Controls on flocculation and accumulation of mud from moving suspensions: experimental insights into the roles of mineralogy, salinity, suspended sediment concentration, flow velocity and microbes

ABSTRACT Understanding how mud moves and deposits is essential for understanding and quantifying the dynamic nature of surface environments and their ancient counterparts. Many factors have been proposed to control the accumulation of mud, including mineralogy, salinity, suspended sediment concentration, flow velocity and microbiota. Flume studies at the IU Shale Research Lab were aimed at investigating the influence of these factors. Earlier work is summarised to provide a snapshot of the current state of understanding. A key parameter that defines mud accumulation is the critical velocity of sedimentation (CVS)—the minimum flow velocity required to keep sediment particles suspended in the water column and prevent them from settling to the bottom. CVS is influenced by a combination of particle and fluid characteristics. Most previous experimental studies have been conducted with sediment composed largely of kaolinite clay in fresh to saline water. Kaolinite, however, is not the dominant clay in natural sedimentary systems on Earth. Recent experiments with the most common clay minerals, illite and smectite, show that these minerals differ markedly in their depositional characteristics from kaolinite. CVSs for illite and smectite are significantly lower (15 and 10 cm/s, respectively) than for kaolinite (25 cm/s) in otherwise identical experiments. Unlike kaolinite, salinity does influence their CVSs. The reasons for these differences are likely related to the morphology of the different clay minerals and the distribution of their surface charges. These significant differences were not observed in earlier experiments, most likely because microbial films formed on the various clay minerals and floccules during extended experimental runs. Particle binding by microbially generated extracellular polymers appears to have counterbalanced the pronounced differences in behaviour among clay minerals observed in intentionally sterile experiments. Ubiquitous microbial coatings, typical for natural environments on Earth since the Archean, apparently make muddy suspensions behave similarly regardless of the dominant clay mineral. In abiogenic settings, however, such as planets devoid of life, mineral controls on flocculation behaviour may result in sediment distribution patterns that are unknown on Earth.