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.

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 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.

Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish.

The complex biogeochemical cycle of Hg makes identifying primary sources of fish tissue Hg problematic. To identify sources and provide insight into this cycle, we combined carbon (δ13C), nitrogen amino acid (δ15NPhe), and Hg isotope (Δ199Hg, Δ201Hg, δ202Hg) data for six species of Hawaiian marine bottomfish. Results from these isotopic systems identified individuals within species that likely fed from separate food webs. Terrestrial freshwater inputs to coastal sediments were identified as the primary source of tissue Hg in the jack species, Caranx ignobilis, which inhabit shallow marine ecosystems. Thus, coastal C. ignobilis were a biological vector transporting Hg from freshwater environments into marine ecosystems. Depth profiles of Hg isotopic compositions for bottomfish (excludung C. ignobilis) were similar, but not identical, to profiles for open-ocean pelagic fishes, suggesting that in both settings inorganic Hg, which was ultimately transformed to monomethylmercury (MeHg) and bioaccumulated, was dominantly from a single source. However, differences between pelagic fish and bottomfish profiles were attributable to mass-dependent fractionation in the benthos prior to incorporation into the food web. Results also confirmed that bottomfish relied, at least in part, on a benthic food web and identified the incorporation of deeper water oceanic MeHg sources into deeper water sediments prior to food web uptake and transfer.

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).

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.

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.

Biogenic carbonate mercury and marine temperature records reveal global influence of Late Cretaceous Deccan Traps.

The climate and environmental significance of the Deccan Traps large igneous province of west-central India has been the subject of debate in paleontological communities. Nearly one million years of semi-continuous Deccan eruptive activity spanned the Cretaceous-Paleogene boundary, which is renowned for the extinction of most dinosaur groups. Whereas the Chicxulub impactor is acknowledged as the principal cause of these extinctions, the Deccan Traps eruptions are believed to have contributed to extinction patterns and/or enhanced ecological pressures on biota during this interval of geologic time. We present the first coupled records of biogenic carbonate clumped isotope paleothermometry and mercury concentrations as measured from a broad geographic distribution of marine mollusk fossils. These fossils preserve evidence of simultaneous increases in coastal marine temperatures and mercury concentrations at a global scale, which appear attributable to volcanic CO2 and mercury emissions. These early findings warrant further investigation with additional records of combined Late Cretaceous temperatures and mercury concentrations of biogenic carbonate.

Mercury accumulation in sea lamprey (Petromyzon marinus) from Lake Huron.

We determined whole-fish total mercury (Hg) concentrations of 40 male and 40 female adult sea lampreys (Petromyzon marinus) captured in the Cheboygan River, a tributary to Lake Huron, during May 2011. In addition, bioenergetics modeling was used to explore the effects of sex-related differences in activity and resting (standard) metabolic rate (SMR) on mercury accumulation. The grand mean for Hg concentrations was 519 ng/g (standard error of the mean=46 ng/g). On average, males were 16% higher in Hg concentration than females. Bioenergetics modeling results indicated that 14% higher activity and SMR in males would account for this observed sex difference in Hg concentrations. We concluded that the higher Hg concentration in males was most likely due to higher rate of energy expenditure in males, stemming from greater activity and SMR. Our findings have implications for estimating the effects of sea lamprey populations on mercury cycling within ecosystems, as well as for the proposed opening of sea lamprey fisheries. Eventually, our results may prove useful in improving control of sea lamprey, a pest responsible for substantial damage to fisheries in lakes where it is not native.

Mercury concentrations, speciation, and isotopic composition in sediment from a cold seep in the northern Gulf of Mexico.

Total-Hg, monomethylmercury (MMHg), and mercury isotopic composition was determined in sediment from a cold seep and background sites in the northern Gulf of Mexico (nGoM). Total-Hg averaged 50 ng/g (n=28), ranged from 31 to 67 ng/g, and decreased with depth (0-15 cm). MMHg averaged 0.91 ng/g (n=18), and ranged from 0.2 to 1.9 ng/g. There was no significant difference for total-Hg or MMHg between cold seep and background sites. δ(202)Hg ranged from -0.5 to -0.8‰ and becomes more negative with depth (r=0.989). Mass independent fractionation (Δ(199)Hg) was small but consistently positive (0.04-0.12‰); there was no difference between cold seeps (Δ(199)Hg = +0.09±0.03; n=7, 1SD) and background sites (Δ(199)Hg=+0.07±0.02; n=5, 1SD). This suggests that releases of hydrocarbons at the cold seep do not significantly alter Hg levels, and that cold seeps are likely not major sources of MMHg to nGoM waters.

Mercury stable isotope fractionation during reduction of Hg(II) to Hg(0) by mercury resistant microorganisms.

Mercury (Hg) undergoes systematic stable isotopic fractionation; therefore, isotopic signatures of Hg may provide a new tool to track sources, sinks, and dominant chemical transformation pathways of Hg in the environment. We investigated the isotopic fractionation of Hg by Hg(II) resistant (HgR) bacteria expressing the mercuric reductase (MerA) enzyme. The isotopic composition of both the reactant Hg(II) added to the growth medium and volatilized product (Hg(0)) was measured using cold vapor generation and multiple collector inductively coupled plasma mass spectrometry. We found that exponentially dividing pure cultures of a gram negative strain Escherichia coli JM109/pPB117 grown with abundant electron donor and high Hg(II) concentrations at 37, 30, and 22 degrees C, and a natural microbial consortium incubated in natural site water at 30 degrees C after enrichment of HgR microbes, preferentially reduced the lighter isotopes of Hg. In all cases, Hg underwent Rayleigh fractionation with the best estimates of alpha202/198 values ranging from 1.0013 to 1.0020. In the cultures grown at 37 degrees C, below a certain threshold Hg(II) concentration, the extent of fractionation decreased progressively. This study demonstrates mass-dependent kinetic fractionation of Hg and could lead to development of a new stable isotopic approach to the study of Hg biogeochemical cycling in the environment.