A FULLY-COUPLED GLOBAL HG ISOTOPE BOX MODEL
The large magnitudes of mass-dependent (MDF) and mass-independent fractionation (MIF) of stable Hg isotopes make it a useful tool to understand Hg biogeochemical cycling on Earth. Accurate understanding of the Hg isotope signatures in various Earth’s reservoirs at a global scale needs to account for all the underlying Hg sources (primary natural/anthropogenic Hg emissions and re-emissions of legacy Hg) and biogeochemical processes that potentially fractionate Hg isotopes.
We integrate source Hg isotope MDF and MIF signatures and processes-based isotope fractionation factors (e.g., Hg0 exchange at the atmosphere-terrestrial and atmosphere-oceanic boundary layers, aqueous dissolved HgII reduction, elemental Hg0 oxidation, sorption of aqueous HgII onto particles) into a fully-coupled, global terrestrial-atmospheric-oceanic Hg isotope box model. The model is used to simulate three isotope signatures: δ202Hg (MDF), Δ199Hg (odd Hg isotope MIF) and Δ200Hg (even Hg isotope MIF). We find that the simulated Hg isotope compositions in the Earth’s reservoirs using the best-available parameters are highly biased relative to several lines of observational constrains.
A series of sensitivity analysis suggests that MDF during atmospheric Hg0 oxidation enrich heavy Hg isotopes in the Hg0, in contrary to the sign indicated by experimental atmospheric Hg0 oxidation by Br atom. A similar MDF sign is also necessary for dissolved Hg0 oxidation in the ocean. To best fit the simulated results to the observational constrains, it requires anthropogenic Hg emissions to have lower δ202Hg than currently thought and a 2 times larger MDF factor for the Hg0 oxidation. Likewise, appropriate odd Hg isotope MIF factors for oxidation of Hg0 are also needed to reasonably simulate the observed Δ199Hg variation in the Earth’s surface reservoirs. We find that an increase of the terrestrial Hg0 re-emission fluxes and associated odd MIF factors gives a closer agreement between model results and observational constrains.
THE MYSTERIOUS MASS-INDEPENDENT FRACTIONATION OF EVEN MERCURY ISOTOPES
Preliminary studies have demonstrated both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in natural samples, and the potential of Hg isotope determination in biochemistry and geochemistry[1, 2]. To date, more than 100 papers have been published on Hg isotope ratios, which demonstrated the potential of Hg isotopes in tracing the source, processes and the fate of Hg in the atmosphere, biosphere, lithosphere, and hydrosphere. A few special processes such as photochemical reduction of Hg(II) and photochemical degradation of methylmercury (MeHg) can produce mass-independent fractionation (MIF) of odd Hg isotopes (odd-MIF), which had been largely reported in variable natural samples and laboratory experiments, and was thought to be caused by either nuclear volume effect (NVE) or magnetic isotope effect (MIE). Moreover, recent work reported, unexpectively, intriguing MIF of even Hg isotopes (even-MIF, D200Hg up to 1.24‰) in natural samples mainly related to the atmosphere, rendering Hg as a “three dimentional” isotope tracing system.
Here, we try to give a tentative review of publications on even Hg isotope anomalies, with a main focus on sample strategies and possible processes and mechanisms triggering even-MIF. Given the fact that even isotope anomaly was observed in variable regions with different altitude and latitude in China and in North America, the occurrence of even-MIF is likely a worldwide phenomenon, supported by the positive D200Hg (~ +0.22‰) determined in the tree moss in Sweden. In fact, D200Hg is actually used to refer to the deviation of even Hg isotopes from MDF, whether other even isotopes are subject to the same fractionation remains unclear and needs to be fully elucidated. In general, D200Hg values were mainly determined in samples related to the atmosphere, implying an upper atmosphere origin of even-MIF. If our conceptual model can hold, even-MIF may serve as a useful indicator of upper atmosphere chemistry. The implication of even-MIF as a possible conservative tracer remains to be largely developed. In fact, 200Hg anomaly is likely related to solar irradiation, air mass move and stratosphere incursion, thus even-MIF could provide additional information about atmospheric chemistry, meteorological condition and even related climate changes. Moreover, the conservative behavior of 200Hg anomaly may also be helpful for better understanding the global biogeochemical cycle of Hg, especially the surface-atmosphere exchange.
1) Bergquist, B. A.; Blum, J. D. Sci. 2007, 318, 417-420. 2) Cai and Chen, Sci. Bull. 2016, 61,116-124; 3) Chen et al., GCA 2012, 90, 33-46.
ISOTOPIC FRACTIONATION OF HG DURING GAS-PHASE OXIDATION CAUSED BY CL•, BR•, HO•, O3 AND PHOTO-EXCITATION IN AIR
In this study, we investigated the Hg isotope composition and fractionation during gas-phase oxidation of Hg0 by oxidants considered important in the atmosphere and incorporated in most current chemical Hg transport models (Br•, HO• and O3). In addition, the systems of Hg + Cl• and Hg + hν (UV-C radiation incl. λ=253.7 nm) + O2 (air) were also studied. The importance of former reaction falls within combustion processes and the latter reaction potentially has importance in the upper atmosphere.
Reaction kinetics of Hg0 oxidation by Cl•, Br•, HO• and O3 in air at atmospheric pressure (750±1 Torr) and room temperature (298±1 K) were determined by competitive and absolute kinetic techniques respectively to (1.8±0.5) × 10-11, (1.6±0.8) × 10-12, (2.7±1.89) × 10-12 and (4.6±0.5) × 10-20 cm3 molecule-1 s-1. Significant mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) were observed in all of the reactions studied. Results show that heavier isotopes are preferentially enriched in the remaining Hg0 during Cl•, O3 and UV-C initiated oxidation, whereas being enriched in the Hg(II) products during oxidation by Br• and HO•. Odd-mass-number MIF (∆199Hg and ∆201Hg) is without exception positive for the reactant Hg0 and negative for the reactant Hg (II) pool, although of various magnitude for the specific oxidants falling in the order (Cl• > Br• > HO• > O3). Hg + UV-C light + O2 induces a large magnitude of even-mass-number MIF (∆200Hg down to -39.9‰ in the remaining Hg0). This result has similarities to Hg fractionation pattern previous observed to occur in compact fluorescent lamps and will be further elaborated. Finally, the atmospheric relevance of Hg0 gas-phase oxidation will be discussed in connection to field observations of Hg stable isotope systematics.
HISTORIC CHANGE IN MERCURY SOURCES AND CYCLING RECONSTRUCTED FROM ISOTOPIC RECORDS IN A GLOBALLY DISTRIBUTED SUITE OF LAKE SEDIMENT CORES
The strongest evidence for anthropogenic alteration of the global mercury (Hg) cycle comes from historic records of mercury deposition preserved principally in lake sediments. More recently, high-precision measurement of Hg isotopes has added a new dimension to these sedimentary archives, promising additional insights into Hg source apportionment and cycling. At present, interpretations are constrained by the small number and geographic extent of cores analyzed isotopically. Here we report on newly acquired Hg-isotopic records from a globally distributed suite of well-dated sediment cores taken from remote lakes in the US (Alaska, Minnesota, California), Canada (Newfoundland), Kenya, and China. Most of these cores were analyzed previously for total-Hg and all show the global rise in Hg deposition associated with the industrial revolution. In nearly all cases, this rise is accompanied by an increase in mass dependent fractionation (MDF, reported in δ notation) and a decrease in mass independent fractionation (MIF, reported in ∆ notation) of Hg isotopes. These trends, also noted in previous core studies, are attributed to large scale industrial emission of Hg (with little MIF) into the global atmosphere and are consistent with positive MDF and MIF as measured in modern-day precipitation. Most cores show negative MIF in preindustrial sediments, indicating Hg sources (e.g. organic soils) that have undergone significant recycling, including photochemical reactions that generate MIF. However, other cores show positive MIF in preindustrial times, suggesting that direct atmospheric deposition could be an important Hg source for some lakes, especially those with relatively small watersheds. Despite similar temporal trends among cores, the actual isotopic signatures vary considerably among the different study regions, with some cores showing highly negative MIF (e.g., southeastern Alaska; ∆199Hg = -0.2 to -0.4 per mil) and others highly negative MDF (e.g., northwestern China; δ202Hg = -3 to -5 per mil) throughout the period of record. Such differences may reflect Hg source differences, as well as fractionation effects during atmospheric transport and deposition, and during Hg cycling in the water column (e.g., Hg residence time, sedimentation rate, water clarity). Differences among the study lakes in latitude, precipitation source region, relative watershed size, and distance from Hg emission sources provide an empirical framework for evaluating Hg isotopic signatures and global Hg cycling in recent centuries.
EFFECTS OF CHINESE INDUSTRIALIZATION AND ECONOMIC GROWTH ON MERCURY INPUTS TO MARGINAL SEA SEDIMENTS: INSIGHTS FROM MERCURY STABLE ISOTOPES
Economic development in the past few decades has made China the largest user and emitter of Hg in the world, however, the effect of such economic development on Hg inputs to the nearby oceans is still unclear. In this study, the influx and isotopic composition of Hg in four 210Pb-dated sediment cores was examined to investigate changes in Hg deposition to major marginal seas in China. Our results show a clear increase in Hg influx since the 1950s, during which time China started its First National Five-Year Plan for economic development. Mercury influxes have increased rapidly since the late 1970s, which coincides with the launching of China’s Economic Reform policy. Increased input of industrial Hg is causative for the observed Hg influx increases. Both mass dependent fractionation (MDF) and mass independent fractionation (MIF) of Hg isotopes were observed. Values of δ202Hg in all cores have shifted following the 1950s, and δ202Hg has increased dramatically (-2.0 to -0.7‰). Offshore cores show small but significantly positive MIF signatures (Δ199Hg: 0.1 to 0.3‰), whereas open ocean cores mainly show slightly negative MIF signals (Δ199Hg:-0.2 to 0‰). Negative MIF in the deep layers of coastal cores suggest input by soil Hg; however, open ocean cores exhibit positive MIF, attributed to atmospheric deposition. Uppermost portions of the cores had lower MIF signals than deep sediments, which can be explained by the dilution of industrial Hg. Historical Hg influxes of industrial, watershed, and atmospheric Hg were calculated a triple-mixing isotope model. According to the model output, increases in industrial Hg began in the 1950s and accelerated in the 1970s, in accordance with increases in industrial Hg inputs due to the economic development of China. Interestingly, decreases in watershed Hg influx were observed since the 1960s, and this decline may be attributed to intense dam construction, enhanced water-soil conservation, and increased water consumption during this period.
STABLE ISOTOPIC EVIDENCE FOR MERCURY ACCUMULATION IN THE MONTANE FORESTS IN SOUTHWEST CHINA
Mercury (Hg) accumulation in montane forested areas plays an important role in global Hg cycling. In China, montane forests account for about 90% forested areas, 25% of which located in Southwest China. In this study, we characterized stable Hg isotopic composition at the background forest sites of Mt. Ailao, Mt. Leigong, Mt. Gongga, and 23 forests in Tibetan Plateau. We aimed to identify Hg isotope fractionation caused by the translocation of atmospheric Hg to the forest floor. Similar isotope compositions were observed in foliage of different tree species. At Mt. Ailao, the Hg isotope signature of bulk leaf sprout samples (δ202Hg = 0.08±0.74‰, Δ199Hg = −0.20±0.14‰) was similar to those of gaseous air samples (δ202Hg= 0.37±0.44‰, Δ199Hg = −0.18±0.04‰) suggesting that atmospheric Hg0 is a major source of Hg found in foliage. Negative δ202Hg-shift (3‰) and Δ199Hg-shift (0.10-0.15‰) were observed during the growing season. Hg0 effluxes from mature foliage exhibits a large, positive mass independent fractionation (MIF) signature (Δ199Hg = 0.17±0.40‰) in contrast to ambient air and leaf samples. During a 2-year litter decomposition period, δ202Hg, Δ199Hg and Hg mass varied slightly, suggesting a steady Hg accumulation in decomposing litter biomass. In contrast to those observed in fully decomposed litter (Oa soil) and 0-5 cm mineral soils, positive δ202Hg-shift (0.5-1‰) was observed, and 0-0.2‰ negative Δ199Hg-shift was dependent on the density of forest canopy. The negative Δ199Hg in the surface soil samples were comparable to the observed Δ199Hg in litter, but significantly different from the positive Δ199Hg in throughfall/rainfall at 26 forest sites. This suggests that Hg input from litter is a predominant source for Hg accumulation on high montane forest floor. Enhanced Hg accumulation and more negative MIF in soil was observed at higher elevations at Mt. Leigong while such altitudinal trends were not found at Mt. Ailao and the sites of Tibetan Plateau. Statistical analysis suggests that precipitation and temperature mediated litter biomass production plays an important role in Hg accumulation in forest ecosystems.
PROVING THE FEASIBILITY OF PASSIVE SAMPLING FOR ISOTOPIC FINGERPRINTING OF ATMOSPHERIC MERCURY
Plants play an important role in the biogeochemical cycle of mercury (Hg) in the environment. It is generally considered that roots act as a barrier to the translocation of soil Hg to aboveground tissues, thereby mitigating Hg stress. However, a critical question that remains to be answered is how Hg is assimilated and sequestered in root. To address this question, we collected Houttuynia cordata, a widely distributed plant in southern China, from Wuchuan Mercury Mine, SW China, and measured the isotopic composition of Hg using multi-collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) and its chemical forms using high energy-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy. Houttuynia cordata Thunb can be a potential accumulating plant of soil Hg due to its large root biomass and relatively high Hg concentration. The concentrations of Hg varied from 0.4 to 1.5 mg/kg DW in primary roots, and 2.3 to 16.0 mg/kg DW in secondary roots, and were similar to the Hg concentration in soils (0.2 to 5.2 mg/kg DW). Both mass dependent fractionation (MDF, denoted δ) and mass independent fractionation (MIF, denoted Δ) of Hg isotopes were observed in the plant-soil system. Significant differences in δ202Hg values were observed between soils (-2.3‰≤ δ202Hg ≤ -1.2‰ ±0.1‰) and roots (primary root:-5.1‰ ≤ δ202Hg ≤ -3.7‰ ±0.1‰; secondary root: -5.7 ‰≤ δ202Hg ≤ -3.2‰ ±0.1‰). Soil Hg had no MIF (-0.07‰ ≤ Δ199Hg ≤ 0.04‰ ±0.04‰), in contrast to primary (0.03‰ ≤ Δ199Hg ≤ 0.11‰ ±0.04‰) and secondary roots (0.06‰ ≤ Δ199Hg ≤ 0.14‰ ±0.04‰). Although small, the differences of MIF between soil and roots were statistically significant (P < 0.05). The more negative δ202Hg values in roots relative to the soils suggest that MDF occurs during the transport and diffusion of Hg from the rhizosphere to the root interior. The positive MIF values in roots suggest that Hg may bind to thiol ligands. Complexation of Hg to thiol sulfur in the roots was demonstrated by HR-XANES, which showed that 82% to 100% (± 10%) of Hg was in Hg(SR)2 complex. Small amounts of nano-particulate β-HgS were detected in three root samples (up to 18 ± 10%). In soils, Hg was mainly present as α-HgS and nano-particulate β-HgS. The absence of α-HgS and paucity of nano-particulate β-HgS in roots and lack of detectable Hg(SR)2 in soils suggest that mercury uptake proceeds through the oxidation of mercury sulfide and subsequent solubilization of divalent mercury. The underlying molecular mechanisms leading to mercury release, transfer, and accumulation in roots can be accompanied by the fractionation of the Hg isotopes.
UNDERSTANDING HG UPTAKE AND SEQUESTRATION IN THE ROOT OF HOUTTUYNIA CORDATA USING STABLE HG ISOTOPE TRACER AND HIGH ENERGY-RESOLUTION X-RAY ABSORPTION NEAR EDGE STRUCTURE SPECTROSCOPY
Plants play an important role in the biogeochemical cycle of mercury (Hg) in the environment. It is generally considered that roots act as a barrier to the translocation of soil Hg to aboveground tissues, thereby mitigating Hg stress. However, a critical question that remains to be answered is how Hg is assimilated and sequestered in root. To address this question, we collected Houttuynia cordata, a widely distributed plant in southern China, from Wuchuan Mercury Mine, SW China, and measured the isotopic composition of Hg using multi-collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) and its chemical forms using high energy-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy. Houttuynia cordata Thunb can be a potential accumulating plant of soil Hg due to its large root biomass and relatively high Hg concentration. The concentrations of Hg varied from 0.4 to 1.5 mg/kg DW in primary roots, and 2.3 to 16.0 mg/kg DW in secondary roots, and were similar to the Hg concentration in soils (0.2 to 5.2 mg/kg DW). Both mass dependent fractionation (MDF, denoted ) and mass independent fractionation (MIF, denoted ) of Hg isotopes were observed in the plant-soil system. Significant differences in 202Hg values were observed between soils (-2.3 202Hg -1.2 0.1) and roots (primary root:-5.1 202Hg -3.7 0.1; secondary root: -5.7 202Hg -3.2 0.1). Soil Hg had no MIF (-0.07 199Hg 0.04 0.04), in contrast to primary (0.03 199Hg 0.11 0.04) and secondary roots (0.06 199Hg 0.14 0.04). Although small, the differences of MIF between soil and roots were statistically significant (P < 0.05). The more negative 202Hg values in roots relative to the soils suggest that MDF occurs during the transport and diffusion of Hg from the rhizosphere to the root interior. The positive MIF values in roots suggest that Hg may bind to thiol ligands. Complexation of Hg to thiol sulfur in the roots was demonstrated by HR-XANES, which showed that 82% to 100% ( 10%) of Hg was in Hg(SR)2 complex. Small amounts of nano-particulate -HgS were detected in three root samples (up to 18 10%). In soils, Hg was mainly present as -HgS and nano-particulate -HgS. The absence of -HgS and paucity of nano-particulate -HgS in roots and lack of detectable Hg(SR)2 in soils suggest that mercury uptake proceeds through the oxidation of mercury sulfide and subsequent solubilization of divalent mercury. The underlying molecular mechanisms leading to mercury release, transfer, and accumulation in roots can be accompanied by the fractionation of the Hg isotopes.