Abstract

Sedimentary organic matter (OM) composes 20% of all carbon preserved in the Earth's crust. Marine sediments constitute the major long-term sink of organic carbon (OC) on Earth, although only <0.3% of OM photosynthesized by plants is eventually preserved. While the burial of this small OM fraction affects the global cycles of atmospheric CO 2 and O 2 , the mechanisms that control sedimentary OM preservation are still unclear. Recent studies have suggested a strong relationship between OM preservation and the OM physical forms, chemical compositions and cumulative exposure to O 2 during deposition and burial. However, although organic geochemistry has now progressed to the point where the major hydrolysable biochemicals (proteins, carbohydrates and lipids) can be measured using standard chromatographic methods, >75% of sedimentary OM is still missed chromatographically and remains molecularly uncharacterized. We therefore combined bulk and molecular-level analytical approaches to target the mechanistic understanding of the effects of physical protection, molecular composition and O 2 exposure time (OET) on OM preservation. A wet chemical sequential extraction procedure was developed to quantify and separate OM from diverse marine sediments fractions having different chemical reactivity. This research project then focussed on one fraction believed to be determinant in OM preservation, i.e., non-hydrolysable oxygen-sensitive organic matter (OSOM). An optimized method based on the gentle chemical oxidation of OM using RuO 4 was developed in order to investigate the molecular structure of OSOM and its relationship to OM preservation under varying sedimentary conditions. The relative abundance of the OSOM fraction in marine sediments decreases exponentially with oxygen exposure time, in agreement with our working hypothesis. However, our data suggest a more complex relationship than previously thought in that inputs of terrestrially-derived OSOM most likely affect the degradation rate and relative abundances of the bulk OSOM fraction. RuO 4 treatment on the OSOM fractions revealed a composition consisting mostly of cross-linked aliphatics polyesters, and relative abundances of the oxidation products that are correlated with oxygen exposure time (OET). Further molecular, compound-specific isotopic (GC-IRMS) and spectroscopic (FTIR, solid-state HR-NMR) studies on the OSOM component will help verifying the hypothesis of the major role of oxygen exposure time in OM preservation.