Fine-grained sediment deposition is often conceived to happen in still water conditions and to represent slow sedimentation rates. However, it has been increasingly noted that mud sedimentation commonly occurs under high-energy conditions. The macrotidal Petitcodiac River estuary is used as an example to investigate the significance of flocculation as an important process in the rapid removal of large amounts of sediments from suspension under turbulent flow conditions. A range of physical sedimentary structures was observed at the Petitcodiac River estuary intertidal flats and bars, including low-angle and horizontal planar lamination, current and climbing ripples, surficial fluid mud, soft-sediment deformation, microfaults, and mud rip-up clasts. Fluid mud within the study area contains considerable proportions of clay (21–67%) and contributes to the formation of creeping fluid-mud sheets and streams. Detailed examination of the naturally occurring clay flocs shows that they contain up to 77% of the entangled silt- and sand-sized grains. SEM and microscopic imaging of fluid mud reveal a substantial amount of bioclastic material within the flocs and dispersed among the sediments. These observations show that physicochemical and biological processes influence silt, plankton, and clay aggregation. Water samples and observations from the Petitcodiac River estuary confirm that flocs form in the water column and then settle to the tidal-channel floor and flanking intertidal flats. Laboratory experiments, using Petitcodiac sediment, show how the clay flocs have the potential to sweep the water of silt-sized suspended grains. Additionally, diatoms and their associated extracellular polymeric substance (EPS) sheaths might play a role in mineral aggregation and increased sediment cohesiveness.
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Research funding, under which these data were collected, was generously provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada. We are particularly grateful for the field assistance of David Herbers and the technical and logistical aid of Mark Labbe and Martin Von Dollen. Special thanks are extended to Yeison Garcia Barrera for constructing the flocculator device. We would like to express a great appreciation to Dr. Alex Wolfe for his help in diatom identification and preparation of mud slides. Thanks to Ozlem Suleyman for letting us use the images from her MSc Thesis. We would also like to thank David Ralston and two anonymous reviewers for thoughtful manuscript reviews.
Communicated by David K. Ralston
Electronic supplementary material
SOM 2 a General view of the laboratory flocculator, which features a wood frame with a built-in motor, a 500 ml rounded glass beaker, and an adjustable height platform. b A closer view of a motor with an attached stainless steel shaft and plastic paddles for creating turbulence. c A top view of a laboratory flocculator with a solid-state controller. d A close-up of C, showing the details of a solid-state speed controller, which is used to change the paddle stirring capacity (GIF 572 kb).
SOM 3 Video material. Formation of flocs during aquarium experiments with stagnant water and salinity 10 ppt. Rapid introduction of stirred fluid mud into the salt-water tank promotes flocculation (MOV 355 mb).
SOM 4 Video material. Formation of flocs during aquarium experiments with stagnant water and salinity 30 ppt. Rapid introduction of stirred fluid mud into the salt-water tank promotes flocculation (MOV 326 mb).
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Shchepetkina, A., Gingras, M.K., Zonneveld, JP. et al. Silt- and Bioclastic-Rich Flocs and Their Relationship to Sedimentary Structures: Modern Observations from the Petitcodiac River Estuary. Estuaries and Coasts 40, 947–966 (2017). https://doi.org/10.1007/s12237-016-0186-x