The impact of sea ice on the air-sea exchange of mercury in the Arctic Ocean

Abstract

Abstract Atmospheric deposition is the main input of mercury (Hg) to the ocean, even in remote locations such as the Arctic. Furthermore, evasion of elemental Hg (Hg°) is the major sink for oceanic Hg. As a result, air-sea exchange is an important part of the oceanic Hg cycle. To examine the air-sea exchange of Hg in the Arctic Ocean we made high resolution measurements of Hg° in surface waters and Hg speciation in the atmosphere using continuous sampling systems during the 2015 U.S. Arctic GEOTRACES cruise from August 9 to October 12. Additionally, samples were obtained for measurement of total Hg and methylmercury (CH3Hg) in wet deposition and bulk aerosols as well as surface snow and sea ice. We used these measurements made in the water and the atmosphere to estimate fluxes of Hg° from the ocean to the atmosphere. Concentrations of dissolved Hg° (Hg°diss) were near saturation in ice-free waters (32\u202f±\u202f30\u202ffM) and resultant fluxes were low; however, Hg° was highly enriched under contiguous ice (101\u202f±\u202f98\u202ffM, up to 544\u202ffM) suggesting the continual formation of Hg° in waters even when ice covered. Predicted evasion fluxes in these regions (these being potential rates for locations under ice) were as high as 492\u202fpmol\u202fm−2 h−1. Atmospheric Hg° concentrations averaged 1.2\u202f±\u202f0.1\u202fng\u202fm−3 with little variation over the course of the cruise even above waters with elevated Hg°diss, indicating that sea ice acts as a barrier to air-sea exchange. Measurements of Hg in precipitation and aerosols were lower than have been found in more coastal regions of the Arctic. We used these concentrations to estimate deposition of Hg and CH3Hg during the time of the cruise. Overall, wet deposition represented 88% of the CH3Hg flux and 38% of the HgT flux. Our flux estimates confirm the importance of air-sea exchange in Hg cycling in the Arctic and suggest that evasion was greater than deposition, indicating a net loss of Hg from the Arctic during this period or the presence of other sources not measured during this study. Additionally, our results suggest that fluxes for offshore waters are lower than found in coastal regions of the Arctic. From these estimates, we predict how Hg concentrations may respond to future changes in ice cover and other potential impacts of climate change on Hg dynamics and food web bioaccumulation in this important ocean region.