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.