Distribution of mercury species across a zonal section of the Eastern Tropical South Pacific (2013 U.S. GEOTRACES)

Abstract:

Mercury (Hg) speciation in filtered water and suspended particulate matter was measured during the 2013 U.S. GEOTRACES zonal section of the tropical South Pacific Ocean. Filtered and gaseous Hg species were determined in high vertical and horizontal resolution across a narrow continental margin and trench, large gradients in biological productivity and N*, a hydrothermal vent field on the East Pacific Rise (EPR), and an expansive suboxic oxygen minimum zone. Total Hg upwelled from depth near Peru, and in deep water, concentrations were significantly greater east of the EPR in Pacific Deep Water compared to Lower Circumpolar Deep Water west of the EPR. Elemental Hg (Hg⁰) was correlated negatively with N*, suggesting that Hg reduction may be linked to denitrification; however, the highest concentrations of Hg⁰ were found along the Peru margin and within the Peru trench. Concentrations of filtered and particulate monomethyl-Hg (MMHg) and gaseous dimethyl-Hg (DMHg) were increased in oxygen minimum/deficit zones across the section. DMHg:MMHg and DMHg:total Hg ratios were significantly greater in sub-oxic (<2 μmol/kg O₂) waters near the coast of Peru compared to more oxygenated waters (2-275 μmol/kg O₂) at similar depths. Within the hydrothermal plume extending west from the EPR, total Hg concentrations were not elevated compared to surrounding bottom waters, however, MMHg increased and DMHg decreased near the plume, similar to the behavior of methylated-Hg near the TAG hydrothermal vent measured during U.S. GA03.