Conceptual models of early diagenetic processes in sedimentary deposits guide interpretation and investigation of compositional patterns, elemental fluxes, and biogeochemical interactions. The ideas that sediments are open to exchange, are laterally homogeneous, and often accrete steadily upward underlie most diagenetic theory. Net accumulation rate of a deposit is thus a master variable controlling reactions, net fluxes, and sediment properties. These basic ideas and corresponding models have proved extraordinarily robust and useful. Large regions of the seafloor, however, can deviate significantly from some of the common assumptions of traditional diagenetic models, particularly along continent-ocean boundaries, where most sedimentary debris is processed. A spectrum of diagenetic facies representing a wide range of boundary conditions and internal transport-reaction regimes is typically present. Mobile muds are one of the major endmember diagenetic facies found in energetic, high sedimentation environments such as estuaries and deltas. These deposits often behave as episodically-mixed, fluidized batch reactors dominated by microbial biomass rather than,for example, classic advective plug flow reactors or geometrically complex, bioturbated bodies. Redox reaction patterns in mobile muds are unsteady. Suboxic conditions often dominate temporally, reflecting a balance between frequency of seafloor disturbance and the relative abundance and reactivity of recently entrained oxidants and reductants. Sedimentary dynamics, rather than net sedimentation, control the magnitude and nature of elemental fluxes and biogeochemical properties of mobile muds and the lateral exchange of material between diagenetic regimes. The understanding of elemental cycling in continental margins and their evolution as biogeochemical systems require consideration of the different dominant modes and the relative importance of diagenetic processing within and between individual facies.