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This thesis focuses on the impact of burrowing, ingestion, digestion and excretion of marine annelid worm Arenicola marina on modifying the preservation and degradation of sedimentary organic matter. This introductory chapter discusses the diagenesis of marine sediment, particularly organic matter cycling and preservation, and the general role of benthic fauna in such processes. The biology, ecology, anatomy and physiology of A. marina are also discussed. The materials and methods employed in the present study are described in Chapter 2. In Chapter 3 focuses on the impacts ofA. marina upon structural and compositional changes of bulk sedimentary organic matter, as well as sediment organic-mineral interaction, suggesting that A. marina enhances the ;.,i remineralization of organic carbon and nitrogen, induces adsorption of protein peptide onto the acidic sites ofthe minerals, and its digestive enzymes may cause conformational changes of the peptide structure. Chapters 4-5 describe and discuss the impact ofA. marina upon sedimentary lipid degradation and distribution, as well as the selectivity, metabolism and assimilation of the animal. A. marina alters the degradation rates of sedimentary lipids, which consequently affects organic matter diagenesis. The primary algal lipid signals are significantly and rapidly~ltered by the animal via a series of digestive processes. The bacterial biomarkers of the sediment, in particular, are substantially affected. Finally in Chapter 6, the summary of conclusion answers the key questions raised at the end of Chapter 1, and future works are presented. 1.2. THE GLOBAL ORGANIC CARBON CYCLE AND PRESERVATION 1.2.1. OVERVIEW The global organic carbon cycle is summarized in Figure 1.1. It is divided into two parts, the biological cycle and the geological cycle. The biological cycle starts with photosynthesis of organic matter from atmospheric carbon dioxide or carbon dioxide/bicarbonate in the surface waters of oceans or lakes. It continues through the different trophic levels of the biosphere and ends with the metabolic or chemical oxidation of decayed biomass to carbon dioxide. The geological organic carbon cycle begins with the incorporation of biogenic organic matter into sediments or soils. It then leads to the formation of natural gas, petroleum and coal or metamorphic forms of carbon, which may be reoxidized to carbon dioxide after erosion of sedimentary rocks or by combustion of fossil fuels. Inventories and stable carbon isotope mass balances indicate that the greater than 99.9% of all carbon is stored in sedimentary rocks of the Earth's crust (BERNER, 1980). About 80% of this carbon occurs within carbonates oflimestones and dolomites (HUNT, 1996). The other 20% of this carbon is organic, occurring mainly in the form of kerogen, an insoluble macromolecular material that is disseminated at an average concentration of