Physical Controls on Methane Ebullition from Reservoirs and Lakes

Abstract

Understanding the nature and extent of methane production and flux in aquatic sediments has important geochemical, geotechnical, and global climate change implications. Quantifying these processes is difficult, because much of the methane flux in shallow sediments occurs via ebullition (bubbling). Direct observation of bubble formation is not possible, and bubbling is episodic and dependent upon a number of factors. Whereas previous studies have correlated methane flux with surface wind intensity, detailed study of Lake Gatun in Panama and Lago Loiza in Puerto Rico suggest that methane flux is more closely correlated with the shear stress in sediments caused by bottom currents. Bottom currents in turn are a complex function of wind, internal pressure gradients, and lake bathymetry. A simple physical model of bottom currents and sediments in these lakes suggests that most methane ebullition originated from the upper 10-20 cm of the sediment column. Our data reaffirm previous studies showing that ebullitive methane flux is minor in water deeper than~5 m.

Public Review Draft: A Method for Assessing Carbon Stocks, Carbon Sequestration, and Greenhouse-Gas Fluxes in Ecosystems of the United States Under Present Conditions and Future Scenarios

“…Further, the importance of ebullitive fluxes is magnified by the fact that such fluxes largely escape oxidation in the sediment and water column (for instance, Martens and Klump (1980) estimated that 85 percent of CH 4 in bubbles survived transit through 7.5 m of water), whereas much of the diffusive flux from sediments is consumed by oxidation before flux to the atmosphere. Fluxes owing to the release of bubbles are likely to be particularly important in shallow (less than 5 m) waters (Joyce and Jewell, 2003), and releases are episodic on tidal and seasonal time scales (Chanton and others, 1989), making them difficult to measure and likely to be missed by oceanographic cruises (Hovland, 1993). Rajkumar and others (2008) provide an example of a study where diffusive and ebullitive fluxes were measured and scaled to an entire estuary.…”

Leakage and Sepage of CO2 from Geologic Carbon SequestrationSites: CO2 Migration into Surface Water

“…Numerous investigations have focused on describing and quantifying the production and transport of CH 4 and CO 2 in lakes and reservoirs (e.g., Sigurdsson et al, 1987;Oskarsson, 1990;Giggenbach, 1990;Giggenbach et al, 1991;Keller and Stallard, 1994;Woods and Philips, 1999;Casper et al, 2000;Joyce and Jewell, 2003). In non-volcanic/magmatic lake environments, CO 2 and CH 4 are primarily derived from biologic processes.…”

“…Ebullition is the primary release mechanism of methane from lakes and other shallow water environments (i.e., where CH 4 production is high and water is shallow). However, bubbling is episodic and dependent on a variety of factors such as temperature, water depth, barometric pressure variations, winds, and related bottom shear stress (e.g., Keller and Stallard, 1994;Walter and Heimann, 2000;Rosenberry et al, 2003;Joyce and Jewell, 2003). Casper et al (2000) measured CO 2 and CH 4 concentration gradients with depth in a small freshwater lake in the U.K. and determined the diffusion and ebullition fluxes of CH 4 and CO 2 to the atmosphere.…”

“…Casper et al (2000) also observed pulses of ebullition to be correlated with periods of falling barometric pressure. Joyce and Jewell (2003) estimated the contribution of bubble to total methane flux, its temporal variation, and relationship to current velocities at several lakes in Panama and Puerto Rico. The authors found a correlation between current velocity and CH 4 flux and also observed temporal correlation of bubbling across sites in the lakes.…”

“…As a result, large bubbling events may occur without trigger after prolonged periods of quiescence because substantial methane has accumulated. Conversely, no bubbling may occur with significant current velocities if no gas has accumulated (Joyce and Jewell, 2003).…”