Chris Barton (Committee Member), William Fitzgerald (Committee Member), Chad Hammerschmidt (Advisor), Steve Higgins (Committee Member), Carl Lamborg (Committee Member), Sarah Tebbins (Committee Member)
Doctor of Philosophy (PhD)
The distribution of mercury (Hg) in the ocean is complex as a result of in situ chemical transformations and inputs from natural and anthropogenic sources. Within the ocean, inorganic Hg is methylated to monomethylmercury (MMHg), which bioaccumulates and biomagnifies in marine food webs and poses a health risk to humans who eat fish. The biogeochemistry of Hg in the ocean has been studied for decades, however, recently improved sampling and analytical techniques have allowed for an enhanced understanding of global distributions of different Hg species. This dissertation uses a newly developed method for the analysis of MMHg that improves detection limits 10-fold over previous methods and allows for separation and analysis of dissolved gaseous dimethylmercury (DMHg) in the same sample. Filtered total Hg (HgT), MMHg, DMHg, and elemental Hg (Hg(0)) were measured in high vertical and horizontal resolution in the water column of the North Atlantic (GA03) and eastern tropical South Pacific (GP16) Oceans, using vetted methods for the trace-metal clean sampling and analysis of Hg through the U.S. GEOTRACES program. Total Hg and MMHg were also measured in suspended particles across both sections. A wide range of oceanographic features important to Hg chemistry were sampled including oligotrophic waters in the Atlantic, productive upwelling waters in the Pacific, hydrothermal vent plumes, and deep and intermediate water masses of varying ages and source regions. The subsurface distribution of Hg(0) was connected to the nitrogen cycle, with nutrient-like vertical distributions, similar to nitrate, in the Atlantic basin and increasing Hg(0) concentrations with denitrification in the Pacific. Filtered total Hg exhibited both scavenged- and nutrient-type vertical distributions in the Atlantic and nutrient-type distributions in the Pacific. Elevated concentrations of HgT were observed in a hydrothermal vent plume stemming from the Mid-Atlantic Ridge; however, Hg was not increased in a plume extending from the East Pacific Rise. Total Hg concentrations increased from younger to older Pacific deep waters but were anomalously high in Atlantic deep waters subducted within the past 200 y due to anthropogenic inputs. Young deep water impacted by anthropogenic Hg in the Atlantic contained 1.4x more methylated Hg (MMHg + DMHg) on average compared to unimpacted deep water in the Pacific. Dimethylmercury was often the dominant form of methylated Hg in deep water and concentrations of both MMHg and DMHg increased in aging Pacific deep water. Vertically stratified maxima of MMHg and DMHg were observed often near the subsurface chlorophyll maximum and frequently in low-oxygen thermocline waters where MMHg concentrations were 2x greater than DMHg. Methylated Hg was weakly positively correlated with apparent oxygen utilization, however, methylated Hg concentrations decreased with greater oxygen consumption. Concentrations of MMHg and DMHg were similar between Atlantic thermocline waters affected by anthropogenic Hg inputs and thermocline waters underlying the highly productive upwelling region in the eastern Pacific, despite substantial differences in oxygen concentrations. Analytical separation of methylated Hg species revealed unique and independent distributions of MMHg and DMHg. Data from both cruise sections suggests that MMHg and DMHg are produced throughout the water column in oxygenated subsurface waters, low-oxygen thermocline waters, and likely in deep water masses. Comparison of oceanic sections following thermohaline circulation revealed the impact of anthropogenic Hg inputs with increased concentrations of HgT, MMHg, and DMHg in young (< 200 y) deep and subsurface Atlantic waters.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
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