Occurrence, distribution, and sea-air fluxes of volatile halocarbons in the upper ocean off the northern Antarctic Peninsula in summer

Highlights: • Anthropogenic CCl₄ are identified in the upper waters off the NAP. • A dicothermal layer can be a significant reservoir of VHCs due to two pycnoclines. • Biogenic CHBrCl₂, CH₂Br₂, and CH₃I concentrations were elevated in ice melt zones. • Waters off the NAP are an important source of VHCs during summer. We studied the spatial variations of six volatile halocarbons (VHCs), namely, iodomethane (CH₃I), chloroform (CHCl₃), tetrachloroethylene (C₂Cl₄), bromodichloromethane (CHBrCl₂), dibromomethane (CH₂Br₂), and carbon tetrachloride (CCl₄), and the environmental influencing factors involved in the cycling of VHCs in the upper ocean (0–500 m) off the Northern Antarctic Peninsula (NAP) during the summer of 2018. About 5%–10% of the total biogenic VHCs in the upper ocean were accumulated in the assemblage layer (AL) with high chlorophyll a. However, higher VHCs levels were observed in the dicothermal layer (DL) compared with the AL because of the preservation from winter and production from dinoflagellates and chlorophytes. Owing to the co-existence occurrence of sharp seasonal pycnocline and thick permanent pycnocline, DL could be an important VHCs reservoir in the upper water column during summer. In response to melting of sea ice and glacier, decreased salinity was responsible for ca. 50% of the variation in the CH₂Br₂ and CCl₄ concentrations, which corresponded with increased CH₂Br₂ and CCl₄ concentrations in the less saline water mass. Anthropogenic CCl₄ was found with an average concentration of 44.9 pmol/L, and there was a strong positive relationship between CCl₄ and CHCl₃ in the upper water, indicating their similar source of pollutant transport caused by anthropogenic activities. Calculated sea-to-air fluxes of CCl₄, C₂Cl₄, CHBrCl₂, and CH₂Br₂ averaged 478.7, 93.7, 33.7, and 61.8 nmol/(m²·d) in summer, respectively, indicating that the waters off the NAP are important sources of VHCs for the atmosphere and exert potentially adverse impacts on the Antarctica ozone depletion.