The Effect of Particle Composition and Concentration on the Partitioning Coefficient for Mercury in Three Ocean Basins

The downward flux of sinking particles is a prominent Hg removal and redistribution process in the ocean; however, it is not well-constrained. Using data from three U.S. GEOTRACES cruises including the Pacific, Atlantic, and Arctic Oceans, we examined the mercury partitioning coefficient, K-d, in the water column. The data suggest that the K-d varies widely over three ocean basins. We also investigated the effect of particle concentration and composition on K-d by comparing the concentration of small-sized (1-51 mu m) suspended particulate mass (SPM) as well as its compositional fractions in six different phases to the partitioning coefficient. We observed an inverse relationship between K-d and suspended particulate mass, as has been observed for other metals and known as the "particle concentration effect," that explains much of the variation in K-d. Particulate organic matter (POM) and calcium carbonate (CaCO3) dominated the Hg partitioning in all three ocean basins while Fe and Mn could make a difference in some places where their concentrations are elevated, such as in hydrothermal plumes. Finally, our estimated Hg residence time has a strong negative correlation with average log bulk K-d, indicating that K-d has significant effect on Hg residence time.