Beschreibung: This study applies three classification methods exploiting the angular dependence of acoustic seafloor backscatter along with high resolution sub-bottom profiling for seafloor sediment characterization in the Eckernförde Bay, Baltic Sea Germany. This area is well suited for acoustic backscatter studies due to its shallowness, its smooth bathymetry and the presence of a wide range of sediment types. Backscatter data were acquired using a Seabeam1180 (180 kHz) multibeam echosounder and sub-bottom profiler data were recorded using a SES-2000 parametric sonar transmitting 6 and 12 kHz. The high density of seafloor soundings allowed extracting backscatter layers for five beam angles over a large part of the surveyed area. A Bayesian probability method was employed for sediment classification based on the backscatter variability at a single incidence angle, whereas Maximum Likelihood Classification (MLC) and Principal Components Analysis (PCA) were applied to the multi-angle layers. The Bayesian approach was used for identifying the optimum number of acoustic classes because cluster validation is carried out prior to class assignment and class outputs are ordinal categorical values. The method is based on the principle that backscatter values from a single incidence angle express a normal distribution for a particular sediment type. The resulting Bayesian classes were well correlated to median grain sizes and the percentage of coarse material. The MLC method uses angular response information from five layers of training areas extracted from the Bayesian classification map. The subsequent PCA analysis is based on the transformation of these five layers into two principal components that comprise most of the data variability. These principal components were clustered in five classes after running an external cluster validation test. In general both methods MLC and PCA, separated the various sediment types effectively, showing good agreement (kappa 〉0.7) with the Bayesian approach which also correlates well with ground truth data (r2 〉 0.7). In addition, sub-bottom data were used in conjunction with the Bayesian classification results to characterize acoustic classes with respect to their geological and stratigraphic interpretation. The joined interpretation of seafloor and sub-seafloor data sets proved to be an efficient approach for a better understanding of seafloor backscatter patchiness and to discriminate acoustically similar classes in different geological/bathymetric settings.

Beschreibung: Seafloor elongated depressions are indicators of gas seepage or slope instability. Here we report a sequence of slope-parallel elongated depressions that link to headwalls of sediment slides on upper slope. The depressions of about 250 m in width and several kilometers in length are areas of focused gas discharge indicated by bubble-release into the water column and methane enriched pore waters. Sparker seismic profiles running perpendicular and parallel to the coast, show gas migration pathways and trapped gas underneath these depressions with bright spots and seismic blanking. The data indicate that upward gas migration is the initial reason for fracturing sedimentary layers. In the top sediment where two young stages of landslides can be detected, the slope-parallel sediment weakening lengthens and deepens the surficial fractures, creating the elongated depressions in the seafloor supported by sediment erosion due to slope-parallel water currents.

Beschreibung: The current topographic maps of the Rhone Delta—and of Lake Geneva in general—are mainly based on hydrographic data that were acquired during the time of F.-A. Forel at the end of the nineteenth century. In this paper we present results of a new bathymetric survey, based on single- and multi-beam echosounder data. The new data, presented as a digital terrain model, show a well-structured lake bottom morphology, reflecting depositional and erosional processes that shape the lake floor. As a major geomorphologic element, the sub-aquatic Rhone Delta extends from the coastal platform to the depositional fans of the central plain of the lake at 310 m depth. 9 canyons cut the platform edge of the delta. These are sinuous (“meandering”) channels formed by erosional and depositional processes, as indicated by the steep erosional canyon walls and the depositional levees on the canyon shoulders. Ripples or dune-like morphologies wrinkle the canyon bottoms and some slope areas. Subaquatic mass movements are apparently missing on the delta and are of minor importance on the lateral lake slopes. Morphologies of the underlying bedrock and small local river deltas are located along the lateral slopes of Lake Geneva. Based on historical maps, the recent history of the Rhone River connection to the sub-aquatic delta and the canyons is reconstructed. The transition from three to two river branches dates to 1830–1840, when the river branch to the Le Bouveret lake bay was cut. The transition from two to one river branch corresponds to the achievement of the correction and dam construction work on the modern Rhone River channel between 1870 and 1880.

Beschreibung: Hydroacoustic methods are particularly suitable for investigations of the occurrence, cyclicity and amount of bubbles released at cold seeps without disturbing them. Experiments with a horizontally looking single beam transducer (40 and 300 kHz) directed towards artificially produced bubbles show that the backscattering strength of the bubbles increases with the gas flux rate independently of the bubble radii distribution. It is demonstrated that an acoustic system can be calibrated in such a way that gas flux rates of bubble-size spectra, as observed at natural seeps, can be directly related to the echo level of a known, acoustically insonified volume. No system-specific parameters have to be known except the beam width.

Beschreibung: Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low δ13C values. This indicates the presence of methane-consuming archaea, and δ13C values of –42 to –51‰ PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.

Beschreibung: Results of the analysis of mineralogy, geochemistry, and isotope parameters (δ13C, δ34S, δ18O, and 87Sr/86Sr) of carbonates and barites from sediments of the Deryugin Basin in the Sea of Okhotsk are presented. The diagenetic nature of carbonates and barites, which are formed as a result of the prolonged activity of cold seeps acting along a fracture zone and supplying methane–and barium-saturated fluids, is determined. Any signs for hydrothermal activity were not observed.

Beschreibung: The chemical composition (alkalinity, pH, NH4+, PO43-, Si, H2S, Cl-, Ca2+,and SO42-) of interstitial water was studied in the sediments of the Sea of Okhotsk at sites of methane emission. Variations in alkalinity were observed in the sediments from a typical seawater value (2.3 mM/kg) to 63 mM/kg. It is demonstrated that they are caused by the processes of sulfate reduction and methane generation. Based on the balance relationships, an equationwas constructed connecting changes in alkalinity with variations of Ca2+, SO42- and NH4+ in interstitial solutions.

Beschreibung: Filament intrusions are observed in high-resolution temperature (T) measurements from a 100-m and several month-long mooring in the Fram Strait in around 400-m water depth at the continental slope West of Svalbard (Spitsbergen, Norway). In this dynamic environment, a wide variety of intrusive layers are observed with thicknesses between 5 and 80 m with warmer water between cooler waters above and below. The layers typically last from several hours up to 1 day, exceeding the local buoyancy period but not lasting as long as intrusive layers in the open ocean. The intrusions are a result of an intermingling of Arctic and North-Atlantic waters and generated in the basins interior and locally via internal wave steepening upon the sloping bottom. Freely propagating semidiurnal lunar internal tides cannot exist without background vorticity at these high latitudes. Strongly nonlinear turbulent bores are not observed at the tidal periodicity, but wave fronts occur at the sub-inertial frequency of dominant baroclinic instability. The fronts are in part associated with near-buoyancy frequency internal waves (breaking). The details of the moored T observations and their spectral content demonstrate the non-smooth, relatively turbulent development including convective overturning and shear-induced instabilities when intrusions disperse in presumably salinity-compensated isopycnal layers.

Beschreibung: Water Column Imaging Multibeam Echosounder Systems (MBES) are effective and sensitive tools for investigating free gas (bubble) release and its rise through the water column. The main advantages of MBES are the detection range and lateral coverage in the water column and at the seafloor; furthermore, they are becoming increasingly available on research vessels worldwide. However, high noise levels and systematic artefacts due to side-lobe induced signal interference degrade MBES Water Column Images (WCIs) and hampered automated bubble detection and related gas seepage investigations. We present a new technique advancing automated detection of bubble streams and moving toward a quantitative gas-release assessment. It is shown that bubble streams can be detected reliably by their spatio-temporal behavior even when they are discontinuous in WCI data. Using assumptions about the bubble rising trajectories, bubble release spots at the seafloor can be traced even if the source location is obscured by acoustic noise or unwanted acoustic targets. A map with acoustic response and source locations of bubbles being released can be produced and serves as a starting point for more detailed quantitative analyses. The efficiency of the method has been assessed at a methane seep site in the Dutch North Sea. Multiple survey lines are merged to a detailed acoustic map of the area. Processed results are in good agreement with manual investigations of the WCI data as well as ROV-based video analysis.

Beschreibung: Hydrothermal activity in the Central Bransfield Basin revealed an active low-temperature vent field on top of a submarine volcanic structure. A temperature anomaly was detected and the sea floor showed various patches of white silica (opal-A) precipitate exposures and some yellow–brown Fe-oxyhydroxide crusts. Enriched dissolved methane concentrations were encountered. Sediment was near 24°C just after the grab came on deck. No dense population of chemosynthetically based macrofauna known from other hydrothermal venting areas was present, except for pogonophora. The observations suggest that the sedimented hydrothermal field at Hook Ridge is a low-temperature end-member branch from a deeper hydrothermal source.