Mercury Stable Isotopes Identify Past and Present Mercury Sources and Cycling

Abstract

Mercury (Hg) is a neurotoxic trace metal that is globally distributed and has important implications for human health. The anthropogenic use of Hg has caused the concentration of Hg in the environment to approximately triple since ~1850. It is therefore imperative to understand the historical deposition trends and modern biogeochemical cycling of Hg to better inform future policy actions regarding the release of Hg to the environment. In this dissertation, measurements of Hg stable isotopes were applied to an environmental record of historical Hg deposition and to remote, low Hg concentration atmospheric samples to answer outstanding questions regarding both historical and modern biogeochemical conditions and processes driving Hg cycling in the environment. To discern past sources of Hg and conditions controlling Hg isotope fractionation in the atmosphere, a sediment core from a remote, high elevation lake near Jackson, Wyoming was collected. Lake sediments were dated to understand temporal changes in the deposition of Hg to the sediments, revealing an approximate 3.8-fold increase in Hg flux from 1850 to the present. Additionally, measurements of Hg stable isotope ratios in the sediments indicated a shift in atmospheric chemical reactions over the same period. Analyses of wet precipitation and snow collected in the lake’s vicinity were utilized to explain the modern Hg isotopic composition observed in the lake sediments and motivated further investigation into Hg isotope fractionation in snow. Measurements of 12-hour atmospheric gaseous Hg samples were collected continuously for one week from two high elevation sites (Mount Bachelor, Oregon and Camp Davis, Wyoming) with contrasting geographic characteristics to understand both the vertical gradient of Hg in the atmosphere and better understand the role of vegetation in Hg isotope fractionation in the atmosphere. Analyses of Hg isotope ratios from samples at Mount Bachelor (mountaintop site) revealed diel variation in the isotopic composition of Hg. Nightly measurements indicated a dominant influence from the free troposphere with a distinct isotopic composition. Near the end of the sampling period, the diel variation dissipated due to a nearby forest fire that came to dominate both daytime and nighttime samples. At Camp Davis (valley site), diel variation in the isotopic composition of Hg was also observed, however, the variation at this site contrasted with observations at Mount Bachelor. Nightly inversions trapping Hg in the valley at Camp Davis and the subsequent uptake of Hg from the atmosphere by vegetation explains the fractionation observed in the residual Hg. Motivated by the analysis of Hg isotopes in snow from Lost Lake, WY, five time series of snow samples (with sampling every 12 hours for 48 hours) were collected at two sites in Michigan (Dexter and Pellston). A time series collected in Dexter during a polar vortex revealed progressively more negative odd-mass independent fractionation (MIF), similar to observations in Arctic snow. In contrast, the fractionation of Hg isotopes in all of the other snow samples progressed towards more positive odd-MIF, indicating a difference in oxidants and binding ligands associated with the Hg in snow. Finally, snow samples indicative of snowmelt were used to estimate the Hg isotopic composition of Hg deposited to mid-latitude ecosystems via snow. As a whole, this dissertation begins to answer outstanding questions concerning processes that control the fractionation and distribution of Hg in the environment and creates a platform for future researchers to expand upon.