Vertical transport of anthropogenic mercury in the ocean

We investigate the vertical transport of mercury (Hg) within the ocean using a simple box diffusion model to represent vertical water transport coupled with a particulate Hg flux. The particulate flux assumes that the Hg content of marine particles is proportional to the Hg concentration of surface waters via a sorption equilibrium constant, K-d. The model is forced with the observed factor of 3 increase in atmospheric Hg deposition over the industrial era. The modeled vertical profile of oceanic Hg shows a subsurface maximum at similar to 500 m depth due to remineralization of Hg bound to sinking organic particles, consistent with observations. Model results indicate that surface (top 100 m) concentrations of Hg have increased by 150% since preindustrial times. Over the past 150 years, 280 Mmol of anthropogenic Hg have accumulated in the ocean, representing a 18% increase in the total oceanic Hg content. We find that 36% of the anthropogenic Hg occurs in the top 400 m and only 7% occurs below 1500 m. Over the industrial era, we find that 14% of cumulative anthropogenic emissions have accumulated in the ocean. Our model results show that half of the accumulation of anthropogenic Hg in the ocean is due to sinking on particulates. A sensitivity analysis indicates that the model results are most dependent on the value of K-d.