Recent Results

Several sites of natural hydrocarbon seepage at continental margins are investigated in detail, including mud volcanoes and pockmarks of the Nordic margin, the Eastern Mediterranean, the Black Sea and the West African margin. Data from different cold seep systems clearly show that these are heterogeneous, and different habitats with high oxygen consumption rates (a measure for the overall benthic activity) and diffuse as well as focused methane effluxes were identified. Abiotic as well as biotic processes are strongly influenced by fluid and gas fluxes varying in space and time. The amount of methane released varies strongly depending on the velocity of upward fluid flow. The obtained results improve our understanding of the oxidation of methane, the development and distribution of different habitats (e.g. bacterial mats), and the emission of methane into the hydrosphere. The data also enhance our understanding of the role of cold seeps in the global budget of methane emission from the deep sea floor.

The Haakon Mosby Mud Volcano is a highly active methane seep hosting different chemosynthetic communities such as thiotrophic bacterial mats and siboglinid tubeworm assemblages. Targeted in situ quantification of oxygen, methane, and sulfide fluxes are used to develop a spatial budget for the four main habitats on a scale ranging from 10 m to 1000 m. The dissolved methane efflux at HMMV of up to 777 mmol m-2 d-1 (mud volcano center) is so far the highest emission recorded in cold seep habitats, indicating also the importance of diffuse fluxes for budget estimates. For the entire mud volcano, the total dissolved methane efflux sums up to 14 x 106 mol yr-1, compared to a gaseous emission of 8–35 x 106 mol yr-1.

Evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in the 18 kHz echosounder records (‘flares’) were obtained at the convergent Makran continental margin. The hydroacoustic anomalies released from hydrocarbon seeps at water depth between 575 and 2870 m disappeared in the water column. Dives with the remotely operated vehicle ‘Quest 4000m’ revealed that several individual bubble emissions contributed to a hydroacoustic anomaly. The volume flux was estimated by counting the emitted bubbles and using their average volume. We found that a low volume flux caused a weak hydroacoustic anomaly in the echosounder record whereas high volume fluxes caused strong anomalies. The total flux of methane bubbles in the study area may be estimated multiplying the average flux of methane causing a strong hydroacoustic anomaly in the echosounder record with the number of recorded strong anomalies. Our results on the fate of bubbles and the order-of-magnitude estimation suggest that all of the ~43 ± 27 x 10^6mol methane emitted per year from emissions within the gas hydrate stability zone remain in the ocean interior.

Carbonate slabs at the seafloor are one specific manifestation of fluid seepage at mid slope depths in the central province of the Nile Deep Sea Fan (NDSF). Motivated by earlier results we conduced research during R/V Maria S. Merian cruise 13/3 and 13/4 at the carbonate slabs that show up as high backscatter patches in multibeam data. The flux of gas bubbles was quantified at one particular carbonate slab termed C-1 by combining autonomous underwater vehicle (AUV)-based multibeam data showing individual bubble streams with remotely operated vehicle (ROV)-based observations of particular emission sites resulting in an estimated flux somewhere in the range between 0.37 to 1.5 x 10^6 mol methane per year. The ultra-high resolution maps obtained by AUV-based multibeam showed that the carbonate slabs project above the very smooth surrounding seafloor by a few decimeters up to nearly two meters in the central part. We calculated the volume and estimated the amount of carbon stored as carbonates in the slabs, e.g. C-1 is composed of at least 327 x 10^6 mol carbon. Taking into account published ages of a carbonate crust and the quantitative values obtained in our study we conclude that the flux of methane transported as gas bubbles is significantly higher than the flux of methane that is anaerobically oxidized and precipitates as carbonates.