Hg Isotopes Research Articles

Appalachian Basin mercury enrichments during the Late Devonian Kellwasser Events and comparison to global records

The Late Devonian Kellwasser Events are two major biodiversity crises recorded at and just below the Frasnian-Famennian stage boundary for which the ultimate cause or causes of diversity depletion remain unclear. The discovery of anomalous mercury (Hg) enrichments in Upper Kellwasser Event (UKE) strata has led to increased interest in a potential volcanic trigger for one or both events. However, UKE Hg anomalies are found mainly on the continental shelves of the Palaeotethys Ocean, and Hg isotope values from at least one distal basin indicate a non-volcanic Hg source. These findings suggest volcanism during the UKE may have had regional rather than global impacts. In comparison to the UKE, few works have investigated the sedimentary Hg record of the Lower Kellwasser Event (LKE). Here, we contribute new Hg data from the LKE of the Appalachian Basin and compare this to a compilation of global Hg data from both events. The Appalachian Basin contains no sedimentary Hg enrichments consistent with massive volcanism during the LKE, and Hg concentrations are strongly correlated with total organic carbon. We argue that organic matter drawdown exerted a primary control upon Hg concentration in the Appalachian Basin. While basins more proximal to the Palaeotethys Ocean preserve potentially volcanogenic LKE Hg anomalies, the absence of such anomalies here reflects volcanism during the LKE that was at most only regionally significant, as hypothesized for the UKE. Volcanism is therefore an unlikely trigger of the global Kellwasser diversity depletions. Broader sampling of the Kellwasser record will assist in the development of an extinction model that accounts for variations in global Hg concentrations, as the depressed origination rates, prolonged background environmental stress, and variations in intra-basinal expression that characterize both events may indicate that the trigger of diversity loss in this interval is unique among the “Big Five” mass extinctions.

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.

Paleo‐Pacific Plate Subduction and Basement Mobilization Triggered Large‐Scale Formation of Mesozoic Gold Deposits in the Northern Margin of the North China Craton

AbstractThe northern margin of the North China Craton (NNCC) hosts Mesozoic gold deposits of significant economic importance. The metal sources of these gold systems have long been debated with mainly magmatic‐hydrothermal versus metamorphic fluid models. Mercury (Hg) isotopes, which undergo unique mass‐independent fractionation, can provide important insights into the source of metals in gold deposits due to the close association between Hg and Au in such systems. Here, we investigated the Hg isotopic composition of six gold deposits of the Middle Jurassic to Early Cretaceous age and potential source rocks in the NNCC. Variable ∆199Hg values were observed in bulk ore and pyrite samples (−0.28‰ to 0.34‰, n = 51) and coeval granites (−0.21‰ to 0.13‰; n = 25). The negative ∆199Hg values of bulk ore and pyrite samples mostly agree with that observed for metamorphic basement rocks (−0.37‰ to 0.11‰; n = 32). The positive ∆199Hg values of bulk ore and pyrite samples agree with those reported in marine sediments (0‰–0.3‰). So the data suggest binary mixing of ocean‐recycled Hg originated from the subducted oceanic slab, and terrestrial‐recycled Hg from the Precambrian basement in Mesozoic gold deposits. The contribution of Hg (and Au, by analogy) from both reservoirs varies depending on active continental arc versus intracontinental setting and is ultimately controlled by translithospheric heat flow driven by paleo‐Pacific plate subduction.

Using Mercury Stable Isotopes to Quantify Bidirectional Water-Atmosphere Hg(0) Exchange Fluxes and Explore Controlling Factors.

In this study, exchange fluxes and Hg isotope fractionation during water-atmosphere Hg(0) exchange were investigated at three lakes in China. Water-atmosphere exchange was overall characterized by net Hg(0) emissions, with lake-specific mean exchange fluxes ranging from 0.9 to 1.8 ng m-2 h-1, which produced negative δ202Hg (mean: -1.61 to -0.03‰) and Δ199Hg (-0.34 to -0.16‰) values. Emission-controlled experiments conducted using Hg-free air over the water surface at Hongfeng lake (HFL) showed negative δ202Hg and Δ199Hg in Hg(0) emitted from water, and similar values were observed between daytime (mean δ202Hg: -0.95‰, Δ199Hg: -0.25‰) and nighttime (δ202Hg: -1.00‰, Δ199Hg: -0.26‰). Results of the Hg isotope suggest that Hg(0) emission from water is mainly controlled by photochemical Hg(0) production in water. Deposition-controlled experiments at HFL showed that heavier Hg(0) isotopes (mean ε202Hg: -0.38‰) preferentially deposited to water, likely indicating an important role of aqueous Hg(0) oxidation played during the deposition process. A Δ200Hg mixing model showed that lake-specific mean emission fluxes from water surfaces were 2.1-4.1 ng m-2 h-1 and deposition fluxes to water surfaces were 1.2-2.3 ng m-2 h-1 at the three lakes. Results from the this study indicate that atmospheric Hg(0) deposition to water surfaces indeed plays an important role in Hg cycling between atmosphere and water bodies.

Timing and Provenance of Volcanic Fluxes Around the Permian‐Triassic Boundary Mass Extinction in South China: U‐Pb Zircon Geochronology, Volcanic Ash Geochemistry and Mercury Isotopes

AbstractAnomalous mercury (Hg) contents recorded near the Permian‐Triassic boundary (PTB) are often linked to Siberian Traps Large Igneous Province (STLIP) volcanism and the Permian‐Triassic boundary mass extinction (PTBME). However, mounting evidence indicates that the relation between STLIP volcanism and Hg “anomalies” is not straightforward. This study focuses on the timing and provenance of volcanic fluxes around the PTBME in South China. We constrain carbon isotope (δ13C) and Hg concentration and isotope records by utilizing high‐precision U‐Pb zircon ages from two expanded deep‐water marine sections spanning the Late Permian to Early Triassic in the Nanpanjiang Basin. Results reveal two episodes of Hg enrichment. The oldest episode predates the onset of a large negative δ13C excursion, which is documented to be older than 252.07 ± 0.130 Ma. The second episode occurred between 251.822 ± 0.060 and 251.589 ± 0.062 Ma, coinciding with the nadir of the δ13C excursion. Volcanic ash geochemistry and Hg isotope compositions suggest that mercury was mainly sourced from subduction‐related volcanic arc magmatism in the Tethys region, which peaked between 251.668 ± 0.079 and 251.589 ± 0.052 Ma. These results are compatible with suggestions that regional arc volcanism contributed to the causes of the PTBME in South China and provide evidence that Hg anomalies close to the PTB are not a reliable stratigraphic marker for the PTB extinction event. This study demonstrates that relations between volcanism, environmental perturbations and mass extinction during the Permian‐Triassic transition are better resolved with the aid of high‐precision U‐Pb zircon ages.

Highly fractionated Hg isotope evidence for dynamic euxinia in shallow waters of the Mesoproterozoic ocean

Studies of Mesoproterozoic ocean chemistry have converged on a widely accepted scenario where a stable shallow layer of oxic waters occurred over deeper anoxic ferruginous waters, with mid-depth euxinic waters restricted to continental margins. However, several lines of evidence suggest shallow waters may not always have remained oxic, but fluctuated between oxic, anoxic, and euxinic conditions. Here, we present new Hg-C-S geochemistry data of ca. 1.5–1.4 Ga sediments in the Yanliao Basin to better understand the redox structure of shallow water environments, especially for the photic zone. In this study, positive Δ199Hg values suggest atmospheric Hg(II) deposition into oxic surface waters, whereas abnormally negative Δ199Hg values indicate enhanced sequestration of gaseous Hg(0) and photo-reduction of reduced sulfur-bound Hg(II) in response to photic zone euxinia. Our highly fractionated Δ199Hg values reveal a new scenario, in which the extent of euxinia is more dynamic than generally thought, and may expand from the mid-depth to the photic zone, likely linked to the superimposed operation of enhanced oxidative weathering input of SO42− and high OM flux in the water column. Given that euxinic waters may accelerate the sequestration of bio-essential elements, it is suggested that the dynamic euxinia of shallow water played a significant role in delaying the diversification of eukaryotes in the Mesoproterozoic oceans. Our study also highlights that Hg isotopes may act as a promising new proxy to trace redox changes in the surface ocean over geological time.

Low-δ18O and negative-Δ199Hg felsic igneous rocks in NE China: Implications for Early Cretaceous orogenic thinning

Magmatic rocks carry key information to understand the processes of material cycling in the Earth's interior and record crucial clues to reconstruct the important tectonic evolution of the Earth. Therefore, it is of great importance to constrain the magma source (such as origin from crust or mantle, oceanic crust or continental crust, lower or upper continental crust) accurately, which however remains to be unraveled because of the variety magmatic sources and complex magmatic processes. High-precision stable isotopic analyses provide new tools to decipher the genesis of igneous rocks and to reconstruct their associated tectonic settings. Here, we report a pioneering work using integrated O-Hg-Hf-Nd isotopes to understand the genesis of the Early Cretaceous A-type granites and rhyolites from the Paleozoic-Mesozoic accretionary orogen in NE China. Low δ18O zircon values (3.2–5.7‰) of the A-type rocks reveal a magma source affected by meteoric fluids. Their whole-rock negative Δ199Hg values (−0.24 to −0.01‰), which are similar with the terrestrial systems but clearly different from the marine systems, indicate that hydrothermal alteration took place in the continental setting. Positive zircon εHf(t) (4.2–10.7) and mainly positive whole-rock εNd(t) (−0.5 to 2.4, mean = 1.2 ± 1.0, 1SD) values indicate that the magma source was dominated by juvenile crustal materials. This study suggests an elevated geothermal gradient due to the lithospheric thinning triggered by rollback and foundering of a Paleo-oceanic slab. The specific thermal anomaly caused the remelting of the altered upper continental crust, resulting in the formation of the A-type granites and rhyolites, with the characters of low-δ18O and negative-Δ199Hg values. Our study shows that Hg isotope, coupled with O and NdHf isotopes, is a promising tool for petrogenetic studies.

Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.

Mercury Biogeochemical Cycle in Yanwuping Hg Mine and Source Apportionment by Hg Isotopes.

Although mercury (Hg) mining activities in the Wanshan area have ceased, mine wastes remain the primary source of Hg pollution in the local environment. To prevent and control Hg pollution, it is crucial to estimate the contribution of Hg contamination from mine wastes. This study aimed to investigate Hg pollution in the mine wastes, river water, air, and paddy fields around the Yanwuping Mine and to quantify the pollution sources using the Hg isotopes approach. The Hg contamination at the study site was still severe, and the total Hg concentrations in the mine wastes ranged from 1.60 to 358 mg/kg. The binary mixing model showed that, concerning the relative contributions of the mine wastes to the river water, dissolved Hg and particulate Hg were 48.6% and 90.5%, respectively. The mine wastes directly contributed 89.3% to the river water Hg contamination, which was the main Hg pollution source in the surface water. The ternary mixing model showed that the contribution was highest from the river water to paddy soil and that the mean contribution was 46.3%. In addition to mine wastes, paddy soil is also impacted by domestic sources, with a boundary of 5.5 km to the river source. This study demonstrated that Hg isotopes can be used as an effective tool for tracing environmental Hg contamination in typical Hg-polluted areas.

Inconsistent mercury records from terrestrial upland to coastal lowland across the Permian-Triassic transition

The enrichment of mercury (Hg) in sediments has been increasingly utilized as a proxy for large-scale volcanism. However, a causal link between these two remains ambiguous, as Hg enrichments can be caused by many non-volcanic factors, e.g., changes in local depositional environments or terrestrial runoffs. In this study, we present high-resolution Hg concentration and isotope records of two drill cores, one located in a coastal lowland (HK-1) and the other in a terrestrial upland (TK-1), spanning the Permian–Triassic boundary (PTB). Interestingly, the coastal lowland core exhibits significant Hg enrichments over a stratigraphic horizon of approximately 200 meters (corresponding to a duration of ∼2 Myrs), with dominating negative Δ199Hg values. In contrast, the terrestrial upland core primarily displays background Hg levels with near-zero Δ199Hg values. Given the spatial proximity of the two cores, separated by only about 70 km, we interpret these discrepant Hg signatures to indicate that terrestrial upland underwent minimal influence from terrestrial and/or atmospheric depositions rather than volcanic input, while the coastal lowland was primarily sourced from enhanced terrestrial influxes, depositing under oxygen-deficient conditions. This interpretation is supported by the high concentrations of organic carbon and sulfur, substantial pyrite layers, and positive correlations between Hg and chalcophile elements, such as Mo, Se, and Co, in the HK-1 core. Our study highlights the need to examine multiple Hg enrichment factors in sedimentary records before comprehensively establishing a robust causal link with volcanic activities. A thorough evaluation of multiple lines of evidence, including Hg isotopes, local environmental proxies, and sedimentology investigations, should be conducted first, particularly necessary for the non-marine sediments.

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

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

Mercury isotope evidence for recurrent photic-zone euxinia triggered by enhanced terrestrial nutrient inputs during the Late Devonian mass extinction

Widespread oceanic anoxia marked by globally extensive deposition of organic-rich black shale during the Late Devonian was a major factor in the mass extinctions at the Frasnian-Famennian (FFB, ∼372 million years ago) and Devonian-Carboniferous boundaries (DCB, ∼359 million years ago), although the triggers for these deoxygenation events are still under debate. Here, we apply a novel paleoredox proxy, Hg isotopes, to investigate Late Devonian ocean redox variation and its causes. We found no Hg enrichments in North America across either the FFB or DCB, thus arguing against the hypothesis of global-scale volcanism as the trigger for Late Devonian environmental and biotic crises. Gradual negative shifts of both mass-independent fractionation (Δ199Hg) and mass-dependent fractionation (δ202Hg) occurred between the FFB and DCB, suggesting a progressive increase of Hg inputs associated with terrestrial organic matter. Moreover, multiple abrupt negative excursions of Δ199Hg (down to −0.19‰) along with concurrent positive shifts of δ202Hg occurred just above the FFB and across the DCB, providing strong evidence for recurrent photic-zone euxinia (PZE) that was preceded by increasing terrestrial inputs in the epicontinental seas of North America. We suggest that the increase of terrestrial inputs of nutrients, probably via expansion of vascular land plants, stimulated marine primary productivity and eventually PZE, which may have been a key kill mechanism for the Late Devonian mass extinction.

Alkaline extraction: An optimal approach for extracting methylmercury from paddy soils

Accurately measuring the concentration of methylmercury (MeHg) is a critical part of Hg research. While analytical methods of MeHg have not been validated for paddy soils, which are one of the most important and active sites of MeHg production. Here we compared two methods most widely used to extract MeHg from paddy soils, i.e., CuSO4/KBr/H2SO4-CH2Cl2 (referred to as acid extraction) and KOH-CH3OH (referred to as alkaline extraction). By evaluating the formation of MeHg artifact using Hg isotope amendments and quantifying the extraction efficiency using the standard spike in 14 paddy soils, we propose that alkaline extraction is an optimal choice for paddy soils, with negligible MeHg artifact (accounting for 0.62–8.11 % of the background MeHg) and consistently high extraction efficiency (81.4–114.6 % for alkaline extraction compared with 21.3–70.8 % for acid extraction). Our finding highlights the importance of suitable pretreatment and appropriate quality controls during the measurement of MeHg concentrations.

Mercury isotopes show vascular plants had colonized land extensively by the early Silurian

The colonization and expansion of plants on land is considered one of the most profound ecological revolutions, yet the precise timing remains controversial. Because land vegetation can enhance weathering intensity and affect terrigenous input to the ocean, changes in terrestrial plant biomass with distinct negative Δ199Hg and Δ200Hg signatures may overwrite the positive Hg isotope signatures commonly found in marine sediments. By investigating secular Hg isotopic variations in the Paleozoic marine sediments from South China and peripheral paleocontinents, we highlight distinct negative excursions in both Δ199Hg and Δ200Hg at Stage level starting in the early Silurian and again in the Carboniferous. These geochemical signatures were driven by increased terrestrial contribution of Hg due to the rapid expansion of vascular plants. These excursions broadly coincide with rising atmospheric oxygen concentrations and global cooling. Therefore, vascular plants were widely distributed on land during the Ordovician-Silurian transition (~444 million years), long before the earliest reported vascular plant fossil, Cooksonia (~430 million years).

National-Scale Assessment of Total Gaseous Mercury Isotopes Across the United States.

With the 2011 promulgation of the Mercury and Air Toxics Standards by the U.S. Environmental Protection Agency, and the successful negotiation by the United Nations Environment Program of the Minamata Convention, emissions of mercury (Hg) have declined in the United States. While the declines in atmospheric Hg concentrations in North America are encouraging, linking the declines to changing domestic and global source portfolios remains challenging. To address these research gaps, the U.S. Geological Survey initiated the first national-scale effort to establish a baseline of total gaseous mercury stable isotope values at 31 sites distributed across the United States. Results indicated that unique Hg sources, such as Hg evasion from an elemental Hg contaminated site or free tropospheric intrusions in high altitude sites, were distinguishable from background atmospheric values. Minor gradients were observed across the nation, with regions of heavy industrial activity demonstrating lower , but no consistent changes in other isotopes such as and were observed. Furthermore, was impacted by foliar uptake and senescence but trends varied between forested regions in the northeastern and midwestern United States. These data demonstrate regional emission sources and other environmental variables can impact total gaseous Hg (TGM) isotope values, highlighting the need to characterize atmospheric Hg isotopes over larger geographical areas to evaluate changes related to national and international Hg regulations.

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs.

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ199Hg values. These Δ199Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

Aquatic methylmercury is a significant subsidy for terrestrial songbirds: Evidence from the odd mass-independent fractionation of mercury isotopes

In contrast to aquatic food chains, knowledge of the origins and transfer of mercury (Hg) and methylmercury (MeHg) in terrestrial food chains is relatively limited, especially in songbirds. We collected soil, rice plants, aquatic and terrestrial invertebrates, small wild fish, and resident songbird feathers from an Hg-contaminated rice paddy ecosystem for an analysis of stable Hg isotopes to clarify the sources of Hg and its transfer in songbirds and their prey. Significant mass-dependent fractionation (MDF, δ202Hg), but no mass-independent fractionation (MIF, ∆199Hg) occurred in the trophic transfers in terrestrial food chains. Piscivorous, granivorous, and frugivorous songbirds and aquatic invertebrates were all characterized by elevated Δ199Hg values. The estimated MeHg isotopic compositions obtained using linear fitting and a binary mixing model explained both the terrestrial and aquatic origins of MeHg in the terrestrial food chains. We found that MeHg from aquatic habitats is an important subsidy for terrestrial songbirds, even those that feed mainly on seeds, fruits, or cereals. The results show that MIF of the MeHg isotope is a reliable tool to reveal MeHg sources in songbirds. Because the MeHg isotopic compositions was calculated with a binary mixing model or directly estimated from the high proportions of MeHg, compound-specific isotope analysis of Hg would be more useful for the interpretation of the Hg sources, and is highly recommended for future studies.

Large-scale basement mobilization endows the giant Carlin-type gold mineralization in the Youjiang Basin, South China: Insights from mercury isotopes

The metal source and genesis of hydrothermal mercury-rich gold metallogenic systems occurring far away from active continental margins remain puzzling. The Youjiang Basin of South China, where exists numerous Carlin-type gold deposits and some synmineralization hidden intrusions, is a natural laboratory to address this issue due to it was up to 1000 km inward from the active continental margins of South China when mineralization. Here, we use mass-independent fractionation of mercury isotope ratios (reported as ∆199Hg), which is predominantly generated during Hg photochemical reactions on Earth’s surface and has superiority of isotopic inheritance during hydrothermal processes, to address the metal source of the Youjiang Carlin-type gold deposits. Ore-associated sulfides from seven representative deposits display negative to near-zero ∆199Hg values (−0.29‰ to 0.04‰), which fall in between that of the regional Precambrian basement rocks (−0.21‰ to 0.06‰) and deep magmatic-hydrothermal systems (∼0‰), suggesting a binary mixing of Hg from these two sources. An isotope mixing model and mass balance calculations demonstrate that ∼1000 km3 of the basement rocks, which contributed to 86% of Hg budget, were leached and remobilized by magmatic-hydrothermal fluids and deep-circulating crustal fluids to endow the gold reserves of these deposits. Given that traditional S, Pb, C, and O isotopic data yielded indirect and ambiguous constraints on metal source due to their complex evolution processes and isotope fractionation during the fluids ascended. Our results, therefore, highlight the great advantage of using Hg isotope as a new tracer to understand metal sources of hydrothermal deposits.

Isotopic Fractionation Characteristics of Speciated Mercury from Local Biomass Combustion in the Tibetan Plateau.

As the Third Pole of the world, the Tibetan Plateau (TP) is sensitive to anthropogenic influences. Biomass combustion is one of the most important anthropogenic sources of mercury (Hg) emissions in the TP. However, due to the lack of knowledge about Hg emission characteristics and activity levels in the plateau, atmospheric Hg emissions from biomass combustion in the TP are under large uncertainties. Here, based on pilot-scale experiments, we found that particle-bound mercury (PBM; mean of 83.1-87.7 ng/m3) occupied 17.93-49.31% of the total emitted Hg and the PBM δ202Hg values (average -1.65‰ to -0.77‰) were significantly higher than those of the corresponding feeding biomass. The Δ200Hg values of total gaseous mercury and PBM were more negative (-0.08‰ to -0.05‰) than other anthropogenic emissions, providing unique isotopic fingerprints for this sector. Together with the investigated local activity levels, Hg emissions from biomass combustion reached 402 ± 74 kg/a, which were dozens of times higher than previous estimates. The emissions were characterized by conspicuous spatial heterogeneity, concentrated in the northern and central TP. Specialized Hg emissions and the Hg isotope fingerprint of local biomass combustion can aid in evaluating the influence of this sector on the fragile ecosystems of the TP.

Chemostratigraphy and source of mercury in the Tropical Western Pacific over the past 600 kyr

To characterize the geochemical records of mercury (Hg) in the western tropical Pacific in the last 600 ka, the total concentration of Hg, and its isotopic compositions in Hole B at Site U1486 from the Western Pacific Warm Pool (WPWP) were determined. Some marked positive values of Δ199Hg in this core, and some Δ200Hg also exhibit significant positive, which resultes from the photoreduction of the volcanic Hg deposits through the long-range atmospheric transportation. In addition, dual odd-even Hg isotopes (Δ199Hg and Δ200Hg) database was also compiled to account for quantitative contributions to potential Hg sources, including marine Hg source, atmospheric Hg deposition, volcanic Hg source, and terrigenous Hg inputs. This study presents that sedimentary Hg in the core Hole B at Site U1486 originates mainly from the marine Hg source with a mean of 64.13%, which is similar to the changes of paleo-productivity in the WPWP with a character of the glacial-interglacial variation. The contributions of atmospheric Hg deposition, volcanic Hg source, and terrigenous Hg inputs represented approximately 17.88%, 10.37%, and 7.87% of the total Hg burden in this core, respectively. During the last 100 ka, the contribution of volcanic Hg source has increased overall, suggesting the increase of volcanic activity. And the amount of atmospheric Hg deposition in this core also generally increased during this period, which was in resonance with the eolian dust in the western Pacific. Furthermore, terrigenous Hg inputs in this study could be related to intensity of monsoon precipitation with an overall increasing trend. In this study, volcanic activities in sediment cores from the western Pacific can provide an additional and independent tool for the precise dating and synchronization of paleoenvironmental changes in these climatically significant regions.