Mercury stable isotopes constrain atmospheric sources to the ocean.

Human exposure to toxic mercury (Hg) is dominated by the consumption of seafood1,2. Earth system models suggest that Hg in marine ecosystems is supplied by atmospheric wet and dry Hg(II) deposition, with a three times smaller contribution from gaseous Hg(0) uptake3,4. Observations of marine Hg(II) deposition and Hg(0) gas exchange are sparse, however5, leaving the suggested importance of Hg(II) deposition6 ill-constrained. Here we present the first Hg stable isotope measurements of total Hg (tHg) in surface and deep Atlantic and Mediterranean seawater and use them to quantify atmospheric Hg deposition pathways. We observe overall similar tHg isotope compositions, with median Δ200Hg signatures of 0.02‰, lying in between atmospheric Hg(0) and Hg(II) deposition end-members. We use a Δ200Hg isotope mass balance to estimate that seawater tHg can be explained by the mixing of 42% (median; interquartile range, 24-50%) atmospheric Hg(II) gross deposition and 58% (50-76%) Hg(0) gross uptake. We measure and compile additional, global marine Hg isotope data including particulate Hg, sediments and biota and observe a latitudinal Δ200Hg gradient that indicates larger ocean Hg(0) uptake at high latitudes. Our findings suggest that global atmospheric Hg(0) uptake by the oceans is equal to Hg(II) deposition, which has implications for our understanding of atmospheric Hg dispersal and marine ecosystem recovery.

Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs.

Pacific Ocean tuna is among the most-consumed seafood products but contains relatively high levels of the neurotoxin methylmercury. Limited observations suggest tuna mercury levels vary in space and time, yet the drivers are not well understood. Here, we map mercury concentrations in skipjack tuna across the Pacific Ocean and build generalized additive models to quantify the anthropogenic, ecological, and biogeochemical drivers. Skipjack mercury levels display a fivefold spatial gradient, with maximum concentrations in the northwest near Asia, intermediate values in the east, and the lowest levels in the west, southwest, and central Pacific. Large spatial differences can be explained by the depth of the seawater methylmercury peak near low-oxygen zones, leading to enhanced tuna mercury concentrations in regions where oxygen depletion is shallow. Despite this natural biogeochemical control, the mercury hotspot in tuna caught near Asia is explained by elevated atmospheric mercury concentrations and/or mercury river inputs to the coastal shelf. While we cannot ignore the legacy mercury contribution from other regions to the Pacific Ocean (e.g., North America and Europe), our results suggest that recent anthropogenic mercury release, which is currently largest in Asia, contributes directly to present-day human mercury exposure.

Distinct dynamics in mountain watersheds: Exploring mercury and microplastic pollution-Unraveling the influence of atmospheric deposition, human activities, and hydrology.

The intensification of human activities all around the globe has led to the spread of micropollutants in high-mountain freshwater environments. We therefore aimed to assess the geospatial distribution and determine the potential sources of (total-) mercury (THg) and microplastics (MPs) in mountain freshwater ecosystems. To do so, we analyzed THg and MP concentrations in brown trout, biofilm, and sediments from lotic and lentic ecosystems in the Pyrenees - all subjected to different types of human pressure. Additionally, we assessed the potential impacts of these pollutants on fish, and explored the bioindication capacity of brown trout (Salmo trutta fario) and biofilm regarding THg and MP pollution. For the first time, we measured concentrations of MPs trapped in the matrix of freshwater biofilm. Our results suggest that THg in the Pyrenees might be explained by both legacy (regional) and distant sources, in combination with environmental characteristics such as the presence of peatlands or streamwater physicochemistry, while MPs in fish are linked to recent local pollution sources such as single-use plastics. In contrast, MPs in biofilm matrix and sediments indicate a combination of distant (i.e., atmospheric deposition) and recent local pollution sources. Moreover, hydrodynamics and plastic density likely control MP distribution in rivers. Based on Fulton's condition factor, we also found that higher THg concentrations caused a negative impact on fish health (K < 1), while no impact of MPs could be seen. Therefore, we suggest that brown trout and biofilm can serve as bioindicators of atmospheric deposition of THg in high-altitude lakes and that biofilm is a reliable bioindicator to assess MP pollution in remote environments. Brown trout may also act as a bioindicator of MP pollution, but only efficiently in more polluted areas.

Tundra uptake of atmospheric elemental mercury drives Arctic mercury pollution.

Anthropogenic activities have led to large-scale mercury (Hg) pollution in the Arctic. It has been suggested that sea-salt-induced chemical cycling of Hg (through 'atmospheric mercury depletion events', or AMDEs) and wet deposition via precipitation are sources of Hg to the Arctic in its oxidized form (Hg(ii)). However, there is little evidence for the occurrence of AMDEs outside of coastal regions, and their importance to net Hg deposition has been questioned. Furthermore, wet-deposition measurements in the Arctic showed some of the lowest levels of Hg deposition via precipitation worldwide, raising questions as to the sources of high Arctic Hg loading. Here we present a comprehensive Hg-deposition mass-balance study, and show that most of the Hg (about 70%) in the interior Arctic tundra is derived from gaseous elemental Hg (Hg(0)) deposition, with only minor contributions from the deposition of Hg(ii) via precipitation or AMDEs. We find that deposition of Hg(0)-the form ubiquitously present in the global atmosphere-occurs throughout the year, and that it is enhanced in summer through the uptake of Hg(0) by vegetation. Tundra uptake of gaseous Hg(0) leads to high soil Hg concentrations, with Hg masses greatly exceeding the levels found in temperate soils. Our concurrent Hg stable isotope measurements in the atmosphere, snowpack, vegetation and soils support our finding that Hg(0) dominates as a source to the tundra. Hg concentration and stable isotope data from an inland-to-coastal transect show high soil Hg concentrations consistently derived from Hg(0), suggesting that the Arctic tundra might be a globally important Hg sink. We suggest that the high tundra soil Hg concentrations might also explain why Arctic rivers annually transport large amounts of Hg to the Arctic Ocean.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg0 to the atmosphere where it is oxidized to reactive HgII compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgI and HgII species postulate their photodissociation back to Hg0 as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg0, HgI, and HgII species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgI and HgII leads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgI and HgII to Hg0 competes with thermal oxidation of Hg0, resulting in a large model overestimation of 99% of measured Hg0 and underestimation of 51% of oxidized Hg and ∼66% of HgII wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the HgI and HgII photoreduction processes largely offset the efficiency of bromine-initiated Hg0 oxidation and reveal missing Hg oxidation processes in the troposphere.

Mediterranean Mercury Assessment 2022: An Updated Budget, Health Consequences, and Research Perspectives.

Mercury (Hg) and especially its methylated species (MeHg) are toxic chemicals that contaminate humans via the consumption of seafood. The most recent UNEP Global Mercury Assessment stressed that Mediterranean populations have higher Hg levels than people elsewhere in Europe. The present Critical Review updates current knowledge on the sources, biogeochemical cycling, and mass balance of Hg in the Mediterranean and identifies perspectives for future research especially in the context of global change. Concentrations of Hg in the Western Mediterranean average 0.86 ± 0.27 pmol L-1 in the upper water layer and 1.02 ± 0.12 pmol L-1 in intermediate and deep waters. In the Eastern Mediterranean, Hg measurements are in the same range but are too few to determine any consistent oceanographical pattern. The Mediterranean waters have a high methylation capacity, with MeHg representing up to 86% of the total Hg, and constitute a source of MeHg for the adjacent North Atlantic Ocean. The highest MeHg concentrations are associated with low oxygen water masses, suggesting a microbiological control on Hg methylation, consistent with the identification of hgcA-like genes in Mediterranean waters. MeHg concentrations are twice as high in the waters of the Western Basin compared to the ultra-oligotrophic Eastern Basin waters. This difference appears to be transferred through the food webs and the Hg content in predators to be ultimately controlled by MeHg concentrations of the waters of their foraging zones. Many Mediterranean top-predatory fish still exceed European Union regulatory Hg thresholds. This emphasizes the necessity of monitoring the exposure of Mediterranean populations, to formulate adequate mitigation strategies and recommendations, without advising against seafood consumption. This review also points out other insufficiencies of knowledge of Hg cycling in the Mediterranean Sea, including temporal variations in air-sea exchange, hydrothermal and cold seep inputs, point sources, submarine groundwater discharge, and exchanges between margins and the open sea. Future assessment of global change impacts under the Minamata Convention Hg policy requires long-term observations and dedicated high-resolution Earth System Models for the Mediterranean region.

Mass-Independent Fractionation of Even and Odd Mercury Isotopes during Atmospheric Mercury Redox Reactions.

Mass-independent fractionation (MIF) of stable even mass number mercury (Hg) isotopes is observed in rainfall and gaseous elemental Hg0 globally and is used to quantify atmospheric Hg deposition pathways. The chemical reaction and underlying even-Hg MIF mechanism are unknown however and speculated to be caused by Hg photo-oxidation on aerosols at the tropopause. Here, we investigate the Hg isotope composition of free tropospheric Hg0 and oxidized HgII forms at the high-altitude Pic du Midi Observatory. We find that gaseous oxidized Hg has positive Δ199Hg, Δ201Hg, and Δ200Hg and negative Δ204Hg signatures, similar to rainfall Hg, and we document rainfall Hg Δ196Hg to be near zero. Cloud water and rainfall Hg show an enhanced odd-Hg MIF of 0.3‰ compared to gaseous oxidized HgII, potentially indicating the occurrence of in-cloud aqueous HgII photoreduction. Diurnal MIF observations of free tropospheric Hg0 show how net Hg0 oxidation in high-altitude air masses leads to opposite even- and odd-MIF in Hg0 and oxidized HgII. We speculate that even-Hg MIF takes place by a molecular magnetic isotope effect during HgII photoreduction on aerosols that involves magnetic halogen nuclei. A Δ200Hg mass balance suggests that global Hg deposition pathways in models are likely biased toward HgII deposition. We propose that Hg cycling models could accommodate the Hg-isotope constraints on emission and deposition fluxes.

Compound-Specific Stable Isotope Analysis Provides New Insights for Tracking Human Monomethylmercury Exposure Sources.

Monomethylmercury (MMHg) exposure can induce adverse neurodevelopmental effects in humans and is a global environmental health concern. Human exposure to MMHg occurs predominately through the consumption of fishery foods and rice in Asia, but it is challenging to quantify these two exposure sources. Here, we innovatively utilized MMHg compound-specific stable isotope analyses (MMHg-CSIA) of the hair to quantify the human MMHg sources in coastal and inland areas, where fishery foods and rice are routinely consumed. Our data showed that the fishery foods and rice end members had distinct Δ199HgMMHg values in both coastal and inland areas. The Δ199HgMMHg values of the human hair were comparable to those of fishery foods but not those of rice. Positive shifts in the δ202HgMMHg values of the hair from diet were observed in the study areas. Additionally, significant differences in δ202Hg versus Δ199Hg were detected between MMHg and inorganic Hg (IHg) in the human hair but not in fishery foods and rice. A binary mixing model was developed to estimate the human MMHg exposures from fishery foods and rice using Δ199HgMMHg data. The model results suggested that human MMHg exposures were dominated (>80%) by fishery food consumption and were less affected by rice consumption in both the coastal and inland areas. This study demonstrated that the MMHg-CSIA method can provide unique information for tracking human MMHg exposure sources by excluding the deviations from dietary surveys, individual MMHg absorption/demethylation efficiencies, and the confounding effects of IHg.

ENSO Climate Forcing of the Marine Mercury Cycle in the Peruvian Upwelling Zone Does Not Affect Methylmercury Levels of Marine Avian Top Predators.

Climate change is expected to affect marine mercury (Hg) biogeochemistry and biomagnification. Recent modeling work suggested that ocean warming increases methylmercury (MeHg) levels in fish. Here, we studied the influence of El Niño Southern Oscillations (ENSO) on Hg concentrations and stable isotopes in time series of seabird blood from the Peruvian upwelling and oxygen minimum zone. Between 2009 and 2016, La Niña (2011) and El Niño conditions (2015-2016) were accompanied by sea surface temperature anomalies up to 3 °C, oxycline depth change (20-100 m), and strong primary production gradients. Seabird Hg levels were stable and did not co-vary significantly with oceanographic parameters, nor with anchovy biomass, the primary dietary source to seabirds (90%). In contrast, seabird Δ199Hg, proxy for marine photochemical MeHg breakdown, and δ15N showed strong interannual variability (up to 0.8 and 3‰, respectively) and sharply decreased during El Niño. We suggest that lower Δ199Hg during El Niño represents reduced MeHg photodegradation due to the deepening of the oxycline. This process was balanced by equally reduced Hg methylation due to reduced productivity, carbon export, and remineralization. The non-dependence of seabird MeHg levels on strong ENSO variability suggests that marine predator MeHg levels may not be as sensitive to climate change as is currently thought.

Eurasian river spring flood observations support net Arctic Ocean mercury export to the atmosphere and Atlantic Ocean.

Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ± 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments.

Concentrations and stable isotopes of mercury in sharks of the Galapagos Marine Reserve: Human health concerns and feeding patterns.

The human ingestion of mercury (Hg) from sea food is of big concern worldwide due to adverse health effects, and more specifically if shark consumption constitutes a regular part of the human diet. In this study, the total mercury (THg) concentration in muscle tissue were determined in six sympatric shark species found in a fishing vessel seized in the Galapagos Marine Reserve in 2017. The THg concentrations in shark muscle samples (n = 73) varied from 0.73 mg kg-1 in bigeye thresher sharks (Alopias superciliosus) to 8.29 mg kg-1 in silky sharks (Carcharhinus falciformis). A typical pattern of Hg bioaccumulation was observed for all shark species, with significant correlation between THg concentration and shark size for bigeye thresher sharks, pelagic thresher sharks (Alopias pelagicus) and silky sharks. Regarding human health concerns, the THg mean concentration exceeded the maximum weekly intake fish serving in all the studied species. Mass-Dependent Fractionation (MDF, δ202Hg values) and Mass-Independent Fractionation (MIF, Δ199Hg values) of Hg in whitetip sharks (Carcharhinus longimanus) and silky sharks, ranged from 0.70‰ to 1.08‰, and from 1.97‰ to 2.89‰, respectively. These high values suggest that both species are feeding in the epipelagic zone (i.e. upper 200 m of the water column). While, blue sharks (Prionace glauca), scalloped hammerhead sharks (Shyrna lewini) and thresher sharks were characterized by lower Δ199Hg and δ202Hg values, indicating that these species may focus their foraging behavior on prey of mesopelagic zone (i.e. between 200 and 1000 m depth). In conclusion, the determination of THg concentration provides straight-forward evidence of the human health risks associated with shark consumption, while mercury isotopic compositions constitute a powerful tool to trace the foraging strategies of these marine predators. CAPSULE: A double approach combining Hg concentrations with stable isotopes ratios allowed to assess ontogeny in common shark species in the area of the Galapagos Marine Reserve and the human health risks concern associated to their consumption.

The Twilight Zone as a Major Foraging Habitat and Mercury Source for the Great White Shark.

The twilight zone contains the largest biomass of the world's ocean. Identifying its role in the trophic supply and contaminant exposure of marine megafauna constitutes a critical challenge in the context of global change. The white shark (Carcharodon carcharias) is a threatened species with some of the highest concentrations of neurotoxin methylmercury (MeHg) among marine top predators. Large white sharks migrate seasonally from coastal habitats, where they primarily forage on pinnipeds, to oceanic offshore habitats. Tagging studies suggest that while offshore, white sharks may forage at depth on mesopelagic species, yet no biochemical evidence exists. Here, we used mercury isotopic composition to assess the dietary origin of MeHg contamination in white sharks from the Northeast Pacific Ocean. We estimated that a minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators, which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.

Automated Stable Isotope Sampling of Gaseous Elemental Mercury (ISO-GEM): Insights into GEM Emissions from Building Surfaces.

Atmospheric monitoring networks quantify gaseous elemental mercury (GEM) concentrations, but not isotopic compositions. Here, we present a new method for automated and quantitative stable isotope sampling of GEM (ISO-GEM) at the outlet of a commercial Hg analyzer. A programmable multivalve manifold selects Hg at the analyzer inlet and outlet based on specific criteria (location, time, GEM concentration, auxiliary threshold). Outlet Hg recovery was tested for gold traps, oxidizing acidic solution traps, and activated carbon traps. We illustrate the ISO-GEM method in an exploratory study on the effect of building walls on local GEM. We find that GEM concentrations directly at the building surface (wall inlet) are significantly enhanced (mean 3.8 ± 1.8 ng/m3) compared to 3 m from the building wall (free inlet) (mean 1.5 ± 0.4 ng/m3). GEM δ202Hg (-1.26‰ ± 0.41‰, 1 SD, n = 16) and Δ199Hg (-0.05‰ ± 0.10‰, 1 SD, n = 16) at the wall inlet were different from ambient GEM δ202Hg (0.76‰ ± 0.09‰, 1 SD, n = 16) and Δ199Hg (-0.21‰ ± 0.05‰, 1 SD, n = 16) at the free inlet. The isotopic fingerprint of GEM at the wall inlet suggests that GEM emission from the aluminum building surface affected local GEM concentration measurements. These results illustrate the versatility of the automated Hg isotope sampling.

Marine versus Continental Sources of Iodine and Selenium in Rainfall at Two European High-Altitude Locations.

The essential elements selenium (Se) and iodine (I) are often present in low levels in terrestrial diets, leading to potential deficiencies. Marine I and Se emissions and subsequent atmospheric wet deposition has been suggested to be an important source of I and Se to soils and terrestrial food chains. However, the contribution of recycled moisture of continental origin to I and Se to precipitation has never been analyzed. Here we report concentrations and speciation of I and Se, as well as of bromine (Br), sulfur (S), and DOC-δ13C signatures for weekly collected precipitation samples (in the period of April 2015 to September 2016) at two high altitude sites, i.e., Jungfraujoch (JFJ; Switzerland) and Pic du Midi (PDM; France). Analysis of precipitation chemistry and moisture sources indicate combined marine and continental sources of precipitation and Se, I, Br, and S at both sites. At JFJ, concentrations of I and Se were highest when continental moisture sources were dominant, indicating important terrestrial sources for these elements. Furthermore, correlations between investigated elements and DOC-δ13C, particularly when continental moisture source contributions were high, indicate a link between these elements and the source of dissolved organic matter, especially for I (JFJ and PDM) and Se (JFJ).

Use of Mercury Isotopes to Quantify Mercury Exposure Sources in Inland Populations, China.

Mercury (Hg) isotopic compositions in hair and dietary sources from Wanshan (WS) Hg mining area, Guiyang (GY) urban area, and Changshun (CS) rural area were determined to identify the major Hg exposure sources of local residents. Rice and vegetables displayed low δ202Hg and small negative to zero Δ199Hg, and are isotopically distinguishable from fish which showed relatively higher δ202Hg and positive Δ199Hg. Distinct isotopic signatures were also observed for human hair from the three areas. Shifts of 2 to 3‰ in δ202Hg between hair and dietary sources confirmed mass dependent fractionation of Hg isotopes occurs during metabolic processes. Near zero Δ199Hg of hair from WS and CS suggested rice is the major exposure source. Positive Δ199Hg of hair from GY was likely caused by consumption of fish. A binary mixing model based on Δ199Hg showed that rice and fish consumption accounted for 59% and 41% of dietary Hg source for GY residents, respectively, whereas rice is the major source for WS and CS residents. The model output was validated by calculation of probable daily intake of Hg. Our study suggests that Hg isotopes can be a useful tracer for quantifying exposure sources and understanding metabolic processes of Hg in humans.

Tropospheric GOM at the Pic du Midi Observatory-Correcting Bias in Denuder Based Observations.

Gaseous elemental mercury (GEM, Hg) emissions are transformed to divalent reactive Hg (RM) forms throughout the troposphere and stratosphere. RM is often operationally quantified as the sum of particle bound Hg (PBM) and gaseous oxidized Hg (GOM). The measurement of GOM and PBM is challenging and under mounting criticism. Here we intercompare six months of automated GOM and PBM measurements using a Tekran (TK) KCl-coated denuder and quartz regenerable particulate filter method (GOMTK, PBMTK, and RMTK) with RMCEM collected on cation exchange membranes (CEMs) at the high altitude Pic du Midi Observatory. We find that RMTK is systematically lower by a factor of 1.3 than RMCEM. We observe a significant relationship between GOMTK (but not PBMTK) and Tekran flushTK blanks suggesting significant loss (36%) of labile GOMTK from the denuder or inlet. Adding the flushTK blank to RMTK results in good agreement with RMCEM (slope = 1.01, r2 = 0.90) suggesting we can correct bias in RMTK and GOMTK. We provide a bias corrected (*) Pic du Midi data set for 2012-2014 that shows GOM* and RM* levels in dry free tropospheric air of 198 ± 57 and 229 ± 58 pg m-3 which agree well with in-flight observed RM and with model based GOM and RM estimates.

Holocene Atmospheric Mercury Levels Reconstructed from Peat Bog Mercury Stable Isotopes.

Environmental regulations on mercury (Hg) emissions and associated ecosystem restoration are closely linked to what Hg levels we consider natural. It is widely accepted that atmospheric Hg deposition has increased by a factor 3 ± 1 since preindustrial times. However, no long-term historical records of actual atmospheric gaseous elemental Hg (GEM) concentrations exist. In this study we report Hg stable isotope signatures in Pyrenean peat records (southwestern Europe) that are used as tracers of Hg deposition pathway (Δ200Hg, wet vs dry Hg deposition) and atmospheric Hg sources and cycling (δ202Hg, Δ199Hg). By anchoring peat-derived GEM dry deposition to modern atmospheric GEM levels we are able to reconstruct the first millennial-scale atmospheric GEM concentration record. Reconstructed GEM levels from 1970 to 2010 agree with monitoring data, and maximum 20th century GEM levels of 3.9 ± 0.5 ng m-3 were 15 ± 4 times the natural Holocene background of 0.27 ± 0.11 ng m-3. We suggest that a -0.7‰ shift in δ202Hg during the medieval and Renaissance periods is caused by deforestation and associated biomass burning Hg emissions. Our findings suggest therefore that human impacts on the global mercury cycle are subtler and substantially larger than currently thought.

Seasonal and Annual Variations in Atmospheric Hg and Pb Isotopes in Xi'an, China.

We present a 3-year time series of lead (Pb) and mercury (Hg) concentrations and isotope signatures in total suspended particulate (TSP) matter and as total gaseous Hg (TGM) in Xi'an, Northwestern China. Mean concentrations of TSP (299 ± 120 µg m-3), PbTSP (0.33 ± 0.15 µg m-3) and HgTSP (0.64 ± 0.54 ng m-3), and TGM (5.7 ± 2.7 ng m-3) were elevated. We find that atmospheric Pb levels in winter in Xi'an have decreased by 4.6% per year since 2003, yet remain elevated relative to air quality guidelines and therefore a major health concern. δ202HgTSP and Δ199HgTSP averaged -0.80 ± 0.30‰ (1σ) and -0.02 ± 0.10‰ (1σ) and δ202HgTGM and Δ199HgTGM averaged -0.08 ± 0.41‰ (1σ) and 0.00 ± 0.04‰ (1σ). Relative to raw coal from Shaanxi and surrounding provinces, δ202HgTSP is enriched in the light Hg isotopes, whereas δ202HgTGM is enriched in the heavy isotopes. TSP and TGM Δ199Hg signatures are indistinguishable from raw coal, indicating little photochemical mass independent fractionation of atmospheric Hg in the near-field urban-industrial environment. δ202HgTGM correlates significantly with TGM levels (r2 = 0.3, p < 0.01) and likely reflects binary mixing of local industrial TGM emissions with global background TGM.

Mercury Isotope Signatures of Methylmercury in Rice Samples from the Wanshan Mercury Mining Area, China: Environmental Implications.

Rice consumption is the primary pathway of methylmercury (MeHg) exposure for residents in mercury-mining areas of Guizhou Province, China. In this study, compound-specific stable isotope analysis (CSIA) of MeHg was performed on rice samples collected in the Wanshan mercury mining area. An enrichment of 2.25‰ in total Hg (THg) δ202Hg was observed between rice and human hair, and THg Δ199Hg in hair was 0.12‰ higher than the value in rice. Rice and human hair samples in this study show distinct Hg isotope signatures compared to those of fish and human hair of fish consumers collected in China and other areas. Distinct Hg isotope signatures were observed between IHg and MeHg in rice samples (in mean ± standard deviation: δ202HgIHg at -2.30‰ ± 0.49‰, Δ199HgIHg at -0.08‰ ± 0.04‰, n = 7; δ202HgMeHg at -0.80‰ ± 0.25‰, Δ199HgMeHg at 0.08‰ ± 0.04‰, n = 7). Using a binary mixing model, it is estimated that the atmospheric Hg contributed 31% ± 16% of IHg and 17% ± 11% of THg in the rice samples and the IHg in soil caused by past mining activities contributed to the remaining Hg. This study demonstrated that Hg stable isotopes are good tracers of human MeHg exposure to fish and rice consumption, and the isotope data can be used for identifying the sources of IHg and MeHg in rice.

Isotopic Composition of Gaseous Elemental Mercury in the Free Troposphere of the Pic du Midi Observatory, France.

Understanding the sources and transformations of mercury (Hg) in the free troposphere is a critical aspect of global Hg research. Here we present one year of observations of atmospheric Hg speciation and gaseous elemental Hg (GEM) isotopic composition at the high-altitude Pic du Midi Observatory (2860 m above sea level) in France. Biweekly integrated GEM from February 2012 to January 2013 revealed significant variations in δ(202)HgGEM (-0.04‰ to 0.52‰) but not in Δ(199)HgGEM (-0.17‰ to -0.27‰) or Δ(200)HgGEM (-0.10‰ to 0.05‰). δ(202)HgGEM was negatively correlated with CO and reflected air mass origins from Europe (high CO, low δ(202)HgGEM) and from the Atlantic Ocean (low CO, high δ(202)HgGEM). We suggest that the δ(202)HgGEM variations represent mixing of recent low δ(202)HgGEM European anthropogenic emissions with high δ(202)HgGEM northern hemispheric background GEM. In addition, Atlantic Ocean free troposphere air masses showed a positive correlation between δ(202)HgGEM and gaseous oxidized Hg (GOM) concentrations, indicative of mass-dependent Hg isotope fractionation during GEM oxidation. On the basis of atmospheric δ(202)HgGEM and speciated Hg observations, we suggest that the oceanic free troposphere is a reservoir within which GEM is readily oxidized to GOM.