Hydrothermal mercury – the natural story of a contaminant

Mercury exposure through fish consumption

The UNEP Minamata Convention on Mercury, adopted in 2017, aims to reduce human exposure to toxic mercury through the reduction of anthropogenic emissions. As it happens, the general population is primarily exposed to it via the oceans, through the consumption of fish that bioaccumulate mercury.

Anthropogenic vs natural mercury

The current paradigm is that anthropogenic mercury emissions (present-day 3,100 t y-1) have increased the global oceanic mercury reservoir by 21%. This estimate is flawed because it is not known how much natural mercury was in the ocean before anthropogenic emissions began. It is similarly impossible to quantify how anthropogenic emissions have affected fish mercury levels.

Hydrothermal venting is the only direct source of natural mercury to the ocean. Previous studies, based on vent fluid measurements alone, suggested that hydrothermal mercury inputs could range from 20 and 2,000 t y-1.

Low hydrothermal mercury flux

The new study used measurements of vent fluids, plume, seawater, and rock cores from the Trans-Atlantic Geotraverse (TAG) hydrothermal vent at the Mid-Atlantic ridge. The combined observations suggest that the majority of the mercury enriched in the vent fluids is diluted into seawater and a small fraction is scavenged locally. An extrapolation of the results suggests that the global hydrothermal mercury flux from mid-ocean ridges is small (1.5 - 65 t y-1) compared to anthropogenic mercury missions.

The goal: reduce anthropogenic emissions

While this study suggests that most of the mercury present in the ocean is of anthropogenic origin, it also gives hope that the strict implementation emission reductions in the framework of the Minamata Convention will reduce fish mercury levels and human exposure. 
 

This article has been made available through open access. It can be downloaded in its entirety.

This project was supported by the French National Funding Agency project HydrOThermal Mercury (ANR-21-CE34-0026, PI Lars-Eric Heimbürger-Boavida) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie GMOS-Train, GA grant agreement number 860497.