Congratulations to Laura Motta Medina who defended her dissertation on Tuesday, November 12, 2019
Advisor: Joel Blum
In our quest to understand the environmental chemistry of mercury (Hg) in aquatic ecosystems and expand our understanding of stable isotope fractionation signatures of heavy elements, I conducted studies designed to improve our knowledge of the marine biogeochemical cycle of Hg. My emphasis was on determining the photochemical reaction involving Hg that is behind the mass independent isotope fractionation (MIF; measured as Δ199Hg), preserved in pelagic fish. 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. My research is divided in three core sections:
I. Field observations:
I will start by discussing the first Hg concentrations, speciation, and stable isotopes of marine zooplankton in the Pacific Ocean (Chapter 2,5). Then, I will talk about the advantages of utilizing Hg stable isotopes to study methylmercury (MeHg) photochemistry in the ocean over traditional laboratory or field experiments (Chapter 3).
From the field observations, it was demonstrated that Hg stable isotopes are excellent proxies for investigating Hg in marine ecosystems and monitor the degree of photochemical degradation in open ocean waters. However, the mechanism behind MIF and the role of HgX2 and MeHgX (X = S, N, or O organic ligands, and Cl) remained unclear and limited our interpretation of MIF. I conducted laboratory HgX2 in deionized water and demonstrated that, unlike traditionally isotope fractionation, MIF is controlled by the photophysical aspects of the reaction, such as intersystem crossing and hyperfine coupling (Chapter 4). Then, I investigated the photolysis of MeHg in artificial and natural seawater. The Hg stable isotope photochemistry experiments demonstrated that in surface marine waters the photolysis of MeHg is controlled by reactive inorganic CH3Hg+ and CH3HgOH and not by MeHg coordinated to dissolved organic matter (Chapter 5).
In Chapters 2 through 5 it was shown that Hg stable isotope measurements can elucidate reaction mechanisms, ligands associated with Hg, and many complex environmental processes involving Hg. However, one big question remained: what is the underlying chemical mechanism of the MIF that produces Δ199Hg anomalies? I will discuss computational approaches used to explore the possibility of the magnetic isotope effect by investigating excitation energies and relativistic properties of Hg complexes (Chapter 6).
I conclude by discussing critical questions that remain concerning the chemistry of this global pollutant and I make recommendations for future research directions (Chapter 7).