High methylmercury (MeHg) concentrations in Arctic marine biota have been linked to a large MeHg uptake driven by shallow MeHg peaks in the Arctic Ocean, but the factors controlling the spatial variability of MeHg across the Arctic Ocean remain uncertain. Using observations from two new transects in the southern Beaufort Sea during ARA13C and six previously published cruises (GN01, GN03, GN04, ARK-XXVI/3, COMIDA CAB, and SWEDARCTIC 2016), we describe the physiochemical properties controlling basin-wide total Hg (THg) and MeHg patterns in seawater. The largest spatial variation is seen in the polar mixed layer, where THg increases and MeHg decreases from the Nansen Basin in the Atlantic sector across the Amundsen and Makarov Basins to the Canada Basin in the Pacific sector. In the halocline water, the Canadian Basin has the highest THg and lowest MeHg/THg. We show that the spatial variability of THg is mainly controlled by THg transport via modified Pacific water inflow, whereas that of MeHg (and MeHg/THg) is controlled by biological production and seawater temperature through inflows of cold Pacific water and warm Atlantic water. Our results suggest that the physiochemical properties of seawater affected by the inflow of Pacific and Atlantic water are critical factors governing THg and MeHg distributions in the upper layer of the Arctic Ocean.

methylmercury, Beaufort Sea, Arctic Ocean, Canada Basin, Pacific water, Atlantic water