An international team of researchers, coordinated by the CNRS, has produced the world's first estimate of hydrothermal mercury (Hg) emissions from mid-ocean ridges. The UN Minamata Convention on mercury aims to reduce human exposure to toxic mercury by reducing anthropogenic emissions. We are mainly exposed through the consumption of fish, which bioaccumulate Hg from the ocean. The current paradigm is that anthropogenic mercury emissions (currently 3,100 t yr-1) are responsible for increasing the global oceanic mercury reservoir by 21 %. This estimate is incorrect because we do not know how much natural mercury resided in the ocean before anthropogenic emissions began.
We are also unable to quantify the impact of anthropogenic emissions on Hg levels in fish. Hydrothermalism is the only direct source of natural Hg to the ocean. Previous studies, based solely on measurements of hydrothermal fluids, suggested that hydrothermal Hg inputs could be between 20 and 2,000 t yr-1. In addition to fluid measurements, this new study used measurements of hydrothermal plumes, seawater and rock cores from the Trans-Atlantic Geotraverse (TAG) hydrothermal spring on the Mid-Atlantic Ridge.
The combined observations suggest that most of the Hg enriched in the fluids would be diluted in the seawater and that a small fraction would precipitate locally. Extrapolation of the results indicates that the global hydrothermal flux of Hg from mid-ocean ridges is low (1.5 to 65 t yr-1) compared with anthropogenic Hg missions. While this suggests that most of the Hg in the ocean is of anthropogenic origin, it also gives hope that strict implementation of emission reductions under the Minamata Convention will reduce fish mercury levels and human exposure.
Contact: Lars-Éric Heimbürger-Boavida
CNRS researcher at the Mediterranean Institute of Oceanography (MIO - PYTHEAS)
Supervisory bodies: CNRS / CNES / IRD / UT3 Paul Sabatier
Marine Environment Sciences Laboratory (LEMAR - IUEM)
Supervisory bodies: CNRS / IFREMER / IRD / Univ. Bretagne Occidentale
Geo-Ocean Laboratory (GO-IUEM)
Supervising bodies: CNRS / Ifremer / UBO / UBS
CNRS Chemistry Laboratory :
Institute of Analytical Sciences and Physical Chemistry for the Environment and Materials (IPREM)
Supervisors: CNRS / Univ. Pau Pays de l'Adour