Friedrich Abegg

The acoustic turbid layer in muddy sediments of Eckernfoerde Bay, Western Baltic: methane concentration, saturation and bubble characteristics

In Eckernfoerde Bay in the western Baltic, an enclosed basin with organic-rich mud, acoustic turbidity has been observed since 1952. To investigate the relationships between acoustic properties and free gas we developed a method for rapid sub-sampling of gravity cores to determine the total methane content. From temperature measurements, determinations of the salinity of the pore water and data from the literature, saturation limits were calculated for each core. It is demonstrated that there is a certain amount of free gas in the mud of Eckernfoerde Bay but subbottom depths of the oversaturation and of the acoustic turbidity do not correspond. The existence of free gas implies the existence of bubbles in the sediment which are regarded to be responsible for the acoustic turbidity. Bubble sizes observed by X-ray computed tomography while still under in situ pressure and temperature vary between 1 and 10 mm equivalent diameter. The distribution of bubbles is discontinuous, occurring in depth zones from 2 to 20 cm thickness.

Marine gas hydrate: Fabric, quantification and free gas content. Examples from Hydrate Ridge–Cascadia margin

A recent advance in the investigation of internal gas hydrate structure and in small scale quantification is based on a combination of a new coring device and computerized x‐ray tomography (CT). This approach has been chosen because gas hydrate is only stable within a special (low) temperature and (high) pressure field. A MultiAutoclaveCorer, developed by the Technical University of Berlin, is in principle similar to a piston corer and has the size of a multiple corer. During the deployments on Hydrate Ridge, two pressure vessels filled with seafloor samples could be recovered. They were CT‐investigated after the cruise in a medical clinic close to San Francisco. As a result, a 3‐D‐dataset of the cores is available which allows to quantify the components’ gas hydrate, sediment and free gas and also shows the distribution and orientation of the gas bubbles. One of the pressure vessels showed a distinct gas hydrate horizon where the gas hydrate content reached close to 50 vol%. Within this horizon there was...