Methane and Halomethane Fluxes in the Northern Alaskan Coastal Tundra: Influence of Microtopography, Soil Redox Conditions and Temperature

Robert C. Rhew, Yit Arn Teh and Triffid Abel
Department of Geography
University of California, Berkeley

Climate change and stratospheric ozone loss are global issues with enhanced manifestations over the Arctic latitudes. The Arctic coastal zone is especially vulnerable to change owing to the complex interactions between ecosystems, geomorphology, hydrology, geochemistry, and climate. Arctic terrestrial ecosystems are undergoing climatic warming, leading to positive feedbacks with emissions of radiatively active greenhouse gases such as CO₂ and CH₄ [1-3]. In addition, Arctic warming will affect the biogeochemical cycling of ozone-destroying compounds, leading to a complex feedback with ozone loss over the arctic. Four such compounds that are known to have active land-atmosphere exchange are the methyl halides (CH₃Br, CH₃Cl, and CH₃I) and chloroform (CHCl₃).

Tundra fluxes of methane, the methyl halides, and chloroform were measured near Barrow, Alaska (71° N, 157° W) during the 2005 growing season. Two-component flux chambers (195 L) were deployed at 20 sites in the coastal tundra, covering a range of microtopographic features: drained lake basins, channels, and high- and low-center ice-wedge polygons [4]. While all sites emitted CH₄, CH₃I, and CHCl₃, most sites also acted as sinks for CH₃Br and CH₃Cl. The magnitude of CH₄ emissions is strongly linked to the microtopography and can be used as a proxy for soil column redox conditions. We find that the more aerobic the conditions, the lower the CH₄ emission rates and the higher the CH₃Br and CH₃Cl uptake rates. The correlation between methane and methyl halide fluxes in the Arctic tundra may be related to the degree of oxidation of these compounds in the aerobic peat layer overlying waterlogged soils. CHCl₃ and CH₃I emissions, on the other hand, appear to be more influenced by temperature than microtopography.

These flux measurements yield several surprising results: 1) while tropical and temperate coastal ecosystems are major sources of CH₃Br and CH₃Cl, the Arctic coastal tundra appears to be a large sink for these compounds; 2) CHCl₃ emissions from the tundra were large, despite the cold temperatures and shallow permafrost, suggesting that the tundra at lower latitudes may be an even larger source; 3) CH₃Br and CH₃Cl uptake rates are highly correlated, with a ratio of uptake rates that is similar to that observed in temperate shrublands [5] and boreal forest soils [6], suggesting that the microbial uptake mechanism of methyl halides is similar between these diverse ecosystems.

Tundra-atmosphere gas exchange of these compounds may influence the seasonality of their background concentrations at high latitudes of the Northern Hemisphere. If the high latitude Arctic tundra continues to experience warming, an important consequence will be how the distribution of microtopographic features changes. This will have implications on whether the tundra becomes a larger source or sink for these radiatively and chemically active compounds.

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[4] J. Brown et al., 1980, in: An Arctic Ecosystem: The Coastal Tundra at Barrow, Alaska, J. Brown et al., Eds. (US/IBP Synthesis Series: 12), pp. 1-29.
[5] R.C. Rhew et al., 2001, J. Geophys. Res., 106 (D18), 20875-20882.
[6] R.C. Rhew et al., 2003, Geophys. Res. Lett., 30 (21), 2103.