Understanding the Modern and Past Mercury Biogeochemical Cycle Using Hg Stable Isotopes

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The long-term goal of my research program is to use Hg isotopes to improve our understanding of the Hg biogeochemical cycle and to develop Hg isotopes as a proxy of past environmental changes. Mercury is a globally distributed metal that bioaccumulates in aquatic food webs leading to dangerous exposure in humans and wildlife. Mercury is often emitted to the atmosphere in its reduced form, gaseous elemental Hg (GEM), by both natural and anthropogenic sources. GEM is relatively stable and has a long residence time (~1 yr), which allows it to be distributed globally. Eventually GEM is oxidized and deposited to terrestrial and aquatic systems quickly (hours to weeks). One of the major complexities of the Hg cycle is that after deposition, Hg can be reduced back to GEM and emitted back to the atmosphere. This results in primary sources making up less than half of the Hg emissions to the atmosphere (30% anthropogenic, 10% natural) with 60% of inputs to the atmosphere being Hg re-emissions dominantly from soils and the ocean. Many key knowledge gaps still hinder our understanding of both the modern and past Hg cycle and make it challenging to predict how changes in emissions and climate will affect the Hg cycle. For example, assessing the relative contributions of different anthropogenic sources versus re-emissions on local, regional and global scales is a huge challenge and is often dependent on models without enough constraints from data. Also, a recent assessment of GEM exchange to and from soils highlighted that very large uncertainties still exist especially in forested ecosystems. A rapidly growing tool to study Hg is Hg stable isotope geochemistry, which was largely driven by the discovery of large mass independent fractionation (MIF) of Hg isotopes. My Discovery program will focus on three goals that are exciting new areas where Hg isotopes are likely to make large impacts on the field of Hg research: (1) the development and application of new methods to measure Hg concentrations and isotopic compositions of atmospheric Hg species, (2) the use of Hg isotopes to quantify and understand sources and remobilization of Hg in soils, and (3) the development and testing of Hg and Hg isotopes as a proxy of ancient volcanic emissions. Objectives 1 and 2 will advance modelling and assessment of sources of Hg to the atmosphere and to soils through much needed experimental studies, novel development of samplers for collection and isotopic analysis of Hg, and fieldwork to verify and apply new techniques. The third objective will investigate a primary natural source of Hg to the atmosphere, volcanism, and the isotopic signatures of volcanic Hg species along with how these signatures are preserved in geological records. The isotopic characterization of this geogenic source of Hg will contribute to our understanding of the modern Hg cycle and will establish the baseline for Hg isotopes as a reliable tracer of ancient volcanic emissions.