Transformations of mercury in the marine water column
Citable URI
https://hdl.handle.net/1912/6434DOI
10.1575/1912/6434Keyword
Seawater; Mercury content; Kilo Moana (Ship) CruiseAbstract
Methylation of mercury (Hg) in the marine water column has been hypothesized to serve
as the primary source of the bioaccumulating chemical species monomethylmercury (MMHg) to
marine food webs. Despite decades of research describing mercury methylation in anoxic
sediments by anaerobic bacteria, mechanistic studies of water column methylation are severely
limited. These essential studies have faced analytical challenges associated with quantifying
femtomolar concentrations of the methylated Hg species dimethylmercury (DMHg) and MMHg
in marine systems. In addition, the complex biogeochemical cycling of Hg in natural systems
require consideration of gaseous, dissolved, and particulate species of Hg in order to probe
potential controls on its ultimate transfer into marine food webs.
The presented work provides a comprehensive study of Hg chemical speciation and
transformations in Tropical Pacific waters. We developed an analytical method for MMHg
determination from seawater that has the potential to ease measurements of MMHg distributions,
as well as mechanistic studies of Hg species transformations.
We used this method, in addition to previously established methods, to measure dissolved
and particulate Hg species distributions and fluxes along a transect of the Pacific Ocean. Over
significant gradients in oxygen utilization and primary productivity, we observed a region of
methylated Hg species focused in the Equatorial Pacific that appeared spatially separated from
higher concentrations in North Pacific Intermediate Waters. From the first full water column
depth profiles of this region, we also observed the intrusion of elevated Hg into deep waters of the
Equatorial and South Pacific Ocean.
In addition we observed substantial potential rates of mercury methylation in subsurface
and low oxygen waters along the Pacific transect as well as the Sargasso Sea using Hg isotope
tracers. We observed dynamic production and decomposition of methylated Hg in low
productivity waters, despite low ambient methylated Hg concentrations. From the addition of
bulk organic matter as well as individual compounds important for methylation in anaerobic
bacteria, we observe no simple limitation of Hg methylation in marine waters but highly dynamic
conversion of Hg between methylated and inorganic species.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2014
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Suggested Citation
Thesis: Munson, Kathleen M., "Transformations of mercury in the marine water column", 2014-02, DOI:10.1575/1912/6434, https://hdl.handle.net/1912/6434Related items
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