ARTIFICIAL MILD THERMAL MATURATION OF A TYPE II KEROGEN SAMPLE: POTENTIAL GENESIS OF PRECURSORS FOR SUSTAINING THE DEEP BIOSPHERE

Abstract

Organic diagenesis represents the processes altering the quantity and the quality of the sedimentary organic matter (SOM) during the first stages of burial, mainly under the action of heterotrophic microorganisms. Once anoxia is established and sulfate is completely consumed from pore water, methanogenesis takes over on the degradation processes, leading to the production of CH4. This process is critical as it is responsible for the production of about 20 % of the global gas resources. Methanogenesis lasts until sediment temperature no longer allows microbial survival or the residual MOS becomes too refractory. Traces of life in the subsurface are found at deeper depths and there are questions about how these microorganisms can survive were the SOM is known to be refractory. Wellsburry et al. (1997) and Parkes et al. (2007) have addressed this phenomenon and revisited the concept of thermal maturation of the SOM. They observed, that low thermal stress (30 90 ° C), over periods spanning from weeks to months, could increase microbial degradation rates of the SOM in laboratory experiments. Following the same hypothesis, Ravin et al. (2017) observed, on a natural sedimentary series, that sediment temperatures around 30 °C could activate the SOM, otherwise refractory, fueling the deep biosphere. There is therefore a hypothesis that abiotic reactions induced by heat stress can lead to the production of labile organic compounds to feed the deep biosphere. In this context, the objective of this work is to characterize the thermolabile potential of the SOM and to determine the its kinetic parameters. We chose a very immature recent marine sediment sample from the ODP Site 1082 located offshore of Namibia, an area influenced by the Benguela upwelling current system. To investigate the kinetics of the thermal degradation of the MOS, the kerogen was pyrolyzed in sealed gold tubes under isothermal temperatures ranging from 150 to 225 °C during 9 to 72 h. The kerogen and the pyrolysis residues were characterized using solid state CP-MAS 13C NMR.