Earth system modelling of mercury using CESM2 – Part 3: Oceanic model POP2/Hg v1.0

Abstract. Mercury (Hg) is a globally distributed toxicant with complex cycling in the ocean, involving redox reactions, air–sea exchange, and microbial methylation. We present POP-Hg v1.0, a new global ocean mercury model developed within the ocean component of CESM2 (POP2) and coupled to the Marine Biogeochemistry Library (MARBL). POP2/Hg v1.0 simulates dissolved elemental Hg (Hg0), oxidized Hg (HgII), particulate-bound Hg (Hgp), and Methylmercury (MeHg = MMHg + DMHg), linking their transformation to ecosystem processes and dynamics of particulate organic carbon (POC). The model captures observed large-scale features of marine Hg. Surface Hg0 concentrations range from 10 to 120 fmol L-1(fM, 1 fM = 10-15 mol L-1) and are elevated in tropical and subtropical oceans due to strong photoreduction and limited evasion. Subsurface Hg0 maxima emerge in upwelling zones, reflecting remineralization-driven HgII reduction. Surface HgII concentrations peak in regions of high atmospheric deposition and productivity and rise at depth in oxygen-deficient zones. Hgp is closely associated with high POC and HgII levels in the surface ocean but attenuates rapidly with depth due to remineralization. MeHg accumulates in the subsurface and deep sea, broadly consistent with observations. Total Hg (HgT) ranges to above 2.0 pmol L-1 (pM, 1 pM = 10-12 mol L-1) in the surface, with open-ocean surface concentrations matching measurements. Deep-ocean HgT accumulation reflects sustained particle flux and remineralization. The biological pump strongly shapes vertical Hg transport. Above 1000 m, soft POC carries 98.6 % of Hgp flux, driving export and remineralization. Below 1000 m, hard POC becomes the dominant carrier, contributing over 99 % of Hgp flux to sediments, although the total flux is small (0.19 t yr-1). Sensitivity tests show that altering redox rates significantly affects the vertical distribution of Hg transformations. Reduced surface reduction and oxidation rates suppress Hg⁰ production in the upper ocean, weaken seasonal variability, and shift net reduction deeper into the water column. By embedding mercury chemistry into a fully coupled biogeochemical and physical ocean model, POP2/Hg v1.0 offers a process-resolving platform for assessing mercury cycling and its response to environmental change. It enables integrated Earth system simulations and improves predictive capacity for marine mercury under future climate and policy scenarios.