NANOSCALE MODELS OF MINERAL SURFACE - ORGANIC MATTER RELATIONSHIPS AS POSSIBLE PATHWAYS OF ORGANIC CARBON PRESERVATION ON GEOLOGIC TIME SCALES

Many factors control the rate of supply and decay of organic material in marine sediments including: primary productivity, concentration of oxidants in the water column and shallow sediment, the rate of removal from oxidant source controlled by rates of sinking and burial, the association with mineral surfaces, and the availability of abiogenic reductants such as hydrogen sulfide. Increasing evidence suggests a mineral surface-controlled pathway of organic carbon burial. Strong interactions between organic matter (OM) and clays (showing the largest natural surface area and electrochemical charge) as well as their spatial relationships, can protect OM from both biogenic and abiogenic oxidation. We applied findings and advanced techniques from nanocomposite research to test 5 models and investigate the mechanistic controls of OM preservation, these include: 1) Microcomposite – clays occur as crystallites. This is common relationship between OM and clays with OM occurring in mesopores and macropores of sediment. The model may be limited to large (micrometric scale) OM particles. 2) Exfoliated nanocomposite – clays occur at least partially as fundamental particles dispersed in OM, interacting with OM with strong physicochemical reactions. This can be one of the most efficient mechanism of OM protection. The model excludes simple compaction or hydrodynamic sorting. 3) Thin film. OM covers external surfaces of minerals (including clays). This model can be applied only to explain small quantities of TOC. Simple coating of external surface area can not account for TOC values >2% in sediments. 4) Polar molecules intercalated nanocomposite – polar OM molecules substitute water layers in interlayer space, preserving crystallite structure; less probable. 5) Charged molecules intercalated nanocomposites. Charged OM molecules can substitute inorganic exchangeable ions on all sites showing electric charge (dominated by smectite interlayer space). That is also the first step for adsorption of other OM molecules, which can not be adsorbed directly on clay without agents, as hydrophobic or non-polar OM molecules. Nanoscale relationships have not previously been considered in models of OM preservation so our studies may lend some new insight by unifying these fields of science.