Methyl halide and methane fluxes in the northern Alaskan coastal tundra

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 (CH 3 Cl, CH 3 Br, and CH 3 I) and methane (CH 4 ) were measured near Barrow, Alaska (71°N, 157°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. CH 3 Cl and CH 3 Br fluxes varied significantly with hydrologic conditions, with progressively higher net uptake rates observed with decreasing soil saturation. Drained tundra sites averaged −620 nmol CH 3 Cl m −2 d −1 and −9.8 nmol CH 3 Br m −2 d −1 while flooded tundra sites averaged −14 nmol CH 3 Cl m −2 d −1 and +1.1 nmol CH 3 Br m −2 d −1 . CH 3 Cl and CH 3 Br fluxes were positively correlated with each other as well as with CH 4 emissions, suggesting that consumption of both compounds occurs primarily in aerobic environments. Average CH 3 I 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 CH 3 Cl and CH 3 Br but a trivial source of CH 3 I.