Benthic deposition and burial of total mercury and methylmercury estimated using thorium isotopes in the high-latitude North Atlantic

The high-latitude oceans are regions of high mercury (Hg) bioaccumulation, especially in the form of methylmercury (MeHg), which is of great concern in terms of human and ecosystem health. In the high-latitude North Atlantic (60-80°N), the deep water entrains the Hg-enriched Arctic water southwards, consequently influencing global Hg cycling. Whilst Hg removal has been proposed en route from the Arctic Ocean to the Atlantic Ocean, the factors and the mechanism underpinning this loss are poorly studied. Here, we constrained Hg behavior at the sediment–water interface during the GEOTRACES process study GApr16 in 2021 using radionuclide approaches. Excess thorium isotopes (²³⁴Th_ex and ²²⁸Th_ex) in the sediment provided evidence of significant deposition of suspended particles on top of the shallow shelves and ridges due to the flowing waters with intense nepheloid layers. The benthic deposition and burial fluxes of Hg species were then evaluated based on the ²³⁴Th_ex and ²²⁸Th_ex, respectively. This analysis showed that most of the total Hg deposited from the bottom water was buried into the sediment. The net scavenging flux was 14.8 ± 7.0 Mg y^-1, explaining half of the Hg removal proposed in previous studies. In comparison, the benthic scavenging flux of MeHg was 27 ± 10 kg y^-1. Due to the lower particle activity, MeHg deposited from the bottom water was not retained well in the sediment, and over half was estimated to be released again, mainly by diffusion and advection, to the water column. This efflux might account for the elevated MeHg in the overflow waters during their advection. Overall, benthic scavenging dominated the Hg loss from the water column of the high-latitude North Atlantic with the shallow shelves and ridge regions being the primary deposition zones.