Measurements of diffusivity near the sediment-water interface with a fine-scale resistivity probe

Fine scale profiles of the sedimentary formation factor, F, were obtained from measurements of resistivity at millimetre intervals in marine sediments. These profiles demonstrated that rapid decreases in diffusivity occurred in the upper few centimeters of both compact intertidal sands and fine silty muds. In the compact sands, a 70% decrease in the formation factor was observed within 2 cm of the interface. Close correlations were found between the formation factor and sediment porosity for both sediment types. Profiles of $\text{1}\text{F}$ predicted from these correlations and porosity profiles were very similar to those obtained from measurements of resistivity. These correlations also suggested that φ^?3 is a better approximation of the formation factor in fine muds than the Archie relation φ^?2. In both sediments, the interfacial value of the diffusion coefficient was estimated to be between ~74 and ~87% of its value in free solution, a value much higher than is normally assumed.