Microtopographic and Hydrological Controls over Respiratory Efflux and Late-Season Arctic Methane Emissions

Abstract:

In recent years, Arctic peatlands have released approximately 35 Tg (3.5 x 10¹²g) of CH₄ annually, corresponding to around 1/3 of the aggregate wetland CH₄ fluxes and 16% of all natural emissions. As climate models increasingly suggest that current warming trends in the Arctic (4-8 °C higher annual surface air temperatures) will continue by century’s end, carbon (C) cycling in these northern climes may be further amplified. Although much has been learned in recent decades, uncertainty remains in regard to the spatial and temporal extent of CO₂ and CH₄ emissions from these systems.

Chamber based carbon flux measurements were gathered for three growing seasons from June 2007 to September 2013 in Barrow, Alaska to investigate the diurnal, weekly, and monthly patterns of CO₂ and CH₄ flux in the North American Arctic. For the 2007 and 2008 growing seasons, high temporal frequency auto-chambers (LI-8100A Automated Soil Flux System, LI-COR Biosciences) were used to gather over 18,000 individual flux measurements. From July to September 2013 an Ultraportable Greenhouse Gas Analyzer (Los Gatos Research Inc.) was deployed in concert with this soil flux system to gather high temporal frequency soil CO₂ and CH₄ fluxes. Nearby eddy covariance towers provided auxiliary meteorological and environmental data, while weekly transects amassed further surficial hydrological measures (pH, thaw depth, water table).

For earlier periods of data, respiratory fluxes were partitioned into five microtopographic classes (polygon rims and troughs, low centered basins, high ridges, and flat mesic terrain). Conversely, for the later periods of data covered chamber fluxes were partitioned into three ‘habitat’ types (High, Medium, Wet) based on corresponding aboveground average water table extent.

Marked dissimilarities were noted across habitat types and microtopographic classes. In general more mesic, waterlogged regions released greater quantities of CO₂ across the growing season, while intermediate (Medium) water table regimes dominated CH₄ release in the fall. Additionally, temperature generally delimited CO₂ release throughout the growing season, while CH₄ release was strongly tied to thaw depth expansion. This large dataset thus greatly underscores the importance of microscale heterogeneity on C flux in the Arctic.