Biogeochemical processes controlling gas bubble production and distribution in organic-rich sediments

Abstract

Biogeochemical processes in organic-rich, muddy sediments often result in the net production of biogenic gases including methane. In coastal sediments, methane production ultimately leads to near saturation gas concentrations and bubble formation. Rates of production, oxidation, and transport processes, together with insitu temperature and pressure (depth), combine to determine the actual sediment column depth of methane bubble occurrence. Recent studies along North Carolina’s Outer Banks and Eckernfoerde Bay in the Baltic Sea reveal how these processes combine to control saturation gas concentrations and bubble distributions in the upper few meters of coastal sediments. At the North Carolina site, gas production depths vary seasonally, resulting in a bubble layer whose shallowest depth oscillates between 10- and 30-cm depth from summer to winter, respectively. Large quantities of gas escape the sediments via diffusion and bubble ebullition during the warm months. Similar oscillations in the depth of the bubble (acoustic absorption) layer appear to occur in the sediments of Eckernfoerde Bay; however, competing microbial processes prevent saturation methane concentrations at depths above approximately 50 cm. Stable isotope measurements reveal that microbial methane oxidation consumes methane transported above the bubble layer, resulting in little release of gas into the water column.