Global 3-D model of oceanic mercury coupled to carbon biogeochemistry and particle dynamics: application to the transport and fate or riverine mercury

Abstract:

Rivers are estimated to deliver 27 Mmol a^-1 of mercury (Hg) to ocean margins, which is comparable to the global atmospheric deposition flux of Hg to the ocean. Previous studies presumed that most of this riverine Hg is sequestered by settling to the coastal regions. However, there has been little investigation of the mechanism and efficiency with which this sequestration takes place, and the implications for riverine influence in different ocean regions. Here we develop a global 3-D chemical transport model for Hg in the ocean (MITgcm-Hg) with ecology (DARWIN model). We track offshore export of the discharged Hg from heterogeneous river systems over different ocean regions, and how it is influenced by the interaction of Hg in a variety of geochemical forms with carbon and suspended particles. We constrain our model assumptions with available offshore observations that bear strong riverine signals. Modeling results suggest that some of the riverine Hg is highly refractory, sorbs strongly to particles and does not follow equilibrium partitioning with the dissolved phase. Simulated global Hg evasion from riverine sources is 50 times larger without this refractory particulate pool, which results in a total evasion flux two times larger than our current best estimate. Based on a typology system of global rivers, we calculate that 10% to 60% of the particulate Hg from different rivers settles in ocean margin sediments because of subgrid sedimentation processes. The remaining 7.5 Mmol a^-1 (28% of total river discharge) is available for offshore transport, where it undergoes further sedimentation to the shelf (5.3 Mmol a^-1) as well as evasion to the atmosphere (0.44 Mmol a^-1). Only 1.7 Mmol a^-1 (6.4% of the global riverine Hg) reaches the open ocean, although that fraction varies from 2.6% in East Asia because of the blockage of Korean Peninsula to 25% in east North America facilitated by the Gulf Stream. We find large riverine influences over coastal oceans off East Asia, and the contributions elsewhere are much smaller due to less riverine Hg discharge. We find the transport of riverine Hg is most influenced by its release rates from organic carbon pools and particle sinking velocities. Varying these parameters changes the contribution of riverine sources to the Hg concentrations over the open ocean for a factor of 2.