Coupling of nitric acid digestion and anion-exchange resin separation for the determination of methylmercury isotopic composition within organisms.

Isotope ratios of methylmercury (MeHg) within organisms can be used to identify sources of MeHg that have accumulated in food webs, but these isotopic compositions are masked in organisms at lower trophic levels by the presence of inorganic mercury (iHg). To facilitate measurement of MeHg isotope ratios in organisms, we developed a method of extracting and isolating MeHg from fish and aquatic invertebrates for compound-specific isotopic analysis involving nitric acid digestion, batch anion-exchange resin separation, and pre-concentration by purge and trap. Recovery of MeHg was quantified after each step in the procedure, and the average cumulative recovery of MeHg was 93.4 ± 2.9% (1 SD, n = 28) for biological reference materials and natural biota samples and 96.9 ± 1.8% (1 SD, n = 5) for aqueous MeHgCl standards. The amount of iHg impurities was also quantified after each step, and the average MeHg purity was 97.8 ± 4.3% (1 SD, n = 28) across all reference materials and natural biota samples after the final separation step. Measured MeHg isotopic compositions of reference materials agreed with literature values obtained using other MeHg separation techniques, and MeHg isotope ratios of aqueous standards, reference materials, and natural biota samples were reproducible. On average, the reproducibility associated with reference material process replicates (2 SD) was 0.10‰ for δ202MeHg and 0.04‰ for Δ199MeHg. This new method provides a streamlined, reliable technique that utilizes a single sample aliquot for MeHg concentration and isotopic analysis. This promotes a tight coupling between MeHg concentration, %MeHg, and Hg isotopic composition, which may be especially beneficial for studying complex food webs with multiple isotopically distinct sources of iHg and/or MeHg.

Mercury isotopic evidence for the importance of particles as a source of mercury to marine organisms.

The origin of methylmercury in pelagic fish remains unclear, with many unanswered questions regarding the production and degradation of this neurotoxin in the water column. We used mercury (Hg) stable isotope ratios of marine particles and biota to elucidate the cycling of methylmercury prior to incorporation into the marine food web. The Hg isotopic composition of particles, zooplankton, and fish reveals preferential methylation of Hg within small (< 53 µm) marine particles in the upper 400 m of the North Pacific Ocean. Mass-dependent Hg isotope ratios (δ202Hg) recorded in small particles overlap with previously estimated δ202Hg values for methylmercury sources to Pacific and Atlantic Ocean food webs. Particulate compound specific isotope analysis of amino acids (CSIA-AA) yield δ15N values that indicate more-significant microbial decomposition in small particles compared to larger particles. CSIA-AA and Hg isotope data also suggest that large particles (> 53 µm) collected in the equatorial ocean are distinct from small particles and resemble fecal pellets. Additional evidence for Hg methylation within small particles is provided by a statistical mixing model of even mass-independent (Δ200Hg and Δ204Hg) isotope values, which demonstrates that Hg within near-surface marine organisms (0-150 m) originates from a combination of rainfall and marine particles. In contrast, in meso- and upper bathypelagic organisms (200-1,400 m), the majority of Hg originates from marine particles with little input from wet deposition. The occurrence of methylation within marine particles is supported further by a correlation between Δ200Hg and Δ199Hg values, demonstrating greater overlap in the Hg isotopic composition of marine organisms with marine particles than with total gaseous Hg or wet deposition.

Investigation of the biochemical controls on mercury uptake and mobility in trees.

Atmospheric elemental mercury (Hg(0)) enters plant stomata, becomes oxidized, and is then transferred to annual growth rings providing an archive of air Hg(0) concentrations. To better understand the processes of Hg accumulation and translocation, the foliage of quaking aspen and Austrian pine were exposed to Hg(0), and methylmercury (MeHg) or Me198Hg via roots, in controlled exposures during the summer. Isotopic measurements demonstrated, in a laboratory setting, that the natural mass-dependent fractionation observed was the same as that measured in field studies, with the lighter isotopes being preferentially taken up by the leaves. Hg was measured in plant tissues across seasons. Aspen trees moved Hg into new growth immediately after exposure, resorbed Hg in the fall, and then distributed Hg to new growth tissues in the spring. Austrian pine did not reallocate Hg. Mercury measured in aspen leaf fractions of trees exposed to Hg(0) demonstrated that 85 % of Hg was in the cell wall. It was also found that redox-active molecules, such as H2O2, could potentiate the release of cell wall-bound Hg from aspen leaves, providing a potential mechanism for remobilization. Regardless of the mechanism, the ability of aspen to reallocate Hg to new tissues indicates that Hg distribution in tree rings from aspen do not provide a reliable record of yearly changes in atmospheric Hg(0).

Mantle Hg isotopic heterogeneity and evidence of oceanic Hg recycling into the mantle.

The geochemical cycle of mercury in Earth's surface environment (atmosphere, hydrosphere, biosphere) has been extensively studied; however, the deep geological cycling of this element is less well known. Here we document distinct mass-independent mercury isotope fractionation (expressed as Δ199Hg) in island arc basalts and mid-ocean ridge basalts. Both rock groups show positive Δ199Hg values up to 0.34‰ and 0.22‰, respectively, which deviate from recent estimates of the primitive mantle (Δ199Hg: 0.00 ± 0.10‰, 2 SD)1. The positive Δ199Hg values indicate recycling of marine Hg into the asthenospheric mantle. Such a crustal Hg isotope signature was not observed in our samples of ocean island basalts and continental flood basalts, but has recently been identified in canonical end-member samples of the deep mantle1, therefore demonstrating that recycling of mercury can affect both the upper and lower mantle. Our study reveals large-scale translithospheric Hg recycling via plate tectonics.

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

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

Isotopic composition of mercury deposited via snow into mid-latitude ecosystems.

Atmospheric deposition of mercury (Hg) to terrestrial and aquatic ecosystems has significant implications for human and animal exposure. Measurements of Hg isotopic composition can be utilized to trace sources of Hg, but outside of the Arctic there has been little Hg isotopic characterization of snow. To better understand deposition pathways at mid-latitudes, five time series of snowfall were collected at two sites (Dexter and Pellston, Michigan, USA) to investigate the Hg isotopic composition of snowfall, how it changes after deposition, and how it compares to rain. The Hg isotopic composition of a subset of fresh snow samples revealed the influence of reactive surface uptake of atmospheric Hg(0). The first time series collected at Dexter occurred during a polar vortex, demonstrating Hg isotopic fractionation dynamics similar to those in Arctic snow, with increasingly negative Δ199Hg as snow aged with exposure to sunlight. All other time series revealed an increase in Δ199Hg as snow aged, with values reaching up to 3.5‰. This characterization of Hg isotopes in snow suggests a strong influence of oxidants and binding ligands in snow that may mediate Hg isotope fractionation. Additionally, isotopic characterization of Hg in snow deposited to natural ecosystems at mid-latitudes allows for better understanding of atmospheric mercury sources that are deposited to lakes and forests and that may become available for methylation and transfer to food webs.

Use of sequential extraction and mercury stable isotope analysis to assess remobilization of sediment-bound legacy mercury.

The goal of this project was to assess how anthropogenic legacy mercury (Hg) retained in streambed sediment may be remobilized to stream water. To do this, we performed sequential extractions and Hg isotope analyses on streambed sediment collected along the length of East Fork Poplar Creek, a point-source contaminated stream in Oak Ridge, Tennessee, USA. Legacy Hg within streambed sediment appears to have been isotopically fractionated by equilibrium isotope effects driven by isotope exchange between co-existing Hg(0) and Hg(ii) species, potentially over-printing fractionation patterns that would have been imparted by kinetic redox reactions. Weakly-bound and recalcitrant sediment Hg pools were isotopically similar to one another, suggesting that small amounts of recalcitrant Hg may be released and then rapidly and weakly re-adsorbed onto the sediment. This weakly-bound Hg pool appears to contribute dissolved Hg to the hyporheic pore water, which may subsequently enter the surface flow. The isotopic composition of the organically-bound sediment Hg pools, as well as biofilm and suspended particulates, converged with that of the weakly-bound and recalcitrant sediment Hg pools along the flow path. This appears to be indicative of both physical mixing with streambed sediment and the transfer of weakly-bound sediment Hg into biofilm and suspended particulates, followed by re-incorporation into the organically-bound sediment Hg pool. Overall, these results provide evidence that legacy Hg in the streambed is remobilized, enters the stream water as dissolved Hg, and may be incorporated into streambed biofilm, which constitutes a basal resource within the stream ecosystem.

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

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

Contrasting Controls on the Diel Isotopic Variation of Hg0 at Two High Elevation Sites in the Western United States.

The atmosphere is a significant global reservoir for mercury (Hg) and its isotopic characterization is important to understand sources, distribution, and deposition of Hg to the Earth's surface. To better understand Hg isotope variability in the remote background atmosphere, we collected continuous 12-h Hg0 samples for 1 week from two high elevation sites, Camp Davis, Wyoming (valley), and Mount Bachelor, Oregon (mountaintop). The samples collected at Camp Davis displayed strong diel variation in δ202Hg values of Hg0, but not in Δ199Hg or Δ200Hg values. We attribute this pattern to nightly atmospheric inversions trapping Hg in the valley and the subsequent nighttime uptake of Hg by vegetation, which depletes Hg from the atmosphere. At Mount Bachelor, the samples displayed diel variation in both δ202Hg and Δ199Hg, but not Δ200Hg. We attribute this pattern to differences in the vertical distribution of Hg in the atmosphere as Mount Bachelor received free tropospheric air masses on certain nights during the sampling period. Near the end of the sampling period at Mount Bachelor, the observed diel pattern dissipated due to the influence of a nearby forest fire. The processes governing the Hg isotopic fractionation differ across sites depending on mixing, topography, and vegetation cover.