Physicochemical Processes in Turbidity Currents

Sediment transport in turbidity currents is normally analyzed by considering only the physical mechanisms of density-induced flow and particle transport. However, the very fine sediments commonly found in turbidity currents are also subject to significant particle-particle interactions due to chemical and electrostatic forces. These physicochemical processes are known to influence the behavior of fine particles in natural waters. For example, increasing salinity in estuaries causes a reduction of inter-particle electrostatic interactions, which generally encourages particle flocculation and deposition. Similar processes should influence the behavior of turbidity currents, but available models do not consider the effect of the different chemical properties of fresh and marine waters on sediment transport in turbidity currents. To improve understanding of these processes, a set of laboratory experiments was conducted with different sediment types and background water compositions. Clay and silica particles were used as representative natural sediments. The background ionic strength of the turbidity current and surrounding water were varied from freshwater to seawater conditions (10 to 600 mM, respectively). Considerably different sediment deposition behavior was observed for clay turbidity currents under different water conditions. Clay deposition was observed to increase with increasing salt concentration, and this behavior was related to changes in the size distribution of the particles that comprised the turbidity current. Additional variation in clay deposition was observed with a change from sodium to calcium ionic composition. The mechanism of this secondary variation is not clear, but we believe that it is due to differences in the structure of the flocculated particles in the turbidity current. Unlike clay, the deposition of silica turbidity currents did not change with background salt concentration. This behavior was expected since silica colloids are generally much more stable than clay colloids. Measurement of silica size distributions with different background water compositions confirmed that silica did not flocculate under any of the conditions used in the turbidity current experiments. These results emphasize that turbidity current behavior can change due to physicochemical particle-particle interactions that depend on the compositions of the sediments and the surrounding water.