Potential environmental control of free shallow gas in the seafloor of Eckernförde Bay, Germany

Abstract

The depth of the free methane gas horizon in Eckernförde Bay, Western Baltic Sea, was monitored over 4 months in 1-h intervals with an echo sounder mounted on a tower at the seafloor. The depth to the top of the free gas varied between 50 cm and 100 cm below seafloor. Short-term fluctuations of free gas depth are related for the most part to changes of total pressure (air pressure and water level), the observation period was too short to demonstrate the effect of the annual temperature cycle. The correlation of gas depth changes and total pressure agrees well with theory which predicts a linear influence of pressure fluctuations on gas solubility. Since the methane content in the pore water of Eckernförde Bay is at the saturation limit environmental changes (pressure and temperature variations) cannot be buffered and have an instant impact on free gas presence. The implications of this observation are manifold. Geo-technically, the bearing strength of the seafloor is altered by the presence of gas bubbles. Similarly, sonar systems are influenced by the presence of gas bubbles which alter the acoustic seafloor properties. Of ecological interest is that pressure drops can trigger a sudden ebullition of the greenhouse gas methane from the seafloor into the atmosphere.