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

Abstract

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, m-2 d-l. At a single site, cbullition made up 50% of the total CH, 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 lo-30% of the total sedimentary CH, 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,.Factors controlling the composition of sedimentary gas bubbles were investigated by collecting samples from scvcral environments. These bubbles consisted primarily of CH,, N2, and CO,. The ratio of CH, to N, was found to bc a useful indicator of mechanisms transporting gases from sediments and was controlled by both the ebullition rate and the presence of rooted emergent macrophytes.

Seasonal variability of methane in the rivers and lagoons of Ivory Coast (West Africa)

“…In rivers and estuarine environments, water currents strongly enhance water turbulence and k (Zappa et al 2003(Zappa et al , 2007Borges et al 2004a, b), although it is probable that this not adequately quantified in k values derived from tracer methods that have a characteristic time scale (*1 day) that is incompatible with the water current characteristic time scale (*1 min). We did not quantify ebullition CH 4 fluxes, which in macrotidal estuarine systems can represent *50% or more of the total emission of CH 4 to the atmosphere (Chanton et al 1989;Kelley et al 1990;Shalini et al 2006). Finally, direct emission of CH 4 from intertidal sediments to the atmosphere strongly contribute in estuarine environments to the overall CH 4 emission at ecosystem scale ranging from *7000 lmol m -2 day -1 in oligohaline regions to Linear regression for rivers (black dotted line) yields r 2 = 0.42 and P = 0.549 and for lagoons (grey dotted line) yields r 2 = 0.25 and P = 0.393.…”

“…In estuarine channels, net CH 4 inputs from the sediments to the water column and CH 4 production in the water column are generally low because oxic and suboxic respiration dominate (Abril and Borges 2004). Consequently, CH 4 in estuarine waters originates from two major sources: (1) rivers, which receive CH 4 from soils, groundwater, wetlands and floodplains on the watershed (De Angelis and Lilley 1987;Richey et al 1988) and (2) tidal wetlands and mud flats, which are generally vegetated and enriched in organic matter to support methanogenesis (Bartlett et al 1987;Chanton et al 1989;Kelley et al 1995;Middelburg et al 2002;Abril and Borges 2004). Majors sinks of CH 4 in estuarine channels are the export to the adjacent coastal zone that dominates in the case of estuaries with a high freshwater discharge and a short residence time (Scranton and McShane 1991;Middelburg et al 2002), the emission to the atmosphere and the bacterial oxidation in the water column and sediment.…”

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland

“…To monitor fine-scale pore water CH 4 concentrations through the top 100 cm of the peat column, a "peeper" (pore water equilio bration sampler fitted with dialysis membrane) was installed at each site [Chanton et al, 1989;Hesslein, 1976]. These samplers were left in the peat for 3-4 weeks.…”

Methane emissions from beaver ponds: Rates, patterns, and transport mechanisms

“…The net amount of methane released to the atmosphere depends on the ratio of methane production versus methane consumption, which in turn is largely dependent on the release mechanism of methane from the sediments (i.e., molecular diffusion versus gas bubble flux). Larger bubble flux to diffusive flux ratios result in a higher proportion of the produced methane reaching the water-atmosphere interface, since gas bubbles are known to "bypass" the oxidizing effect of the sediments and overlying water column [Martens and Klump, 1980;Crill et al, 1988a;Chanton et al, 1989]. The ratio of methane emissions in the form of gas bubbles to diffusive methane flux is therefore expected to govern the portion of the produced methane that eventually will be released from a wetland system.…”