Bacterial Methane Formation and Distribution in Marine Environments.

Thießen, Olaf (2005) Bacterial Methane Formation and Distribution in Marine Environments. Open Access (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 145 pp.

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Abstract

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.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Schmidt, Mark
Keywords: Ostsee, Pazifischer Ozean, Biogenes Sediment, Methan, Arkona Becken, Energie- und CO2-Deponierungstechnologien, Methangärung, Hotspots, Südpazifik
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: No
Date Deposited: 03 Dec 2008 16:52
Last Modified: 19 Jan 2023 10:49
URI: https://oceanrep.geomar.de/id/eprint/5790

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