Isotopic composition and fractionation of mercury in Great Lakes precipitation and ambient air

Mercury compound-specific stable isotope fractionation in high-altitude lake ecosystems of the Bolivian Altiplano

A combination of mercury (Hg) speciation and compound-specific stable isotope analyses was employed to trace the origin and fate of monomethylmercury (MMHg) in the high-altitude food webs of Lakes Titicaca (TTKK) and Uru Uru (UU). Significant MMHg biomagnification was observed, with concentrations reaching up to 2 μg.g-1 Hg in top predators. Hg isotopes lake-specific trends were identified in relation to trophic position (δ15N) and MMHg fractions. In particular, Δ199MMHg increased from 0 ‰ in UU epiphytic biofilm to ≈2 ‰ and ≈4 ‰ in UU and TTKK piscivorous fish, respectively. Both δ202MMHg and Δ199MMHg signatures indicate that the sediment and/or the epibenthic environment is the primary MMHg source in both food webs. However, an additional MMHg pool, associated with photodegraded MMHg, was identified entering the two food webs at a different trophic level. Photodemethylation was estimated to account for 21 % of MMHg degradation before it entered TTKK Lake food web at the fish level, and 16 % before reaching UU Lake food web at the invertebrate level. Even-Hg MIF (Δ200Hg) shows that both Hg(0) atmospheric deposition and geogenic inputs contributed to Hg accumulation in sediments, while the food web adds significant atmospheric Hg(II) signatures, with up to 94 % of Hg(II) contributing to Lake TTKK fish MMHg. These findings underscore the high potential of Hg-CSIA revealing the dominant role of atmospheric Hg(II) deposition and distinct MMHg pools in driving MMHg bioaccumulation in high-altitude lake food webs.

Mercury Transport, Transformation and Accumulation Recorded by Stable Isotopes during Retreated Glacier Chronosequence of 250 Years

Vegetative development in regions where glaciers retreated due to global warming forces the mercury (Hg) cycle in the cryosphere. This study depicts the fate of Hg in a glacier-retreated chronosequence over the last 250 years recorded by signals of stable Hg isotopes. Results show that the Hg storage in surface soil increases by 3.2 times over 250 years after the glacier retreated. 53 ± 11% of Hg in grass shoots is from the uptake of atmospheric Hg0 and 47 ± 11%, from the uptake of soil Hg. Atmospheric Hg2+ is the primary source of surface soil Hg (54 ± 13%), followed by atmospheric Hg0 (40 ± 10%) and geogenic Hg. The Hg accumulation in soils increased by a factor of 5 at an accelerating rate from the 1870s to 2010s. The Hg release flux from melting glaciers is 3.51 ± 0.01 μg m-2 yr-1. The highly positive Δ199Hg (1.03 ± 0.49‰) in precipitation due to photoreduction of Hg2+ in water droplets causes all samples in ecosystems to have positive Δ199Hg values. Isotopic evidence suggests that photolytic and abiotic dark reduction processes have driven Hg0 re-emission from glacier and underlying soil after melting. The accelerated Hg release from melting glaciers and soil Hg accumulation caused by global warming alter Hg cycling in the cryosphere.

Tracing Atmospheric Mercury from Artisanal and Small-Scale Gold Mining

Artisanal and small-scale gold mining (ASGM) is one of the largest primary sources of mercury (Hg) pollution in the atmosphere globally; however, there is a paucity of atmospheric Hg data in ASGM areas. We measured atmospheric gaseous elemental mercury (GEM) concentrations and stable Hg isotopes at fine spatial resolution in the Madre de Dios region of Peru, where ASGM is a major source of Hg. This study employed new passive air samplers that overcome logistical challenges in measuring atmospheric Hg in remote locations. Regional GEM concentrations were elevated (∼1.3 to 11 ng m-3) compared to the background (<1 ng m-3), with very high GEM levels (∼10 to >5000 ng m-3) associated with mining areas and gold shops. Because ASGM-derived GEM is isotopically distinct, its contribution to regional and local atmospheric Hg was estimated using an isotope mixing model and found to be generally over 70%. We also show that vegetation is taking up ASGM-derived GEM, affecting both the concentrations and isotope compositions of GEM as well as in foliage and litter samples. This supports vegetation uptake as a key removal process of GEM from the atmosphere and therefore a major source of Hg to terrestrial ecosystems and soils, which is heightened in ASGM regions.

Quantifying Altitudinal Mercury Accumulation in Biomonitors along Himalayan Valleys Using Mercury Isotopes

The Himalayan valleys are important transport channels of atmospheric pollutants from South Asia to the Tibetan Plateau. This study aims to demonstrate the use of biomonitors (i.e., tree foliage, bark, mosses, and lichens) in the Himalayas to understand the sources and accumulation of mercury (Hg), including the transboundary atmospheric Hg transport across the Himalayas. Results showed that the significant variability in the physiological characteristics and nutrient uptake pathways, coupled with rapid changes in topography and climate-forced precipitation, led to significant differences in concentrations and isotopic compositions among biomonitor species. Δ199Hg values (-0.32 to -0.10‰) at the lower altitudes were slightly more positive than values at upper altitudes, likely reflecting signals of transboundary transport of anthropogenic Hg from South Asia. The isotope mixing model determined atmospheric Hg0 as the main source of Hg in most biomonitors (67 ± 13% to 88 ± 13%), except for Usnea longissimas (i.e., a unique type of lichen) with 61 ± 16% contribution of atmospheric Hg2+. Additionally, the morphological structure and epiphytic environment of U. longissimas facilitate aqueous Hg secondary reactions. Our results suggest that the Hg cycling in the Himalayan valleys could mix multiple impacts from montane environments and signals of transboundary transport of anthropogenic Hg from South Asia.

Atmospheric Hg(0) dry deposition over environmental surfaces: Insights from mercury isotope fractionation

Atmospheric Hg(0) dry deposition is a vital process that significantly affects the global distribution and cycling of Hg. However, significant knowledge gaps and challenges remain in understanding atmospheric Hg(0) deposition and its subsequent post-deposition processes. Hg isotope fractionation has emerged as the most powerful tool for evaluating the impact of atmospheric Hg(0) deposition and unraveling key processes associated with it. By focusing on Hg isotope fractionation processes, Hg isotopic compositions, and influencing factors, this review presents current knowledge, recent advances, and new insights into atmospheric Hg(0) deposition and post-deposition processes over vegetation, soil, snow, and water surfaces. This review also points out the knowledge gaps pertaining to atmospheric Hg(0) deposition and highlights the need for further investigation into the associated processes, mechanisms, isotope fractionation, and modeling. Further research into Hg isotope fractionation in atmospheric Hg(0) deposition and post-deposition processes will advance source and process tracing, paleoclimate reconstruction, and the modeling of Hg isotope distribution on regional and global scales.

Hair mercury isotopes, a noninvasive biomarker for dietary methylmercury exposure and biological uptake

Background. Fish and rice are the main dietary sources of methylmercury (MeHg); however, rice does not contain the same beneficial nutrients as fish, and these differences can impact the observed health effects of MeHg. Hence, it is important to validate a biomarker, which can distinguish among dietary MeHg sources. Methods. Mercury (Hg) stable isotopes were analyzed in hair samples from peripartum mothers in China (n = 265). Associations between mass dependent fractionation (MDF) (δ202Hg) and mass independent fractionation (MIF) (Δ199Hg) (dependent variables) and dietary MeHg intake (independent variable) were investigated using multivariable regression models. Results. In adjusted models, hair Δ199Hg was positively correlated with serum omega-3 fatty acids (a biomarker for fish consumption) and negatively correlated with maternal rice MeHg intake, indicating MIF recorded in hair can be used to distinguish MeHg intake predominantly from fish versus rice. Conversely, in adjusted models, hair δ202Hg was not correlated with measures of dietary measures of MeHg intake. Instead, hair δ202Hg was strongly, negatively correlated with hair Hg, which explained 27-29% of the variability in hair δ202Hg. Conclusions. Our results indicated that hair Δ199Hg can be used to distinguish MeHg intake from fish versus rice. Results also suggested that lighter isotopes were preferentially accumulated in hair, potentially reflecting Hg binding to thiols (i.e., cysteine); however, more research is needed to elucidate this hypothesis. Broader impacts include 1) validation of a non-invasive biomarker to distinguish MeHg intake from rice versus fish, and 2) the potential to use Hg isotopes to investigate Hg binding in tissues.

Unraveling atmospheric mercury dynamics at Mauna Loa through the isotopic analysis of total gaseous mercury

Our investigation seeks to uncover the intricate nature of mercury dynamics in the free troposphere through analysis of the isotopic composition of total gaseous elemental mercury (TGM) at the high altitude Mauna Loa Observatory (MLO, 3397 m) in Hawaii, USA. By focusing on this unique site, we aim to provide essential insights into the behavior and cycling of mercury, contributing valuable data to a deeper understanding of its global distribution and environmental impacts. Forty-eight hours of TGM sampling from January to September 2022 revealed significant variations in δ202Hg (-1.86 % to -0.32 %; mean = -1.17 ± 0.65 %, 2 SD, n = 34) and small variations in Δ199Hg (-0.27 % to 0.04 %; mean = -0.13 ± 0.14 %, 2 SD, n = 34) and Δ200Hg (-0.20 % to 0.06 %; mean = -0.05 ± 0.13 %, 2 SD, n = 34). During the sampling period, GEM was negatively correlated with gaseous oxidized mercury (GOM). However, the GOM/GEM ratio was not -1, suggesting that GEM oxidation and subsequent scavenging occurred previously. The δ202Hg isotopic compositions of TGM at MLO were different from those of reported values of high-altitude mountains; the δ202Hg of TGM at MLO was lower than the isotopic ratios that were obtained from other mountain regions. The unique atmospheric conditions at Mauna Loa, with (upslope winds during the day and downslope winds at night, likely result in the) possibly mixing of GEMs from terrestrial (and possibly oceanic GEM emission) sources with and tropospheric sources, influencing and affect the isotopic composition. During the late summer to early fall (September 14-28), negative correlations were found between relative humidity and GOM and between particle number concentrations and Δ199Hg, indicating the gas-to-particle partitioning of the atmospheric mercury during this period. This study will improve our understanding on mercury dynamics of marine origin and high altitudes and shed light on its complex interactions with environmental factors.

Low mercury risks in paddy soils across the Pakistan

Mercury (Hg) is a globally distributed heavy metal. Here, we study Hg concentration and isotopic composition to understand the status of Hg pollution and its sources in Pakistan's paddy soil. The collected paddy soils (n = 500) across the country have an average THg concentration of 22.30 ± 21.74 ng/g. This low mean concentration suggests Hg pollution in Pakistan was not as severe as previously thought. Meanwhile, samples collected near brick kilns and industrial areas were significantly higher in THg than others, suggesting the influence of Hg emitted from point sources in certain areas. Soil physicochemical properties showed typical characteristic of mineral soils due to the study area's arid to semi-arid climate. Hg stable isotopes analysis, depicted mean Δ199Hg of -0.05 ± 0.12‰ and mean δ202Hg -0.45 ± 0.35‰, respectively, for contaminated sites, depicting Hg was primarily sourced from coal combustion by local anthropogenic sources. While uncontaminated sites show mean Δ199Hg of 0.15 ± 0.08‰, mean Δ200Hg of 0.06 ± 0.07‰ and mean δ202Hg of -0.32 ± 0.28‰, implying long-range transboundry Hg transport through wet Hg(II) deposition as a dominant Hg source. This study fills a significant knowledge gap regarding the Hg pollution status in Pakistan and suggests that the Hg risk in Pakistan paddies is generally low.

Atmospheric Mercury Concentrations and Isotopic Compositions Impacted by Typical Anthropogenic Mercury Emissions Sources

Coal-fired power plants (CFPPs) and cement plants (CPs) are important anthropogenic mercury (Hg) emission sources. Mercury speciation profiles in flue gas are different among these sources, leading to significant variations in local atmospheric Hg deposition. To quantify the impacts of Hg emissions from CFPPs and CPs on local-scale atmospheric Hg deposition, this study determined concentrations and isotopes of ambient gaseous elemental mercury (GEM), particulate-bound mercury (PBM), and precipitation total Hg (THg) at multiple locations with different distances away from a CFPP and a CP. Higher concentrations of GEM and precipitation THg in the CFPP area in summer were caused by higher Hg emission from the CFPP, resulting from higher electricity demand. Higher concentrations of GEM, PBM, and precipitation THg in the CP area in winter compared to those in summer were related to the higher output of cement. Atmospheric Hg concentration peaked near the CFPP and CP and decreased with distance from the plants. Elevated GEM concentration in the CFPP area was due to flue gas Hg0 emissions, and high PBM and precipitation Hg concentrations in the CP area were attributed to divalent Hg emissions. It was estimated that Hg emissions from the CFPP contributed 58.3 ± 20.9 and 52.3 ± 25.9% to local GEM and PBM, respectively, and those from the CP contributed 47.0 ± 16.7 and 60.0 ± 25.9% to local GEM and PBM, respectively. This study demonstrates that speciated Hg from anthropogenic emissions posed distinct impacts on the local atmospheric Hg cycle, indicating that Hg speciation profiles from these sources should be considered for evaluating the effectiveness of emission reduction policies. This study also highlights the Hg isotope as a useful tool for monitoring environmental Hg emissions.

Photoreduction and origin of dissolved and particulate mercury in cloud water: Insights from stable mercury isotopes

The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as Δ199Hg), with averaged Δ199Hg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.

Isotopic insights and integrated analysis for heavy metal levels, ecological risks, and source apportionment in river sediments of the Qinghai-Tibet Plateau

The research was carried out to examine the pollution characteristics, ecological risk, and origins of seven heavy metals (Hg, As, Pb, Cu, Cd, Zn, and Ni) in 51 sediment samples gathered from 8 rivers located on the Qinghai-Tibet Plateau (QTP) in China. The contents of Hg and Cd were 5.0 and 1.1 times higher than their background values, respectively. The mean levels of other measured heavy metals were below those found naturally in the local soil. The enrichment factor showed that the study area exhibited significantly enriched Hg with 70.6% sampling sites. The Cd contents at 19.6% of sampling sites were moderately enriched. The other sampling sites were at a less enriched level. The sediments of all the rivers had a medium level of potential ecological risk. Hg was the major ecological risk factor in all sampling sites, followed by Cd. The findings from the positive matrix factorization (PMF) analysis shown agricultural activities, industrial activities, traffic emissions, and parent material were the major sources. The upper, middle, and low reaches of the Quanji river had different Hg isotope compositions, while sediments near the middle reaches were similar to the δ202Hg of the industrial source. At the upstream sampling sites, the Hg isotope content was very close to the background level. The results of this research can establish a strong scientific sound to improve the safety of the natural circumstances of rivers on the QTP.

Atmospheric mercury uptake and accumulation in forests dependent on climatic factors

The environmental and climatic factors dictating atmospheric mercury (Hg) uptake by foliage and accumulation within the forest floor are evaluated across six mountain sites, South Korea, using Hg concentration and Hg stable isotope analyses. The isotope ratios of total gaseous Hg (TGM) at six mountains are explained by local anthropogenic Hg emission influence and partly by mountain elevation and wind speed. The extent to which TGM is taken up by foliage is not dependent on the site-specific TGM concentration, but by the local wind speed, which facilitates TGM passage through dense deciduous canopies in the Korean forests. This is depicted by the significant positive relationship between wind speed and foliage Hg concentration (r2 = 0.92, p < 0.05) and the magnitude of δ202Hg shift from TGM to foliage (r2 = 0.37, p > 0.05), associated with TGM uptake and oxidation by foliar tissues. The litter and topsoil Hg concentrations and isotope ratios reveal relationships with a wide range of factors, revealing lower Hg level and greater isotopic fractionation at sites with low elevation, high wind speed, and high mean warmest temperature. We attribute this phenomenon to active TGM re-emission from the forest floor at sites with high wind speed and high temperature, caused by turnover of labile organic matter and decomposition. In contrast to prior studies, we observe no significant effect of precipitation on forest Hg accumulation but precipitation appears to reduce foliage-level Hg uptake by scavenging atmospheric Hg species available for stomata uptake. The results of this study would enable better prediction of future atmospheric and forest Hg influence under climate change.

Fate and Transport of Mercury through Waterflows in a Tropical Rainforest

Knowledge gaps of mercury (Hg) biogeochemical processes in the tropical rainforest limit our understanding of the global Hg mass budget. In this study, we applied Hg stable isotope tracing techniques to quantitatively understand the Hg fate and transport during the waterflows in a tropical rainforest including open-field precipitation, throughfall, and runoff. Hg concentrations in throughfall are 1.5-2 times of the levels in open-field rainfall. However, Hg deposition contributed by throughfall and open-field rainfall is comparable due to the water interception by vegetative biomasses. Runoff from the forest shows nearly one order of magnitude lower Hg concentration than those in throughfall. In contrast to the positive Δ199Hg and Δ200Hg signatures in open-field rainfall, throughfall water exhibits nearly zero signals of Δ199Hg and Δ200Hg, while runoff shows negative Δ199Hg and Δ200Hg signals. Using a binary mixing model, Hg in throughfall and runoff is primarily derived from atmospheric Hg0 inputs, with average contributions of 65 ± 18 and 91 ± 6%, respectively. The combination of flux and isotopic modeling suggests that two-thirds of atmospheric Hg2+ input is intercepted by vegetative biomass, with the remaining atmospheric Hg2+ input captured by the forest floor. Overall, these findings shed light on simulation of Hg cycle in tropical forests.

Continuous Flow-Double Purge and Trap Method for Preconcentrating Mercury in Large Volumes of Seawater for Stable Isotope Analysis

Mercury (Hg) isotopes provide a useful tool to understand Hg sources and processes in the environment. The Hg isotopic composition of seawater remains poorly constrained due to the lack of an efficient method to process large volumes of low-Hg-concentration seawater samples. Here, we develop a continuous flow-double purge and trap device for the in situ preconcentration of Hg in seawater. This method yielded a good Hg recovery of 91.7 ± 3.3% (n = 4, 1SD) for spiked seawater samples and gave reasonably similar Hg isotope ratios of NIST 8610, indicating a limited matrix effect and limited Hg isotope fractionation during processing of seawater. NIST 8610 δ202Hg (-0.55 ± 0.09‰, n = 4, 1SD) and Δ199Hg (0.07 ± 0.02‰, n = 4, 1SD) were similar to previously published data. The method was successfully applied to seawater collected from the Xiamen Bay and the South China Sea. The seawater samples showed a Hg recovery of 91.6 ± 5.4% (n = 12, 1SD). Seawater Δ199Hg (-0.04 ± 0.05‰, n = 7, 1SD) in the Xiamen Bay was different from seawater Δ199Hg (0.05 ± 0.07‰, n = 5, 1SD) in the South China Sea, which implies distinct Hg sources to coastal and open ocean areas and highlights the robustness of our method in understanding the Hg isotopic composition of seawater.

Mercury deposition and redox transformation processes in peatland constrained by mercury stable isotopes

Peatland vegetation takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remains, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg redox transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that above the water table, dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations. Below the water table, supersaturation of gaseous Hg is likely created more by direct photoreduction of rainfall rather than by reduction and release of Hg from the peat soil. Identification and quantification of these light-driven and dark redox processes advance our understanding of the fate of Hg in peatlands, including the potential for mobilization and methylation of HgII.

Elevated Mercury Deposition, Accumulation, and Migration in a Karst Forest

The karst forest is one of the extremely sensitive and fragile ecosystems in southwest China, where the biogeochemical cycling of mercury (Hg) is largely unknown. In this study, we investigated the litterfall deposition, accumulation, and soil migration of Hg in an evergreen-deciduous broadleaf karst forest using high-resolution sampling and stable isotope techniques. Results show that elevated litterfall Hg concentrations and fluxes in spring are due to the longer lifespan of evergreen tree foliage exposed to atmospheric Hg0. The hillslope has 1-2 times higher litterfall Hg concentration compared to the low-lying land due to the elevated atmospheric Hg levels induced by topographical and physiological factors. The Hg isotopic model suggests that litterfall Hg depositions account for ∼80% of the Hg source contribution in surface soil. The spatial trend of litterfall Hg deposition cannot solely explain the trend of Hg accumulation in the surface soil. Indeed, soil erosion enhances Hg accumulation in soil of low-lying land, with soil Hg concentration up to 5-times greater than the concentration on the hillslope. The high level of soil Hg migration in the karst forest poses significant ecological risks to groundwater and downstream aquatic ecosystems.

Mercury Accumulation and Sequestration in a Deglaciated Forest Chronosequence: Insights from Particulate and Mineral-Associated Forms of Organic Matter

Understanding mercury (Hg) complexation with soil organic matter is important in assessing atmospheric Hg accumulation and sequestration processes in forest ecosystems. Separating soil organic matter into particulate organic matter (POM) and mineral-associated organic matter (MAOM) can help in the understanding of Hg dynamics and cycling due to their very different chemical constituents and associated formation and functioning mechanisms. The concentration of Hg, carbon, and nitrogen contents and isotopic signatures of POM and MAOM in a deglaciated forest chronosequence were determined to construct the processes of Hg accumulation and sequestration. The results show that Hg in POM and MAOM are mainly derived from atmospheric Hg0 deposition. Hg concentration in MAOM is up to 76% higher than that in POM of broadleaf forests and up to 60% higher than that in POM of coniferous forests. Hg accumulation and sequestration in organic soil vary with the vegetation succession. Variations of δ202Hg and Δ199Hg are controlled by source mixing in the broadleaf forest and by Hg sequestration processes in the coniferous forest. Accumulation of atmospheric Hg and subsequent microbial reduction enrich heavier Hg isotopes in MAOM compared to POM due to the specific chemical constituents and nutritional role of MAOM.

Isotopic Characterization of Mercury Atmosphere-Foliage and Atmosphere-Soil Exchange in a Swiss Subalpine Coniferous Forest

To understand the role of vegetation and soil in regulating atmospheric Hg0, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere-foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg0 sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δ202Hg values. The soil served mostly as net sources of atmospheric Hg0 with higher Hg0 emission from the moss-covered soils than from bare soils. The negative shift of δ202Hg and Δ199Hg values of the efflux air relative to ambient air and the Δ199Hg/Δ201Hg ratio among ambient air, efflux air, and soil pore gas highlight that Hg0 re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg0 emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg0 to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg0 isotope signatures. This study improves our understanding of Hg atmosphere-foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.

Tracking the multiple Hg sources in sediments in a typical river-lake basin by isotope compositions and mixing models

In this study, total mercury (THg) contents and Hg isotope compositions in sediments were investigated in the Lianxi River, Zijiang River and South Dongting Lake to identify and quantify multiple Hg sources and evaluate the Hg environmental processes. The THg contents, δ202Hg and Δ199Hg values in sediments were 48.22 ∼ 4284.32 µg/kg, - 1.33 ∼ 0.04‰ and - 0.25 ∼ 0.03‰, respectively. Relatively distinct Hg isotope characteristics of sediments were presented in the Lianxi River, Zijiang River and South Dongting Lake, indicating the dominant Hg sources considerably varied in these regions. Source apportionment based on MixSIAR proved that Hg in sediments mainly originated from industrial activities, and the ternary mixing model concluded non-ferrous metal smelting was the dominant industrial Hg contributor in the Lianxi River. Compared with the Lianxi River, the relative contribution of Hg in sediments from industrial activities significantly decreased, while the relative contributions of Hg from background releases significantly increased in the Zijiang River and South Dongting Lake. Nonetheless, the contribution of industrial Hg in this study area deserves more attention. These results are conducive to further manage Hg pollution.

Mercury Sources and Fate in a Large Brackish Ecosystem (the Baltic Sea) Depicted by Stable Isotopes

Identifying Hg sources to aquatic ecosystems and processes controlling the levels of monomethylmercury (MMHg) is critical for developing efficient policies of Hg emissions reduction. Here we measured Hg concentrations and stable isotopes in sediment, seston, and fishes from the various basins of the Baltic Sea, a large brackish ecosystem presenting extensive gradients in salinity, redox conditions, dissolved organic matter (DOM) composition, and biological activities. We found that Hg mass dependent fractionation (Hg-MDF) values in sediments mostly reflect a mixing between light terrestrial Hg and heavier industrial sources, whereas odd Hg isotope mass independent fractionation (odd Hg-MIF) reveals atmospheric inputs. Seston presents intermediate Hg-MDF and odd Hg-MIF values falling between sediments and fish, but in northern basins, high even Hg-MIF values suggest the preferential accumulation of wet-deposited Hg. Odd Hg-MIF values in fish indicate an overall low extent of MMHg photodegradation due to limited sunlight exposure and penetration but also reveal large spatial differences. The photodegradation extent is lowest in the central basin with recurrent algal blooms due to their shading effect and is highest in the northern, least saline basin with high concentrations of terrestrial DOM. As increased loads of terrestrial DOM are expected in many coastal areas due to global changes, its impact on MMHg photodegradation needs to be better understood and accounted for when predicting future MMHg concentrations in aquatic ecosystems.

Mercury stable isotopes in the ocean: Analytical methods, cycling, and application as tracers

Mercury (Hg) has seven stable isotopes that can be utilized to trace the sources of Hg and evaluate the importance of transport and transformation processes in the cycling of Hg in the environment. The ocean is an integral part of the Earth and plays an important role in the global mercury cycle. However, there is a lack of a systematic review of Hg stable isotopes in marine environments. This review is divided into four sections: a) advances in Hg stable isotope analysis, b) the isotope ratios of Hg in various marine environmental matrices (seawater, sediment, and organisms), c) processes governing stable Hg isotope ratios in the ocean, and d) application of Hg stable isotopes to understand biotic uptake and migration. Mercury isotopes have provided much useful information on marine Hg cycling that cannot be given by Hg concentrations alone. This includes (i) sources of Hg in coastal or estuarine environments, (ii) transformation pathways and mechanisms of different forms of Hg in marine environments, (iii) trophic levels and feeding guilds of marine fish, and (iv) migration/habitat changes of marine fish. With the improvement of methods for seawater Hg isotope analysis (especially species-specific methods) and the measurement of Hg isotope fractionation during natural biogeochemical processes in the ocean, Hg stable isotopes will advance our understanding of the marine Hg cycle in the future, e.g., mercury exchange at the sea-atmosphere interface and seawater-sediment interface, contributions of different water masses to Hg in the ocean, fractionation mechanisms of Hg and MeHg transformation in seawater.

Atmospheric Mercury Isotope Shifts in Response to Mercury Emissions from Underground Coal Fires

Pollutant emissions from coal fires have caused serious concerns in major coal-producing countries. Great efforts have been devoted to suppressing them in China, notably at the notorious Wuda Coalfield in Inner Mongolia. Recent surveys revealed that while fires in this coalfield have been nearly extinguished near the surface, they persist underground. However, the impacts of Hg volatilized from underground coal fires remain unclear. Here, we measured concentrations and isotope compositions of atmospheric Hg in both gaseous and particulate phases at an urban site near the Wuda Coalfield. The atmospheric Hg displayed strong seasonality in terms of both Hg concentrations (5-7-fold higher in fall than in winter) and isotope compositions. Combining characteristic isotope compositions of potential Hg sources and air mass trajectories, we conclude that underground coal fires were still emitting large amounts of Hg into the atmosphere that have been transported to the adjacent urban area in the prevailing downwind direction. The other local anthropogenic Hg emissions were only evident in the urban atmosphere when the arriving air masses did not pass directly through the coalfield. Our study demonstrates that atmospheric Hg isotope measurement is a useful tool for detecting concealed underground coal fires.

Identification of sources and their potential health risk of potential toxic elements in soils from a mercury‑thallium polymetallic mining area in Southwest China: Insight from mercury isotopes and PMF model

Identification of potential toxic element (PTE) sources and their specific human health risk is critical to the management of PTEs in soils. In this study, multi-medium were collected from a mercury‑thallium polymetallic mining area in Southwestern China. Hg isotope technique together with positive matrix factorization (PMF) model was used to identify PTE sources and assess their source-oriented health risk. Results showed that among the studied PTEs, this study area presented high pollution of Hg, Tl and As, with higher concentrations than their corresponding background values of Guizhou province, yet their average concentrations in covering soils were significantly lower than those in the natural soils. The Tl in coix grains should also be paid more attention due to its high concentration. Both natural and covering soils had different Hg isotope composition with tailings, while sediments have similar Hg isotope fractionation with covering soils. According to the PMF model, three sources in both natural and covering soils were apportioned and Hg, Tl and As were mainly influenced by the historical mining activities, which also confirmed by their Hg isotope signatures. The contributions of historical mining activities accounted for 40 % and 20 % of the PTEs in natural and covering soils, respectively. The assessment of source-specific health risks suggested that the non-carcinogenic risk of Hg, Tl and As was much higher than other elements. Historical mining activities were regarded as the major contributor to health risks (79 % and 76 % for natural soils and 50 % and 59 % for covering soils, respectively). This indicated that the restoration of coveing soils indeed decreased the health risk in this study area. These findings thus highlight the importance of ongoing monitoring of covering soils in the polymetallic mining area, which is imperative for preferably assessing the health risk of PTEs in similar mining area worldwide.

Quantifying Mercury Distribution and Source Contribution in Surface Soil of Qinghai-Tibetan Plateau Using Mercury Isotopes

Long-range transport and atmospheric deposition of gaseous mercury (Hg⁰) result in significant accumulation of Hg in the Qinghai-Tibetan Plateau (QTP). However, there are significant knowledge gaps in understanding the spatial distribution and source contribution of Hg in the surface soil of the QTP and factors influencing Hg accumulation. In this study, we comprehensively investigated Hg concentrations and isotopic signatures in the QTP to address these knowledge gaps. Results show that the average Hg concentration in the surface soil ranks as follows: forest (53.9 ± 36.9 ng g^-1) > meadow (30.7 ± 14.3 ng g^-1) > steppe (24.5 ± 16.1 ng g^-1) > shrub (21.0 ± 11.6 ng g^-1). Hg isotopic mass mixing and structural equation models demonstrate that vegetation-mediated atmospheric Hg⁰ deposition dominates the Hg source in the surface soil, with an average contribution of 62 ± 12% in forests, followed by 51 ± 10% in shrub, 50 ± 13% in steppe, and 45 ± 11% in meadow. Additionally, geogenic sources contribute 28-37% of surface soil Hg accumulation, and atmospheric Hg²⁺ inputs contribute 10-18% among the four types of biomes. The Hg pool in 0-10 cm surface soil over the QTP is estimated as 8200 ± 3292 Mg. Global warming, permafrost degradation, and anthropogenic influences have likely perturbed Hg accumulation in the soil of QTP.

Tracing the transboundary transport of atmospheric Particulate Bound Mercury driven by the East Asian monsoon

Taking Beijing-Tianjin-Hebei (BTH) with severe atmospheric mercury (Hg) and PM_2.5 pollution as a typical region, this study clarified the characteristics and transboundary transport of atmospheric Particulate Bound Mercury (PBM_2.5) affected by the East Asian monsoon. Five sampling sites were conducted in rural, suburban, urban, industrial, and coastal areas of BTH from northwest to southeast along the East Asian monsoon direction. PBM_2.5 showed increasing concentrations from northwest to southeast and negative δ²⁰²Hg values, indicating significant contributions from anthropogenic sources. However, the mean Δ¹⁹⁹Hg values of PBM_2.5 at the five sites were significantly positive, probably triggered by the photoreduction of Hg(II) during long-range transport driven by the East Asian monsoon. Apart from local anthropogenic emissions as the primary sources, the transboundary transport of PBM_2.5, driven by west and northwest air masses originating in Central Asia and Russia, contributed significantly to the PBM_2.5 pollution of BTH. Moreover, these air masses reaching BTH would carry elevated PBM_2.5 concentrations further transported to the ocean by the East Asian monsoon. In contrast, the southeast air masses transported from the ocean by the East Asian monsoon in summer diluted inland PBM_2.5 pollution. This study provides insight into the atmospheric Hg circulation affected by the East Asian monsoon.

Fractionation of mercury stable isotopes in lichens

Bio-monitoring of mercury (Hg) in air using transplanted and in-situ lichens was conducted at three locations in Slovenia: (I) the town of Idrija in the area of the former Hg mine, where Hg contamination is well known; (II) Anhovo, a settlement with a cement production plant, which is a source of Hg contamination, and (III) Pokljuka, a part of a national park. Lichens from Pokljuka were transplanted to different sites and sampled four times-once per season, from January 2020 to February 2021. Lichens were set on tree branches, fences, and under cover, allowing them to be exposed to different environmental conditions (e.g., light and rain). The in-situ lichens were sampled at the beginning and the end of the sampling period. The highest concentrations were in the Idrija area, which was consistent with previous research. Significant mass-dependent fractionation has been observed in transplanted lichens during summer period. The δ²⁰²Hg changed from -3.0‰ in winter to -1.0‰ in summer and dropped again to the same value in winter the following year. This trend was observed in all samples, except those from the most polluted Idrija sampling site, which was in the vicinity of the former Hg ore-smelting plant. This was likely due to large amounts of Hg originating from polluted soil close to the former smelting plant with a distinct isotopic fingerprint in this local area. The Δ¹⁹⁹Hg in transplanted lichens ranged from -0.5‰ to -0.1‰ and showed no seasonal trends. These findings imply that seasonality, particularly in summer months, may affect the isotopic fractionation of Hg and should be considered in the sampling design and data interpretation. This trend was thus described in lichens for the first time. The mechanism behind such change is not yet fully understood.

Mercury isotopic evidence for the importance of particles as a source of mercury to marine organisms

The origin of methylmercury in pelagic fish remains unclear, with many unanswered questions regarding the production and degradation of this neurotoxin in the water column. We used mercury (Hg) stable isotope ratios of marine particles and biota to elucidate the cycling of methylmercury prior to incorporation into the marine food web. The Hg isotopic composition of particles, zooplankton, and fish reveals preferential methylation of Hg within small (< 53 µm) marine particles in the upper 400 m of the North Pacific Ocean. Mass-dependent Hg isotope ratios (δ 202 Hg) recorded in small particles overlap with previously estimated δ 202 Hg values for methylmercury sources to Pacific and Atlantic Ocean food webs. Particulate compound specific isotope analysis of amino acids (CSIA-AA) yield δ 15 N values that indicate more-significant microbial decomposition in small particles compared to larger particles. CSIA-AA and Hg isotope data also suggest that large particles (> 53 µm) collected in the equatorial ocean are distinct from small particles and resemble fecal pellets. Additional evidence for Hg methylation within small particles is provided by a statistical mixing model of even mass–independent (Δ 200 Hg and Δ 204 Hg) isotope values, which demonstrates that Hg within near-surface marine organisms (0–150 m) originates from a combination of rainfall and marine particles. In contrast, in meso- and upper bathypelagic organisms (200–1,400 m), the majority of Hg originates from marine particles with little input from wet deposition. The occurrence of methylation within marine particles is supported further by a correlation between Δ 200 Hg and Δ 199 Hg values, demonstrating greater overlap in the Hg isotopic composition of marine organisms with marine particles than with total gaseous Hg or wet deposition.

Tracing the sources and depositional history of mercury to coastal northeastern U.S. lakes

Mercury (Hg) deposition was reconstructed in sediment cores from lakes in two coastal U.S. National Parks: Acadia National Park (ANP) and Cape Cod National Seashore (CCNS), to fill an important spatial gap in Hg deposition records and to explore changing sources of Hg and processes affecting Hg accumulation in these coastal sites. Recent Hg deposition chronology was assessed using (1) a newly developed lead-210 (²¹⁰Pb) based sediment age model which employs ⁷Be to constrain deposition and sediment mixing of ²¹⁰Pb-excess, (2) coinciding Pb flux and isotope ratios (²⁰⁶Pb/²⁰⁷Pb), and (3) Hg isotope ratios and their response to changes in Hg flux. At both sites, Hg flux increased substantially from pre-1850 levels, with accumulation in ANP peaking in the 1970s, whereas in CCNS, Hg levels were highest in recent sediments. Negative values of δ²⁰²Hg and Δ¹⁹⁹Hg indicated terrestrially-derived Hg was a major constituent of Hg flux to Sargent Mountain Pond, ANP, although recent decreases in Hg flux were in agreement with precipitation Hg records, indicating a rapid watershed response. By contrast, δ²⁰²Hg and Δ¹⁹⁹Hg profiles in Long Pond, CNNS reflect direct Hg deposition, but disturbances in the sedimentary record were indicated by bomb fallout radionuclide inventories and by peaks in both Pb and Hg isotope depth profiles. These cores provided poor reconstructions of atmospheric deposition and reveal responses that are decoupled from emissions reduction due to complex post-depositional redistribution of atmospheric metals including Hg. The application of multiple tracers of Hg deposition provide insight into the sources and pathways governing Hg accumulation in these lakes.

Mercury Uptake, Accumulation, and Translocation in Roots of Subtropical Forest: Implications of Global Mercury Budget

Plant roots are responsible for transporting large quantities of nutrients in forest ecosystems and yet are frequently overlooked in global assessments of Hg cycling budgets. In this study, we systematically determined the distribution of total Hg mass and its stable isotopic signatures in a subtropical evergreen forest to elucidate sources of Hg in plant root tissues and the associated translocation mechanisms. Hg stored in roots and its isotopic signatures show significant correlations to those found in surrounding soil at various soil depths. The odd mass-independent fractionation (MIF) of root Hg at a shallow soil depth displays a -0.10‰ to -0.50‰ negative transition compared to the values in aboveground woody biomass. The evidence suggests that root Hg is predominantly derived from surrounding soil, rather than translocation of atmospheric uptake via aboveground tissues. The cortex has a more negative mass-dependent fractionation (MDF) of -0.10‰ to -1.20‰ compared to the soil samples, indicating a preferential uptake of lighter isotopes by roots. The similar MDF and odd-MIF signals found in root components imply limited Hg transport in roots. This work highlights that Hg stored in plant roots is not a significant sink of atmospheric Hg. The heterogeneous distribution of Hg mass in roots of various sizes represents a significant uncertainty of current estimates of Hg pool size in forest ecosystems.

Mercury isotope fractionation during methylmercury transport and transformation: A review focusing on analytical method, fractionation characteristics, and its application

Methylmercury (MeHg), a potent neurotoxin, can be formed, migrated and transformed in environmental compartments, accompanying with unique mass-dependent and mass-independent fractionation of mercury (Hg). These Hg isotope fractionation signals have great potential to probe the transformation and transport of MeHg in aquatic environments. However, the majority of studies to date have focused on total Hg isotopic composition, with less attention to the isotopic fractionation of MeHg due to technical difficulties in analysis, which severely hinders the understanding of MeHg isotopic fractionation and its applications. This review a) evaluates the reported analytical methods for Hg isotopic composition of MeHg, including online and offline measurement techniques; b) summarizes the extent and characteristics of Hg isotopic fractionation during MeHg transport and transformation, focusing on methylation, demethylation, trophic transfer and internal metabolism; and c) briefly discusses several applications of MeHg isotopic fractionation signatures in estimating the extent of photodemethylation, tracing the source of Hg species, and diagnosing reaction mechanisms. Additionally, the existing problems and future directions in MeHg isotope fractionation are highlighted to improve the analytical protocol for Hg isotope fractionation and deepen our understanding of Hg isotope fractionation in the biogeochemical cycling of MeHg.

Dissociation of Mercuric Oxides Drives Anomalous Isotope Fractionation during Net Photo-oxidation of Mercury Vapor in Air

The atmosphere is the primary medium for long-distance transport and transformation of elemental mercury (Hg), a potent neurotoxin. The recent discovery of mass-independent fractionation (MIF) of even-mass Hg isotopes (even-MIF, measured as Δ²⁰⁰Hg and Δ²⁰⁴Hg) in the atmosphere is surprising and can potentially serve as a powerful tracer in understanding Hg biogeochemistry. Far-ultraviolet (UVC) light-induced gas-phase reactions have been suspected as a likely cause for even-MIF, yet the mechanism remains unknown. Here, we present the first experimental evidence of large-scale even-MIF caused by UVC-induced (wavelength: 254 nm) Hg oxidation in synthetic air at the pressure (46-88 kPa) and temperature (233-298 K) resembling those of the lower atmosphere. We observe negatively correlated Δ²⁰⁰Hg and Δ²⁰⁴Hg signatures with values as low as -50‰ and as high as 550‰, respectively, in the remaining atomic Hg pool. The magnitude of even-MIF signatures decreases with decreasing pressure with the Δ²⁰⁰Hg/Δ²⁰⁴Hg ratio being similar to that observed in global precipitation. This even-MIF can be explained by photodissociation of mercuric oxides that are photochemically formed in the UVC-irradiated Hg-O₂ system. We propose that similar processes occurring in the atmosphere, where mercuric oxide species serve as intermediates, are responsible for the observed even-MIF in the environment.

Event-Based Atmospheric Precipitation Uncovers Significant Even and Odd Hg Isotope Anomalies Associated with the Circumpolar Vortex

The determination of the mass-independent fractionation of even Hg isotopes (even-MIF, Δ²⁰⁰Hg) in atmospheric samples adds another intriguing feature to the Hg isotope system. Despite our lack of sufficient experimental verification and the momentary absence of a valid mechanism to explain its occurrence, even-MIF could be instrumental in understanding the cycle and deposition of atmospheric Hg. In contrast to slightly positive Δ²⁰⁰Hg values (<0.30‰) frequently observed in most atmospheric samples, large Δ²⁰⁰Hg values (up to 1.24‰) determined in precipitation from Peterborough (Ontario, Canada) stand out and could provide valuable information for the origin of the even-MIF mystery. We now report a systematic analysis of high-resolution rainfall and snowfall samples collected in winter during cold weather at Peterborough, Canada. Dissolved and particulate Hg both displayed large variations of odd-MIF (from -0.93‰ to 2.02‰ for Δ¹⁹⁹Hg), which may result from long-range transportation, as the negative odd-MIF in particulate Hg is likely a result of long-distance transport of arctic atmospheric Hg(II). Dissolved Hg revealed significant even-MIF values (from 0.25‰ to 1.19‰ for Δ²⁰⁰Hg) and a negative relationship between Δ²⁰⁰Hg and Δ²⁰⁴Hg, which provide further evidence for the previously proposed conceptual model of Δ²⁰⁰Hg. Disconnected odd-MIF and even-MIF trends were detected in sequentially collected precipitation samples, which further suggests different sources or mechanisms for Δ¹⁹⁹Hg and Δ²⁰⁰Hg. Particularly, the high Δ²⁰⁰Hg values highlight the transport of stratospheric Hg through a polar vortex to the sampling region, stimulating further systematic investigation. The new Δ²⁰⁰Hg data for particulate Hg add to existing information on atmospheric Hg(II) worldwide, suggesting a global distribution of Hg characterized by even-MIF in the atmosphere, and further constrain the model of atmospheric Hg deposition.

Mercury transformation processes in nature: Critical knowledge gaps and perspectives for moving forward

The transformation of mercury (Hg) in the environment plays a vital role in the cycling of Hg and its risk to the ecosystem and human health. Of particular importance are Hg oxidation/reduction and methylation/demethylation processes driven or mediated by the dynamics of light, microorganisms, and organic carbon, among others. Advances in understanding those Hg transformation processes determine our capacity of projecting and mitigating Hg risk. Here, we provide a critical analysis of major knowledge gaps in our understanding of Hg transformation in nature, with perspectives on approaches moving forward. Our analysis focuses on Hg transformation processes in the environment, as well as emerging methodology in exploring these processes. Future avenues for improving the understanding of Hg transformation processes to protect ecosystem and human health are also explored.

Modeling mercury isotopic fractionation in the atmosphere

Mercury (Hg) stable isotope analysis has become a powerful tool to identify Hg sources and to understand its biogeochemical processes. However, it is challenging to link the observed Hg isotope fractionation to its global cycling. Here, we integrate source Hg isotope signatures and process-based Hg isotope fractionation into a three-dimensional isotope model based on the GEOS-Chem model platform. Our simulated isotope compositions of total gaseous Hg (TGM) are broadly comparable with available observations across global regions. The isotope compositions of global TGM, potentially distinguishable over different regions, are caused by the atmospheric mixture of anthropogenic, natural, and re-emitted Hg sources, superimposed with competing processes, notably gaseous Hg(0) dry deposition and Hg redox transformations. We find that Hg(0) dry deposition has a great impact on the isotope compositions of global TGM and drives the seasonal variation of δ²⁰²Hg in forest-covered regions. The atmospheric photo-reduction of Hg(Ⅱ) dominates over Hg(0) oxidation in driving the global Δ¹⁹⁹Hg (and Δ²⁰¹Hg) distribution patterns in TGM. We suggest that the magnitude of isotope fractionation associated with atmospheric aqueous-phase Hg(Ⅱ) reduction is likely close to aquatic Hg(Ⅱ) reduction. Our model provides a vital tool for coupling the global atmospheric Hg cycle and its isotope fractionation at various scales and advances our understanding of atmospheric Hg transfer and transformation mechanisms.

Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

Haze with high loading of particles may result in significant enrichment of particle-bound Hg (PBM), potentially impacting the atmospheric Hg transformation and transport. However, the dynamics of Hg transformation and the relative environmental effect during severe haze episodes remain unclear. Here, we report Hg isotopic compositions of atmospheric particles (PM_2.5, PM₁₀, and TSP) collected during a severe haze episode in Tianjin, China, to investigate the transformation and fate of Hg during haze events. All severe haze samples display significantly higher Δ¹⁹⁹Hg (up to 1.50‰) than global urban PBM, which cannot be explained by primary anthropogenic emissions. The high Δ¹⁹⁹Hg is likely caused by photoreduction of PBM promoted by water-soluble organic carbon (WSOC) during the particle accumulation period, as demonstrated by the positive correlations of Δ¹⁹⁹Hg with WSOC and relative humidity and confirmed by our laboratory-controlled photoreduction experiment. The results show that, on average, 21% of PBM are likely photoreduced and re-emitted back to the atmosphere as Hg(0), potentially requiring revision of atmospheric Hg budgeting and modeling. This study highlights the release of large portions of PBM back to the gas phase through photoreduction, which needs to be taken into account while evaluating the atmospheric Hg cycle and the relative ecological effects.

Latitudinal gradient for mercury accumulation and isotopic evidence for post-depositional processes among three tropical forests in Southwest China

Tropical forest contributes to > 50% of global litterfall mercury (Hg) inputs and surface soil Hg storage, while with limited understanding of Hg biogeochemical processes. In this study, we displayed the 5-m resolution of Hg spatial distribution in three 1-ha tropical forest plots across the latitudinal gradient in Southwest China, and determined Hg isotopic signatures to understand factors driving Hg spatial distribution and sequestration processes. Our results show that tropical forest at the lowest latitude has the highest litterfall Hg input (74.95 versus 34.14-56.59 μg m^-2 yr^-1 at higher latitude plots), but the smallest surface soil Hg concentration (2-3 times smaller than at higher latitude sites). Hg isotopic evidence indicates that the decreasing climate mediated microbial Hg reduction in forest floor leads to the increasing Hg accumulation along the latitudinal gradient in three tropical forests. The terrain induced indirect effects by influencing litterfall Hg inputs, soil organic matters distribution and interplays between surface and deep soils drive the heterogeneity of surface soil Hg distribution within each sampling plot. Our results highlight though the elevated litterfall Hg inputs, the distinct post-depositional reductions induced Hg loss would remarkedly decrease atmospheric Hg net sink in tropical forest.

Sources and Transformation Mechanisms of Atmospheric Particulate Bound Mercury Revealed by Mercury Stable Isotopes

This study examined the isotopic composition of particulate bound mercury (PBM) in 10 Chinese megacities and explored the associated sources and transformation mechanisms. PBM in these cities was characterized by negative δ²⁰²Hg (mean: -2.00 to -0.78‰), slightly negative to highly positive Δ¹⁹⁹Hg (mean: -0.04 to 0.47‰), and slightly positive Δ²⁰⁰Hg (mean: 0.02 to 0.06‰) values. The positive PBM Δ¹⁹⁹Hg signatures were likely caused by physiochemical reactions in aerosols. The Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio varied from 0.94 to 1.39 in the cities and increased with the increase in the corresponding mean Δ¹⁹⁹Hg_PBM value. We speculate that, in addition to the photoreduction of oxidized Hg, other transformation mechanisms in aerosols (e.g., isotope exchange, complexation, and oxidation, which express nuclear volume effects) also shape the Δ¹⁹⁹Hg_PBM signatures in the present study. These processes are likely enhanced in the presence of strong gas-particle partitioning of gaseous oxidized Hg (GOM) and elevated levels of redox active metals (e.g., Fe), halides, and elemental carbon. Based on Δ²⁰⁰Hg_PBM data presented in this and previous studies, we estimate that large proportions (∼47 ± 22%) of PBM were sourced from the oxidation of gaseous elemental Hg followed by the partitioning of GOM onto aerosols globally, indicating the transformation of Hg(0) to PBM as an important sink of atmospheric Hg(0).

The Hg behaviors in mangrove ecosystems revealed by Hg stable isotopes: a case study of Maowei mangrove

As one of the most productive marine ecosystems in the tropics and subtropics, mangroves are an important part of the global mercury (Hg) cycling. The environmental processes and effects of Hg in mangroves are complex and affect human Hg exposure, and it is crucial to understand Hg behaviors in the mangrove ecosystem. However, clarifying Hg behaviors in the mangrove ecosystem remains difficult because of an insufficient understanding of the dominant pathways. In this study, measurements of mercury (Hg) concentration and isotope ratios in sediment and plant tissues from a mangrove wetland were used to investigate Hg isotope fractionation in mangrove plants and sediments. Spatial patterns in Hg concentration and isotope signatures indicate that Hg re-emission in the sediment was suppressed by mangrove plants. The ratio of Δ¹⁹⁹Hg/Δ²⁰¹Hg was 0.93 for all sediments, indicating that Hg mass-independent fractionation in the mangrove ecosystem was primarily affected by photoreduction, while the ratios of Δ¹⁹⁹Hg/Δ²⁰¹Hg and Δ¹⁹⁹Hg/δ²⁰²Hg for plant tissues suggested that natural organic matter reduction of Hg(II) was occurred in the plants. The distinct positive Δ¹⁹⁹Hg values found in mangrove plants were supposed to be the results of the unique physiological characteristics of mangroves. The exterior Hg sources from atmosphere and seawater emphasize the role of mangrove ecosystems in the global Hg biogeochemistry. Our study highlights the distinct Hg isotope signatures in the mangrove from that in forests and indicates unique Hg behaviors in the mangrove ecosystem.

Elucidating sources of mercury in the west coast of Korea and the Chinese marginal seas using mercury stable isotopes

Nearshore systems play an important role as mercury (Hg) sources to the open ocean and to human health via fish consumption. The nearshore system along East Asia is of particular concern given the rapid industrialization, which contributes to significant anthropogenic Hg emissions and releases. We used Hg stable isotopes to characterize Hg sources in the sediment and fish along the west coast of Korea, located at the northeast of the East China Sea. The Hg isotope ratios of the west coast sediments (δ²⁰²Hg; -0.89 to -0.27‰, Δ¹⁹⁹Hg; -0.04 to 0.14‰) were statistically similar with other nearshore sediments (δ²⁰²Hg; -0.99 to -0.30‰, Δ¹⁹⁹Hg; -0.04 to 0.19‰) and overlapped with the industrial Hg source end-member (δ²⁰²Hg; -0.5‰, Δ¹⁹⁹Hg; 0.01‰) estimated from the Chinese marginal seas. Using a ternary mixing model, we estimated that industrial Hg sources contribute 83-97% in the west coast of Korea, and riverine and atmospheric Hg sources play minor roles in the Korean west coast and the Chinese marginal seas. The comparison between Hg isotope ratios of the sediment and nearshore fish revealed that the fish in the most west coast sites are exposed to MeHg produced in the sediment. At a few southwest coast sites, external MeHg produced in rivers and the open ocean water column appears to be more important as a source in fish. This is supported by much higher δ²⁰²Hg (0.74‰; similar to oceanic fish) and lower δ²⁰²Hg (-0.71‰; similar to riverine sources) compared to fish collected from other west coast sites influenced by sedimentary MeHg. The substantial Hg contributions from industrial activities suggest the national policies regulating anthropogenic Hg releases can directly mitigate human Hg exposure originating via local fish consumption. This study contributes to the growing regional and global inventories of Hg fluxes and sources exported into coastal oceans.

Using carbon, nitrogen, and mercury isotope values to distinguish mercury sources to Alaskan lake trout

Lake trout (Salvelinus namaycush), collected from 13 remote lakes located in southwestern Alaska, were analyzed for carbon, nitrogen, and mercury (Hg) stable isotope values to assess the importance of migrating oceanic salmon, volcanic activity, and atmospheric deposition to fish Hg burden. Methylmercury (MeHg) bioaccumulation in phytoplankton (5.0 - 6.9 kg L^-1) was also measured to quantify the basal uptake of MeHg to these aquatic food webs. Hg isotope values in lake trout revealed that while the extent of precipitation-delivered Hg was similar across the entire study area, volcanic Hg is likely an important additional source to lake trout in proximate lakes. In contrast, migratory salmon (Oncorhynchus nerka) deliver little MeHg to lake trout directly, although indirect delivery processes via decay could exist. A high level of variability in carbon, nitrogen, and Hg isotope values indicate niche partitioning in lake trout populations within each lake and that a complex suite of ecological interactions is occurring, complicating the conceptually linear assessment of contaminant source to receiving organism. Without connecting energy and contaminant isotope axes, we would not have understood why lake trout from these pristine lakes have highly variable Hg burdens despite consistently low water Hg and comparable age-length dynamics.

Mercury evidence for combustion of organic-rich sediments during the end-Triassic crisis

The sources of isotopically light carbon released during the end-Triassic mass extinction remain in debate. Here, we use mercury (Hg) concentrations and isotopes from a pelagic Triassic–Jurassic boundary section (Katsuyama, Japan) to track changes in Hg cycling. Because of its location in the central Panthalassa, far from terrigenous runoff, Hg enrichments at Katsuyama record atmospheric Hg deposition. These enrichments are characterized by negative mass independent fractionation (MIF) of odd Hg isotopes, providing evidence of their derivation from terrestrial organic-rich sediments (Δ¹⁹⁹Hg < 0‰) rather than from deep-Earth volcanic gases (Δ¹⁹⁹Hg ~ 0‰). Our data thus provide evidence that combustion of sedimentary organic matter by igneous intrusions and/or wildfires played a significant role in the environmental perturbations accompanying the event. This process has a modern analog in anthropogenic combustion of fossil fuels from crustal reservoirs.

Mercury (Hg) concentrations and isotopes from a deep-ocean Triassic–Jurassic (~201 Ma) boundary section provide evidence of large inputs from terrestrial organic-rich sources through combustion by magmatic sills and wildfires.

Distribution and Transformation of Mercury in Subtropical Wild-Caught Seafood from the Southern Taiwan Strait

Wild-caught seafood contains significant amounts of mercury. Investigating the mercury accumulation levels in wild-caught seafood and analyzing its migration and transformation are of great value for assessing the health risks of mercury intake and for the tracking of mercury sources. We determined the concentrations and stable mercury isotopic compositions (δ²⁰²Hg, Δ¹⁹⁹Hg, Δ²⁰⁰Hg, and Δ²⁰¹Hg) of 104 muscle samples collected from 38 species of seafood typically harvested from the Taiwan Shallow Fishing Ground (TSFG), Southern Taiwan Strait. Overall, the concentrations of total mercury (THg) and methylmercury (MeHg) ranged from 11 to 479 ng/g (dry weight, dw) and 10 to 363 ng/g (dw), respectively, and were below the threshold value established by the USEPA and the Chinese government. Demersal and near-benthic species accumulated more mercury than pelagic or mesopelagic species. The characteristics of mercury isotopes in wild-caught marine species differed in terms of vertical and horizontal distribution. Considering the known peripheral land sources of mercury (Δ¹⁹⁹Hg ≈ 0), the mercury in seafood from the TSFG (Δ¹⁹⁹Hg > 0) did not originate from anthropogenic emissions. The ratio of Δ¹⁹⁹Hg and Δ²⁰¹Hg (1.18 ± 0.03) suggested that the photoreduction of Hg (II) and the photo-degradation of MeHg equally contributed to mass-independent fractionation. Based on the values of Δ¹⁹⁹Hg/δ²⁰²Hg (1.18 ± 0.03), about 67% of the mercury in seawater had undergone microbial demethylation prior to methylation and entering the seafood. Additionally, the vertical distribution of Δ²⁰⁰Hg in seafood from different water depths implies that mercury input was in part caused by atmospheric deposition. Our results provide detailed information on the sources of mercury and its transfer in the food web in offshore fishing grounds.

Tracing the Transboundary Transport of Mercury to the Tibetan Plateau Using Atmospheric Mercury Isotopes

Deposition of atmospheric mercury (Hg) is the most important Hg source on the high-altitude Himalayas and Tibetan Plateau. Herein, total gaseous Hg (TGM) at an urban and a forest site on the Tibetan Plateau was collected respectively from May 2017 to October 2018, and isotopic compositions were measured to clarify the influences of landforms and monsoons on the transboundary transport of atmospheric Hg to the Tibetan Plateau. The transboundary transported anthropogenic emissions mainly originated over Indo-Gangetic Plain and carried over the Himalayas by convective storms and mid-tropospheric circulation, contributing over 50% to the TGM at the Lhasa urban site, based on the binary mixing model of isotopes. In contrast, during the transport of TGM from South Asia with low altitude, the uptake by evergreen forest in Yarlung Zangbo Grand Canyon largely decreased the TGM level and shifted isotopic compositions in TGM at the Nyingchi forest site, which are located at the high-altitude end of the canyon. Our results provided direct evidence from Hg isotopes to reveal the distinct patterns of transboundary transport to the Tibetan Plateau shaped by landforms and climates, which is critical to fully understand the biogeochemical cycling of Hg in the high-altitude regions.

Coprecipitation of Mercury from Natural Iodine-Containing Seawater for Accurate Isotope Measurement

Oceans play a key role in the global mercury (Hg) cycle, but studies on Hg isotopes in seawater are rare due to the extremely low Hg concentration and the lack of a good preconcentration method. Here, we introduce a new coprecipitation method for separating and preconcentrating Hg from seawater for accurate isotope measurement. The coprecipitation was achieved by sequential addition of 0.5 mL of 0.5 M CuSO₄, 1 mL of 0.5 M Na₂S, and 1 mL of 0.5 M CuSO₄ reagents, which allowed for quantitatively precipitating Hg from up to 10 L of seawater. The protocol was validated by testing synthetic solutions with varying Hg and iodide (I^-) concentrations and by comparing the reaction times of various reagents added. The method resulted in a quantitative recovery of 98 ± 12% (n = 32, two standard deviations, 2 SD) and a relatively low procedure blank (103 pg of Hg, n = 8). The precipitates were filtrated and analyzed for Hg isotopes. Repeated measurements of synthetic seawaters spiked with certificated standard materials (NIST 3133 and 3177) using the entire method gave identical Hg isotope ratios with near-quantitative Hg recovery, indicating no isotope fractionation during preconcentration. A total of six nearshore seawater samples from the Yellow Sea and the Bohai Sea (China) were analyzed using the coprecipitation method. The data showed a large fractionation of Hg isotopes and revealed the possible impact of both atmospheric and anthropogenic inputs to the coastal seawater Hg budget, implying the potential application of this method in studying marine Hg systematics and global Hg cycling.

Hg concentrations and stable isotope variations in tropical fish species of a gold-mining-impacted watershed in French Guiana

The aim of the study was to determine if gold-mining activities could impact the mercury (Hg) concentrations and isotopic signatures in freshwater fish consumed by riparian people in French Guiana. Total Hg, MeHg concentrations, and Hg stable isotopes ratios were analyzed in fish muscles from different species belonging to three feeding patterns (herbivorous, periphytophagous, and piscivorous). We compared tributaries impacted by gold-mining activities (Camopi, CR) with a pristine area upstream (Trois-Sauts, TS), along the Oyapock River. We measured δ¹⁵N and δ ¹³C to examine whether Hg patterns are due to differences in trophic level. Differences in δ ¹⁵N and δ ¹³C values between both studied sites were only observed for periphytophagous fish, due to difference of CN baselines, with enriched values at TS. Total Hg concentrations and Hg stable isotope signatures showed that Hg accumulated in fish from both areas has undergone different biogeochemical processes. Δ¹⁹⁹Hg variation in fish (-0.5 to 0.2‰) was higher than the ecosystem baseline defined by a Δ¹⁹⁹Hg of -0.66‰ in sediments, and suggested limited aqueous photochemical MeHg degradation. Photochemistry-corrected δ²⁰²Hg in fish was 0.7‰ higher than the baseline, consistent with biophysical and chemical isotope fractionation in the aquatic environment. While THg concentrations in periphytophagous fish were higher in the gold-mining area, disturbed by inputs of suspended particles, than in TS, the ensemble of Hg isotope shifts in fish is affected by the difference of biotic (methylation/demethylation) and abiotic (photochemistry) processes between both areas and did therefore not allow to resolve the contribution of gold-mining-related liquid Hg(0) in fish tissues. Mercury isotopes of MeHg in fish and lower trophic level organisms can be complementary to light stable isotope tracers.

Mass-Independent Fractionation of Mercury Isotopes during Photoreduction of Soot Particle Bound Hg(II)

Soot and mercury (Hg) are two notorious air pollutants, and the fate and transport of Hg may be affected by soot at various scales in the environment as soot may be both a carrier and a reactant for active Hg species. This study was designed to quantify photoreduction of Hg(II) in the presence of soot and the associated Hg isotope fractionation under both atmospheric aerosol and aqueous conditions (water-saturated). Photoreduction experiments were conducted with diesel soot particulate matter under controlled temperature and relative humidity (RH) conditions using a flow-through semibatch reactor system. Mass-dependent fractionation resulted in the enrichment of heavier Hg isotopes in the remaining Hg(II) with enrichment factors (ε²⁰²Hg) of 1.48 ± 0.02‰ (±2 standard deviation) to 1.75 ± 0.05‰ for aerosol-phase reactions (RH 28-68%) and from 1.26 ± 0.11 to 1.50 ± 0.04‰ for aqueous-phase reactions. Positive odd mass-independent fractionation (MIF) was observed in aqueous-phase reactions, resulting in Δ¹⁹⁹Hg values for reactant Hg(II) as high as 5.29‰, but negative odd-MIF occurred in aerosol-phase reactions, in which Δ¹⁹⁹Hg values of reactant Hg(II) varied from -1.02 to 0‰. The average ratio of Δ¹⁹⁹Hg/Δ²⁰¹Hg (1.1) indicated that under all conditions, MIF was dominated by magnetic isotope effects during photoreduction of Hg(II). Increasing RH resulted in higher reduction rates but lower extents of negative MIF in the aerosol-phase experiments, suggesting that the reduction of soot particle-bound Hg(II) was responsible for the observed negative odd-MIF. Our results suggest that mass-independent Hg isotope fractionation during Hg(II) photoreduction varies with soot aerosol water content and that Hg-stable isotope ratios may be used to understand the transformational histories of aerosol-bound Hg(II) in the environment.

Hg isotopic composition of one-year-old spruce shoots: Application to long-term Hg atmospheric monitoring in Germany

The Hg isotopic composition of 1-year-old Norway spruce (Picea abies) shoots collected from Saarland cornurbation Warndt, Germany, since 1985 by the German Environmental Specimen Bank, were measured for a better understanding of the temporal trends of Hg sources. The isotopic data showed that Hg was mainly taken up as gaseous element mercury (GEM) and underwent oxidation in the spruce needles; this led to a significant decrease in the δ²⁰²Hg compared with the atmospheric Hg isotopic composition observed for deciduous leaves and epiphytic lichens. Observation of the odd mass-independent isotopic fractionation (MIF) indicated that Δ¹⁹⁹Hg and Δ²⁰¹Hg were close to but slightly lower than the actual values recorded from the atmospheric measurement of the GEM isotopic composition in non-contaminated sites in U.S. and Europe, whereas observation of the even-MIF indicated almost no differences for Δ²⁰⁰Hg. This confirmed that GEM is a major source of Hg accumulation in spruce shoots. Interestingly, the Hg isotopic composition in the spruce shoots did not change very significantly during the study period of >30 years, even as the Hg concentration decreased significantly. Even-MIF (Δ²⁰⁰Hg) and mass-dependent fractionation (MDF) (δ²⁰²Hg) of the Hg isotopes exhibited slight decrease with time, whereas odd-MIF did not show any clear trend. These results suggest a close link between the long-term evolution of GEM isotopic composition in the air and the isotopic composition of bioaccumulated Hg altered by mass-dependent fraction in the spruce shoots.

Isotopic composition of mercury deposited via snow into mid-latitude ecosystems

Atmospheric deposition of mercury (Hg) to terrestrial and aquatic ecosystems has significant implications for human and animal exposure. Measurements of Hg isotopic composition can be utilized to trace sources of Hg, but outside of the Arctic there has been little Hg isotopic characterization of snow. To better understand deposition pathways at mid-latitudes, five time series of snowfall were collected at two sites (Dexter and Pellston, Michigan, USA) to investigate the Hg isotopic composition of snowfall, how it changes after deposition, and how it compares to rain. The Hg isotopic composition of a subset of fresh snow samples revealed the influence of reactive surface uptake of atmospheric Hg(0). The first time series collected at Dexter occurred during a polar vortex, demonstrating Hg isotopic fractionation dynamics similar to those in Arctic snow, with increasingly negative Δ¹⁹⁹Hg as snow aged with exposure to sunlight. All other time series revealed an increase in Δ¹⁹⁹Hg as snow aged, with values reaching up to 3.5‰. This characterization of Hg isotopes in snow suggests a strong influence of oxidants and binding ligands in snow that may mediate Hg isotope fractionation. Additionally, isotopic characterization of Hg in snow deposited to natural ecosystems at mid-latitudes allows for better understanding of atmospheric mercury sources that are deposited to lakes and forests and that may become available for methylation and transfer to food webs.

Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes

The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ¹⁹⁹Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ¹³C, δ¹⁵N) alone or in combination with Δ¹⁹⁹Hg values, to assess resource partitioning between the three shark species. Adding Δ¹⁹⁹Hg resulted in a decrease in trophic overlap estimates compared to those based on δ¹³C/δ¹⁵N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.