (−0.74 to 0.59‰) and hot spring water (−1.29 to 0.52‰) could reflect limited fluids boiling in geothermal fields. This study, thus, fills an important knowledge gap regarding Hg global cycling.


Introduction
Mercury (Hg) is a highly volatile heavy metal and global pollutant that could transport over long distances in the atmosphere as gaseous Hg(0) species and deposits into ecosystems via wet and dry deposition.Mercury is released into the atmosphere through natural (e.g., volcanic/geothermal emissions, and biomass burning) and anthropogenic processes (e.g., fossil fuel combustion, mining, and chemical industry) (Pirrone et al., 2010).The amount of Hg released via anthropogenic (∼2,000 t yr 1 ) (Pacyna et al., 2016;Streets et al., 2017) and natural processes (4,600-5,300 t yr 1 ) (Driscoll et al., 2013;Pirrone et al., 2010) remain poorly constrained and are associated with large uncertainties, respectively.Past studies on Hg natural emissions have mainly focused on volcanic emissions (Edwards et al., 2021;Ferrara et al., 2000;Pyle & Mather, 2003;Sherman et al., 2009;Si et al., 2020;Tassi et al., 2016) and Hg concentrations, Hg exchanges fluxes in geothermal fields (Bagnato et al., 2015(Bagnato et al., , 2020;;Engle et al., 2006;Nuvolone et al., 2023;Parisi et al., 2019), while little is known about Hg isotopes in geothermal emissions.The lack of Hg isotopes data on geothermal emissions has largely hindered our knowledge of the global Hg cycle and subsequent assessment of its health impacts.
Mercury isotope geochemistry offers a new tool to study the geochemical fate of Hg.The seven natural isotopes of Hg display large mass dependent fractionation (Hg-MDF, reported as δ 202 Hg) and mass-independent fractionation (Hg-MIF, typically denoted as Δ 199 Hg and Δ 200 Hg), which provide useful constraints on Hg sources and processes during Hg biogeochemical cycle.Available studies have established the general framework of Hg isotopic systematics on Earth's major geochemical pools, demonstrating a large variation of ∼10‰ in δ 202  Hg and  Δ 199 Hg values, and a relatively smaller variation of ∼1‰ in Δ 200 Hg values (Blum et al., 2014).The largest Hg-MIF signals were observed in the atmosphere-land-ocean system, believed to be caused by Hg photochemical reactions on Earth's surface.In contrast, deep reservoirs (e.g., crust and mantle) display less pronounced or no Hg-MIF signals, which are thought to be caused by the recycling of Hg from the atmosphere-land-ocean system into Earth's interior via plate tectonics (Deng et al., 2021; R. S. Yin et al., 2022).
Yet, our knowledge of the Hg isotopic composition and the geochemical fate of Hg is still lacking, despite the few studies conducted in the Yellowstone geothermal field in USA (Sherman et al., 2009) and New Zealand (Si et al., 2020).Southwestern China (SW-China), especially in the Yunnan-Guizhou Plateau and the Tibetan Plateau, hosts abundant Cenozoic volcanoes and associated geothermal springs, which provide a natural laboratory to fill this gap.As a result, here, we investigated the Hg abundance and isotopic composition of fumarole gas and hot spring samples, to gain insights into the source, fate and geochemical cycle of Hg in geothermal systems.

Site Description
This study was conducted in two geothermal fields (Dagejia and Rehai) in the Yunnan-Tibet belt (YT) (Figure 1a).The Dagejia geothermal field is located in the Xigazê Region, Tibet.The Dagejia geothermal field is located in the Indus-Brahmaputra suture zone Segment, the southern margin of the magmatic arc of the Gangdis continental margin (X.G. Wang et al., 2011).It is widely covered by the Quaternary sediments.Jurassic sandstone, Cretaceous sandstone and mudstone, and tertiary sandstone and conglomerate are distributed in Dagejia (Figure 1b).The main outcropping stratigraphic units in the study area are: Gradual-Miocene Dazhu Card Set (E 3 N 1 d) sandstone interbedded with mudstone, Palaeocene Eocene Middle Formation 2 (E 1-2 d 2 ) rhyolite volcanic rock interbedded with tuffaceous sandstone and rhyolite.Early to middle Cretaceous Angren Formation 1st member (K 1-2 a 1 ).Among them, the Dianzhong Formation has the characteristics of typical intra-arc basin sedimentation (X.G. Wang et al., 2011).The magmatic rocks outcropped in the area are mainly medium-grained porphyritic mafic monzogranite Hot springs, geysers, hot spring water ponds and fumaroles are widely distributed.The entire geothermal zone is located within a small north-south oriented graben, and its tensional structure provides good conditions for the transportation of hot spring water (Shen et al., 2011).
The Tengchong geothermal field lies in the Tengchong block.It is in western margin of the Sanjiang tectonic belt of southeastern Tibetan-Qinghai Plateau, and separated by the Bangong Lake-Nujiang Suture Zone with the Baoshan block to the east and the Myitkyina Suture Zone with the Indian plate to the west.The Rehai geothermal field, located in Tengchong, Yunnan Province, has one of the youngest volcanic groups in China, whose magma chambers have been well identified (M.K. Li et al., 2018;Ye et al., 2018;Zhao et al., 2021).The eastward subduction of the Myanmar plate below the Tengchong massif has led to the formation of a large number of active hot spring and fumaroles in Rehai (Guo & Wang, 2012).Numerous nearly north-south stretching faults act as conduits for the transport of the hot spring water and heat (M.Wang et al., 2020).The Rehai reservoirs are composed of Yanshanian granites and Proterozoic metamorphic bedrocks, and their caprocks are composed of strongly altered gravelstone and sandstone (Figure 1c) (Guo et al., 2020).Fumaroles, hydrothermal explosion craters, steaming ground, and hot springs with temperatures ranging from 38°to 95°C are representative geothermal features in the Rehai area, and is affected by Quaternary volcanism (Bai et al., 2001).

Samples Collection of Hg Concentration Measurement
We use 500 ml Teflon bottle to collect all water samples (hot spring water and rainwater) in Rehai and Dagejia, all Teflon bottles were soaked in 4‰ nitric acid for 24 hr and rinsed with deionized water for 3 times before use.The Hg concentration of fumarole gas was determined in situ via a Lumex RA 915+ Hg analyzer.Before each measurement, the instrument was calibrated with an internal Hg standard to reach a recovery higher than 90%.Data was measured every 10 seconds; six data were measured at each point and the average was taken as the data for that point.We use a portable water quality analyzer to measure the temperature and Pondus Hydrogenii of the water.wide and the height of the front and rear legs supported at the bottom is 0.8 and 1 m, respectively (see Figure S1 in Supporting Information S1).Teflon film was placed on the shelf, and rainwater flowed into a clean 5 L Teflon bottle along the groove in the middle.All glass bottles were soaked in 4‰ nitric acid for 24 hr and rinsed with deionized water for 3 times before use.Chlorine-impregnated activated carbon (CIC) was used to trap Hg in the aforementioned samples, and CIC was prepared following Fu et al. (2014).The method for total Hg (THg) preconcentration in hot spring water and rainwater following the method by K. Li et al. (2019).A desiccant tube, PFA filter membrane and Tekran Zero air filter were attached in the front of the CIC trap to remove water vapor, particles, and Hg in air, respectively.A silica rubber heater kept at a temperature of 70°C was attached to the surface of the CIC trap to prevent possible water vapor entry.Vacuum pump was connected to the CIC trap to move the purging gas (see Figure S2 in Supporting Information S1).

Fumarole Gases Sampling for Hg Isotope Analysis
Nineteen fumarole gases were collected in Rehai from August 13 to 17 in 2020 and August 22 to September 11 in 2023, and 17 fumarole gases were taken in Dagejia during June 28 to July 8 in 2021 and July 27 to August 7 in 2023.Teflon tube (1/4 inch) was extended into the fumarole for TGM isotope sample collection, and the gas emitted from the fumarole was pumped at 2.5 L min 1 (KNF, Germany) into ClC trap via Teflon tubes.A desiccant tube, PFA filter membrane were attached in the front of the CIC trap to remove water vapor and particles in air, respectively (see Figures S3 and S4 in Supporting Information S1).GOM and PBM are principally filtered out, however, considering GEM is typically >95% of TGM then the loss can be ignored here.

Analysis of Hg Concentrations and Hg Isotopes
Total Hg concentrations in all liquid samples were measured with a cold vapor atomic fluorescence spectrometry (CVAFS, Tekran 2500, Canada).Data assurance during total Hg concentration analysis was performed via blank experiments, parallel samples, and spiked recovery.The total Hg blank was less than 0.01 ng L 1 .The relative variability of geothermal sample duplicates (n = 3) was 10%.Recoveries of THg from certificated Hg standards (1 ng L 1 ) are 95%-105%.The Hg isotopic compositions were measured by Nu-Plasma II MC-ICP-MS, following a previous method (R. S. Yin et al., 2016).Hg-MDF was reported in delta notation (δ) in unit of per mil referenced to the bracketed NIST-3133 Hg standard (Bergquist & Blum, 2007), Mercury-MIF values were represented by capital delta (Δ) notation (‰), which represents the difference between the measured value and the theoretically predicted value (Bergquist & Blum, 2007).NIST SRM 8610 secondary standard solutions were also measured once in every ten samples (Table S1 in Supporting Information S1).The measured values for NIST SRM 8610 (δ 202 Hg = 0.54 ± 0.07(‰); ∆ 199 Hg = 0.01 ± 0.04(‰); ∆ 200 Hg = 0.00 ± 0.05(‰); ∆ 201 Hg = 0.00 ± 0.06(‰) ; 2SD, n = 8) agreed well with those reported in literature (Bergquist & Blum, 2007;R. S. Yin et al., 2016).Repeated analysis of the samples could not be performed due to insufficient sample volume.Hence, the higher 2SD values of the NIST SRM 8610 repeated analyses (n = 8) and spring water spiked Hg (see Tables S1 and S3 in Supporting Information S1) are taken as the analytical error of the data.All aforementioned experiments were conducted at the State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences.(average: 79.6 ± 15.0°C, SD, n = 8) and from 47.9 to 74.0°C (average: 63.9 ± 9.00°C, SD, n = 11), respectively.

Geothermal Fields as an Important Natural Hg Emission Source
TGM concentrations in the fumarole gas measured in the Rehai and Dagejia geothermal fields (56.5 ± 50.7 and 24.0 ± 11.7 ng m 3 , respectively) were higher than the background TGM level in the Tibetan plateau (Nam Co  2020) and data for fumarole gas in Indonesia from Sun et al. (2016); Data for fumarole gas in Italy are from Zambardi et al. (2009).Analytical uncertainties reported here are based on the higher values in NIST SRM 8610 solution and spring water spiked Hg experiment.station: 1.33 ± 0.24 ng m 3 ; X. Yin et al. (2018)) and comparable with previously measured data in other geothermal fields, for example, 15-7,265 ng m 3 in Italy, 1.80-781 μg m 3 in Japan, 1.00-40.7 μg m 3 in US Hawaiian, 4.0-110 ng m 3 in New Zealand and 1.3-33 ng m 3 in Iceland (Bagnato et al., 2014;Edwards et al., 2023;Nakagawa, 1999;Si et al., 2020;Siegel & Siegel, 1975).Thus, our results suggest that geothermal emission of Hg from the Rehai and Dagejia geothermal fields can be an important point source of Hg to the atmosphere from a long-term perspective, considering geothermal activities in the two geothermal fields may have lasted for up to million years (Du et al., 2005;Shen et al., 2011).
Notably, large differences in TGM concentrations in the fumarole gas and THg concentrations in hot spring water between the Rehai and Dagejia geothermal fields were observed.This potentially reflects different Hg sources, enrichment mechanisms, or fractionation processes between these two geothermal fields.Below, we applied Hg isotope compositions to gain a better understanding of such differences.

Multiple Sources of Hg for Geothermal Fields Reflected by Hg-MIF Signals
Positive to near-zero ∆ 199 Hg values were observed in hot spring water from Rehai ( 0.06 to 0.17‰) and Dagejia ( 0.04 to 0.23‰), which were opposite to terrestrial soil signatures that mainly show negative ∆ 199 Hg values (Blum et al., 2014), but were between that of rainwater (Rehai: 0.42-0.44‰;Dagejia: 0.47-0.73‰)and endogenous Hg (Figure 3).These results may be explained by two alternative cases: (1) the ∆ 199 Hg values in hot spring waters resulted from rainwater; or (2) During the volcanic degassing processes in the Earth's interior, Hg from endogenous sources entered the hot spring water during the rising process, and subsequent Hg(II) photoreduction in hot spring water near surface resulted in the mass independent fractionation.In this study, we observed positive ∆ 200 Hg values in one hot spring water sample (up to 0.19‰, RH-LGG-2), indicating the likely contribution of Hg from rainwater, given that rainwater from Rehai and Dagejia have positive ∆ 200 Hg values (Figure 3b).However, most of the hot spring water display near-zero ∆ 200 Hg values (Figure 3b), inconsistent with that of rainwater.Thus, the commonly observed positive ∆ 199 Hg values in hot spring water likely mainly resulted from Hg(II) photoreduction in near-surface hot spring water, similar to the case in the Yellowstone geothermal field, USA (Sherman et al., 2009).Guo and Wu (2020) think the neutral hot spring is the excretion of deep geothermal fluids through different cooling processes, while acid hot springs is shallow groundwater with a short seepage path in Rehai.In our findings, RH-LGG hot spring water samples were acidic and exhibited the same positive ∆ 200 Hg as the rainwater, providing evidence that Hg in the hot spring water could originate from rainwater.2020) and data for fumarole gas in Indonesia from Sun et al. (2016); Data for fumarole gas in Italy are from Zambardi et al. (2009).Analytical uncertainties reported here are based on the higher values in NIST SRM 8610 solution and spring water spiked Hg experiment.
In comparison with hot spring water, fumarole gases provide more direct insights into the multiple Hg sources in the geothermal field.In both Rehai and Dagejia, partial fumarole gases display negative δ 202 Hg and nearly zero ∆ 199 Hg values in the range of previously reported volcanic Hg isotope signatures (Si et al., 2020;Sun et al., 2016;Zambardi et al., 2009), suggesting that endogenous volcanic degassing of Hg contributes in both geothermal fields studied, and confirms that photochemical reactions produce MIF.The other samples exhibited slightly negative ∆ 199 Hg values, which is similar to the background atmospheric signal values, suggesting that Hg in these fumarole gases could come from the background atmosphere.

Limited δ 202 Hg Variation Suggest the Lack of Geothermal Fluid Boiling
The variation of δ 202 Hg can provide some useful insights into the geochemical behavior of Hg in systems (Sherman et al., 2009;Smith et al., 2005Smith et al., , 2008)).Previous studies have demonstrated that at high temperatures, limited changes in δ 202 Hg (within ±0.5‰) could occur during magmatic processes and geothermal leaching of Hg from source rocks (Bergquist & Blum, 2007;Sherman et al., 2009;Smith et al., 2008).Redox reactions, mineral precipitation and boiling of geothermal fluids, however, may cause a large variation of up to 5‰ in 2Hgδ 20 (Sherman et al., 2009;Smith et al., 2005Smith et al., , 2008;;Tang et al., 2017;Zambardi et al., 2009).In two fossil geothermal systems in Nevada, USA, the large variation of δ 202 Hg (up to 5‰) was attributed to boiling of magmatic fluids and associated loss of isotopically lighter Hg(0) from the geothermal system (Smith et al., 2005).On the contrary, narrower δ 202 Hg were found in the geothermal systems where geothermal fluid boiling is limited, such as two sediment-hosted Pb-Zn geothermal systems in SW-China (Cuona: ∼1.2‰; Lanuoma: ∼1.6‰) (Xu et al., 2018) and seafloor geothermal fields in SW-India Ridge (Duanqiao: ∼0.4‰; Yuhuang: 1.2‰) (Zhu et al., 2020).Accordingly, the similar δ 202 Hg variations of the Rehai (hot spring water: 1.29‰ to 0.48‰; fumarole gas: 1.46‰-1.04‰)and Dagejia (hot spring water: 0.27‰ to +0.52‰; fumarole gas: 0.74‰-0.16‰)samples possibly suggest the limited geothermal fluid boiling, due to the lack of strong magmatic activities in both geothermal fields.The larger variations of δ 202 Hg for the Rehai fumarole gases (2.50‰) relative to Dagejia (0.90‰) suggest the more loss of Hg(0) from geothermal fluid.Sherman et al. (2009) observed that δ 202 Hg of the fluid was preferentially enriched with heavy Hg and suggested that light Hg preferentially escaped into atmosphere in the form of Hg(0) and heavy Hg remained in the fluid.We observed a similar phenomenon in Dagejia, where hot spring water samples show heavy isotopes of Hg, and the Hg concentration in the fumarole gas was significantly higher than that of the background zone.Another possible explanation for these observations is the co-precipitation of light Hg, which requires more in-depth research in the future.

Conclusions and Environmental Implications
This study highlighted elevated Hg concentrations and variable Hg isotopic signals in hot spring water and fumarole gases from two large geothermal fields in SW-China, highlighting that (a) these two geothermal fields are important local Hg emission point sources to the environment; (b) geothermal fields could be receive Hg from endogenous volcanic degassing and crustal circulation of rainwater; and (c) there is likely the limited boiling in the two geothermal fields.As shown in Figure 4, these insights allow us to better understand the geochemical cycle of Hg in geothermal fields: (a) the presence of magma chamber beneath the two geothermal fields may release a substantial amount of Hg source in fumaroles through magmatic outgassing; (b) Hg in the hot spring water could be originated from endogenous sources or rainwater; (c) the deep-seating magma chambers also served as a heating source, triggering crustal fluid circulation and resulting in the involvement of Hg from rainwater into the geothermal system.Nonetheless, this study, based on Hg isotopes, provide new constraints on Hg sources, fates and geochemical cycle in geothermal fields.

Geophysical Research Letters
Hot spring water (n = 8) and rainwater (n = 2) were collected in Rehai from August 13 to 17 in 2020, and hot spring water (n = 11) and rainwater (n = 3) were taken in Dagejia during June 28-July 8 in 2021 and July 27-August 7 in 2023.Hot spring water sampling was done with opaque borosilicate glass bottles near the outlet.Rainwater was sampled in open fields via iron shelves.The cross section of each shelf was 1.6 m long and 1 m

Figure 2 .
Figure 2. Plot of ∆ 201 Hg versus ∆ 199 Hg diagrams for Hg in fumarole gas, rainwater and hot spring water in Dagejia and Tengchong compared with the potential Hg sources and Hg in fumarole gas worldwide.Data for fumarole gas in New Zealand are from Si et al. (2020) and data for fumarole gas in Indonesia fromSun et al. (2016); Data for fumarole gas in Italy are fromZambardi et al. (2009).Analytical uncertainties reported here are based on the higher values in NIST SRM 8610 solution and spring water spiked Hg experiment.

Figure 3 .
Figure 3. Plot of ∆ 199 Hg versus δ 202 Hg and ∆ 200 Hg versus ∆ 199 Hg diagrams of Dagejia and Rehai fumarole gas, rainwater and hot spring water in comparison with the potential Hg sources and Hg of fumarole gas worldwide.Data for terrestrial Hg and background air are from Blum et al. (2014); Data for primitive mantle is from Moynier et al. (2021); Data for fumarole gas in New Zealand are from Si et al. (2020) and data for fumarole gas in Indonesia fromSun et al. (2016); Data for fumarole gas in Italy are fromZambardi et al. (2009).Analytical uncertainties reported here are based on the higher values in NIST SRM 8610 solution and spring water spiked Hg experiment.

Figure 4 .
Figure 4. Conceptual presentation of Hg cycling in the geothermal environment of the Rehai and Dagejia.MIF: mass independent fraction.

Table S2
summarizes the THg concentrations of all samples investigated.Generally, THg concentrations of hot spring water from Rehai and Dagejia ranged from 3.44 to 20.1 ng L 1 (average: 8.60 ± 5.98 ng L 1 , SD, n = 8) PAN ET AL.