Non-linear processes affecting the deposition and accumulation of mud on continental margins

The deposition and accumulation of mud is affected by nonlinear processes associated with particle adhesion. Specifically, particle adhesion in suspension produces aggregate particles that increase the deposition rate of muds. Particle adhesion in the seabed reduces erosion rates and limits the sorting of fine particles, which enhances accumulation rate of muds. This talk brings together results from a decade of work on these processes, synthesizing how they affect the position of mud depo-centers on river-influenced continental margins. While in suspension, silts and clays collide and adhere in a process called "flocculation". The resulting aggregate particles, called "flocs", sink faster than their component grains and are the primary vector for delivery of clays, very fine silts and fine silts to the seabed. The rate of aggregation scales generally with the square of particle concentration, so deposition rates of the finest sediment particles are greater where concentrations are larger. This general trend is modulated by turbulence, which can break flocs when it is energetic. The ideal conditions for flocculation, therefore, are large sediment concentrations and weak turbulence, conditions which typically do not co-occur. Riverine sediment plumes entering quiet receiving basins and bottom boundary layers where turbulence has been damped by suspended sediment density stratification are two important environments where large concentrations and weak turbulence co-occur. Deposition of sediments rich in fines occurs when there is rapid delivery of riverine sediment to sections of the seabed where the stress is imparted primarily by waves rather than currents. The deposition of clays, very fine silts and fine silts to the seabed is important to accumulation of mud because the presence of these finest fractions decreases the erodibility and sorting of a sediment deposit. Once the clay content of a deposit exceeds 5-10%, the erosion rate at a given applied stress decreases, but more importantly, particles finer than 16 micrometers adhere to one another, making it challenging to winnow these fine sediments from the bed. In summary, clays, very fine silts and silts deposit due to flocculation. The presence of these sizes in sediment decreases the erodibility and sortability of that sediment, leading to the accumulation of mud. These non-linear processes must be considered when predicting eventual depo-centers of mud on continental margins.