Methyl halide and methane fluxes in the northern Alaskan coastal tundra

[ 1] The Arctic tundra is a major source and sink of carbon-containing gases, but the biogeochemical cycling of halocarbons in this ecosystem has been largely unexplored. In this study, coastal tundra fluxes of methyl halides (CH3Cl, CH3Br, and CH3I) and methane (CH4) were measured near Barrow, Alaska (71 degrees N, 157 degrees W) during the 2005 growing season. Sites covered a range of microtopographic features including drained lake basins, channels, and high- and low-centered ice-wedge polygons. CH3Cl and CH3Br fluxes varied significantly with hydrologic conditions, with progressively higher net uptake rates observed with decreasing soil saturation. Drained tundra sites averaged - 620 nmol CH3Cl m(-2) d(-1) and - 9.8 nmol CH3Br m(-2) d(-1) while flooded tundra sites averaged -14 nmol CH3Cl m(-2) d(-1) and + 1.1 nmol CH3Br m(-2) d(-1). CH3Cl and CH3Br fluxes were positively correlated with each other as well as with CH4 emissions, suggesting that consumption of both compounds occurs primarily in aerobic environments. Average CH3I net emissions were relatively weak (4.0 nmol m(-2) d(-1)). Average methane fluxes (2.0 mmol m(-2) d(-1)) and their relationship with soil moisture were comparable to tundra emissions reported by prior studies. Methane fluxes showed a marked seasonality, with emissions tripling between early and late in the growing season, but methyl halide fluxes did not show a similar temporal trend. If these measurements are representative of the Arctic tundra, then the Arctic tundra is a regionally important sink for CH3Cl and CH3Br but a trivial source of CH3I.