Mercury deposition to snow
Mercury (Hg) is a persistent neurotoxin of concern to human health particularly in its methylated form. Hg biogeochemical cycling involves multiple complex processes and some have not been adequately characterized. With improvements in multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS), Hg stable isotope measurements have proven to be a powerful tool in elucidating the cycling of Hg.
Hg deposition in snow is an understudied aspect of the biogeochemical cycle of mercury. Recent preliminary studies have shown that Hg deposition with snow can be supplemented by gaseous elemental Hg (GEM) deposition to the snowpack while Hg can be lost from snow by photochemical reduction and evasion of GEM (Douglas et al. 2018). Each of these processes have distinctive Hg isotope fractionations associated with them. Snow can be an important vector for Hg deposition to both aquatic and terrestrial ecosystems, providing a source of Hg that can then be transformed into the toxic form of Hg. Better characterization of Hg deposition in snow would improve catchment scale mass balances and the use of Hg isotopes to reconstruct historic Hg deposition to forests and lakes.
A previous study conducted at the University of Michigan Biological Station (UMBS) demonstrated the transfer of Hg between Douglas Lake and the adjacent forest ecosystem (Kwon et al. 2015). With the use of Hg stable isotopes, they investigated the relative importance of methyl mercury transfer between these ecosystems and characterized the different pathways of Hg transport. This study exemplifies the use of the UMBS site for studies of Hg cycling. Furthermore, the UMBS is a site for the collection of precipitation for Hg concentration analyses by the National Mercury Deposition Network.
Another previous study (in northern Wisconsin) investigated the Hg isotopic fractionation associated with GEM deposition to forest vegetation (Demers et al 2013). This work fundamentally changed the way we think of Hg deposition to forests, but it was conducted during summer and provided no information on the influence of snowfall on deposition amounts or isotopic composition.
Combining the interdisciplinary faculty perspectives and expertise of the LSA departments of Earth and Environmental Sciences and Ecology and Evolutionary Biology and the School of the Environment and Sustainability, we propose to use Hg isotopes to further understand the atmospheric deposition of Hg to terrestrial and aquatic ecosystems. To perform this study, we will collect snow samples directly after a large snow events and then continue to collect snow samples on consecutive days after snow events to better understand the influence of photochemical reactions in snow on the isotopic evolution of Hg as it sits exposed to sunlight.
PhD candidate Aaron Kurz will work on this project as part of his dissertation work. In collaboration with UMBS staff he will collect snowfall before exposure to sunlight, and then several times per day for five days following snowfall. Samples will be preserved and transported to Ann Arbor. Kurz helped develop an ion exchange separation technique for Hg low concentration water samples and he will use this technique to separate Hg from the samples. He will then use (MC-ICP-MS) for the high precision measurement of Hg isotope ratios in these samples.