Characterization of Mercury Particle Flux Using 238U:234Th Disequilibria in the North Pacific Subtropical Gyre

A critical part of understanding the biogeochemistry of mercury (Hg) in marine systems is characterizing its export to depth on sinking particles. Seasonal samples were collected in the North Pacific Subtropical Gyre on several cruises in 2014-2015. Using high resolution water column measurements of ²³⁸U:²³⁴Th disequilibria, we determined total particulate Hg fluxes on both small (1-53 µm) and large (> 53 µm) particles. Preliminary data show distinct seasonal differences in water column derived ²³⁴Th fluxes measured at 150 m, ranging from a low of 493 ± 15 dpm m^-2 day^-1 in February to as high as 1890 ± 116 dpm m^-2 day^-1 in September 2014. Total Hg/Th ratios in small particles ranged from 0.21 pmol/dpm to 0.27 pmol/dpm. This results in downward small particulate total Hg fluxes at 150 m of 102 pmol m^-2 day^-1 in February to 502 pmol m^-2 day^-1 in September, consistent with patterns observed for particulate organic carbon fluxes. In comparison, large particle total Hg fluxes were 50% lower (estimated at ~61 pmol m^-2 day^-1) and were similar between seasons, whereas organic carbon fluxes increased significantly in September. We argue that differences in small and large particles fluxes in total Hg are likely due to surface adsorption of inorganic Hg onto particles that scales with surface area to volume ratios, while organic carbon flux increases are due to changes in phytoplankton speciation and abundance (presence of Trichodesmium spp) and zooplankton grazing and export based on compound specific amino acid isotope data.