Sources and melt flux of methylmercury in sea ice on the Chukchi Plateau, Arctic Ocean

To identify the major source and melt flux of Hg in sea ice, we measured total Hg (THg) and methylmercury (MeHg) concentrations from sea ice cores collected from the Chukchi Plateau, Arctic Ocean. The THg maxima were found in the surface section (0-40 cm) of the cold cores (-2.3 +/- 0.19 degrees C) and middle section (30-75 cm) of the warm cores (-1.9 +/- 0.38 degrees C), whereas MeHg concentrations were nearly constant by core depth in the cold and warm cores. Using the enrichment factor mixing model, we estimated seawater to be the dominant source of THg over atmospheric deposition, both in the cold (89.1-99.6 %) and warm cores (98.4-99.7 %). The preliminary input estimation for MeHg showed that in situ methylation, instead of seawater input, was the potential source of MeHg in sea ice. The sea ice melt flux of MeHg in the Arctic Ocean was estimated to be 231 +/- 77 kg yr- 1 in the period of 2010-2020, which is insignificant compared with other external sources, such as atmospheric deposition, river discharge, and coastal erosion. Based on the results of the present study, the ongoing decline of sea ice extent related to polar amplification is likely to enhance MeHg production in sea ice in late spring and summer as a result of the improved light availability and consequent promotion of algal and microbial activities.