第九届青年地学论坛

The high-latitude oceans feature high mercury (Hg) bioaccumulation which is of great concern in terms of human and ecosystem health. In the far North Atlantic (60-70 ºN), the deep water entrains the Hg-enriched Arctic water southwards, consequently influencing global Hg cycling. Whilst Hg removal has been found en route from the Arctic Ocean to the Atlantic Ocean, the factors and the mechanism underpinning this loss are poorly studied. Here, we constrain 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 evidence significant sediment resuspension and redeposition on top of the shallow ridges due to the intense overflows. The benthic redeposition and burial rates of Hg species were then evaluated with a constant rate of supply model and particulate Hg/Th ratios. We show that resuspended sediment on the ridges scavenged total Hg from the bottom water at a rate of 14.8 ± 0.3 Mg y^-1, and this Hg was fully retained in the sediment, accounting for half of the Hg loss between the Arctic and Atlantic Oceans. In contrast, owing to the different particle reactivities, half of the scavenged methylmercury (MeHg) was re-released to the water column, forming a concentration maximum in the mid depth (~400 m); and the remaining was buried in the sediment at a rate of 38 ± 21 kg y^-1.  In particular, benthic Hg efflux was observed on the continental slope of the Irminger Sea, indicating that some Hg to the high-latitude North Atlantic was source from the benthic exchange other than the direct input from the polar water.

thorium isotopes,benthic process,mercury cycling