Mercury isotope evidence for Arctic summertime re-emission of mercury from the cryosphere.

During Arctic springtime, halogen radicals oxidize atmospheric elemental mercury (Hg0), which deposits to the cryosphere. This is followed by a summertime atmospheric Hg0 peak that is thought to result mostly from terrestrial Hg inputs to the Arctic Ocean, followed by photoreduction and emission to air. The large terrestrial Hg contribution to the Arctic Ocean and global atmosphere has raised concern over the potential release of permafrost Hg, via rivers and coastal erosion, with Arctic warming. Here we investigate Hg isotope variability of Arctic atmospheric, marine, and terrestrial Hg. We observe highly characteristic Hg isotope signatures during the summertime peak that reflect re-emission of Hg deposited to the cryosphere during spring. Air mass back trajectories support a cryospheric Hg emission source but no major terrestrial source. This implies that terrestrial Hg inputs to the Arctic Ocean remain in the marine ecosystem, without substantial loss to the global atmosphere, but with possible effects on food webs.

Chemistry of surface water, precipitation, throughfall, leaves, sediment, soil, and air near a gold mining region in Peru.

Artisanal and small-scale gold mining (ASGM) is the primary global source of anthropogenic mercury (Hg) emissions and a large source of landscape change. ASGM occurs throughout the world, including in the Peruvian Amazon. This data set contains measurements of surface water, precipitation, throughfall, leaves, sediment, soil, and air samples from across the Madre de Dios region of Peru, in locations near and remote from ASGM. These data were collected to determine the fate and transport of Hg across the landscape. Samples were collected in 2018 and 2019. Data predominantly included total Hg and methyl Hg concentrations in surface water, precipitation, throughfall, leaves, sediment, soil, and air. Additional water and soil parameters were also measured to better characterize their chemistry. There are no copyright restrictions; please cite this data paper when the data are used in publication.

Amazon forests capture high levels of atmospheric mercury pollution from artisanal gold mining.

Mercury emissions from artisanal and small-scale gold mining throughout the Global South exceed coal combustion as the largest global source of mercury. We examined mercury deposition and storage in an area of the Peruvian Amazon heavily impacted by artisanal gold mining. Intact forests in the Peruvian Amazon near gold mining receive extremely high inputs of mercury and experience elevated total mercury and methylmercury in the atmosphere, canopy foliage, and soils. Here we show for the first time that an intact forest canopy near artisanal gold mining intercepts large amounts of particulate and gaseous mercury, at a rate proportional with total leaf area. We document substantial mercury accumulation in soils, biomass, and resident songbirds in some of the Amazon's most protected and biodiverse areas, raising important questions about how mercury pollution may constrain modern and future conservation efforts in these tropical ecosystems.

Anthropogenic lead pervasive in Canadian Arctic seawater.

Anthropogenic Pb is widespread in the environment including remote places. However, its presence in Canadian Arctic seawater is thought to be negligible based on low dissolved Pb (dPb) concentrations and proxy data. Here, we measured dPb isotopes in Arctic seawater with very low dPb concentrations (average ∼5 pmol ⋅ kg-1) and show that anthropogenic Pb is pervasive and often dominant in the western Arctic Ocean. Pb isotopes further reveal that historic aerosol Pb from Europe and Russia (Eurasia) deposited to the Arctic during the 20th century, and subsequently remobilized, is a significant source of dPb, particularly in water layers with relatively higher dPb concentrations (up to 16 pmol ⋅ kg-1). The 20th century Eurasian Pb is present predominantly in the upper 1,000 m near the shelf but is also detected in older deep water (2,000 to 2,500 m). These findings highlight the importance of the remobilization of anthropogenic Pb associated with previously deposited aerosols, especially those that were emitted during the peak of Pb emissions in the 20th century. This remobilization might be further enhanced because of accelerated melting of permafrost and ice along with increased coastal erosion in the Arctic. Additionally, the detection of 20th century Eurasian Pb in deep water helps constrain ventilation ages. Overall, this study shows that Pb isotopes in Arctic seawater are useful as a gauge of changing particulate and contaminant sources, such as those resulting from increased remobilization (e.g., coastal erosion) and potentially also those associated with increased human activities (e.g., mining and shipping).

Isotopic Characterization of Atmospheric Gaseous Elemental Mercury by Passive Air Sampling.

Tracing emission sources and transformations of atmospheric mercury with Hg stable isotopes depends on the ability to collect amounts sufficient for reliable quantification. Commonly employed active sampling methods require power and long pumping times, which limits the ability to deploy in remote locations and at high spatial resolution and can lead to compromised traps. In order to overcome these limitations, we conducted field and laboratory experiments to assess the preservation of isotopic composition during sampling of gaseous elemental mercury (GEM) with a passive air sampler (PAS) that uses a sulfur-impregnated carbon sorbent and a diffusive barrier. Whereas no mass independent fractionation (MIF) was observed during sampling, the mass dependent fractionation (MDF, δ202Hg) of GEM taken up by the PAS was lower than that of actively pumped samples by 1.14 ± 0.24‰ (2SD). Because the MDF offset was consistent across field studies and laboratory experiments conducted at 5, 20, and 30 °C, the PAS can be used for reliable isotopic characterization of GEM (±0.3‰ for MDF, ±0.05‰ for MIF, 2SD). The MDF offset occurred more during the sorption of GEM rather than during diffusion. PAS field deployments confirm the ability to record differences in the isotopic composition of GEM (i) with distance from point sources and (ii) sampled at different background locations globally.

Mercury isotopic signatures of tailings from artisanal and small-scale gold mining (ASGM) in southwestern Ecuador.

Artisanal and small-scale gold mining (ASGM) is currently one of the largest anthropogenic sources of mercury (Hg) to the atmosphere and a significant source to downstream terrestrial aquatic systems. Given the potential toxicity of Hg when it is released to aquatic ecosystems, both scientific and regulatory communities have interest in the development of tools and methods for understanding the fate and transport of Hg in the environment. Although Hg isotopes are applied extensively to trace other anthropogenic point sources of Hg in aquatic ecosystems, few studies have used isotopic analyses to investigate the downstream impacts of ASGM. Furthermore, these studies are currently limited by relatively sparse Hg isotopic analyses on ASGM materials. In this study, ASGM samples from Portovelo-Zaruma (Ecuador), representing various stages of the processing of low-grade gold-bearing ores, were analyzed for Hg concentrations and stable isotope ratios. These data were used to assess the isotopic variability of the ASGM endmember and confirm the results of prior isotopic analyses of ASGM materials and downstream sediments from Portovelo-Zaruma. While the Hg concentrations of the ASGM samples varied significantly, isotopic analyses revealed a limited range of signatures that was characterized by relatively heavy mass-dependent fractionation (MDF) with little to no significant mass-independent fractionation (MIF). The signatures of ASGM samples analyzed in this study matched well with previously reported ASGM samples from Portovelo-Zaruma as well as downstream sediments (for up to 120 km). Furthermore, the overall Hg isotopic compositions of potential ASGM endmembers are distinct from typical compositions observed in freshwater ecosystems (e.g., from soil erosion, atmospheric deposition), allowing Hg isotopes to be a powerful tool in tracing downstream Hg contamination from ASGM.

Mercury Stable Isotope Fractionation during Abiotic Dark Oxidation in the Presence of Thiols and Natural Organic Matter.

Mercury (Hg) stable isotope fractionation has been widely used to trace Hg sources and transformations in the environment, although many important fractionation processes remain unknown. Here, we describe Hg isotope fractionation during the abiotic dark oxidation of dissolved elemental Hg(0) in the presence of thiol compounds and natural humic acid. We observe equilibrium mass-dependent fractionation (MDF) with enrichment of heavier isotopes in the oxidized Hg(II) and a small negative mass-independent fractionation (MIF) owing to nuclear volume effects. The measured enrichment factors for MDF and MIF (ε202Hg and E199Hg) ranged from 1.10‰ to 1.56‰ and from -0.16‰ to -0.18‰, respectively, and agreed well with theoretically predicted values for equilibrium fractionation between Hg(0) and thiol-bound Hg(II). We suggest that the observed equilibrium fractionation was likely controlled by isotope exchange between Hg(0) and Hg(II) following the production of the Hg(II)-thiol complex. However, significantly attenuated isotope fractionation was observed during the initial stage of Hg(0) oxidation by humic acid and attributed to the kinetic isotope effect (KIE). This research provides additional experimental constraints on interpreting Hg isotope signatures with important implications for the use of Hg isotope fractionation as a tracer of the Hg biogeochemical cycle.

An investigation of mercury sources in the Puyango-Tumbes River: Using stable Hg isotopes to characterize transboundary Hg pollution.

Mercury (Hg) concentrations and stable isotopes along with other trace metals were examined in environmental samples from Ecuador and Peru's shared Puyango-Tumbes River in order to determine the extent to which artisanal- and small-scale gold mining (ASGM) in Portovelo-Zaruma, Ecuador contributes to Hg pollution in the downstream aquatic ecosystem. Prior studies investigated the relationship between ASGM activities and downstream Hg pollution relying primarily on Hg concentration data. In this study, Hg isotopes revealed an isotopically heavy Hg signature with negligible mass independent fractionation (MIF) in downstream sediments, which was consistent with the signature observed in the ASGM source endmember. This signature was traced as far as ∼120 km downstream of Portovelo-Zaruma, demonstrating that Hg stable isotopes can be used as a tool to fingerprint and trace sources of Hg over vast distances in freshwater environments. The success of Hg isotopes as a source tracer in fresh waters is largely due to the particle-reactive nature of Hg. Furthermore, the magnitude and extent of downstream Hg, lead, copper and zinc contamination coupled with the Hg isotopes suggest that it is unlikely that the smaller artisanal-scale activities, which do not use cyanidation, are responsible for the pollution. More likely it is the scale of ores processed and the cyanide leaching, which can release other metals and enhance Hg transport, used during small-scale gold mining that is responsible. Thus, although artisanal- and small-scale gold mining occur in tandem in Portovelo-Zaruma, a distinction should be made between these two activities.

Evidence of transboundary mercury and other pollutants in the Puyango-Tumbes River basin, Ecuador-Peru.

In Portovelo in southern Ecuador, 87 gold processing centers along the Puyango-Tumbes River produce an estimated 6 tonnes of gold per annum using a combination of mercury amalgamation and/or cyanidation and processing poly-metallic ores. We analysed total Hg, Hg isotopes, total arsenic, cadmium, copper, lead and zinc in water and sediment along the Puyango in 2012-2014. The highest total mercury (THg) concentrations in sediments were found within a 40 km stretch downriver from the processing plants, with levels varying between 0.78-30.8 mg kg-1 during the dry season and 1.80-70.7 mg kg-1 during the wet season, with most concentrations above the CCME (Canadian Council of Ministers of the Environment) Probable Effect Level (PEL) of 0.5 mg kg-1. Data from mercury isotopic analyses support the conclusion that mercury use during gold processing in Portovelo is the source of Hg pollution found downstream in the Tumbes Delta in Peru, 160 km away. The majority of the water and sediment samples collected from the Puyango-Tumbes River had elevated concentrations of, arsenic, cadmium, copper, lead and zinc exceeding the CCME thresholds for the Protection of Aquatic Life. At monitoring points immediately below the processing plants, total dissolved concentrations of these metals exceeded the thresholds by 156-3567 times in surface waters and by 19-740 times in sediment. The results illustrate a significant transboundary pollution problem involving Hg and other toxic metals, amplified by the fact that the Puyango-Tumbes River is the only available water source in the semi-arid region of northern Peru.

Disconnecting the Estrogen Receptor Binding Properties and Antimicrobial Properties of Parabens through 3,5-Substitution.

Commercially utilized parabens are employed for their antimicrobial properties, but a weak binding to the estrogen receptor alpha (ERα) may lead to breast cancer in some applications. Modification of the paraben scaffold should allow for a disconnection of these observed properties. Toward this goal, various 3,5-substituted parabens were synthesized and assessed for antimicrobial properties against S. aureus as well as competitive binding to the ERα. The minimum inhibitory concentration assay confirmed retention of antimicrobial activity in many of these derivatives, while all compounds exhibited decreased xenoestrogen activity as determined by a combination of competitive enzyme linked immunosorbent assay (ELISA), proliferation, and estrogen receptor binding assay. Thus, these changes to the paraben scaffold have led to a multitude of paraben derivatives with antimicrobial properties up to 16 times more active than the parent paraben and that are devoid or significantly diminished of potential breast cancer causing properties.

Mercury anomalies and the timing of biotic recovery following the end-Triassic mass extinction.

The end-Triassic mass extinction overlapped with the eruption of the Central Atlantic Magmatic Province (CAMP), and release of CO2 and other volcanic volatiles has been implicated in the extinction. However, the timing of marine biotic recovery versus CAMP eruptions remains uncertain. Here we use Hg concentrations and isotopes as indicators of CAMP volcanism in continental shelf sediments, the primary archive of faunal data. In Triassic-Jurassic strata, Muller Canyon, Nevada, Hg levels rise in the extinction interval, peak before the appearance of the first Jurassic ammonite, remain above background in association with a depauperate fauna, and fall to pre-extinction levels during significant pelagic and benthic faunal recovery. Hg isotopes display no significant mass independent fractionation within the extinction and depauperate intervals, consistent with a volcanic origin for the Hg. The Hg and palaeontological evidence from the same archive indicate that significant biotic recovery did not begin until CAMP eruptions ceased.

Mercury Stable Isotopes in Ornithogenic Deposits As Tracers of Historical Cycling of Mercury in Ross Sea, Antarctica.

Production of methylmercury (MeHg) in ocean waters and its bioaccumulation in marine organisms are critical processes controlling the fate and toxicity of mercury (Hg). However, these processes are not well understood in the Antarctic, where high levels of MeHg are observed in the subsurface ocean (100-1000 m). We explored the use of Hg stable isotope compositions in historical and modern biological deposits as a new approach for discerning Hg sources and tracing MeHg cycling in the ocean and bioaccumulation in marine biota. We found similar mass independent isotope fractionation (MIF) of Hg between a sediment profile containing historical penguin and seal feces deposits from coastal Antarctica and modern penguin and seal feces, suggesting that penguin and seal feces were the dominant sources of Hg to the sediments at different time periods. Furthermore, sediments dominated by seal feces displayed a significantly lower MIF slope (Δ(199)Hg/Δ(201)Hg) than those dominated by penguin feces despite similar extents of MIF. Since seals forage at greater depths (>400 m) than penguins (<100 m), the high MIF values and lower Δ(199)Hg/Δ(201)Hg in seal feces suggest that a significant fraction of MeHg accumulated by seals was produced in situ in the subsurface ocean from residual inorganic Hg(II) that sank from the euphotic zone after partial photoreduction. Our results suggest that in situ Hg methylation can be an important source of MeHg for marine biota, and Hg isotope compositions in biological archives can be valuable tracers of MeHg cycling.

Mercury isotope fractionation during aqueous photoreduction of monomethylmercury in the presence of dissolved organic matter.

Monomethylmercury (MMHg) is a toxic pollutant that bioaccumulates in aquatic food webs. A major mechanism that limits MMHg uptake by biota is photodemethylation in surface waters. Recently, the extent of mass-independent fractionation (MIF) of Hg isotopes preserved in fish is being used to quantify this MMHg sink. Here, the effects of different types and amounts of DOM on Hg MIF during MMHg photodemethylation were investigated to assess how variable MIF enrichment factors may be with respect to changing DOM binding sites. From experiments conducted with varying amounts of reduced organic sulfur (S(red)-DOM), the extent and signature of MIF is likely dependent on whether MMHg is dominantly bound to S(red)-DOM. Similar enrichment factors were observed for low MMHg:S(red)-DOM experiments, where S(red)-DOM was in far excess of MMHg. In contrast, significantly lower and variable enrichment factors were observed for experiments with higher MMHg:S(red)-DOM ratios. Additionally the relationship between the two odd Hg isotopes that display MIF (Δ(199)Hg/Δ(201)Hg) was consistent for the low MMHg:S(red)-DOM experiments, while lower Δ(199)Hg/Δ(201)Hg relationships were observed for the higher MMHg:S(red)-DOM experiments. These results suggest that both the extent and signature of MMHg MIF are sensitive to different ligands that bind MMHg in nature.

Natural mercury isotope variation in coal deposits and organic soils.

There is a need to distinguish among sources of Hg to the atmosphere in order to more fully understand global Hg pollution. In this study we investigate whether coal deposits within the United States, China, and Russia-Kazakhstan, which are three of the five greatest coal-producing regions, have diagnostic Hg isotopic fingerprints that can be used to discriminate among Hg sources. We also investigate the Hg isotopic composition of modern organic soil horizons developed in areas distant from point sources of Hg in North America. Mercury stored in coal deposits displays a wide range of both mass dependent fractionation (MDF, delta202Hg) and mass independent fractionation (MIF, delta201Hg). delta202Hg varies in coals by 3 per thousand and delta201Hg varies by 0.9 per thousand. Combining these two Hg isotope signals results in what may be a unique isotopic "fingerprint" for many coal deposits. Mass independent fractionation of mercury has been demonstrated to occur during photochemical reactions of mercury. This suggests that Hg found in most coal deposits was subjected to photochemical reduction near the Earth's surface prior to deposition. The similarity in MDF and MIF of modern organic soils and coals from North America suggests that Hg deposition from coal may have imprinted an isotopic signature on soils. This research offers a new tool for characterizing mercury inputs from natural and anthropogenic sources to the atmosphere and provides new insights into the geochemistry of mercury in coal and soils.

Mass-dependent and -independent fractionation of hg isotopes by photoreduction in aquatic systems.

Mercury (Hg) isotopes can be used as tracers of Hg biogeochemical pathways in the environment. The photochemical reduction of aqueous Hg species by natural sunlight leads to both mass-dependent fractionation (MDF) of Hg isotopes and mass-independent fractionation (MIF) of the odd-mass isotopes, with the relation between the MIF for the two odd isotopes being distinct for different photoreduction pathways. Large variations in MDF and MIF are observed in fish and provide new insights into the sources and bioaccumulation of Hg in food webs. MIF in fish can also be used to estimate the loss of methylmercury via photoreduction in aquatic ecosystems.

Mercury stable isotope fractionation during reduction of Hg(II) to Hg(0) by mercury resistant microorganisms.

Mercury (Hg) undergoes systematic stable isotopic fractionation; therefore, isotopic signatures of Hg may provide a new tool to track sources, sinks, and dominant chemical transformation pathways of Hg in the environment. We investigated the isotopic fractionation of Hg by Hg(II) resistant (HgR) bacteria expressing the mercuric reductase (MerA) enzyme. The isotopic composition of both the reactant Hg(II) added to the growth medium and volatilized product (Hg(0)) was measured using cold vapor generation and multiple collector inductively coupled plasma mass spectrometry. We found that exponentially dividing pure cultures of a gram negative strain Escherichia coli JM109/pPB117 grown with abundant electron donor and high Hg(II) concentrations at 37, 30, and 22 degrees C, and a natural microbial consortium incubated in natural site water at 30 degrees C after enrichment of HgR microbes, preferentially reduced the lighter isotopes of Hg. In all cases, Hg underwent Rayleigh fractionation with the best estimates of alpha202/198 values ranging from 1.0013 to 1.0020. In the cultures grown at 37 degrees C, below a certain threshold Hg(II) concentration, the extent of fractionation decreased progressively. This study demonstrates mass-dependent kinetic fractionation of Hg and could lead to development of a new stable isotopic approach to the study of Hg biogeochemical cycling in the environment.

Reporting of variations in the natural isotopic composition of mercury.

High-precision measurements of natural variations in the stable isotopic composition of mercury show great promise as a new tracer of mercury sources and chemical transformations in the environment. We strongly suggest that all laboratories adopt a common means of data correction, standardization, and nomenclature in order to ensure that data from various laboratories can be easily evaluated and compared. We make suggestions for mass bias correction, reporting of mass-dependent and mass-independent isotope variations, and a standard protocol for reporting analytical uncertainties. We also present our measured values for isotope ratios in several mercury standard solutions.