We review recent progress in our understanding of the global cycling of mercury (Hg), including best estimates of Hg concentrations and pool sizes in major environmental compartments and exchange processes within and between these reservoirs. Recent advances include the availability of new global datasets covering areas of the world where environmental Hg data were previously lacking; integration of these data into global and regional models is continually improving estimates of global Hg cycling. New analytical techniques, such as Hg stable isotope characterization, provide novel constraints of sources and transformation processes. The major global Hg reservoirs that are, and continue to be, affected by anthropogenic activities include the atmosphere (4.4–5.3 Gt), terrestrial environments (particularly soils: 250–1000 Gg), and aquatic ecosystems (e.g., oceans: 270–450 Gg). Declines in anthropogenic Hg emissions between 1990 and 2010 have led to declines in atmospheric Hg0 concentrations and HgII wet deposition in Europe and the US (− 1.5 to − 2.2% per year). Smaller atmospheric Hg0 declines (− 0.2% per year) have been reported in high northern latitudes, but not in the southern hemisphere, while increasing atmospheric Hg loads are still reported in East Asia. New observations and updated models now suggest high concentrations of oxidized HgII in the tropical and subtropical free troposphere where deep convection can scavenge these HgII reservoirs. As a result, up to 50% of total global wet HgII deposition has been predicted to occur to tropical oceans. Ocean Hg0 evasion is a large source of present-day atmospheric Hg (approximately 2900 Mg/year; range 1900–4200 Mg/year). Enhanced seawater Hg0 levels suggest enhanced Hg0 ocean evasion in the intertropical convergence zone, which may be linked to high HgII deposition. Estimates of gaseous Hg0 emissions to the atmosphere over land, long considered a critical Hg source, have been revised downward, and most terrestrial environments now are considered net sinks of atmospheric Hg due to substantial Hg uptake by plants. Litterfall deposition by plants is now estimated at 1020–1230 Mg/year globally. Stable isotope analysis and direct flux measurements provide evidence that in many ecosystems Hg0 deposition via plant inputs dominates, accounting for 57–94% of Hg in soils. Of global aquatic Hg releases, around 50% are estimated to occur in China and India, where Hg drains into the West Pacific and North Indian Oceans. A first inventory of global freshwater Hg suggests that inland freshwater Hg releases may be dominated by artisanal and small-scale gold mining (ASGM; approximately 880 Mg/year), industrial and wastewater releases (220 Mg/year), and terrestrial mobilization (170–300 Mg/year). For pelagic ocean regions, the dominant source of Hg is atmospheric deposition; an exception is the Arctic Ocean, where riverine and coastal erosion is likely the dominant source. Ocean water Hg concentrations in the North Atlantic appear to have declined during the last several decades but have increased since the mid-1980s in the Pacific due to enhanced atmospheric deposition from the Asian continent. Finally, we provide examples of ongoing and anticipated changes in Hg cycling due to emission, climate, and land use changes. It is anticipated that future emissions changes will be strongly dependent on ASGM, as well as energy use scenarios and technology requirements implemented under the Minamata Convention. We predict that land use and climate change impacts on Hg cycling will be large and inherently linked to changes in ecosystem function and global atmospheric and ocean circulations. Our ability to predict multiple and simultaneous changes in future Hg global cycling and human exposure is rapidly developing but requires further enhancement.

/pdf/a-review-of-global-environmental-mercury-processes-in-2yqze7953z.pdf

A review of global environmental mercury processes in response to human and natural perturbations: Changes of emissions, climate, and land use

Forest biomass and soils represent some of the largest reservoirs of actively cycling mercury (Hg) on Earth, but many uncertainties exist regarding the source and fate of Hg in forest ecosystems. We systematically characterized stable isotope compositions of Hg in foliage, litter, and mineral soil horizons across 10 forest sites in the contiguous United States. The mass independent isotope signatures in all forest depth profiles are more consistent with those of atmospheric Hg(0) than those of atmospheric Hg(II), indicating that atmospheric Hg(0) is the larger source of Hg to forest ecosystems. Within litter horizons, we observed significant enrichment in Hg concentration and heavier isotopes along the depth, which we hypothesize to result from additional deposition of atmospheric Hg(0) during litter decomposition. Furthermore, Hg isotope signatures in mineral soils closely resemble those of the overlying litter horizons suggesting incorporation of Hg from litter as a key source of soil Hg.

The spatial distribution of Hg isotope compositions in mineral soils across all sites is modeled by isotopic mixing assuming atmospheric Hg(II), atmospheric Hg(0) and geogenic Hg as major sources. This model shows that northern sites with higher precipitation tend to have higher atmospheric Hg(0) deposition than other sites, whereas drier sites in the western U.S. tend to have higher atmospheric Hg(II) deposition than the rest. We attribute these differences primarily to the higher litterfall Hg input at northern wetter sites due to increased plant productivity by precipitation. These results allow for a better understanding of Hg cycling across the atmosphere-forest-soil interface.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015GB005323

Mercury isotope compositions across North American forests

The latest Permian mass extinction, the most devastating biocrisis of the Phanerozoic, has been widely attributed to eruptions of the Siberian Traps Large Igneous Province, although evidence of a direct link has been scant to date. Here, we measure mercury (Hg), assumed to reflect shifts in volcanic activity, across the Permian-Triassic boundary in ten marine sections across the Northern Hemisphere. Hg concentration peaks close to the Permian-Triassic boundary suggest coupling of biotic extinction and increased volcanic activity. Additionally, Hg isotopic data for a subset of these sections provide evidence for largely atmospheric rather than terrestrial Hg sources, further linking Hg enrichment to increased volcanic activity. Hg peaks in shallow-water sections were nearly synchronous with the end-Permian extinction horizon, while those in deep-water sections occurred tens of thousands of years before the main extinction, possibly supporting a globally diachronous biotic turnover and protracted mass extinction event. Previously, little direct evidence has been found to link large volcanic eruption events with the end-Permian mass extinction. Here, the authors find that mercury enrichment and isotope records in marine sections across the globe can be linked to increased volcanic activity, which resulted in the protracted Permian-Triassic biocrisis

/pdf/evidence-for-a-prolonged-permian-triassic-extinction-3esasgmut9.pdf

Evidence for a prolonged Permian-Triassic extinction interval from global marine mercury records.

The isotopic composition of atmospheric total gaseous mercury (TGM) and particle-bound mercury (PBM) and mercury (Hg) in litterfall samples have been determined at urban/industrialized and rural sites distributed over mainland China for identifying Hg sources and transformation processes. TGM and PBM near anthropogenic emission sources display negative δ202Hg and near-zero Δ199Hg in contrast to relatively positive δ202Hg and negative Δ199Hg observed in remote regions, suggesting that different sources and atmospheric processes force the mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) in the air samples. Both MDF and MIF occur during the uptake of atmospheric Hg by plants, resulting in negative δ202Hg and Δ199Hg observed in litter-bound Hg. The linear regression resulting from the scatter plot relating the δ202Hg to Δ199Hg data in the TGM samples indicates distinct anthropogenic or natural influences at the three study sites. A similar trend was also observed for Hg accumulated ...

Isotopic Composition of Atmospheric Mercury in China: New Evidence for Sources and Transformation Processes in Air and in Vegetation.

The first Reviews in Mineralogy volume on the Geochemistry of Non-Traditional Stable Isotopes was compiled before it was appropriate to include a chapter on mercury (Hg) stable isotope geochemistry. At that time there were only a few papers on this new topic (Jackson 2001; Lauretta et al. 2001; Hintelmann and Lu 2003), and there were still some important analytical issues that needed to be resolved. But the field has come a long way in a decade. Now we have a different problem; at our last count there were well over 100 publications utilizing mercury stable isotopes and it is becoming very difficult to synthesize this vast amount of exciting and rapidly developing research. Experimental studies have expanded our knowledge of the mechanisms of mercury isotope fractionation and applications of mercury isotope measurements have touched virtually every area of research in mercury biogeochemistry. There have been a number of previous reviews of the mercury stable isotope literature as it has developed (Ridley and Stetson 2006; Bergquist and Blum 2009; Yin et al. 2010; Blum 2011; Hintelmann 2012; Blum et al. 2014).

It is our view that the field has become too large to comprehensively review the entire literature on mercury stable isotopes. Ten years ago Hg isotope researchers were just beginning to explore the boundaries of natural Hg isotope variation and the mechanisms that cause this variation in the environment. At that time large and relatively easily measured isotope signals were of great interest and mercury isotope researchers were beginning to develop theories to explain mass dependent isotope fractionation (MDF) and mass independent isotope fractionation of the odd mass-numbered isotopes of mercury (odd-MIF). More recently researchers have discovered a wider range of types of isotopic variability (even-MIF), some of which are subtle and …

Recent Developments in Mercury Stable Isotope Analysis

In order to evaluate the distribution and partitioning characteristics of heavy metals in the large anthropogenically impacted Pearl River Basin, the contents of “anthropophile” elements (Cr, Ni, Cu, Zn, Cd and Pb, which are clearly influenced by human activities) were determined, and their partitioning coefficients (Kd) between water and sediments and enrichment factors (EF) were calculated for samples collected at different locations along the Pearl River main stream. The modified BCR sequential extraction procedure (proposed by the European Community Bureau of Reference in 1993), which involves the successive extraction of metals in a decreasing order of reactivity, was applied. Sediment samples from the upper, middle, and lower reaches were included in this study. The results showed that the content of most metals in water and sediment samples gradually increases from upstream to downstream, suggesting a possible input from human activities as shown by their increasing high EF, ranged from 1.4 to 3.9 for Cu, from 1.4 to 6.7 for Zn, from 2.5 to 59.1 for Cd, and from 1.7 to 8.9 for Pb, respectively. The higher partition coefficients (Kd) for Cr, Zn, and Pb (105–106) indicated that they were mainly transported in solid phase, while parts of Ni, Cu, and Cd were transported in dissolved phase as they display relatively lower Kd in the range of 104–105. According to the results of the BCR leaching, the percentage of non-residual fraction of heavy metals in the sediments showed a decreasing order of Cd > Pb > Zn > Cu > Ni > Cr, implying that Cd and Pb were more active and bioavailable compared to the other four metals, and thus would be potentially more harmful to the watershed ecosystem.

Distribution and partitioning of heavy metals in large anthropogenically impacted river, the Pearl River, China

Mercury isotope compositions in large anthropogenically impacted Pearl River, South China

Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Trace elements (and their isotopes) in carbonates are commonly used to reconstruct paleoenvironment and paleoclimate. Understanding the processes and mechanisms of element incorporation into carbonates is thus crucial for using such geochemical parameters as paleoclimate proxies. In contrast to laboratory-based experimental results, the partitioning of trace metals between solid and solution phases in natural carbonate precipitation systems has rarely been reported. In this study, we investigated the partition coefficients of metal ions between solid and solution in the channel of the natural Baishuitai travertine system, Yunnan, China. Our results show that the partition coefficients of Li+, Na+, Mg2+, Sr2+ and Ba2+ are   1, consistent with the results found in previous experimental studies. Although the substitution for Ca2+ is likely the main uptake process of these metals into calcite, depending on their ionic radius and charge, trace elements may also be incorporated by adsorption or physical entrapment. Our study shows that unlike laboratory experiments performed under specific conditions, the partitioning of metals between two phases in the natural travertine system could be controlled by several, even multiple, environmental factors (e.g., carbonate deposition rate, temperature, and pH), which should be taken into account when using trace metals (and their isotopes) in carbonate archives as a paleoclimate proxy.

Zhongwei Wang

Papers

Distribution and partitioning of heavy metals in large anthropogenically impacted river, the Pearl River, China

Mercury isotope compositions in large anthropogenically impacted Pearl River, South China

Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Coprecipitation of metal ions into calcite: an estimation of partition coefficients based on field investigation