Mass-independent Fractionation Research Articles

A Catalyst Tube-Equipped Dual-Stage Tube Furnace System for Accurate Hg Isotopic Determination of Ore Samples Using Neptune Plus Multicollector Inductively Coupled Plasma Mass Spectrometry.

Mercury (Hg) isotopes, which display mass-dependent fractionation and mass-independent fractionation, provide a multidimensional tracer to decipher the source of metals in mineral deposits. However, mineral ore samples usually contain abundant interfering elements (e.g., Te) that can cause inaccurate Hg isotopic analysis. Available acid digestion and combustion methods failed to remove these interfering elements, hindering the application of Hg isotopes for metallogenetic tracing. Here, we developed a new dual-stage tube furnace system employing a Mn-containing catalyst tube to pretreat mineral ore samples. This method yielded good Hg recoveries (100.5 ± 3.8%, 1SD, n = 15) and low levels of interfering elements in sample solutions, allowing for accurate analysis of a series of ore standard reference materials (GBW-11108v: coal; GSO-3: Cu-Ag sulfide ore; GBW 07859: Au-Te sulfide ore). The new method was also successfully applied to measure the Hg isotopic composition of magmatic and hydrothermal ore deposits, which yielded a large range in Δ199Hg value (-0.19 to 0.22‰) for ore deposits formed in different geological settings, highlighting the future applications of this method for metallogenic tracing, especially tracing the source of metals in mineral ore deposits.

Phasing and climate forcing potential of the Millennium Eruption of Mt. Baekdu

The Millennium Eruption of Mt. Baekdu, one of the largest volcanic eruptions in the Common Era, initiated in late 946. It remains uncertain whether its two main compositional phases, rhyolite and trachyte, were expelled in a single eruption or in two. Investigations based on proximal and medial ash have not resolved this question, prompting us to turn to high-resolution ice-core evidence. Here, we report a suite of glaciochemical and tephra analyses of a Greenlandic ice core, identifying the transition from rhyolitic to trachytic tephra with corresponding spikes in insoluble particle fallout. By modeling annual snow accumulation, we estimate an interval of one to two months between these spikes, which approximates the hiatus between two eruptive phases. Additionally, negligible sulfur mass-independent fractionation, near-synchroneity between particle and sulfate deposition, and peak sulfur fallout in winter all indicate an ephemeral aerosol veil. These factors limited the climate forcing potential of the Millennium Eruption.

Mercury isotope fractionation and mercury source analysis in coal

Understanding the sources of mercury (Hg) in coal is crucial for understanding the natural Hg cycle in the Earth's system, as coal is a natural Hg reservoir. We conducted analyses on the mass-dependent fractionation (MDF), reported as δ202Hg, and mass-independent fractionation (MIF), reported as Δ199Hg, of Hg isotopes among individual Hg species and total Hg (THg) in Chinese coal samples. This data, supplemented by a review of prior research, allowed us to discern the varying trend of THg isotope fractionation with coal THg content. The Hg isotopic composition among identified Hg species in coal manifests notable disparities, with species exhibiting higher thermal stability tending to have heavier δ202Hg values, whereas HgS species typically display the most negative Δ199Hg values. The sources of Hg in coal are predominantly attributed to Hg accumulation from the original plant material and subsequent input from hydrothermal activity. Hg infiltrates peat swamps via vegetation debris, thus acquiring a negative Δ199Hg isotopic signature. Large-scale lithospheric Hg recycling via plate tectonics facilitates the transfer of Hg with a positive Δ199Hg from marine reservoirs to the deep crust. The later-stage hydrothermal input of Hg with a positive Δ199Hg enhances coal Hg content. This process has resulted in an upward trend of Δ199Hg values corresponding with the increase in coal THg content, ultimately leading to near-zero Δ199Hg in high-Hg coals. Coal Hg reservoirs are affected by large-scale natural Hg cycling, which involves the exchange of Hg between continents and seas.

Isotope-Based Characterization of Soil Elemental Mercury Emissions from Historical Mercury Mining Areas: Driving Pathways and Relative Contributions.

Photo-, microbial, and abiotic dark reduction of soil mercury (Hg) may all lead to elemental mercury (Hg(0)) emissions. Utilizing lab incubations, isotope signatures of Hg(0) emitted from mining soils were characterized to quantify the interplay and contributions of various Hg reduction pathways, which have been scarcely studied. At 15 °C, microbial reduced Hg(0) showed a negative mass-dependent fractionation (MDF) (δ202Hg = -0.30 ± 0.08‰, 1SD) and near-zero mass-independent fractionation (MIF) (Δ199Hg = 0.01 ± 0.04‰, 1SD), closely resembling dark reduced Hg(0) (δ202Hg = -0.18 ± 0.05‰, Δ199Hg = -0.01 ± 0.03‰, 1SD). In comparison, photoreduced Hg(0) exhibited significant MDF and MIF (δ202Hg = -0.55 ± 0.05‰, Δ199Hg = -0.20 ± 0.07‰, 1SD). In the dark, Hg isotopic signatures remained constant over the temperature range of 15-35 °C. Nonetheless, light exposure and temperature changes together altered Hg(0) MIF signatures significantly. Isotope mixing models along with Hg(0) emission flux data highlighted photo- and microbial reduction contributing 79-88 and 12-21%, respectively, of the total Hg(0) emissions from mining soils, with negligible abiotic dark reduction. Microorganisms are the key driver of soil Hg(0) emissions by first dissolving HgS and then promoting ionic Hg formation, followed by facilitating the photo- and microbial reduction of organically bound Hg. These insights deepen our understanding of the biogeochemical processes that influence Hg(0) releases from surface soils.

A new Re-Os age constraint informs the dynamics of the Great Oxidation Event

The early Paleoproterozoic (ca. 2.5−2.2 Ga) represents a critical juncture in Earth history, marking the inception of an oxygenated atmosphere while bearing witness to potentially multiple widespread and severe glaciations. Deciphering the nature of this glacial epoch and its connection with atmospheric oxygenation has, however, proven difficult, hindered by a reliance on disputed stratigraphic correlations given the paucity of direct radiometric age constraints. Nowhere is this more acute than within the South African Transvaal Supergroup: Here, while the loss of oxygen-sensitive mass-independent sulfur isotope fractionation (S-MIF) has been reported from both the Duitschland and Rooihoogte formations, divided opinion surrounding the time-equivalence of these units has prompted authors to argue for vastly different oxygenation trajectories. Addressing this debate, we present a depositional Re-Os age (2443 ± 33 Ma) from diamictite samples preserved in drillcore of the upper Duitschland Formation. The 100-million-year separation between the Duitschland Formation and its previously presumed equivalent reveals at least two isolated disappearances of S-MIF, requiring that the Great Oxidation Event was dynamic and proceeded via discrete oxygenation episodes whose structure remains incompletely understood. Importantly, our revised framework aligns the lower Duitschland diamictite with the low-latitude glacigenic Makganyene Formation, supporting hypotheses of widespread regional, and potentially global, early Paleoproterozoic glaciation.

Hair mercury isotopes, a noninvasive biomarker for dietary methylmercury exposure and biological uptake.

Background. Fish and rice are the main dietary sources of methylmercury (MeHg); however, rice does not contain the same beneficial nutrients as fish, and these differences can impact the observed health effects of MeHg. Hence, it is important to validate a biomarker, which can distinguish among dietary MeHg sources. Methods. Mercury (Hg) stable isotopes were analyzed in hair samples from peripartum mothers in China (n = 265). Associations between mass dependent fractionation (MDF) (δ202Hg) and mass independent fractionation (MIF) (Δ199Hg) (dependent variables) and dietary MeHg intake (independent variable) were investigated using multivariable regression models. Results. In adjusted models, hair Δ199Hg was positively correlated with serum omega-3 fatty acids (a biomarker for fish consumption) and negatively correlated with maternal rice MeHg intake, indicating MIF recorded in hair can be used to distinguish MeHg intake predominantly from fish versus rice. Conversely, in adjusted models, hair δ202Hg was not correlated with measures of dietary measures of MeHg intake. Instead, hair δ202Hg was strongly, negatively correlated with hair Hg, which explained 27-29% of the variability in hair δ202Hg. Conclusions. Our results indicated that hair Δ199Hg can be used to distinguish MeHg intake from fish versus rice. Results also suggested that lighter isotopes were preferentially accumulated in hair, potentially reflecting Hg binding to thiols (i.e., cysteine); however, more research is needed to elucidate this hypothesis. Broader impacts include 1) validation of a non-invasive biomarker to distinguish MeHg intake from rice versus fish, and 2) the potential to use Hg isotopes to investigate Hg binding in tissues.

Mercury isotope evidence for the importance of recycled fluids in collisional ore systems.

The sources of fluids and metals in porphyry systems of continental-collision settings are poorly constrained. Mercury isotopes display unique mass-independent fractionation (expressed as Δ199Hg) and may provide important constraints on metal and volatile sources given that Hg is a highly volatile metal. Here, we report Hg isotope data on ore-forming porphyries, barren magmatic rocks, and mantle-derived mafic magmas from southern Tibet. The fertile porphyries and coeval mafic magmas display mainly positive Δ199Hg values (up to +0.25 per mil), while Δ199Hg values in barren magmatic rocks and mafic magmas are largely negative (-0.54 to 0.00 per mil). The positive Δ199Hg values observed here are consistent with seawater and marine sediments, suggesting that the ultimate source of fluids involved in the genesis of post-subduction porphyry copper deposits was the mantle lithosphere metasomatized by previous oceanic plate subduction. Our Hg isotope data provide an alternative view to current metallogenetic models on collisional porphyry systems that focus on melting of the lower continental crust.

A newly recognised mafic sill-hosted Ni-sulfide deposit emplaced during the 2.4 Ga Widgiemooltha dike swarm event, Eastern Goldfields, Western Australia

AbstractThe Cathedrals Ni-Cu prospect, located at the western margin of the Eastern Goldfields of the Yilgarn Craton, is hosted within a mafic intrusion interpreted as a sill complex. U-Pb dating of apatite from the sill yielded a crystallisation age of 2336 ± 64 Ma, inferring an association of sill emplacement and Ni mineralisation related to emplacement of the c. 2400 Ma Widgiemooltha dike swarm. The sill is typically differentiated into a lower olivine orthocumulate layer overlain by a dolerite unit containing xenoliths of partially assimilated granitoids in its upper portion. The latter is interpreted to be the result of stoping and melting of the granitic hanging wall, thereby creating a gravitationally stable buoyant melt layer beneath the top contact. Ni-Cu-Fe sulfides are increasingly abundant towards the base of the sill, ranging from globular disseminated sulfides to net-textured and massive sulfides at the basal contact. The presence and orientation of sulfide globule-bubble pairs indicates a primary near-horizontal orientation. Massive sulfides commonly exhibit a loop texture with pyrrhotite grains surrounded by pentlandite and chalcopyrite. Despite the variety of sulfide textures, sulfur isotopes have a homogeneous mantle-like signature without significant mass independent fractionation. Mineral chemistries that indicate sulfide prospectivity in larger intrusions do not work as effectively in this small sill, therefore new indicators may need to be developed to explore for similar deposits. To date, there are no other known magmatic deposits of this age in Australia. Sills of this age may be more prospective than previously recognised.

Evidence of both molecular cloud and fluid chemistry in Ryugu regolith.

The sulfur chemistry of (162173) Ryugu particles can be a powerful tracer of molecular cloud chemistry and small body processes, but it has not been well explored. We report identification of organosulfurs and a sulfate grain in two Ryugu particles, A0070 and A0093. The sulfate grain shows oxygen isotope ratios (δ17O = -11.0 ± 4.3 per mil, δ18O = -7.8 ± 2.3 per mil) that are akin to silicates in Ryugu but exhibit mass-independent sulfur isotopic fractionation (Δ33S = +5 ± 2 per mil). A methionine-like coating on the sulfate grain is isotopically anomalous (δ15N = +62 ± 2 per mil). Both the sulfate and organosulfurs can simultaneously form and survive during aqueous alteration within Ryugu's parent body, under reduced conditions, low temperature, and a pH >7 in the presence of N-rich organic molecules. This work extends the heliocentric zone where anomalous sulfur, formed by selective photodissociation of H2S gas in the molecular cloud, is found.

Mercury compound distribution and stable isotope composition in the different compartments of seabird eggs: The case of three species breeding in East Greenland

Mercury (Hg) is a toxic contaminant of global concern and the impact on Arctic ecosystems, particularly in seabirds, is critical due to large-scale Hg transport towards polar regions and its biomagnification in marine trophic systems. While the adverse effects of Hg on reproductive processes in seabirds are established, the understanding of Hg maternal transfer pathways and their control on Hg reproductive toxicity is limited. The combination of Hg compounds speciation (inorganic mercury and monomethylmercury MMHg) and Hg stable isotope composition in the different egg compartments (yolk, albumen, membrane, and shell) before embryo development was investigated to provide information on (i) Hg maternal transfer mechanisms, (ii) influence of egg biochemical composition on Hg organotropism and (iii) proxies of inputs of Hg contamination. Eggs of three seabird species (the common eider, the black-legged kittiwake and the little auk) collected within the same breeding period (summer 2020) in East Greenland were investigated. For all seabirds, albumen and membrane, the most protein-rich compartments, were the most contaminated (from 1.2 to 2.7 μg g−1 for albumen and from 0.3 to 0.7 μg g−1 for membrane). In these two compartments, more than 82% of the total Hg amount was in the form of MMHg. Additionally, mass-dependent fractionation values (δ202Hg) were higher in albumen and membrane in the three species. This result was mainly due the organotropism of MMHg as influenced by the biochemical properties and chemical binding affinity of these proteinous compartments. Among the different egg compartments, individuals and species, mass-independent fractionation values were comparable (mean ± sd were 0.99 ± 0.11‰, 0.78 ± 0.11‰, 0.03 ± 0.05‰, 0.04 ± 0.10‰ for Δ199Hg, Δ201Hg, Δ200Hg and Δ204Hg, respectively). We conclude that initial MMHg accumulated in the three species originated from Arctic environmental reservoirs exhibiting similar and low photodemethylation extent. This result suggests a unique major source of MMHg in those ecosystems, potentially influenced by sea ice cover.

Mass-independent fractionation of oxygen and sulfur isotopes

Mass-independent fractionation of oxygen and sulfur isotopes

Dynamics of mercury stable isotope compounds in Arctic seals: New insights from a controlled feeding trial on hooded seals Cystophora cristata

Accurate interpretation of mercury (Hg) isotopic data requires the consideration of several biotic factors such as age, diet, geographical range, and tissue metabolic turnover. A priori knowledge of prey-predator isotopic incorporation rates and Hg biomagnification is essential. This study aims to assess Hg stable isotopes incorporation in an Arctic species of Phocidae, the hooded seal Cystophora cristata, kept in human care for 24 months (2012–2014) and fed on a constant diet of Norwegian Spring Spawning herring Clupea harengus. We measured THg, MMHg and iHg levels, as well as Hg stable isotope composition with both mass dependent (MDF) and mass independent (MIF) fractionation (e.g. δ202Hg and Δ199,200,201,204Hg) in hooded seal kidney, liver, hair and muscle, in addition to herring muscle. We then calculated Hg MDF and MIF isotopic fractionation between hooded seals and their prey. We found a significant shift in δ202Hg between hooded seal hair (+0.80‰) and kidney (−0.78‰), and herring muscle. In hooded seals tissues δ202Hg correlated positively with MMHg percentage. These findings suggest that tissue-specific Hg speciation is the major driver of changes in Hg isotopic fractionation rates in this Arctic predator. Δ199Hg, Δ200Hg, Δ201Hg and Δ204Hg values did not vary between herring and hooded seal tissues, confirming their utility as tracers of Hg marine and atmospheric sources in top predators. To our knowledge, this represents the first attempt to assess complex Hg isotope dynamics in the internal system of Arctic Phocidae, controlling the effects of age, diet, and distribution. Our results confirm the validity of Hg stable isotopes as tracers of environmental Hg sources even in top predators, but emphasize the importance of animal age and tissue selection for inter-study and inter-species comparisons.

Forging inner-disk Al-rich chondrules by interactions of CAI-like melt and ambient gas

The mechanism of gas-melt interactions and the compositions of precursors are key to understanding the formation of chondrules. To shed light on the two enigmas, we studied the petrography, chemistry, and oxygen isotopes of six Al-rich chondrules (ARCs, five glassy and one plagioclase-bearing) in unequilibrated ordinary chondrites (OCs, petrologic subtype: 3.05). The plagioclase-bearing ARC was also investigated with Al-Mg chronology. Elemental zonation and inter-element correlations in glassy mesostasis of two ARCs indicate the condensation of gaseous Mg, SiO, Fe, and Na onto chondrule melt. The plagioclase-bearing ARC appears to display internal mass-independent oxygen isotope fractionation with δ18O increasing following the order of mineral crystallization, suggesting partial oxygen isotope exchange with ambient gas during crystallization. Oxygen isotopes of the six ARCs are distributed along a mixing line of slope = 0.99 ± 0.05, which intersects with calcium-aluminum-rich inclusions (CAIs), consistent with a small portion of OC type IA chondrules, but deviates from other OC ferromagnesium chondrules (FMCs) towards higher δ17O, suggesting that OC ARCs and some IA chondrules were established by interactions between CAI-like melts and 16O-poor ambient gas, rather than simply remelting solid mixtures of CAI and FMC materials.All six ARCs have unfractionated refractory lithophile element patterns with bulk concentrations ranging from ∼7 × CI to ∼15 × CI, indicating ∼ 30–100 % of CAI-like materials in their precursors. Their bulk compositions are linearly evolved toward the Mg: SiO ∼ 3:2 to 2:1 (in atomic) apex, consistent with adding gaseous Mg and SiO to the chondrule bulk via gas–melt interactions. The back-calculated compositions of the recycled CAI-like materials closely overlap with pyroxene-anorthite-rich CAIs, suggesting that extensive interactions between the melt of pyroxene-anorthite-rich CAI-like materials and ambient gas could make OC ARCs. The Al-Mg age of the plagioclase-bearing ARC is ∼2.2 Ma after CAIs, similar to typical OC FMCs, suggesting that the refractory component arrived at the OC reservoirs in the late stage of the chondrule heating events.

Archean atmospheric sulfur stocked in the Cretaceous gold deposits

The giant Cretaceous Jiaodong gold province is the world's third largest gold mining district, while the source of the ore-forming fluids and AuS remains controversial. The elemental sulfur is a key carrier for gold and the isotopic composition is an ideal tracer of the sulfur source, which makes it crucial for understanding gold deposit genesis and guiding ore exploration. Here, we show the Au-bearing pyrite in the gold deposits from Jiaodong bears a non-zero Δ33S signature from −0.07 ‰ to 0.31 ‰ showing the mass-independent fractionation (S-MIF). This clearly indicates involvement of Archean atmospheric sulfur during the Cretaceous gold mineralization. The MIF-bearing sulfur was likely remobilized from the Archean supracrustal rocks in the basement of the Jiaodong region via fluid-rock interaction while the hydrothermal ore-forming fluids passed through the crustal-scale fault zones. The Phanerozoic gold deposits from Jiaodong have relatively minor S-MIF signals compared with the Archean orogenic gold deposits due to more material (S) cycling and dilution. This new discovery of S-MIF signal in the Phanerozoic ore deposits implies that the recycling of crustal sulfur may be a more common process and play an important role in forming hydrothermal gold deposits.

Onset of coupled atmosphere-ocean oxygenation 2.3 billion years ago.

The initial rise of molecular oxygen (O2) shortly after the Archaean-Proterozoic transition 2.5 billion years ago was more complex than the single step-change once envisioned. Sulfur mass-independent fractionation records suggest that the rise of atmospheric O2 was oscillatory, with multiple returns to an anoxic state until perhaps 2.2 billion years ago1-3. Yet few constraints exist for contemporaneous marine oxygenation dynamics, precluding a holistic understanding of planetary oxygenation. Here we report thallium (Tl) isotope ratio and redox-sensitive element data for marine shales from the Transvaal Supergroup, South Africa. Synchronous with sulfur isotope evidence of atmospheric oxygenation in the same shales3, we found lower authigenic 205Tl/203Tl ratios indicative of widespread manganese oxide burial on an oxygenated seafloor and higher redox-sensitive element abundances consistent with expanded oxygenated waters. Both signatures disappear when thesulfur isotope data indicate a brief return to an anoxic atmospheric state. Our data connect recently identified atmospheric O2 dynamics on early Earth with the marine realm, marking an important turning point in Earth's redox history away from heterogeneous and highly localized 'oasis'-style oxygenation.

Atmospheric oxygenation at the onset of Earth’s Great Oxidation forced enhanced marine anoxia

Abstract Capturing the loss of mass-independent sulphur isotope fractionation (MIF-S), the correlative South African Duitschland and Rooihoogte formations are widely held to bear the isotopic fingerprint of the first atmospheric oxygenation at the onset of the so-called Great Oxidation Event (GOE). Surprisingly, however, while the multiple sulphur isotope systematics of these formations remain central to our understanding of the GOE, until now, comparatively little work has been done to elucidate the repercussions within the marine realm. Here we present chemostratigraphic records from four drill cores covering a large area of the Transvaal Basin, transcending these crucial units and continuing into the overlying Timeball Hill Formation (TBH), that document the immediate, yet counterintuitive, marine response to atmospheric oxygenation. Specifically, irrespective of the interpretative framework employed, our basin-wide redox-sensitive trace element data document an environmental change from oxic/suboxic conditions within the lower and middle parts of the Duitschland and Rooihoogte formations to suboxic/anoxic conditions within their upper reaches. Interestingly, in concert with a ~35‰ negative δ34S excursion that implicates increased sulphate availability and bacterial sulphate reduction, δ98/95Mo3134+0.25 values increase by ~1.0 to 1.5‰. Combining these observations with increased Fe/Mn ratios, elevated total sulphur and carbon contents and a trend towards lower δ13Corg values imply a shift toward less oxygenated conditions across the Transvaal Basin. The combined observations in the mentioned parameters expose a geobiological feedback-driven causality between the earliest oxygenation of the atmosphere and decreased redox potentials of medium to deep marine environments, at least within the Transvaal Basin.

Sedimentary and stratigraphic architecture of the Duitschland and Rooihoogte formations (Palaeoproterozoic, South Africa): implications for tempo of the Great Oxidation Event

Abstract Determining the tempo and causality of key palaeoclimate events recorded in sedimentary strata depends on high-resolution numerical ages and well-constrained stratigraphic correlations at regional and global scale. This requirement is not necessarily met in Precambrian strata due to poorer age resolution, limited preservation, and secondary overprints. A good example includes the Palaeoproterozoic Rooihoogte and Duitschland formations in South Africa, which document the disappearance of mass-independent fractionation of sulphur isotopes (MIF-S) and contain glacial diamictites at their base. They are thus key records of Earth’s surface oxygenation during the Great Oxidation Event (GOE). However, previous studies have either correlated these units, resulting in a unidirectional oxidation trend; or have regarded them as successive strata, causing an interpretation of oscillating oxidation. This study uses extensive outcrop and new core material to investigate correlation between these two units, and to establish depositional models. Results show that key stratigraphic markers can be traced around the entire Kaapvaal craton both in outcrop and the subsurface. In particular, the basal Bevets breccia and the top Duitschland breccia are here re-interpreted as two separate units that are present at the base and top of both formations, supporting correlation of the formations. Consequently, the base of both formations records a major craton-wide event of uplift and karstification, leading to carbonate dissolution and chert brecciation. Erosion of older rocks from across the craton also delivered material for the basal glacial diamictite. The majority of mixed siliciclastic-carbonate sediments were deposited on a storm- and/or delta-influenced shelf. Depositional packages in both formations reflect post-glacial relative sea level rise, followed by progradation of a deltaic, storm or shoreline depositional system. There is a relatively short-lived depositional hiatus to overlying shales of the Timeball Hill Formation. Both Rooihoogte and Duitschland formations thus record only a single glacial event at their base, and a unidirectional trend of oxidation.

Isotope Data Constrains Redox Chemistry of Atmospheric Mercury.

The redox chemistry of mercury (Hg) in the atmosphere exerts a significant influence on its global cycle. However, our understanding of this important process remains shrouded in uncertainty. In this study, we utilize three-dimensional atmospheric Hg isotope modeling to evaluate the isotopic composition of particle-bound mercury [HgII(P)] in the global atmosphere. We investigate various chemistry mechanisms and find that they induce remarkably disparate odd-number mass-independent fractionation (odd-MIF) in HgII(P) on a global scale. The observed odd-MIF data identify the essential role of sea salt aerosol debromination in the redox chemistry of atmospheric Hg and underscore the predominant influence of Br oxidation in the marine boundary layer. The odd-MIF signatures significantly narrow the uncertainty range of redox chemistry rates and constrain the photoreduction of HgII(P) at a magnitude of 10-3 JNO2 (local photolysis frequency of NO2) in the global atmosphere. This study advances our understanding of atmospheric Hg chemistry processes and provides insights into the potential impacts of climate change on Hg cycling.

Organ-specific mercury stable isotopes, speciation and particle measurements reveal methylmercury detoxification processes in Atlantic Bluefin Tuna

Identifying metabolism and detoxification mechanisms of Hg in biota has important implications for biomonitoring, ecotoxicology, and food safety. Compared to marine mammals and waterbirds, detoxification of MeHg in fish is understudied. Here, we investigated Hg detoxification in Atlantic bluefin tuna Thunnus thynnus using organ-specific Hg and Se speciation data, stable Hg isotope signatures, and Hg and Se particle measurements in multiple tissues. Our results provide evidence for in vivo demethylation and biomineralization of HgSe particles, particularly in spleen and kidney. We observed a maximum range of 1.83‰ for δ202Hg between spleen and lean muscle, whereas Δ199Hg values were similar across all tissues. Mean percent methylmercury ranged from 8% in spleen to 90% in lean muscle. The particulate masses of Hg and Se were higher in spleen and kidney (Hg: 61% and 59%, Se: 12% and 6%, respectively) compared to muscle (Hg: 2%, Se: 0.05%). Our data supports the hypothesis of an organ-specific, two-step detoxification of methylmercury in wild marine fish, consisting of demethylation and biomineralization, like reported for waterbirds. While mass dependent fractionation signatures were highly organ specific, stable mass independent fractionation signatures across all tissues make them potential candidates for source apportionment studies of Hg using ABFT.

Continuous-Flow Stable Sulfur Isotope Analysis of Organic and Inorganic Compounds by EA-MC-ICPMS.

Elemental analysis (EA) coupled to isotope ratio mass spectrometry (IRMS) is a well-established method to derive stable isotope ratios of sulfur (34S/32S). Conversion of sulfur to SO2 by EA and measurement of SO2 isotopologues by IRMS represents the simplest and most versatile method to accomplish isotope measurement of sulfur even in bulk samples. Yet, interferences by oxygen isotopes in SO2 often impair the precision and trueness of measured results. In the current study, we coupled EA to multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) to establish a method that avoids such interferences due to direct measurement of S+ ions. In addition, measurement of the 33S/32S isotope ratios is possible, thus representing the first bulk method that is suitable to study mass-independent isotope fractionation (MIF). Analytical precision (σ) of available Ag2S and BaSO4 reference materials (RMs) was, on average, 0.2 mUr for δ33S and δ34S, never exceeding 0.3 mUr within this study (1 mUr = 1‰ = 0.001). Measured δ34S values of reference materials agreed within ±0.2 mUr of officially reported values. Measurement of wood samples yielded good precision (0.2 mUr) for sulfur amounts as low as 3.5 μg, but precision deteriorated for samples at lower sulfur contents due to poor peak shape. Finally, we explored cross-calibration of organic liquids separated via gas chromatography (GC) against solid RMs combusted via EA that avoids challenging offline conversion of RMs. Results indicate good precision (≤0.08 mUr) and acceptable trueness (≤0.34 mUr) for determination of δ34S, demonstrating the future potential of such an approach.