Identification of Atmospheric Mercury Input to Ecosystems From Precipitation Using Coupled Δ200Hg and Δ204Hg Fractionation

Abstract:

Mercury has seven stable isotopes, and during most biogeochemical reactions all of the isotopes fractionate mass dependently (MDF; relative to δ^202/198Hg). The odd isotopes also fractionate mass independently (MIF) during reactions involving the magnetic isotope and nuclear volume effects. In 2010 we first reported MIF of ²⁰⁰Hg in precipitation (Gratz et al), and in 2013 we reported MIF of ²⁰⁴Hg in precipitation (Demers et al). Measurements of Δ²⁰⁰Hg are becoming more common and several studies have now used Δ²⁰⁰Hg as a tracer of Hg(II) deposited from the atmosphere (Chen et al, 2012; Strok et al 2015). Δ²⁰⁴Hg is much less commonly measured and reported, but sheds additional light on the mechanisms that might cause even isotope MIF. We observe Δ²⁰⁴Hg to be of opposite sign and ~2x the magnitude of Δ²⁰⁰Hg. The presence of coupled Δ²⁰⁰Hg and Δ²⁰⁴Hg is most useful for detecting precipitation inputs of Hg to ecosystems. We have measured Δ²⁰⁰Hg and Δ²⁰⁴Hg in hundreds of samples of precipitation, invertebrates, fish, moss, lichen, ice crystals and GEM at many locations in North America and Hawaii. When average values for each sample type at each study location are plotted as Δ²⁰⁰Hg versus Δ²⁰⁴Hg they fall on a linear array with slope=0.57 and R²=0.92. Samples of sediment, coal, various rock-types, and point-source contaminants have contrasting Δ²⁰⁰Hg and Δ²⁰⁴Hg indistinguishable from zero. We also use combined Δ²⁰⁰Hg and Δ²⁰⁴Hg to explore mechanisms leading to even isotope MIF. Ghosh et al (2012) measured nuclear volume fractionation and found even MIF to be undetectable. Mead et al (2013) calculated even mass MIF from nuclear self-shielding and it does not fit our observations of Δ²⁰⁰Hg/Δ²⁰⁴Hg in natural samples; they also measured MIF caused by implantation of Hg into glass in compact fluorescent lights (CFLs) and this is consistent with Δ²⁰⁰Hg/Δ²⁰⁴Hg in atmospheric Hg(II), suggesting the possibility of a common fractionation mechanism.