Mercury Isotopic Compositions Research Articles

Metal recycling tracked by mercury and helium isotopes in platinum–palladium nuggets from Córrego Bom Sucesso, Brazil

The enigmatic botryoidal nuggets of platinum (Pt) and palladium (Pd) from Córrego Bom Sucesso in the southern Serra do Espinhaço, Minas Gerais, Brazil, are considered to have formed during supergene alteration of placer deposits. This requires hitherto unidentified precious-metal enrichment processes and is inconsistent with Pt–Os age of 180 Ma that entails formation at depth, as recently proposed. Here we report the first mercury (Hg) and helium (He) isotopic determinations of Pt–Pd nuggets. Mercury isotopic compositions have a mass-independent fractionation (MIF) signature with an odd-mass deficit (Δ199Hg = − 0.22 ± 0.04; 1SD, n = 15), which requires aqueous photochemical reduction of Hg (II). Extremely low 3He/4He (<0.001 Ra) and extremely high concentrations of He (up to 1.9 × 1017 at/g) are indicative of nugget formation from He-enriched fluids within the quartzite sequence of the Espinhaço basin, not from meteoric surface water. The data are consistent with a nugget-forming setting in the deep biosphere, as a result of groundwater interaction with Pt–Pd–Hg minerals in Pan-African-Brasiliano post-orogenic veins. We propose that the negative Hg-MIF signature was inherited from the vein minerals that originally acquired their Hg from Earth's surface during the intracratonic sedimentation of the Proterozoic Espinhaço basin.

The relationship between volcanism and global climate changes in the Tropical Western Pacific over the mid-Pleistocene transition: Evidence from mercury concentration and isotopic composition

Volcanoes are a significant component of the Earth system, influencing the interaction between oceans and the atmosphere over large spatial and temporal scales. Being a volcanically dynamic region, the Tropical Western Pacific (TWP) can significantly impact variations in global climate. However, high-resolution continuous records of volcanic activity in this region are lacking, resulting in significant uncertainties regarding the coupling between the deep earth, climate changes, and atmospheric CO2 in the TWP. To address this issue, mercury (Hg) levels, isotopic compositions, and Hg/total organic carbon (Hg/TOC) ratios were determined at site U1486 to track volcanic activity throughout the mid-Pleistocene transition (MPT) from 1.3 Myr to 0.6 Myr. Our results of anomalously high Hg concentrations and Hg/TOC ratios provide evidence of time-varying volcanism throughout the MPT. Mercury isotopes in the Hg-enriched sediments were characterized by near-zero Δ199Hg values, which is consistent with volcanism acting as the primary source of Hg to the sediments. Spectral analysis of the Hg/TOC ratio showed significant periodicity at ~100 kyr and ~ 23 kyr as well as a weaker signal at ~41 kyr consistent with Milankovitch cycles. A cross spectral analysis of Hg/TOC and the LR04 δ18O stack record suggests that the peak in volcanism lags the temperature minimum by ~6 kyr, and occurs prior to the δ18O minimum known as the glacial termination by ~14 ± 2 kyr. The records of volcanic activity in this site are also consistent with a prominent rise in atmospheric CO2 and negative excursion of benthic carbon isotopes throughout the MPT. This study provides direct sedimentary evidence in the TWP of the feedback between volcanic activity, climate change and atmospheric CO2.

Online determination of mercury isotopic compositions at ultratrace levels by automated purge and trap coupled with multicollector inductively coupled plasma-mass spectrometry

This study reports a sensitive online method for Hg isotope analysis at ultratrace levels by automated P&amp;T-MC-ICPMS.

Mercury isotopic compositions in fine particles and offshore surface seawater in a coastal area of East China: implications for Hg sources and atmospheric transformations

Abstract. Isotopic compositions of Mercury (Hg) in atmospheric particles (HgPM) are probably the mixed results of emission sources and atmospheric processes. Here, we present Hg isotopic compositions in daily fine particles (PM2.5) collected from an industrial site (Chunxiao – CX) and a nearby mountain site (Daimeishan – DMS) in a coastal area of East China, and in surface seawater close to the industrial area, to reveal the influence of anthropogenic emission sources and atmospheric transformations on Hg isotopes. The PM2.5 samples displayed a significant spatial difference in δ202Hg. For the CX site, the negative δ202Hg values are similar to those of source materials, and the HgPM contents were well correlated with chemical tracers, indicating the dominant contributions of local industrial activities to HgPM2.5, whereas the observed positive δ202Hg at the DMS site was likely associated with regional emissions and extended atmospheric processes during transport. The Δ199Hg values in PM2.5 from the CX and DMS sites were comparably positive. The unity slope of Δ199Hg versus Δ201Hg over all data suggests that the odd mass independent fractionation (MIF) of HgPM2.5 was primarily induced by the photoreduction of Hg2+ in aerosols. The positive Δ200Hg values with a minor spatial difference were probably associated with the photooxidation of Hg0, which is generally enhanced in the coastal environment. Total Hg in offshore surface seawater was characterized by negative δ202Hg and near-zero Δ199Hg and Δ200Hg values, which are indistinguishable from Hg isotopes of source materials. Overall, the PM2.5 collected from industrial areas had comparable δ202Hg values but more positive Δ199Hg and Δ200Hg as compared to surface seawater. The results indicate that atmospheric transformations would induce the significant fractionation of Hg isotopes and obscure the Hg isotopic signatures of anthropogenic emissions.

The Mercury Isotopic Composition of Earth's Mantle and the Use of Mass Independently Fractionated Hg to Test for Recycled Crust

AbstractThe element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ199Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ199Hg for terrestrial sub‐aerially‐derived materials and positive Δ199Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐3He/4He lavas from Samoa and Iceland, providing an average of Δ199Hg = 0.00 ± 0.10, Δ201Hg = −0.02 ± 0.0.09, δ202Hg = −1.7 ± 1.2; 2SD, N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ199Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ199Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ199Hg which correlate with 87Sr/86Sr, consistent with variable proportions of continental (low Δ199Hg and high 87Sr/86Sr) and oceanic (high Δ199Hg and low 87Sr/86Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions.

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean “whiff” of oxygen

Earth's early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903-1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281-290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied "whiff" of O2 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903-1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative ∆199Hg and near-zero ∆200Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713-716 (2009)], show Hg enrichment coupled with positive ∆199Hg and slightly negative ∆200Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O2 was preceded by subaerial volcanism. The transient interval of O2 accumulation may thus have been triggered by diminished volcanic O2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.

Mercury concentration and isotopic composition on different atmospheric particles (PM10 and PM2.5) in the subtropical coastal suburb of Xiamen Bay, Southern China

The concentration and isotopic composition of particulate-bound mercury (PBM) on both PM2.5 and PM10 were simultaneously measured (24 months) in a typical subtropical coastal suburb of Xiamen bay (SCXB), southern China. The results showed that most mercury was concentrated in PM2.5 and small particulates enriched heavier isotopes of mercury more than lighter ones. Negative Δ199Hg values and slightly positive Δ200Hg values were observed in both PM2.5 and PM10. PBM input sources were identified with isotopic compositions and meteorological data using the NOAA HYSPLIT model. It revealed that biomass combustion is the main source of particulate mercury in SCXB, and the two other sources are urban anthropogenic emissions and potential marine sources. Seasonal variations in isotopic composition and correlation analysis suggested that PBM in SCXB mainly originate from inland after long-distance transportation in spring and under the influence of oceanic air masses in summer. The Δ199Hg/Δ201Hg ratios in all samples (1.01 ± 0.05, 2SE) indicated that the photoreduction of Hg(II) in the aerosol was the main reaction process of PBM transformation in the atmosphere. Seasonal variations in Δ200Hg values were also found, indicating the possibility of even mass independent fractionation in tracing the mercury sources.

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.

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. CapsuleA 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.

Comparison of the Isotopic Composition of Hg and Pb in Two Atmospheric Bioaccumulators in a Pyrenean Beech Forest (Iraty Forest, Western Pyrenees, France/Spain)

Mercury (Hg) and lead (Pb) isotopic compositions were investigated in mosses and lichens collected in a large mountainous beech forest (Iraty Forest) located on the French-Spanish Pyrenean border. Hg isotopic signature in topsoil samples were also analyzed in selected sampling sites. This is the first work that uses the complementary information of both isotopic systems in two distinct atmospheric bioaccumulators. Mosses and lichens present characteristic accumulation due to their integration times, displaying different information on metal pollution over the area. Hg and Pb concentrations in annual moss shoots represent recent atmospheric accumulation, while whole lichen thalli integrates a process of accumulation over a longer period. Lead isotope ratios in mosses are consistent with reported data corresponding to the actual European atmospheric background (206Pb/207Pb ∼ 1.158), while Hg isotopic composition reflects potential uptake of both dry and wet Hg depositions. For lichens, Pb isotopic composition exhibits the contribution of a longer integration period of both industrial Pb emissions and legacy of leaded gasoline pollution. Hg isotopes in lichens discriminate two main groups: a larger one representing the background atmospheric contribution and a second one corresponding to unexpected higher Hg content. The similarities in odd and even Mass-independent fractionation of Hg isotopes between topsoils and lichens from the larger group, support the idea that foliage uptake is the main input of Hg in soils. The second group of lichens exhibits more negative δ202Hg (down to –4.69‰) suggesting a new source of fractionation in this area, probably related to lichens aging and/or stubble and grass fires due to pastoral activities. This study demonstrates that using both Hg and Pb isotopic signature in lichens and mosses allows to trace atmospheric sources and environmental pathways of these metals in forested ecosystems. This original data set in a remote environment provides also new information on the fate of atmospheric Pb and Hg depositions.

Mercury isotopes identify near-surface marine mercury in deep-sea trench biota

Mercury isotopic compositions of amphipods and snailfish from deep-sea trenches reveal information on the sources and transformations of mercury in the deep oceans. Evidence for methyl-mercury subjected to photochemical degradation in the photic zone is provided by odd-mass independent isotope values (Δ199Hg) in amphipods from the Kermadec Trench, which average 1.57‰ (±0.14, n = 12, SD), and amphipods from the Mariana Trench, which average 1.49‰ (±0.28, n = 13). These values are close to the average value of 1.48‰ (±0.34, n = 10) for methyl-mercury in fish that feed at ∼500-m depth in the central Pacific Ocean. Evidence for variable contributions of mercury from rainfall is provided by even-mass independent isotope values (Δ200Hg) in amphipods that average 0.03‰ (±0.02, n = 12) for the Kermadec and 0.07‰ (±0.01, n = 13) for the Mariana Trench compared to the rainfall average of 0.13 (±0.05, n = 8) in the central Pacific. Mass-dependent isotope values (δ202Hg) are elevated in amphipods from the Kermadec Trench (0.91 ±0.22‰, n = 12) compared to the Mariana Trench (0.26 ±0.23‰, n = 13), suggesting a higher level of microbial demethylation of the methyl-mercury pool before incorporation into the base of the foodweb. Our study suggests that mercury in the marine foodweb at ∼500 m, which is predominantly anthropogenic, is transported to deep-sea trenches primarily in carrion, and then incorporated into hadal (6,000-11,000-m) food webs. Anthropogenic Hg added to the surface ocean is, therefore, expected to be rapidly transported to the deepest reaches of the oceans.

Total mercury and mercury isotope signatures in reservoir sediment reflecting the landscape changes and agricultural activities in northeast China

Northeast China possess large areas of agricultural farmlands that were reclaimed from earlier marsh wetlands in the past several decades, while the response of trace element like mercury (Hg) in this process were not fully understood. In the present study, total mercury (THg) and Hg isotope composition were determined in reservoir sediment cores of a reservoir, agricultural soils and forest soils in Northeast China to reveal the possible impacts of agricultural activities on Hg geochemistry in the reservoir. The more negative δ202Hg (↓0.35‰), increased concentrations of THg (↑22.4 ng g−1), and higher organic matter (OM) (↑10%) in the deeper than the upper layer sediments indicate noticeable Hg input about 50–60 years ago, which is coincident with the grand reclamation of marsh wetland in the study area. The gradual decrease in THg content and the increase of δ202Hg value in the upper sediment layer reflect the weakened reclamation and thus less Hg input in the later decades. Using a triple mixing model, it is estimated that more than half of Hg in this reservoir sediment was originated from the reclamation of marsh wetlands into farmlands in the earlier stage and soil erosion in the later period. The study demonstrated that Hg isotope signatures in reservoir sediment core can reflect the landscape changes, such as reclamation.

Mercury and sulfur isotopic composition of sulfides from sediment-hosted lead-zinc deposits in Lanping basin, Southwestern China

The Lanping basin is an important Pb-Zn-Cu-Ag polymetallic province in southwestern (SW) China. Mineralization types in the basin include mainly MVT Pb-Zn deposits, vein-type Cu deposits, and minor quartz vein-type Pb-Zn deposits. In this study, we analyzed the total Hg (THg) content and Hg-S isotopic compositions of sulfides from two types of Pb-Zn deposits in the basin, and investigate the Hg isotope fractionation mechanism and its major controlling factors in the Pb-Zn ore-forming process. At the Shangnuluo quartz vein-type Pb-Zn-Sb deposit, sulfides (especially sphalerite) show a wide δ202Hg range (−0.40 to 2.71‰), suggesting that Hg0 evaporation from hydrothermal fluids occurred during the ore formation. Negative THg vs. δ202Hg correlation in the sphalerites suggests a mixing of Hg from the vapor phase (low δ202Hg, high THg) and the residual aqueous phase (high δ202Hg, low THg). The negative ∆199Hg (−0.42 to −0.08‰) suggests that the Hg was sourced from the metamorphic basement, whereas the negative δ34S values (−13.0 to −2.7‰) suggests that the sulfur was mainly sourced from sedimentary sequences, possibly related to the organic matter decomposition and thermochemical sulfate reduction (TSR). For the Liziping and Fulongchang MVT Pb-Zn deposits, sulfides have relatively narrow δ202Hg range (−1.46 to 0.25‰), suggesting limited Hg0 evaporation during the mineralization. Lower δ202Hg values were observed in the early-stage sulfides from these two deposits, probably due to the preferential precipitation of lighter Hg isotopes in these early-stage sulfides. Sulfides from both deposits have negative ∆199Hg values (−0.24 to 0.01‰), suggesting that the Hg was also derived from the metamorphic basement. Sulfide δ34S values from the two deposits are positive, which also suggests that the sulfur was sourced from sedimentary sequences, but mainly related to TSR under relatively low-temperatures. Sulfides from different Pb-Zn deposit types in the Sanjiang Tethyans Metallogenic Belt fall into distinct fields in the δ34S vs. δ202Hg (or ∆199Hg) diagrams, indicating that the integration of S and Hg isotopes is useful to discriminate different types of Pb-Zn deposits.

Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites

Variations in the abundances of moderately volatile elements (MVE) are one of the most fundamental geochemical differences between the terrestrial planets. Whether these variations are the consequence of nebular processes, planetary volatilization, differentiation or late accretion is still unresolved. The element mercury is the most volatile of the MVE and is a strongly chalcophile element. It is one of the few elements that exhibit large mass-dependent (MDF) and mass-independent (MIF) isotopic fractionations for both odd (odd-MIF, Δ199Hg and Δ201Hg) and even (even-MIF, Δ200Hg) Hg isotopes in nature, which is traditionally used to trace Hg biogeochemical cycling in surface environments. However, the Hg isotopic composition of Earth and meteorites is not well constrained. Here, we present Hg isotopic data for terrestrial basaltic, trachytic and granitic igneous samples. These rocks are isotopically lighter (δ202Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition (δ202Hg=−0.7±0.5‰; 1 standard deviation). We show degassing during magma emplacement induces MIF that are consistent with kinetic fractionation in these samples. Also presented is a more complete dataset for chondritic (carbonaceous, ordinary and enstatite) meteorites, which are consistent with previous work for carbonaceous chondrites (positive odd-MIF) and ordinary chondrites (no MIF), and demonstrate that some enstatite chondrites exhibit positive odd-MIF, similar to carbonaceous chondrites. The terrestrial igneous rocks fall within the range of chondritic compositions for both MIF and MDF. Given the fact that planetary differentiation (core formation, evaporation) would contribute to Hg loss from the silicate portion of Earth and would likely fractionate Hg isotopes from chondritic compositions, we suggest that the budget of the mantle Hg is dominated by late accretion of chondritic materials to Earth, as also suggested for other volatile chalcophile elements (S, Se, Te). Considering the Hg isotopic signatures, materials with compositions similar to CO chondrites or ordinary chondrites are the most likely late accretion source candidates. Finally, eucrite meteorites, which are highly depleted in volatile elements, are isotopically heavier than chondrites and exhibit negative odd-MIF. The origin of volatile depletion in eucrites has been vigorously debated. We show that Δ199Hg versus Δ201Hg relationships point toward an equilibrium nuclear field shift effect, suggesting that volatile loss occurred during a magma ocean phase at the surface of the eucrite parent body, likely the asteroid 4-Vesta.

Mercury isotope signatures in sediments and marine organisms as tracers of historical industrial pollution

Isotopic composition of mercury (Hg) in marine organisms and sediment cores was used to identify sources and reconstruct historical trends of contamination in the coastal-marine area of Rosignano Solvay (Italy), affected by Hg pollution from a chlor-alkali plant on the near land. Sediments show a wide range of Hg concentration and Hg isotope signatures. Particularly, coupled Hg concentration and δ202Hg values trace inputs from different sources. The two depth-profiles clearly indicate three distinct periods: “pre-industrial” (before 1941), “industrial” (between 1941 and 2007) and “post-industrial” (after 2007) ages. This is also corroborated by sediment chronology, using 210Pb dating method, validated through 137Cs. Marine organisms are characterized by Hg isotope signatures comparable to “post-industrial” surface sediments. Notably, specimens of Mullus spp. evidence isotope composition comparable to the “industrial” sediments, thus suggesting a still active role of those sediments as source of Hg for the benthic fish compartment. The small amount of MIF and the Δ199Hg/Δ201Hg ratio recorded in organisms are reasonably consistent with limited processes of MMHg demethylation in the water column.

Trace elements contamination assessment in marine sediments from different regions of the Caribbean Sea

Trace elements (TEs), rare earth elements (REEs), and methylmercury (MeHg) concentrations as well as mercury (Hg) and lead (Pb) isotope compositions in sediment samples collected from strategic locations along the Caribbean Sea were determined. The analyzed sediment samples were collected at different core depths from localities in Colombia, Cuba, Haiti, and the Dominican Republic. The evaluation of pollution assessment indices i.e. enrichment factors and geoaccumulation index revealed significant enrichment of several priority substances, such as Pb, Cd and Hg, in most of the sampling sites. Hg was found in extremely high concentrations (up to 22 ± 3 mg kg−1) in bottom samples of Colombian core, which led the authors to further investigate this area with respect to the source for Hg contamination. The analysis of Hg isotope ratios in Colombian sediments and the Pb isotope ratios in all studied cores, helped in the identification of likely pollution sources and represents a critically important record of anthropogenic influence in the region. Finally, the REEs patterns determined in all samples, also provide a needed baseline for these contaminants in the Caribbean region.

Concentration and isotopic composition of mercury in a blackwater river affected by extreme flooding events

AbstractTorrential rain and extreme flooding caused by Atlantic hurricanes mobilize a large pool of organic matter (OM) from coastal forested watersheds in the southeastern United States. However, the mobilization of toxic metals such as mercury (Hg) that are associated with this vast pool of OM are rarely measured. This study aims to assess the variations of total Hg (THg) and methylmercury (MeHg) levels and the isotopic compositions of Hg in a blackwater river (Waccamaw River, SC, U.S.A.) during two recent extreme flooding events induced by Hurricane Joaquin (October 2015) and Hurricane Matthew (October 2016). We show that extreme flooding considerably increased filtered THg and MeHg concentrations associated with aromatic dissolved organic matter. During a 2‐month sampling window each year (October–November), we estimate that about 27% (2015) and 78% (2016) of the average amount of Hg deposited atmospherically during these 2 months was exported via the river. The isotopic composition of Hg in the river waters was changed only slightly by the substantial inputs of runoff from surrounding landscapes, in which mass‐dependent fractionation (as δ202Hg) decreased from −1.47 to −1.67‰ and mass‐independent fractionation (as ∆199Hg) decreased from −0.15 to −0.37‰. The slight variations in Hg isotopic composition during such extreme flooding events imply that sources of Hg in the river are nearly unchanged even under the very high wet deposition of Hg derived from the intensive rainfall. The majority of Hg exported by the river (74–85%) is estimated to have been derived from dry deposition to the watersheds. An increase in frequency and intensity of Atlantic hurricanes is expected in the next few decades due to further warming of ocean surface waters. We predict that increased hurricanes will mobilize more dry‐deposited Hg and in situ produced MeHg from these coastal watersheds where MeHg can be extensively bioaccumulated and biomagnified in the downstream aquatic food webs.

Assessing mercury distribution using isotopic fractionation of mercury processes and sources adjacent and downstream of a legacy mine district in Tuscany, Italy

Mercury (Hg) concentrations and isotopic compositions in a range of sample types collected from the legacy Abbadia San Salvatore Mine (ASSM) area were used to evaluate the distribution of Hg in the region. The district generated more than 100,000 metric tons of Hg releasing extensive amounts of gaseous Hg emissions and producing large amounts of mine waste calcine from which Hg can be mobilized into the local and regional environments. Direct and indirect impact from cinnabar ore mining and processing resulted in elevated Hg concentrations in the calcine, soils, lake and stream sediments, and fish samples collected at and downstream of ASSM. The contribution of Hg from the different sources and processes resulted in a wide range of δ202Hg and Δ199Hg isotopic compositions (−2.25 to 0.96‰, −0.12 to 0.72‰, respectively). Fish samples resulted in negative values for δ202Hg (−0.53 to −1.21‰) and positive Δ199Hg (0.12–0.73‰). Primary cinnabar ore and present-day geothermal water and precipitate showed distinctly negative δ202Hg (−0.96 to −2.25‰), whereas waste calcines were enriched in 202Hg (δ202Hg from about −1 to +1‰); soils and sediments show intermediate compositions, reflecting different extents of contributions from the various Hg sources and processes. These sources and processes of Hg include weathering cinnabar, gaseous Hg emissions from ore processing, and geothermal activity to a lesser extent. Hg speciation in concert with Hg isotope ratios can better ascertain source attribution and assist in identifying Hg pathways into the environment to distinguish the more bioavailable forms of Hg entering the ecosystem.

Mercury isotope compositions in large anthropogenically impacted Pearl River, South China.

Rivers integrate natural and anthropogenic mercury (Hg), and are important vectors of terrestrial Hg to the oceans. Here, we report the total Hg concentration and Hg isotope compositions of dissolved load in the Pearl River, the second largest river in China, in order to understand the processes and sources affecting Hg systematics in large anthropogenically-impacted river water. The dissolved Hg showed a concentration varying from 0.45 to 2.44ng/L, within the range reported for natural background lake and river waters. All river water samples showed significantly negative δ202Hg (-2.89‰ to -0.57‰), slightly positive Δ200Hg (-0.05‰ to 0.52‰), and mostly positive Δ199Hg (0.10‰ to 0.57‰), except for three extremely negative values (-2.25‰ to -0.76‰). Combined with other geochemical parameters, we suggest that the influence of in-river processes, such as sorption and reduction, on the Hg isotope compositions is very limited, and the dissolved Hg in the Pearl River mainly comes from atmospheric precipitation and surface soil weathering. Although the whole river basin is largely affected by urban, industrial and mining activities, unlike other heavy metals, their direct contributions to dissolved Hg seem limited. It is worth noting that the three samples with very negative Δ199Hg values (down to -2.25‰) are derived from special source which attribute to the input of Hg released from the local incineration of electronic wastes. This study demonstrates that isotope approach is a powerful tool for tracing sources and pathways of Hg in large complex river systems.