Methane Production and Transport in a Tropical Peatland
Tuesday, 16 December 2014: 5:15 PM
Wetlands are the largest source of CH4 to the atmosphere, but emissions measurements are highly uncertain, particularly in the tropics. We examine CH4 production and transport in a pristine tropical peatland in Borneo. We use the carbon isotopic (stable and radioactive) composition of dissolved CH4, DIC and DOC within the peat porewater to identify the source and mechanism of CH4 production in tropical peat. First, we measure 14C in all carbon phases to identify the source of CH4. In contrast to the peat, which ages with depth to nearly 3000 cal BP, DOC is modern throughout the peat column, to depths of 4.5m. The 14C content of CH4 and DIC are nearly identical, and are intermediate between the DOC and peat 14C content. Thus, despite the presence of modern carbon throughout the peat profile, peat decomposition is an important source of CH4 production. Next, we use the δ13C of CH4 and DIC to identify the mechanism of CH4 production. Within the peat profile, CH4 and DIC concentrations increase with depth and DIC becomes increasingly enriched in 13C. The δ13C of CH4 is relatively uniform with depth, resulting in a δ13C fractionation between DIC and CH4 of 55-70‰ (αCCO2-CH4 = 1.06-1.07). This fractionation suggests CO2 reduction is the dominant pathway for CH4 production at the site. We find consistent trends with depth across the peatland, attributable to the unique hydrologic behavior of the dome. These trends are similar to those observed in northern peat bogs. Finally, we use information on site hydrology, CH4 and DIC concentrations, isotopic compositions and fluxes to build a model of CH4 production and transport. This model allows us to partition CH4 losses from the peat due to diffusion, tree-mediated transport, and ebullition.