An Investigation of the Major Transformations and Loss Mechanisms of Mercury and Selenium in the Surface Ocean
Date of Completion
Photodegradation, Mercury, Methylmercury, Demethylation, Gas Exchange, Selenium, Dimethylselenide
Field of Study
Doctor of Philosophy
The importance of methylmercury (CH3Hg) photochemical degradation, an understudied process in marine ecosystems, was investigated in variety of coastal and oceanic waters from the northeastern U.S. as well as the Atlantic, Pacific and Arctic Oceans. Degradation rate constants ranged from 0.87 to 1.67 day-1,but did not correlate with the environmental parameters measured. Further experiments investigating the reaction mechanism observed little effect of nitrate, chloride, and bromide ions. CH3Hg loss per year due to photodegradation was modeled across latitudes from the Equator to the Arctic using water column integrated rates determined for coastal wetlands, estuaries and the open ocean. A global photochemical demethylation rate of 25.3 Mmol yr-1 was calculated, representing an important flux in the biogeochemical cycle of CH3Hg.
Air-sea exchange of elemental mercury (Hg0), another important process in the biogeochemical cycle of Hg, was investigated on the U.S. GEOTRACES cruise in the Arctic Ocean in 2015. High resolution measurements of Hg0 in surface waters and the atmosphere were used to calculate evasional fluxes, and Hg concentrations determined in aerosols and precipitation were used to estimate atmospheric deposition. Overall, concentrations of dissolved Hg0 were near saturation in ice-free waters (32 ± 30 fM), but were highly enriched under contiguous ice (101 ± 98 fM, up to 544 fM). Predicted peaks in Hg0 evasion, although blocked by the sea ice barrier, were as high as 270 pmol m-2 h-1. From these estimates we can better predict the effect of a changing climate on Hg dynamics in the Arctic.
The photochemical cycling of selenium (Se), an essential micronutrient, was also studied in marine waters. Inorganic Se(IV) and Se(VI) were found in nutrient-type distributions in samples collected during the Metzyme cruise on the equatorial Pacific Ocean in 2011. Photochemically mediated redox transformation pathways studied could not explain the enhanced concentrations of Se(IV) observed, indicating that a biological process is likely involved. Photodegradation was an important sink of dimethyl selenide ((CH3)2Se) in a variety of natural waters, with reaction rate constants ranging from 18.1 to 47.0 day-1. The global loss of (CH3)2Se due to photodegradation was estimated at 28.0 Gmol yr-1.
DiMento, Brian P., "An Investigation of the Major Transformations and Loss Mechanisms of Mercury and Selenium in the Surface Ocean" (2017). Doctoral Dissertations. 1447.