Hydroacoustic and geochemical traces of marine gas seepage in the North Sea

Methane is the second most important anthropogenic greenhouse gas on Earth and contributes considerably to global radiative forcing. The last IPCC assessment report 2007 assigns geological methane emissions as a significant source. This thesis therefore concentrates on the quantity and atmospheric implications of methane emissions from the seabed of the North Sea. Sampling of marine seepage is challenging compared to readily accessible terrestrial sites; thus marine seepage sites have scarcely been observed or even yet discovered. Moreover, in terms of atmospheric contribution, the fate of methane after ebullition into the water column is usually not considered. Hydroacoustic systems have proven to be very efficient remote sensing tools for gas seepage analysis even in water depth greater than 2000 m. Technical progress led to much higher remote sensing potential by means of modern multibeam applications for gas bubbles detection in the water column. However, to be effective, these novel multibeam systems require new methods for data analysis. This thesis firstly demonstrates the application of multibeam systems as efficient gas bubble remote sensing tools. Therefore an anthropogenic blowout site was mapped using a multibeam sonar. The advantage of multibeam technology compared to singlebeam is increased efficiency due to larger coverage than singlebeam systems, three dimensional plume mapping, and exact localization of gas sources. Moreover the deployment of the multibeam prototype GasQuant is examined, which is an adapted sounder specifically designed for in situ gas bubble detection. GasQuant was deployed for several days within a gas seep field in the Central North Sea (Tommeliten). Aside from minor system adaptations, major effort was spent to handle the non-standard large datasets by means of various data processing and visualization routines. Taking into account the surrounding tidal current flow field, unique data patterns were extracted to unambiguously detect gas bubbles in the water column. Thus, a total of 52 single seep holes were localized and characterized with respect to their tempo-spatial variability. Recently, water column scanning multibeam mapping systems entered the market. Due to their huge amount of data output, manual processing is no longer feasible. Thus, a generic algorithm for the detection of rising gas bubbles in multibeam data was developed that accounts for the current tidal flow field for detection issues (Appendix A). Incorporation of other disciplines such as geochemistry and oceanography allowed for a methane gas source strength estimate of the Tommeliten gas seepage field in the North Sea. Combined acoustic mapping and in situ sampling revealed a source strength of ~0.8-4.8*106 mol/yr – a considerable quantity compared to prominent gas seep sites around the world (e.g. ~1*106 mol/yr at Vodyanitskii mud volcano, Black Sea; 2.19*106 mol/yr at North Hydrate Ridge offshore Oregon). Obviously previous studies have underestimated the area of active venting at Tommeliten. By modeling gas bubble dissolution and geochemical sampling it was found that the majority of bubble-mediated methane at Tommeliten already dissolves in the ‘deep’ water between the 70 m release depth and 40 m. Thus the methane is trapped below the upper-well mixed summer layer, from which it would readily be degassed by air-sea exchange processes. Given the heavy storm activity during winter, research cruises into the North Sea preferentially take place during the summer, where low atmospheric outgassing/emissions from seabed methane is expected due to stratification. However, considering the distinct hydrographic seasonal cycle of the North Sea, quantitative transport of seepage methane into the atmosphere seems likely during winter after fall mixing. This seasonal bias is not only constrained to the study site, but relevant for the entire Central and Northern North Sea as well as many mid-latitude shallow shelf sea waters showing temporal stratification.