Geophysical properties of surficial sediments : textural and biological controls
The structural properties of surficial sedimentary deposits strongly influence exchange processes across the benthic boundary layer, and hence are important factors controlling sedimentation and biogeochemical cycling. They are governed by a complex combination of hydrodynamic, chemical and biological controls, and therefore exhibit significant spatial and temporal heterogeneity. Geophysical techniques have great potential as tools for investigation of these structural properties. Techniques for determining in situ Electrical Formation Factor (FF) and acoustic shear-wave velocity (Vs) in the upper few centimetres of saturated intertidal deposits were developed: FF was measured by a Wenner electrode array at the sediment surface; Vs was measured by piezoelectric bender transducers inserted to a depth of 40mm. Geophysical properties, porosity, and textural and biological characteristics were determined in situ at a variety of locations, predominantly in sands. The highest variability in all parameters was obtained over large spatial scales, with primary variation in bulk textural composition and benthic infaunal community. Within-location variability was significant over medium spatial scales of tens to hundreds of metres, controlled by local variation in tide-averaged hydrodynamic environment and duration of tidal exposure. In sands, bulk textural variability was of secondary importance over these scales, and was interpreted as due to admixture of relatively small proportions of coarse and fine sub-populations into a uniform sedimentary framework. Localised geophysical variability, while significant, was also of secondary importance, being influenced primarily by properties of this framework. Seasonal variability was also significant, controlled by temporal variation in hydrodynamic environment and temperature. Structural parameters (porosity, FF, Vs) were more responsive than textural parameters to temporal variation in hydrodynamic or biological activity. Porosity was controlled by grain shape and size distribution, and by the depositional environment. It was not apparently directly affected by benthic macrofaunal activity. FF was controlled by porosity, and additionally by factors affecting tortuosity. Tortuosity was increased by increasing both carbonate and mud contents. Burrowing organisms (Arenicola marina and Corophium) and tube-building organisms (Lanice conchilega) reduced FF. Vs was controlled by porosity, and additionally by factors which control intergranular friction. This was increased by increasing carbonate content, and reduced by increasing mud content. Burrowing organisms (Arenicola marina and Corophium) reduced Vs, while tube-building organisms (Lanice conchilega and Pygospio elegans) increased it.