Air-sea gas exchange is a critical driver of the global mercury (Hg) cycle, regulating surface ocean concentrations and extending the residence time of Hg within the biosphere. While recent research has advanced our understanding of ionic Hg (Hg^II) deposition and elemental Hg (Hg⁰) evasion, the dynamics governing methylated Hg species, namely monomethylmercury (MMHg) and volatile dimethylmercury (DMHg), remain poorly constrained. To address these knowledge gaps, high-resolution atmospheric and surface water measurements were conducted during the U.S. GEOTRACES GP17-OCE cruise aboard the R/V Roger Revelle (Nov 2022 – Jan 2023). The transect spanned a diverse range of oceanographic regimes from San Diego to Tahiti and Punta Arenas, Chile, capturing the equatorial Pacific, the oligotrophic South Pacific subtropical gyre, the sub-Antarctic front, major zones of upwelling and water mass formation, and the Antarctic and Chilean continental shelves. Atmospheric Hg speciation (Hg⁰, reactive gaseous Hg, and particulate Hg) was quantified at high resolution using a Tekran 1130/1135/2537X system, while dissolved Hg⁰ and DMHg in surface waters were continuously monitored via automated, high-resolution equilibrium degassing systems. This presentation will evaluate the magnitude and direction of inorganic and methylated Hg air-sea fluxes across the South Pacific and Southern Ocean. We assess the relative importance of gas exchange as a net oceanic sink compared to competing biogeochemical pathways, investigate the potential role of DMHg evasion as an atmospheric precursor contributing to MMHg deposition in remote marine environments, and discuss the broader implications of shifting climate dynamics on these ocean-atmosphere feedback mechanisms.

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