Shedding light on Arctic Mercury

Arctic biota contain higher mercury levels than elsewhere. The Arctic Ocean is key to understand the drivers because bioaccumulating neurotoxin methylmercury is formed from inorganic mercury within the ocean itself. Based on the recent Arctic Monitoring and Assessment Programme (www.amap.no) and new observations, we revise the mercury budget in the Arctic Ocean1. We find that the revised Arctic Ocean mercury budget (~1,870 Mg) is lower than previous estimates (2,847–7,920 tons) and this implies a higher sensitivity to climate change and anthropogenic emissions. Particulate mercury settling (122 ± 55 tons per year) from surface waters to the shelf sediments is the largest mercury removal mechanism in the Arctic Ocean. The revised Arctic Ocean mass balance suggests that mercury burial in shelf sediments (42 ± 31 tons per year) may be underestimated by over 100% (52.2 ± 43.5 tons per year).

We organized several oceanographic expeditions to the Barents Sea to better constrain those numbers. Until now, seawater observations were possible only during summer. We report the first polar night mercury seawater observations on a gradient from the shelf into the Arctic Ocean2. We observe a loss of one third of the total mercury between summer and winter, but no changes in methylmercury concentrations. Persistent methylmercury concentrations are likely driven by a lower affinity for particles and the presence of gaseous species. Our results update the current understanding of Arctic mercury cycling and require budgets and models to be reevaluated with a seasonal aspect. The residence time of methylmercury is longer (25 years) compared to inorganic mercury (3 years), and we expect persisting high methylmercury levels in the future.

References (supervised students, MIO staff)

1 A Dastoor, H Angot, J Bieser, J Christensen, T Douglas, LE Heimbürger-Boavida, M Jiskra, R Mason, D McLagan, D Obrist, P Outridge, M Petrova, A Ryjkov, K St. Pierre, A Schartup, A Soerensen, O Travnikov, K Toyota, S Wilson, C Zdanowicz. Arctic mercury cycling. Nature Reviews Earth & Environment 3 : 270-286, (2022). 2022-03-22

https://www.nature.com/articles/s43017-022-00269-w . https://hal.archives-ouvertes.fr/hal-03619231

2 SG Kohler, LE Heimbürger-Boavida, MV Petrova, MG Digernes, N Sanchez, A Dufour, A Simic, K Ndungu, MV Ardelan. Arctic Ocean’s wintertime mercury concentrations limited by seasonal loss on the shelf. Nature Geoscience in press. 2022-07-18. https://www.nature.com/articles/s41588-022-01127-7