Depth profiles of volatile iodine and bromine-containing halocarbons in coastal Antarctic waters

Abstract

Measurements of bromoform (CHBr 3), diiodomethane (CH 2I 2), chloroiodomethane (CH 2ICl) and bromoiodomethane (CH 2IBr) were made in the water column (5–100 m depth) of the Southern Ocean within 0–40 km of the Antarctic sea ice during the ANTXX1/2 transect of the German R/V Polarstern, at five locations between 70–72°S and 9–11°W in the Antarctic spring/summer of 2003–2004. Some of the profiles exhibited a very pronounced layer of surface sea-ice meltwater, as evidenced by salinity minima and temperature maxima, along with surface maxima in concentrations of CHBr 3, CH 2I 2, CH 2ICl and CH 2IBr. These results are consistent with in situ surface halocarbon production by ice algae liberated from the sea ice, although production within the sea ice followed by transport cannot be entirely ruled out. Additional sub-surface maxima in halocarbons occurred between 20 and 80 m. At a station further from shore and not affected by surface sea-ice meltwater, surface concentrations of CH 2I 2 were decreased whereas CH 2ICl concentrations were increased compared to the stations influenced by meltwater, consistent with photochemical conversion of CH 2I 2 to CH 2ICl, perhaps during upward mixing from a layer at ∼ 70 m enhanced in iodocarbons. Mean surface (5–10 m) water concentrations of halocarbons in these coastal Antarctic waters were 57 pmol l − 1 CHBr 3 (range 44–78 pmol l − 1 ), 4.2 pmol l − 1 CH 2I 2 (range 1.7–8.2 pmol l − 1 ), 0.8 pmol l − 1 CH 2IBr (range 0.2–1.4 pmol l − 1 ), and 0.7 pmol l − 1 CH 2ICl (range 0.2–2.4 pmol l − 1 ). Concurrent measurements in air suggested a sea-air flux of bromoform near the Antarctic coast of between 1 and 100 (mean 32.3, median 10.4) nmol m − 2 day − 1 and saturation anomalies of 557–1082% (mean 783%, median 733%), similar in magnitude to global shelf values. In surface samples affected by meltwater, CH 2I 2 fluxes ranged from 0.02 to 6.1 nmol m − 2 day − 1 , with mean and median values of 1.9 and 1.1 nmol m − 2 day − 1 , respectively.