Program
 
Special Session 4: Biogeochemical cycling of trace elements in the ocean: GEOTRACES and beyond
 
 
 
Poster
Methylmercury flux from sediment: Insights from comparison between 224Ra/228Th disequilibrium and core incubation
SS4-09-S
Xiangming Shi* , Xiamen University
Robert P. Mason, University of Connecticut
Matthew A. Charette, Woods Hole Oceanographic Institution
Presenter Email: kidsxm@stu.xmu.edu.cn
Sediment is the main site of mercury methylation. Accurate quantification for methylmercury (MeHg) flux at the sediment-water interface is vital to better understand the biogeochemical cycling of mercury, especially the toxic MeHg species. Methylmercury is non-conservative in aquatic systems and many factors affect its concentration. Traditional approaches, such as core incubation, are hard to maintain at the in-situ conditions, leading to over/underestimation of benthic fluxes. The 224Ra/228Th disequilibrium method for tracing the transfer of dissolved substances just across the sediment-water interface, has proven to be a reliable proxy to quantify the benthic flux. In this study, both 224Ra/228Th disequilibrium and core incubation methods were applied to examine the benthic fluxes of both 224Ra and MeHg in Barn Island, Connecticut, USA (a wetland) from May to August, 2016. For 224Ra, the two methods presented comparable results. But MeHg fluxes showed significant differences. By the 224Ra/228Th disequilibrium method, benthic fluxes of MeHg ranged from 112¡À53 pmol m-2 d-1 to 2983¡À1627 pmol m-2 d-1. Core incubation produced similar results to the radiotracer method in May and August (1277¡À96 and 98¡À14 pmol m-2 d-1, respectively) but not in June and July as the calculations suggested that the sediment was a sink of MeHg, which was contrary to the evidence of significant MeHg gradients at the sediment-water interface. Overall, we conclude that the 224Ra/228Th disequilibrium approach is preferred for estimating the benthic flux of MeHg and that sediment is indeed an important MeHg source in this wetland.