Gas transport from methane‐saturated, tidal freshwater and wetland sediments

Variations in hydrostatic pressure controlled by diurnal tides triggered ebullition from subtidal freshwater sediments dominated by methanogenesis in the White Oak River estuary, North Carolina. Pulses of gas consisting of 50–80% methane were released when the tidal cycle reached its nadir. In August, site‐to‐site variations in these fluxes ranged from 60 to 650 ml (39–425 mg) CH 4 m −2 d −1 . At a single site, ebullition made up 50% of the total CH 4 flux out of the sediments; the remainder was transported across the sediment‐water interface by molecular diffusion. The sedimentary gas bubble reservoir varied seasonally between 2.6 and 14.8 liters m −3 at two White Oak sites. These quantities represented 10–30% of the total sedimentary CH 4 inventory, the balance of which was dissolved in pore waters. Methane shifted between the two pools with seasonal changes in temperature as bubble methane partial pressure maintained equilibrium with dissolved CH 4 . Factors controlling the composition of sedimentary gas bubbles were investigated by collecting samples from several environments. These bubbles consisted primarily of CH 4 , N 2 , and CO 2 . The ratio of CH 4 to N 2 was found to be a useful indicator of mechanisms transporting gases from sediments and was controlled by both the ebullition rate and the presence of rooted emergent macrophytes.