Abstract
Benthic sediments in continental shelf seas control a variety of biogeochemical processes, yet their composition, especially that of fine sediment, remains difficult to predict. Mechanisms for mud or fine sediment deposition and retention are not fully understood. Using sediment data and a hydrodynamic model of the Northwest European shelf seas, a relationship is shown to exist between fine benthic sediment composition and regions of cyclonic tidal current rotation. The reduced thickness of cyclonic tidal benthic boundary layers compared with the anticyclonic case promotes deposition of fine sediment and trapping of resuspended material. Adding the effects of the benthic boundary layer thickness, as influenced by ellipticity or not, sheds some light on the limitations of approaches only focusing on bed shear stress and sediment pathways to predict the location of mud deposits. A tidal boundary layer predictor that includes ellipticity alongside tidal current magnitude and depth was shown to spatially agree with maps of mud deposits.
Highlights
Coastal and shelf seas cover a small fraction of the ocean but are of utmost importance and value (e.g. Costanza et al, 1997)
We present the hypothesis that the suppressed boundary layer in cyclonic tidal currents aids the deposition and retention of fine sediment, and is an important mechanism to consider in shelf sediment dynamics, and of pollutant, carbon, or nutrient retention
To match sediment spatial polygon data and gridded hydrodynamic model data, the grid points located within each sediment polygon type were selected to compare sediment, stress, ellipticity, and Numerical model results for the Northwest European shelf seas show that the M2 ellipticity across the shelf is often positive at locations with benthic mud deposits (Figs. 2a, 3b)
Summary
Coastal and shelf seas cover a small fraction of the ocean but are of utmost importance and value (e.g. Costanza et al, 1997). Sediment composition (mud, sand, gravel) influences a range of biogeochemical and physical parameters. Biogeochemical processes depend on sediment type, varying between advective sediments (sand, gravel) with low organic content and cohesive sediments (mud) with high organic content (Somerfield et al, 2018). Sediment type influences physical processes in shelf seas through modification of bed friction (van Rijn, 2007), impacting dissipation of energy, and sediment mobility (Hsiao and Shemdin, 1980; Soulsby, 1997; Winterwerp and van Kesteren, 2004). It influences benthic habitats and community structure Understanding the overall structure and functioning of shelf seas, including their response to human and climate pressures, requires an understanding of sediment composition, transport, and deposition mechanisms
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