Temporal Evolution of Gaseous Mercury Across the Sea Ice‐Seawater Interface: A Mesocosm Study.

In the marine cryosphere, seasonal sea ice dynamics affect the behavior of gaseous mercury, yet the mechanism remains poorly understood. By carrying out an outdoor sea ice mesocosm study, we examine primarily the abiotic factors influencing mercury dynamics and show distinct behaviors of gaseous mercury across the sea ice‐seawater interface over the full growth‐melt cycle. The distribution of gaseous mercury in sea ice is influenced by entrapment of gaseous mercury from different sources into sea ice of different textures, transport schemes within sea ice, and in situ cryo‐processes that affect mercury speciation. In the growing sea ice sections where solar radiation penetrated, production of gaseous mercury was observed, supporting the occurrence of in‐ice cryo‐photoreduction of divalent mercury. In under‐ice seawater, concentrations of dissolved gaseous mercury decreased gradually during ice growth and increased rapidly to pre‐freezing levels as ice started to melt, suggesting that the atmosphere‐sea ice‐seawater exchange pathway of gaseous mercury could be re‐established through melting first‐year sea ice. Our results from this unique mesocosm study provide new insights on the dynamics of gaseous mercury in and around sea ice that are primarily driven by abiotic processes, assisting model parameterizations for mercury cycling in polar regions. Plain Language Summary: Mercury is a contaminant of health concern that can be readily redistributed globally in the form of gaseous mercury. The air‐sea exchange of gaseous mercury is the primary pathway for mercury transport between the atmosphere and surface oceans. However, such a process is largely modified in polar regions where the presence of sea ice not only physically impedes the direct air‐sea interaction but also induces cryo‐processes that influence mercury speciation. Despite studies on the distribution of gaseous mercury in the Arctic, little is known about the cryo‐processes controlling the dynamics of gaseous mercury in sea ice. In this study, we report temporal evolution of total and gaseous mercury in experimental first‐year sea ice from an outdoor sea ice mesocosm. We show that the distribution and entrapment of gaseous mercury in sea ice depend on ice textures and vary temporally over the growth‐melt cycle. We further show evidence for in‐ice mercury photoreduction that leads to the production of gaseous mercury during the formation of sea ice. Key Points: The dynamics of gaseous mercury in sea ice and seawater, primarily driven by abiotic processes, were studied over a full growth‐melt cycleThe sources, sinks and dynamics of gaseous mercury in sea ice evolve temporally and are associated with ice textures and cryo‐processesNet mercury photoreduction could occur in sea ice where solar radiation penetrates [ABSTRACT FROM AUTHOR]