Mixing and the bottom nepheloid layer

Abstract

A close relationship between active mixing and the structure of the bottom nepheloid layer on the Oregon shelf is shown, by combining mixing information inferred from temperature microstructure with concentration profiles derived from a transmissometer mounted on the microstructure instrument. Mechanical energy dissipation rates and eddy coefficients are usually consistent with those in an unstratified turbulent boundary layer, but an exception was found when stratification due to suspended sediment was required to explain the observations. Except for this case, the suspended particle concentration profiles, for observations in approximately 100 m of water, are consistent with the assumption of local equilibrium between turbulent diffusion and gravitational settling. Observations made in deeper (180–200 m) water do not show this balance. The high-gradient region above the nepheloid layer in shallow water impedes diffusion sufficiently that escape of particles by this route is not important; winter mixing destroys the layer long before diffusion would. This region of low turbulent mixing was not consistently seen in the deeper water where high levels of turbidity were found throughout the water column. Low-density turbidity currents of approximately 1 cm s −1 can be generated by the concentrations found.