ALBERT

Description: This study was performed to investigate gas formation and gas saturation conditions related to acoustic turbidity in shallow (∼40 m deep) marine basins. The Arkona Basin, Baltic Sea, with its organic-rich fine-grained surface sediment provides an ideal “Natural Laboratory” to characterise free gas using seismic, geoacoustic, and geochemical methods. The area of acoustic turbidity covers about 1500 km2 of the central Arkona Basin, corresponding to areas where organic-rich post-glacial sediments exceed 4–6 m in thickness. The highest concentration of pore water methane (7660 μmol L−1), found in areas of high acoustic turbidity, was near the calculated lower limit of methane solubility for the measured in situ temperature, salinity, and pressure. Pore water methane concentration decreased to near 4 μmol L−1 in areas outside of the zone of high acoustic turbidity. Stable carbon (−70.7‰ to −92.3‰ PDB) and hydrogen (−124‰ to −185‰ SMOW) isotope values of methane indicate that methane is predominantly formed by microbial CO2 reduction in Arkona Basin surface sediments and rules out significant contributions of other sources.

Description: Gas in sediments has become an important subject of research for various reasons. It affects large areas of the sea floor where it is mainly produced. Gas and gas migration have a strong impact on the environmental situation as well as on sea floor stability. Furthermore, large research programs on gas hydrates have been initiated during the last 10 years in order to investigate their potential for future energy production and their climatic impact. These activities require the improvement of geophysical methods for reservoir investigations especially with respect to their physical properties and internal structures. Basic relationships between the physical properties and seismic parameters can be investigated in shallow marine areas as they are more easily accessible than hydrocarbon reservoirs. High-resolution seismic profiles from the Arkona Basin (SW Baltic Sea) show distinct ‘acoustic turbidity’ zones which indicate the presence of free gas in the near surface sediments. Total gas concentrations were determined from cores taken in the study area with mean concentrations of 46.5 ml/l wet sediment in non-acoustic turbidity zones and up to 106.1 ml/l in the basin centre with acoustic turbidity. The expression of gas bubbles on reflection seismic profiles has been investigated in two distinct frequency ranges using a boomer (600–2600 Hz) and an echosounder (38 kHz). A comparison of data from both seismic sources showed strong differences in displaying reflectors. Different compressional wave velocities were observed in acoustic turbidity zones between boomer and echosounder profiles. Furthermore, acoustic turbidity zones were differently characterised with respect to scattering and attenuation of seismic waves. This leads to the conclusion that seismic parameters become strongly frequency dependent due to the dynamic properties of gas bubbles.

Description: Within the framework of this thesis the investigation of methanogenesis and secondary degradation processes of methane in two distinctly different marine environments has been carried out. These two environments were (i) the gassy shallow-marine sediments of the Arkona Basin, western Baltic Sea and (ii) the hydrothermal submarine fluid/gas exhalations at hotspot volcanoes of the central South Pacific. Based on the results of geochemical, sedimentological and seismic investigations as well as geochemical modelling it was possible to reconstruct the occurrence, distribution, genesis and degradation processes of methane in these two environments. Particularly, the analyses of the molecular composition of the hydrocarbons extracted from the fluid/pore water in combination with the stable carbon and hydrogen isotopic signal enabled deductions regarding the methane formation processes in the respective working areas. In the central Arkona Basin an area of about 1500 km2 shows acoustic turbidity of the seismic signal in ~1.5 m depth in the surface sediments. This acoustic turbidity is an indication of free gas in post-glacial surface sediments which may reach about 12 m thickness in the basin centre. These sediments are characterised by a conspicuously high organic carbon content (ca. 5-8 %) and a clayey-silty structure. The degradation of organic matter produces anaerobic conditions in near surface sediments and in about 1 m sediment depth, below the sulphate reduction zone, it results in the accumulation of methane. The stable carbon and hydrogen isotopic results of all gas samples from the post-glacial sediment layer confirm bacterial methane production via CO2 reduction. Solely, the d13C-CH4 values of ~-40‰ of the upper few centimetres of the surface sediments indicate oxidised methane. The late- and post-glacial clay (reddish/brown and grey) that is located beneath the organic-rich sediments, however, shows rather low methane concentrations within the pore-water and the concentration profile as well as the d13C-CH4 values point to diffusion of gas from the surface sediments in to the clay. Overall, the measured methane concentrations in pore-water show distinct differences in methane concentration from trace concentrations at the northern rim of the basin to the point of methane saturation in the central basin. The increase in CH4 concentration is generally accompanied by an increasing thickness of organic-rich surface sediments. Based on the concentrations of organic carbon, methane, and sulphate and the average sedimentation rate a numerical model was developed to characterise sulphate reduction, anaerobic oxidationof methane and methane production. The model results show that a sediment thickness of 3.5 m has to be achieved to obtain CH4 production rates that enable the accumulation of methane in the sediments. Furthermore, it could be shown that a sufficient amount of methane required to cause oversaturation and gas bubble formation can not be generated until a sediment thickness of 5-6 m is reached. This minimum sediment thickness established by the geochemical model correlates nicely with the sediment thickness at which the seismic readings show acoustic turbidity. During research cruises Sonne 65 (1989) and Polynaut (1999) methane analyses were conducted on water column and fluid samples above submarine volcanoes of the Pitcairn, Austral, and Society island chains, central South Pacific. Weak hydrothermal activity was determined at hotspot volcanoes Bounty and Teahitia in 1999. The 1999 results, in comparison with methane results from 1998, point to a rather calm exhalation activity of the hotspot volcanoes. The higher methane concentrations determined in the water column in 1998 could be attributed to the eruption of the Macdonald seamount that occurred at that time. The importance of bacterial methane production by CO2 reduction, also for hot exhalations of submarine volcanoes, could be described the first time by means of the 13C/12C and D/H isotope ratios of methane. At the Bounty seamount the stable isotopic values of methane from the fluids refer to an almost 100% bacterial production of methane at about 88°C. An amount of about 50% bacterially produced methane besides the abiogenic produced methane could be determined at Macdonald seamount.