Release of gas bubbles from lake sediment traced by noble gas isotopes in the sediment pore water

The release of gas bubbles from lacustrine or oceanic sediments into the overlying water (ebullition) is a major mechanism for the discharge of biogenic or geogenic gases into the water body. Ebullition of methane or carbon dioxide, for instance, contributes considerably to the release of these potent greenhouse gases through the sediment/water interface. Depending on the rate of ebullition, the pore water will show a depletion in dissolved atmospheric noble gases, because the poorly soluble noble gases escape from the pore water into the gas bubbles. In this study, the depletion of dissolved noble gases in sediment pore water was analyzed for the first time to study bubble formation and ebullition in sediments. The noble gases in the pore water of the sediments of Soppensee (Switzerland) show a distinct depletion due to ebullition of biologically produced methane. This depletion is lowest in the deep sediment and increases towards the sediment surface. The noble gas isotope ratios in the pore water indicate that vertical diffusion barely affects the observed noble gas profiles. The isotope ratios further show that the methane bubbles remain long enough in the sediment to attain noble gas solubility equilibrium before escaping into the overlying water. The volume of gas released from the sediment by ebullition can therefore be reconstructed from the extent of the noble gas depletion in the pore water using a simple gas-equilibration model. The noble gas profiles in the sediment indicate that ebullition increased in Soppensee during the Holocene, and that ebullition contributed strongly to the release of methane from the sediment. Our case study thus illustrates that noble bases are sensitive proxies for the release of gas from lacustrine and marine sediments or similar aquatic environments. (c) 2005 Elsevier B.V. All rights reserved.