Degassing of Pore Water Methane during Sediment Incubations

Laboratory experiments were used to examine the degassing of CH(4) from a muddy sediment. Sediment containing dissolved CH(4) showed a hyperbolic time course of CH(4) release when allowed to degas in stoppered 20-ml vials. Equilibration required ca. 24 h for 5 ml of sediment. The rate of CH(4) release was found to be dependent on the ratio of exposed sediment surface area to sediment volume. The water content of the sediment was a factor in the total amount of CH(4) released but did not affect the rate of degassing. Addition of water to sediment samples (to form a slurry) accelerated CH(4) release, with a 1:1 dilution giving ca. 80% of maximum release after only 2 min. Shaking (vortexing) the sediments also facilitated CH(4) exchange, with 2 min of vigorous agitation giving 77% of maximum release. The organic content of the sediment did not affect either the amount or the rate of CH(4) degassing. Rubber stoppers exposed to CH(4) were found to absorb CH(4) rapidly and to subsequently release it in proportion to the concentration to which they were exposed. Artifacts may be associated with CH(4) production measurements if sediment and stopper degassing are not considered. It is recommended that any study of methane production or distribution include preliminary experiments to determine the degassing kinetics for the specific sediment system being used.