LATERAL ADVECTION LIMITS METHANE EMMISSIONS FROM TROPICAL PEATLANDS

We characterized methane fluxes through a tropical peatland in Brunei on the island of Borneo. We find that groundwater flushes the peat at a rate sufficient to limit methane concentrations below the threshold for exsolvation, and hence little methane escapes from the peat surface. This finding explains why low emissions from the peat surface have been reported from a variety of tropical peatlands, but also indicates a large flux of methane transported laterally towards rivers. Analysis of the δ¹³C of methane dissolved in river water indicates that most of methane leaving the peat is oxidized before escaping to the atmosphere.

We made these calculations by developing a new framework for estimating methane production rates from vertical profiles of: (1) Dissolved inorganic carbon (DIC); (2) The δ¹³C of DIC, and; (3) Some conservative tracer of flow. We used both sulfur hexafluoride and chloride, which diffuses up from the underlying clay, as tracers of groundwater flow. The concentration and isotopic composition (δ¹³C) of methane may be used as additional data, but are not necessary. We fit the profiles of all constituents simultaneously to estimate the recharge rate and the production rate of methane. We find a production rate (0.05 mM/yr) at the low end of the range observed in northern peatlands, but a recharge rate (0.4 m/yr), substantially higher than northern bogs. This flow does not penetrate the underlying clay and, consequentially, the horizontal flow towards the river accelerates over the four kilometers from the dome center to the river. Nonetheless, the near identical vertical profile of constituents measured at different locations along a flow path towards the river confirms that travel times to particular depths remain uniform; that isochrones are horizontal as predicted by Vogel’s model, and hence that methane fluxes over a large area maybe be accurately estimated by a limited number of profiles.