The chemistry of mercury in the equatorial Pacific Ocean

Date of Completion

January 1991


Biogeochemistry|Chemistry, General|Geochemistry




The upper ocean biogeochemical cycling of mercury (Hg) is an important part of the global Hg cycle because the ocean is both a source and sink for atmospheric Hg. Atmospheric deposition of Hg and gas evasion of elemental Hg (Hg$\sp0)$ are the major routes for Hg exchange at the water-air interface. Improved analytical techniques now allow the determination of dissolved ionic Hg, monomethylmercury (MMHg), gaseous mercury species (Hg$\sp0$ and dimethylmercury $\{$DMHg$\}),$ and particulate Hg species; and thus permit a detailed examination of the aquatic Hg biogeochemical cycle. This dissertation examined the hypothesis that inorganic divalent mercury $\{$Hg(II)$\}$ is the substrate for methylation, and for Hg$\sp0$ formation, in the marine environment. The Hg(II) substrate hypothesis, derived from laboratory methylation experiments, was untested in aquatic systems. The equatorial Pacific Ocean was chosen as the principal aquatic study site because previous work suggested that its highly productive surface waters, and extensive low oxygen region were zones of MMHg, DMHg and Hg$\sp0$ formation. This study provided evidence that DMHg, MMHg and Hg$\sp0$ are formed in the low oxygen waters of the equatorial Pacific Ocean from reactive mercury (Hg$\sb{\rm R}),$ a suitable measure of the available substrate. Reactive Hg is supplied to the sub-thermocline waters principally by particulate dissolution. Upwelling and precipitation are the major sources of Hg$\sb{\rm R}$ to the mixed layer of equatorial Pacific, and the rate of Hg$\sp0$ evasion at the sea surface is related to the supply of Hg substrate. In the mixed layer, Hg$\sp0$ is produced principally by reduction of Hg(II), while in the sub-thermocline waters Hg$\sp0$ is probably formed by demethylation. Atmospheric deposition supplies little MMHg, indicating that MMHg is formed in situ in these aquatic systems. Modelling studies supported these observations about Hg biogeochemical cycling. This work has demonstrated that methylated mercury compounds and Hg$\sp0$ are formed within the upper ocean of the equatorial Pacific, and in other aquatic systems, from ionic mercury $\{$Hg(II)$\}$ and that the rate of supply of labile inorganic mercury is the primary factor controlling the production of DMHg, MMHg, and Hg$\sp0$ in the upper ocean of the equatorial Pacific. ^