The Magnetic Isotope Effect and Anomalous Case of Mercury Stable Isotope Fractionation

Mercury (Hg) is a global pollutant that poses a public health threat because exposure through consumption of fish or rice can lead to adverse neurological and immunological effects. Despite decades of research there are significant gaps in the understanding of Hg biogeochemistry and the measurement of Hg stable isotopes has proven to be an effective tool for answering some of the outstanding questions. Mass-dependent fractionation (MDF, reported as δ202Hg) and mass-independent fractionation (MIF, reported as Δ199Hg) of Hg isotopes occur during many biogeochemical processes and can be diagnostic of specific chemical reactions. This dissertation enhances our understanding of key pathways that control Hg toxicity in marine waters and pinpoints the key dissolved Hg species that may limit Hg methylation and bioaccumulation. The overarching goal of this research was to investigate the biogeochemical cycle of Hg in marine waters with a focus on enhancing the understanding of the photochemical decomposition of Hg in surface marine waters, which reduces the pool of Hg available for incorporation into the marine foodweb. With this goal in mind, Hg stable isotopes were used to investigate Hg cycling in North Pacific Ocean. This resulted in the first Hg stable isotope analysis of marine particles, zooplankton and open ocean precipitation (Chapter 2, 3, 5). In order to fully interpret the Hg stable isotopic composition of natural samples associated with Hg photochemistry, a number of Hg photochemistry studies were conducted on the rooftop of a laboratory building and onboard the research vessel R/V Kilo Moana in the North Pacific Subtropical Gyre (Chapter 4,5). Finally, to connect the Hg stable isotopes studies with the photochemical decomposition mechanism, a series of high-level electronic structure calculations of environmentally relevant complexes were conducted (Chapter 6). The body of research presented in this dissertation highlights the advantages of utilizing Hg stable isotopes to study Hg cycling in the environment, bypassing the limitations of traditional field and laboratory experiments. The environmental isotope measurements indicated that precipitation is the main source of Hg to surface marine waters, that both methylation and demethylation of Hg takes place in the water column, that surface marine zooplankton record a diurnal Δ199Hg variation indicative of methylmercury (MeHg) photochemistry, and that there is seasonal variability in the Hg isotope composition of particulate Hg. The MeHg photochemistry isotope experiments demonstrated that the photodecomposition of MeHg in marine waters may be controlled intracellularly by phytoplankton or by reactive inorganic MeHg complexes (e.g MeHgOH) but not by dissolved organic matter as previously hypothesized. The isotope experiments indicate that direct photolysis of Hg may be the dominant photochemical mechanism for decomposition and reduction in aquatic ecosystems, which is contrary to previous investigations. Finally, the high-level computational simulations demonstrate that spin-orbit coupling may not quench mass-independent fractionation (Δ199Hg) and that the magnetic isotope effect is a plausible mechanism for the Δ199Hg recorded in natural samples around the world. Specifically, the simulations indicate that spin-orbit coupling may limit mass-independent isotope fractionation for divalent inorganic Hg(II) complexes (e.g. Hg(thiol)2), but that spin-orbit coupling does not have any effect on mass-independent isotope fractionation of MeHg complexes.PhDEarth & Environ Sci & Chem PhDUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/153470/1/laumot_1.pd