Mercury Isotopic Compositions Research Articles

Mercury Stable Isotopes Reveal the Vertical Distribution and Trophic Ecology of Deep-Pelagic Organisms over the North-East Atlantic Ocean Continental Slope.

Deep-pelagic species are central to marine ecosystems and increasingly vulnerable to global change and human exploitation. To date, our understanding of these communities remains limited mainly due to the difficulty of observations, calling for complementary innovative tools to better characterize their ecology. We used mercury (Δ199Hg, δ202Hg, Δ201Hg, and Δ200Hg), carbon (δ13C), and nitrogen (δ15N) stable isotope compositions to segregate deep-pelagic species caught on the continental slope of the Bay of Biscay (NE Atlantic) according to their foraging depth and trophic ecology. Decreasing fish Δ199Hg values with corresponding depth estimates from the surface to down to 1,800 m confirmed that mercury isotopes are able to segregate deep species over a large vertical gradient according to their foraging depth. Results from isotopic compositions also identified different mercury sources, likely reflecting different trophic assemblages over the continental slope, in particular, the demersal influence for some species, compared to purely oceanic species. Overall, our results demonstrate how mercury stable isotopes can inform the vertical foraging habitat of little-known species and communities feeding in the deep.

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.

Deep sea cold seeps are a sink for mercury and source for methylmercury

The effect of seafloor cold seeps on the biogeochemical cycling of mercury (Hg) remains enigmatic. Here we demonstrate substantial enrichments of mercury and methylmercury, along with the presence of microbes capable of metabolizing mercury in sediments of the Haima cold seep, South China Sea, by analyzing mercury and methylmercury concentrations, mercury isotopic composition analyses and metagenomic analyses of sediment cores. Compared to the reference area, the sediments in the upper sediment column of the active-seep area were 2.4 times enriched in Hg and 10.5 times in methylmercury. The slope of the capital delta ratio of mercury 199 to mercury 201 (Δ199Hg/Δ201Hg) with 1.23 ± 0.10 in the active-seep area indicate the occurrence of dark redox reactions. Genes related to mercury methylation (hgcA), demethylation (merB) and reduction (merA) were phylogenetically associated with several bacterial and archaeal linages. We roughly estimated an additional 2,835 Mg mercury and 9 Mg methylmercury are stored in cold seep globally. In summary, we propose that cold seeps globally function as a previously unrecognized sink for mercury and source for methylmercury in the deep ocean.

Mercury Isotope Composition in Open Water Corals of South China Sea: Implication for Atmospheric Mercury Deposition Pathways Into Tropical Oceans

AbstractThe deposition of different atmospheric mercury (Hg) species into oceans determines the atmospheric Hg lifetime and the production of neurotoxin methylmercury. Yet, the relative contribution of atmospheric Hg(II) and Hg(0) is largely unconstrained. Here, we report the concentrations of total Hg and methylmercury, as well as Hg isotope composition in living corals collected from the tropical South China Sea (SCS). The results show that the Hg in corals is mainly present as inorganic Hg, exhibiting slightly negative δ202Hg and small yet significant Δ199Hg and Δ200Hg. These isotope features closely resemble those of pelagic waters, suggesting that shallow‐water corals that are widely distributed in oligotrophic waters could be used to trace atmospheric Hg deposition pathways. A mixture model based on coral Δ200Hg indicates that approximately 49% of the seawater Hg in the tropical SCS, a region characterized by high rainfall, originates from atmospheric gaseous Hg(0), much larger than previous estimates.

Insights into seasonal variations in mercury isotope composition of lichens

Lichens are commonly used to assess mercury (Hg) concentrations in air because of their cost-effectiveness. However, recent research has revealed temporal variations in the isotopic composition of Hg. Previous work on this topic leaves open questions about the repeatability of data over multiple seasons, different types of sampling (transplantation or in-situ collection), and diverse locations. This study aims to address these issues by conducting a high-frequency sampling campaign of in-situ and transplanted lichens and atmospheric particulate matter (APM). Sampling sites included a range of areas, from pristine to Hg-contaminated sites. Isotopic analysis showed that the isotopic composition of Hg in lichens undergoes mass-dependent fractionation and changes with time. The heaviest isotopic composition was observed in summer and the lightest in winter. These trends were consistent across polluted and unpolluted environments, as well as in both in-situ and transplanted lichens and in APM. The results further indicated towards a correlation between changes in Hg concentrations and isotopic composition in lichens and environmental factors. All of these variables seem to be changing at the same frequency and may have not just correlation but also causation relationship. Environmental factors seem to be influencing the Hg concentrations and isotopic composition. The summer high temperatures might be influencing the heavier isotopic fingerprint observed in lichens during the same season. Similarities with APM-bound Hg suggest a common underlying mechanism. This study highlights the importance of considering temporal and seasonal trends, as well as the method of lichen sampling, when interpreting results. Researchers using lichens as proxies for atmospheric Hg concentrations or isotope ratios should consider these findings when designing their studies.

Mercury abundance and isotopic composition in granitic rocks: Implications for Hg cycling in the upper continental crust

Mercury (Hg) is a highly volatile metal exhibiting both isotope mass-dependent (MDF, defined as δ202Hg) and mass-independent (MIF, expressed as Δ199Hg) fractionation. Assessing the Hg isotopic composition of the upper continental crust is crucial for the use of this new geochemical tool for petrogenetic tracing, particularly for understanding crust-mantle interaction. Here, we report Hg isotopic compositions of granitic rocks with distinct source affinity (I-, A-, and S-type) and of metasedimentary enclaves from the South China Craton. The results of more than 100 rock samples show large δ202Hg variations of −2.03 to 0.47 ‰ and small Δ199Hg variations of −0.16 to 0.09 ‰. The S-type granites studied here exhibit higher δ202Hg values (−0.58 to 0.00 ‰; n = 57) than their sedimentary protoliths (metasedimentary enclaves; −2.03 to −0.91 ‰; n = 7), suggesting significant Hg-MDF during magmatic processes. The Δ199Hg values of S-type granites (−0.08 to 0.09 ‰) are indistinguishable from their sedimentary protoliths (−0.16 to 0.02 ‰), suggestive of the absence of Hg-MIF during crustal anatexis and subsequent magmatic differentiation. The I- and A-type granites studied have smaller Δ199Hg variations of −0.11 to 0.04 ‰ (n = 28) and −0.07 to 0.04 ‰ (n = 15), respectively, but are within the range of the S-type granites. The compositional range of all granite variants may be explained by the variable intensity of large-scale MASH (melting-assimilation-storage-homogenization) processes. Our new data combined with previously published data of igneous, sedimentary, and metamorphic rocks allow us to estimate that the upper continental crust (UCC) has a Hg abundance of 13.5 ppb and a weighted average δ202Hg of −1.15 ± 1.01 ‰ (2SD), both of which are higher than the corresponding values of the primitive mantle. However, the weighted average Δ199Hg value of the UCC (0.03 ± 0.15 ‰; 2SD) is nondistinguished from the primitive mantle, suggesting no obvious Hg-MIF during the formation and differentiation of the continental crust.

Impact of forest fire on the mercury stable isotope composition in litter and soil in the Amazon

Mercury (Hg) emissions from forest fires, especially tropical forests such as the Amazonian forest, were shown to contribute significantly to the atmospheric mercury budget, but new methods are still necessary to improve the traceability and to reduce the great uncertainties related to this emission source. Recent studies have shown that the combustion process can result in Hg stable isotope fractionation that allows tracking coal combustion Hg emissions, as influenced by different factors such as combustion temperature. The main goal of the present study was, therefore, to investigate for the first time the potential of Hg stable isotopes to trace forest fire Hg emissions and pathways. More specifically, small-scale and a large scale prescribed forest fire experiments were conducted in the Brazilian Amazonian forest to study the impact of fire severity on Hg isotopic composition of litter, soil, and ash samples and associated Hg isotope fractionation pathways. In the small-scale experiment, no difference was found in the mercury isotopic composition of the samples collected before and after burning. In contrast, the larger-scale experiment resulted in significant mass dependent fractionation (MDF δ202Hg) in soils and ash suggesting that higher combustion temperature influence Hg isotopic fractionation with the emission of lighter Hg isotopes to the atmosphere and enrichment with heavier Hg in ashes. As for coal combustion, mass independent fractionation was not observed. To our knowledge, these results are the first to highlight the potential of forest fires to cause Hg isotopic fractionation, depending on the fire severity. The results also allowed to establish an isotopic fingerprint for tropical forest fire Hg emissions that corresponds to a mixture of litter and soil Hg isotopic composition (resulting atmospheric δ202Hg, Δ200Hg and Δ199Hg were −1.79 ± 0.24‰, −0.05 ± 0.04‰ and −0.45 ± 0.12‰, respectively).

Mercury isotope compositions in seawater and marine fish revealed the sources and processes of mercury in the food web within differing marine compartments.

Anthropogenic activities and climate change have significantly increased mercury (Hg) levels in seawater. However, the processes and sources of Hg in differing marine compartments (e.g. estuary, marine continental shelf (MCS) or pelagic area) have not been well studied, which makes it difficult to understand Hg cycling in marine ecosystems. To address this issue, the total Hg (THg) concentration, methylmercury (MeHg) concentration and stable Hg isotopes were determined in seawater and fish samples collected from differing marine compartments of the South China Sea (SCS). The results showed that the estuarine seawater exhibited substantially higher THg and MeHg concentrations than those in the MCS and pelagic seawater. Significantly negative δ202Hg (-1.63‰ ± 0.42‰) in estuarine seawater compared with that in pelagic seawater (-0.58‰ ± 0.08‰) may suggest watershed input and domestic sewage discharge of Hg in the estuarine compartment. The Δ199Hg value in estuarine fish (0.39‰ ± 0.35‰) was obviously lower than that in MCS (1.10‰ ± 0.54‰) and pelagic fish (1.15‰ ± 0.46‰), which showed that relatively little MeHg photodegradation occurred in the estuarine compartment. The Hg isotope binary mixing model based on Δ200Hg revealed that approximately 74% MeHg in pelagic fish is derived from atmospheric Hg(II) deposition, and over 60% MeHg in MCS fish is derived from sediments. MeHg sources for estuarine fish may be highly complex (e.g. sediment or riverine/atmospheric input) and further investigations are warranted to clarify the contribution of each source. Our study showed that Hg stable isotopes in seawater and marine fish can be used to identify the processes and sources of Hg in different marine compartments. This finding is of great relevance to the development of marine Hg food web models and the management of Hg in fish.

A 1500-year record of mercury isotopes in seal feces documents sea ice changes in the Antarctic

Temporal and spatial variations in sea ice coverage at high Northern Hemisphere latitudes have been shown to affect the photodegradation of methylmercury in seawater and the mercury isotope signatures in biological samples, suggesting the potential of mercury isotopes to reconstruct sea ice variability. Here we study the mercury isotopic composition of a 1500-year sediment profile strongly affected by seal activities on the Fildes Peninsula, King George Island, Antarctic Peninsula. The mass independent isotope fractionation of mercury (represented by Δ199Hg) in sediments dominated by seal feces input reflects the Δ199Hg of marine methylmercury before entering the food web, documenting the changes in the degree of photodemethylation. We found much higher Δ199Hg in sediments deposited during a warm period (~700-1000 years ago), suggesting that reduced sea ice promoted greater photodemethylation. Thus, this study demonstrates the modulation of methylmercury photodegradation by sea ice in the Antarctic, and that mercury isotopes can record historical sea ice changes.

Mercury isotopes trace historical mercury pollution from Pb[sbnd]Zn smelter in China

Lead–zinc (PbZn) smelting is one of the largest anthropogenic emission sources of mercury (Hg) in China and the world. In the last few decades, Hg emission control devices have been employed in PbZn smelters to reduce Hg emissions. To evaluate the effectiveness of Hg emission control, this study uses Hg isotopes to trace the sources of Hg in soils and reservoir sediments near the Zhuzhou smelter, the largest Zn production facility in China. The results showed that Hg concentrations in reservoir sediments were higher than those in soils at the three study sites, suggesting that reservoir sediments accumulate more Hg than soils do under the same conditions. Hg concentrations and deposition fluxes in reservoir sediments have dramatically decreased since the year 2000, suggesting the effectiveness of Hg emission control in the Zhuzhou smelter. Mercury isotope compositions in soil suggest binary mixing of Hg from the Zhuzhou smelter and the local background, and the contribution of Hg from the Zhuzhou smelter to soils and sediment is mainly a function of location and distance from the source. The model output suggested that approximately 61–75% of Hg in reservoir sediments was sourced from the Zhuzhou smelter. This study demonstrates that legacy Hg will continue to be a source to local ecosystems, even after Hg emission control has been implemented for decades.

Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran

The Ediacaran Period (~635–539 Ma) is marked by the emergence and diversification of complex metazoans linked to ocean redox changes, but the processes and mechanism of the redox evolution in the Ediacaran ocean are intensely debated. Here we use mercury isotope compositions from multiple black shale sections of the Doushantuo Formation in South China to reconstruct Ediacaran oceanic redox conditions. Mercury isotopes show compelling evidence for recurrent and spatially dynamic photic zone euxinia (PZE) on the continental margin of South China during time intervals coincident with previously identified ocean oxygenation events. We suggest that PZE was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but PZE may have also initiated negative feedbacks that inhibited oxygen production by promoting anoxygenic photosynthesis and limiting the habitable space for eukaryotes, hence abating the long-term rise of oxygen and restricting the Ediacaran expansion of macroscopic oxygen-demanding animals.

Changes in Atmospheric Gaseous Elemental Mercury Concentrations and Isotopic Compositions at Mt. Changbai During 2015–2021 and Mt. Ailao During 2017–2021 in China

AbstractChina is the largest contributor to the global total anthropogenic mercury (Hg) emissions. However, the trend in anthropogenic Hg emissions in recent years in China has not been effectively evaluated due to the lack of long‐term atmospheric Hg observations. This study documents the changes in atmospheric gaseous elemental mercury (GEM) concentrations and isotopic compositions at Mt. Changbai (MCB) in northeastern China during 2015–2021 and Mt. Ailao (MAL) in southwestern China during 2017–2021, and explores the potential factors controlling these changes. GEM concentrations showed continuous declines from 2015 to 2021 (−2.1 ± 0.6% yr−1) at MCB and from 2017 to 2021 (−4.0 ± 1.4% yr−1) at MAL. Accompanied with these GEM declines are positive shifts in δ202Hg (medians: from 0.42 to 0.46‰ at MCB and from 0.17 to 0.57‰ at MAL), and negative shifts in Δ199Hg (medians: from −0.17‰ to −0.21‰ at MCB and from −0.10‰ to −0.17‰ at MAL) and Δ200Hg values (medians: from −0.07‰ to −0.08‰ at MCB and from −0.03‰ to −0.05‰ at MAL) (at significant levels for Δ199Hg at MCB and δ202Hg and Δ199Hg at MAL). These changes were mainly caused by the decreases in regional anthropogenic emissions in the study areas. Based on a ternary mixing model with Δ199Hg and Δ200Hg as input, we estimate decline rates of 5.8 ± 2.8 and 4.8 ± 3.0% yr−1 for the regional anthropogenic GEM emissions in northeastern and southwestern China, respectively.

Volcanism intensity and associated climate-ocean-land dynamics during the Cryogenian interglaciation: Insights from mercury isotopes

The Cryogenian interglaciation (ca. 660 Ma to 650 Ma) was an interlude between the Sturtian glaciation (ca. 717 Ma to 660 Ma) and Marinoan glaciation (ca. 650 Ma to 635 Ma). Recent observations of anomalously high mercury (Hg) concentrations in Cryogenian interglacial sediments at basinal settings in the Nanhua Rift Basin, South China, imply that large volcanism can be a possible driving force of the Cryogenian interglaciation. To test this hypothesis and understand the potential linkage between large volcanism and climate-ocean-land dynamics during the Cryogenian interglaciation, we investigate mercury (Hg) concentrations and isotopic composition of Cryogenian interglacial sediments at shallow water (shelf/slope) settings in the Nanhua Rift Basin, South China. The basal Mn-bearing black shales show an increased pattern in Hg concentrations (318 to 4400 ppb) and Hg/TOC ratios (243 to 1730 ppb/wt%), and overall positive Δ199Hg values (0.00 to 0.10‰), suggesting increasing volcanic Hg input into the ocean via atmospheric Hg(II) deposition in the early Cryogenian interglaciation. The upper siltstone samples show a dramatic decrease in Hg concentrations (from 5110 to 459 ppb) and a drop of Δ199Hg values (from 0.04 to −0.05‰), suggesting weakened volcanism during the middle-late Cryogenian interglaciation. Combined with the analyses of major and trace elements proxies, we demonstrate that (1) greenhouse climate in the early Cryogenian interglaciation induced high oceanic productivity, high organic burial on the seafloor and high terrestrial sulfate input into the ocean, which favors the deposition of Mn-bearing black shales in the ocean; and (2) weakened volcanism and continued continental weathering exhausted atmospheric CO2 and drove the global climate to the Marinoan glaciation. This study, therefore, provides important insights into the climate-ocean-land dynamics during the Cryogenian interglaciation.

Mercury accumulation, distribution, and isotopic composition in tissues of the Collared Scops Owl (Otus lettia)

Mercury accumulation, distribution, and isotopic composition in tissues of the Collared Scops Owl (Otus lettia)

Mercury loss and isotope fractionation during thermal maturation of organic-rich mudrocks

Mercury (Hg) concentration and isotopic composition of organic-rich sedimentary rocks have been widely used as proxies to track ancient volcanism and associated environmental perturbations. However, interpretation of the Hg data is based on the assumption that the thermal maturity of the sedimentary rocks has limited effects on Hg abundance and isotopic composition, which is yet to be evaluated. Here we conduct closed-system anhydrous pyrolysis experiments to investigate the changes of Hg concentration and isotope composition during the thermal mature process, using both marine and lacustrine mudrocks. A siliceous shale section (Luocun, China) with heterogeneous thermal maturity caused by dike intrusion was also analyzed for Hg. Significant Hg loss (over 80%), with variations of δ202Hg (by 0.3 to 0.7‰) but small variations of Δ199Hg (by <0.1 to 0.36‰), were observed during the experiment. The Hg isotopic variation may be caused by the release of isotopically distinct Hg in different mineral phases at different temperatures. Samples from the Luocun section have anomalously low Hg and Hg/TOC values, due to thermal maturity. These findings suggest that (1) thermal maturation of organic-rich shales may be an important factor in distribution of Hg in sediments, and (2) the use of Hg, Hg/TOC and Hg isotopes proxies for paleoclimate and paleoceanography reconstruction should be applied in caution in substrates that may have experienced significant thermal alteration, especially if sill/dike intrusions were present that may cause large varieties of thermal maturity within sedimentary sections.

Mercury evidence of Deccan volcanism driving the Latest Maastrichtian warming event

Abstract The timing and ecological impacts of the Deccan Traps large igneous province eruption are vigorously debated. Pre–Cretaceous-Paleogene (KPg) boundary impacts of Deccan volcanism have been widely identified in marine sediments, but direct evidence of terrestrial impacts remains rare. We used mercury concentrations and isotopic compositions, a proxy for volcanic activity, to assess impacts on terrestrial environments. We studied two drill cores across the KPg boundary in eastern China that represent two different depositional environments: clastic deposits in the Jiaolai Basin and carbonate deposits in the Pingyi Basin. Both drill cores exhibit strong Hg enrichment prior to the KPg boundary. Near consistent mass-independent fractionation (MIF) of odd-Hg isotopes (odd-MIF) in the Jiaolai Basin likely indicates a volcanogenic source of Hg spikes below the KPg boundary. Odd-MIF isotopes in the Pingyi Basin likewise suggest a volcanogenic Hg source but with a terrestrial Hg signature of lower Δ199Hg values before and after the Hg spike interval. The Hg enrichment level can be stratigraphically correlated to the beginning of the Latest Maastrichtian warming event (LMWE) and is consistent with a strong, negative carbon-isotope excursion (CIE) in both δ13Corg (organic matter) and δ13Ccarb (carbonate), suggesting a disturbance of the global carbon cycle induced by a major pulse of Deccan Traps volcanism. Our discovery of a ter-restrial record of pre-KPg boundary Deccan volcanism provides robust evidence of global influence of the Deccan Traps large igneous province during the LMWE.

Tracking the legacy of early industrial activity in sediments of Lake Zurich, Switzerland: using a novel multi-proxy approach to find the source of extensive metal contamination

Historical industrial activities at the Horn Richterwil, on the shore of Lake Zurich (Switzerland), caused widespread metal contamination on land and in the adjacent lake sediments. This study provides an estimation of the age and source of the contamination by using XRF core scanning, ICP-OES, and Hg-AFS for quantitative measurements of trace metals and MC-ICP-MS for the stable isotope analysis of mercury. Radiometric dating (^{137}Cs, ^{210}Pb, and Pu dating) of two proximal cores and varve chronology in a distal core suggest two different contaminations, one stemming from around 1960 (Zn, Cd) and an earlier one from 1880 (Cr, Cu, Hg, Pb, Sn). The XRF data suggest two different contamination pathways: one by landfill of contaminated soil and another one by industrial wastewater effluents. Maximum concentrations found within all samples are in the range of per mil (dry weight) for Cr, Cu, Hg, Pb, Sn, and Zn and lie within the top 10 cm of the sediment cores. The analysis of the mercury isotopic composition (delta ^{202}Hg and Delta ^{199}Hg) shows a significantly different signature for one of the cores, indicating a second mercury source. We could not identify the exact source or process leading to the isotopic fractionation of mercury, but the isotopic data confirm two different sources.

Mercury isotopic composition of igneous rocks from an accretionary orogen: Implications for lithospheric recycling

Abstract Mercury (Hg) provides critical information on terrestrial planet formation and evolution due to its unique physicochemical properties and multiform isotopic compositions. Current knowledge of Hg is mainly limited to Earth's surface environments, and the understanding of Hg in the Earth's interior remains unclear. Accretionary orogens are major settings for continental crustal growth and crust-mantle interactions. We studied the Hg concentration and isotopic composition of igneous rocks in the eastern Central Asian orogenic belt, using Hg as a proxy to trace the recycling of surface materials in Earth's lithosphere. Our results show low Hg abundances in mafic through felsic igneous rocks (4.93 ± 4.35 ppb, standard deviation [SD], n = 267). Mafic rocks show slightly lower δ202Hg (−2.9‰ ± 0.5‰, SD, n = 24) than intermediate (−2.4‰ ± 0.8‰, SD, n = 58) and felsic (−1.5‰ ± 0.8‰, SD, n = 185) rocks, indicating a chemical stratification of Hg isotopic composition in the continental crust with isotopically lighter Hg in the lower part and heavier Hg in the upper part. Slightly positive Δ199Hg values are observed in mantle-derived mafic (0.07‰ ± 0.06‰, SD) and intermediate (0.06‰ ± 0.07‰, SD) rocks, which agree well with those reported for marine sediments, indicating the involvement of fluids or melts from the oceanic crust. Larger variations of Δ199Hg values (−0.26‰ to +0.21‰, average: 0.01‰ ± 0.08‰, SD, n = 185) are observed in felsic rocks, further indicating recycling of surface Hg from the marine reservoir via slab subduction (reflected by positive values) plus magmatic assimilation of terrestrial Hg (reflected by negative values). Our study demonstrates that Hg isotopes can be a promising tracer for the chemical dynamics of Earth's lithosphere.

Mercury isotopic compositions of the Precambrian rocks and implications for tracing mercury cycling in Earth's interior

Mercury isotopes undergo unique mass-independent fractionation (MIF) during photochemical processes on Earth's surface. Studies have observed pronounced Hg-MIF signals in sedimentary and magmatic rocks, suggesting recycling of Hg from Earth's surface systems into the lithosphere via sedimentation and magmatism. However, the isotopic signature of Hg in metamorphic rocks and the geochemical fate of Hg during metamorphism remain unclear. Precambrian basements are important components of cratons or orogenic belts on Earth. Here, we study the Hg concentration and isotopic composition of Precambrian metamorphic and sedimentary rocks from the eastern Central Asian Orogenic Belt, and North and South China cratons. Metamorphic rocks show much lower Hg contents (0.21–7.8 ppb) than sedimentary rocks (2.6–694 ppb), indicating a substantial loss of Hg during metamorphism. The lack of correlation between δ202Hg values (–2.41 to 0.18‰) and metamorphic grades indicates no systematic mass-dependent fractionation (MDF) of Hg isotopes during metamorphism. The Δ199Hg/Δ201Hg ratios of ∼ 1.0 for both metamorphic and sedimentary rocks indicate Hg was sourced from Earth's surface systems. The coupling of Hg-MIF signals between the metasedimentary rocks and the sedimentary settings of their protolith suggests no Hg-MIF during metamorphism. The negative Δ199Hg values (–0.30 to –0.02‰) in the Precambrian coastal sedimentary rocks imply the input of Hg into coastal regions via soil erosion. The positive Δ199Hg values (0.06 to 0.31‰) in the Precambrian marine sedimentary rocks suggest deposition of atmospheric Hg(II) to open oceans via wet deposition. The lack of significant Hg-MIF during metamorphism and other underground geological processes shows that Hg-MIF signals can work as a reliable tracer for indicating material cycling in Earth's interior.

Dynamics of Dietary Mercury Determined by Mercury Speciation and Isotopic Composition in Dicentrarchus labrax

Seafood has a great ecological and nutritional value for human and wildlife communities. However, accumulation of mercury (Hg) in fish is a concern to animal and human health. There is a crucial need to understand Hg speciation in marine organisms through controlled feeding experiments. This study represents a first assessment of the biological processes that may influence Hg bioaccumulation and dynamics in a marine predatory fish. We conducted a feeding experiment to investigate the dynamics of MeHg and iHg, as well as Hg isotopes in the liver and muscles of captive juvenile seabass (Dicentrarchus labrax). Three groups of juvenile seabass were fed in captivity during 3 weeks of acclimatization and 6 weeks of experiment. Each group was fed with pellets containing environmentally relevant MeHg concentrations (Control, 200 and 500 ng g−1 dw). We monitored the evolution of MeHg and iHg concentrations as well as Hg isotopic values in liver and muscle. We determined Hg dynamics with respect to the contamination level in the fish diet. Muscle δ202Hg and Δ199Hg turnover rates ranged between 33 and 14 days (Low diet) to 5 and 9 days (Mod diet). Liver δ202Hg and Δ199Hg turnover rates ranged between 3 and 7 days (Low diet) to 3 and 2 days (Mod diet), respectively. Hg species concentrations and δ202Hg varied over time between diet groups and tissues, showing the occurrence of internal mass-dependent fractionation (MDF). No significant intra-tissue and temporal Hg mass-independent fractionation (MIF) was observed. The results of our experiment are strongly in favor of the existence of MeHg demethylation in a coastal predatory fish exposed to low to moderate concentrations of environmental Hg. The decrease over time of δ202Hg in muscle of seabass from the most contaminated diet was accompanied by a temporal increase in iHg, pointing to possible Hg detoxification processes occurring in this tissue when dietary Hg exposure is high. The absence of Hg MDF and different turnover between muscle and liver in seabass exposed to 500 ng Hg g−1 confirmed that Hg speciation and bioaccumulation in juvenile fish are controlled by Hg levels and speciation in their diet.