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Abstract

During the last decades of biogeochemical and microbiological research the widespread occurrence of microorganisms was demonstrated in deep marine and terrestrial sediments. With this discovery inevitably the question of potential carbon and energy sources for this deep subsurface microbial life rises. In sedimentary systems such a source is provided by the buried organic matter deposited over geological times. During geochemical and geothermal maturation these organic material undergoes biotic and abiotic alteration processes and is suggested to release potential substrates into the surrounding. Previous studies showed that especially oxygen containing compounds are lost from the macromolecular matrix during diagenesis and early catagenesis. Oxygen containing low molecular weight organic acids (LMWOAs) such as formate, acetate and oxalate represent important substrates for microbial metabolism. Thus, lithologies containing accumulated sedimentary organic matter (e.g. lignites and coals) may provide a large feedstock for deep microbial life releasing LMWOAs into the pore water during maturation. In this thesis, lignite and coal samples from sedimentary basins of New Zealand covering a broad and almost continuous maturity range representing diagenetic to catagenetic coalification levels were investigated to estimate their feedstock potential for deep microbial life using a novel developed analytical procedure to analyse kerogen-bound LMWOAs liberated by selective chemical degradation reactions. Formate, acetate and oxalate were found to decrease continously from early diagenesis to early catagenesis. This suggests a constant release of these compounds during this maturation interval providing a suitable feedstock for microbial ecosystems in geological time spans. Investigation of a transect from organic carbon rich (lignite) into poor lithologies (silt and sandstones) from the DEBITS-1 well, suggested that the lignite layers sustain microbial communities inhabiting the adjacent more permeable layers (sandstone) by releasing substrates into the surrounding lithologies. Investigations of kerogen-bound high molecular weight fatty acids show for the long chain fatty acids (C20-C30), representing a terrestrial plant material signal, a constant decrease during diagenesis and early catagenesis. In contrast the short chain fatty acids (mainly C16 and C18) show an increase again during early catagenesis an interval where the release of hydrocarbons slowly starts but temperatures are still compatible with microbial life. These increasing abundance of C16 and C18 fatty acids (being also main constituents in cell membrane phospholipids of bacteria) might suggest an increased microbial community stimulated by the geothermal release of hydrocarbons. Hydrolysis of kerogen-bound LMWOAs is suggested to be the most favorable abiotic process releasing potential substrates into the pore water. Kinetic investigations suggested that the hydrolysis appears to be a relatively fast process and, thus, the observed substrate release from lignites and coals over geological times must be influenced and slowed down by further processes such as e.g. pore space, permeability, pore water flow and diffusion. The calculated kinetic parameters point to structural alteration within the macromolecular network during maturation leading to more sterically protected kerogen-bound LMWOAs and, therefore, to a slower substrate release with ongoing maturation. Additional information about the structure of the macromolecular network were obtained by selective ether-cleavage procedure revealing that aliphatic alcohols with more than one hydroxy groups represent important cross-linkage structures. In contrast to the terminal ether-bound monoalcohols which show a rapid decrease during diagenetic alteration, these compounds show relatively high concentrations even in the more mature coals suggesting that these cross-link bridges are sterically protected within the network structure.