Natural biogeochemical cycle of mercury in a global three‐dimensional ocean tracer model

Abstract

We implement mercury (Hg) biogeochemistry in the offline global 3-D ocean tracer model (OFFTRAC) to investigate the natural Hg cycle, prior to any anthropogenic input. The simulation includes three Hg tracers: dissolved elemental (Hg 0 aq ), dissolved divalent (Hg II aq ), and particle-bound mercury (Hg P aq ). Our Hg parameterization takes into account redox chemistry in ocean waters, air-sea exchange of Hg 0 , scavenging of Hg II aq onto sinking particles, and resupply of Hg II aq at depth by remineralization of sinking particles. Atmospheric boundary conditions are provided by a global simulation of the natural atmospheric Hg cycle in the GEOS-Chem model. In the surface ocean, the OFFTRAC model predicts global mean concentrations of 0.16 pM for total Hg, partitioned as 80% Hg II aq , 14% Hg 0 aq , and 6% Hg P aq . Total Hg concentrations increase to 0.38 pM in the thermocline/intermediate waters (between the mixed layer and 1000 m depth) and 0.82 pM in deep waters (below 1000 m), reflecting removal of Hg from the surface to the subsurface ocean by particle sinking followed by remineralization at depth. Our model predicts that Hg concentrations in the deep North Pacific Ocean (>2000 m) are a factor of 2-3 higher than in the deep North Atlantic Ocean. This is the result of cumulative input of Hg from particle remineralization as deep waters transit from the North Atlantic to the North Pacific on their~2000 year journey. The model is able to reproduce the relatively uniform concentrations of total Hg observed in the old deep waters of the North Pacific Ocean (observations: 1.2 ± 0.4 pM; model: 1.1 ± 0.04 pM) and Southern Ocean (observations: 1.1 ± 0.2 pM; model: 0.8 ± 0.02 pM). However, the modeled concentrations are factors of 5-6 too low compared to observed concentrations in the surface ocean and in the young water masses of the deep North Atlantic Ocean. This large underestimate for these regions implies a factor of 5-6 anthropogenic enhancement in Hg concentrations.

Mercury presence and speciation in the South Atlantic Ocean along the 40°S transect

“…This is not surprising, since Hg exhibits strong adsorptive properties and is generally distributed according to presence and concentration of organic matter (OM), which has usually slow sedimentation flux. Long residence time of THg in the water column therefore leads to uniform mixing, which is reflected in our profiles [ Amos et al , ; Zhang et al , ; Bowman et al , ]. We have also not observed the nutrient‐like distribution of THg and its correlation with phosphate as was shown before [ Lamborg et al , ].…”

Development of a global ocean mercury model with a methylation cycle: Outstanding issues

“…K d was set to the median observed value of 200,000 based on various studies [ Allison and Allison , ]. There was no tuning of this value to adjust the sedimentation burial flux so that geogenic emissions are balanced by ocean sediment burial under equilibrium conditions as in the case of Zhang et al []. We find that the equilibrium state seabed burial is less than the geogenic emission which is physically consistent.…”

“…In this paper we present a new 3‐D ocean mercury transport and biogeochemistry model which includes for the first time an explicit methylation cycle. The redox component of the biogeochemistry is formulated in our model as a bulk process using the bulk rate constants from Zhang et al [, hereinafter ZA]. By contrast, the methylation/demethylation cycle bulk rate constants are adapted from Lehnherr et al [] and assumed to be proportional to the organic carbon remineralization rate (OCRR) associated with the bacterial decomposition of particulate organic carbon (POC) in subsurface waters and proportional to net primary productivity (NPP) in surface waters.…”

Elemental Mercury Concentrations and Fluxes in the Tropical Atmosphere and Ocean

“…55 Figure 4 illustrates the importance of lateral seawater flow in the surface ocean for redistributing enhanced atmospheric mercury deposited in the ITCZ region. Lateral transport of Hg is clearly important for a variety of ocean regions 30, 35 and has not been captured in earlier GEOS-Chem simulations of air− sea exchange. 11,56,57 Ekman pumping is particularly pronounced in the ITCZ region, resulting in strong horizontal outflow of Hg in the surface ocean to other regions of the tropical ocean.…”