Seismic mapping of free methane gas in the Baltic Sea
The marine environment plays a minor role in the Earth's radiation budget, accounting for merely 2% of methane sources to the atmosphere. However, the atmospheric methane content significantly influences radiation absorption in the infrared spectrum as well as photochemical reactions with serious implications for the local environment and future climate changes. Most of the methane originating from marine environments forms in the final stage of microbial degradation of organic matter deposited on the sea floor and buried in ocean margin sediments. In order to obtain reliable estimates of methane escape from the seabed, it is important to have as precise an overview of methane gas occurrences and their proximity to seabed as possible. For this purpose, researchers at the Geological Survey of Denmark and Greenland (GEUS) in Copenhagen examined shallow seismic data from numerous surveys carried out in the Shagerrak and western Baltic Sea region. Information from maps produced on the basis of various reports on past site surveys were put together with new data acquired during cruises carried out within the framework of the METROL project. Based on the seismic profiles collected, a contour map indicating sub-seafloor depths of free methane gas was constructed, giving an accurate overview of all major shallow methane gas occurrences in marine sediments. The shallow gas map was stored digitally as an ESRI® shape file, which offers the possibility of viewing the map at any given scale using suitable Geographic Information System (GIS) software. Although the database of shallow gas depths was constructed to provide better understanding of the geological controls of methane escape from the seabed, it may serve other purposes as well. If properly calibrated with sediment pore water chemistry data, information on the condition of ecologically vulnerable areas can be provided. Such areas can be estuaries and bays subjected to long-term eutrophication due to the increased discharge of nutrients from high-productivity farming.
