Odd Isotope Deficits in Atmospheric Hg Measured in Lichens

Abstract

Redox reactions govern mercury (Hg) concentrations in the atmosphere because fluxes (emissions and deposition), and residence times, are largely controlled by Hg speciation. Recent work on aquatic Hg photoreduction suggested that this reaction produces non-mass dependent fractionation (NMF) and that residual aquatic Hg(II)is characterized by positive delta199Hg and delta201Hg anomalies. Here, we show that atmospheric Hg accumulated in lichens is characterized by NMF with negative delta199Hg and delta201Hg values (-0.3 to -1 per thousand), making the atmosphere and the aquatic environment complementary reservoirs regarding photoreduction and NMF of Hg isotopes. Because few other reactions than aquatic Hg photoreduction induce NMF, photochemical reduction appears to be a key pathway in the global Hg cycle. Based on a NMF isotope mass balance, direct anthropogenic emissions may account for only 50 +/- 10% of atmospheric Hg deposition in an urban area of NE France. Furthermore, isotopic anomalies found in several polluted soils and sediments strongly suggests that an important part of Hg in these samples was affected by photoreactions and has cycled through the atmosphere before being stored in the geological environment. Thus, mercury isotopic anomalies measured in environmental samples may be used to trace and quantify the contribution of source emissions.

Forensic Assessment of Metal Contaminated Rivers in the 21st Century Using Geochemical and Isotopic Tracers

“…The vertical bars represent the estimated crust-and mantel-derived Hg. [229], Gosh et al [230], Carignan et al [234], Bergquist et al [191].…”

Mercury isotope variations in surface soils in different contaminated areas in Guizhou Province, China

“…Fractionation of Hg has been detected in a large variety of environmental compartments, e.g. hydrothermal Hg-bearing ores [8][9][10], coals [11,12], soil/sediments [7,[13][14][15], atmospheric Hg [16] and biological tissues [17]. Above cited studies have unequivocally demonstrated that variations in the Hg isotopic signature could be used in the future as tracers to distinguish between Hg sources and important chemical transformation processes in the environment.…”

“…[16,29], Hg entering the environment through varies industrial activities probably have little to no MIF signatures. However, large variation of MIF was observed in the sub-surface environment compartments such as in aquatic systems [17] and atmospheres [16].…”

“…[16,29], Hg entering the environment through varies industrial activities probably have little to no MIF signatures. However, large variation of MIF was observed in the sub-surface environment compartments such as in aquatic systems [17] and atmospheres [16]. The negative MIF of Hg in the atmospheric samples is inspiring because elemental Hg 0 has an extended tropospheric residence time entailing global-scale transportation and interhemispherical boundary-layer mixing ratio homogeneities [30,31].…”

“…Hence, the isotope composition of Hg in atmosphere can be altered through global and regional cycling of Hg, after Hg is emitted from natural sources and anthropogenic sources. Epiphytic lichens have been used as surrogates for decades in reflecting atmospheric wet/dry Hg deposition [16]. As shown in Figure 3, most lichen samples that obtained Hg after it had been through cycling in the environment have ∆…”