Particulate size spectra, behavior, and origin of nepheloid layers over the Nova Scotian Continental Rise

Abstract

Samples of suspended sediment and light transmissometer observations were taken on two occupations of a transect of the Nova Scotian continental rise in 1980. Initial low suspended sediment concentrations were followed by high values 10 days later. Patches of high concentration were found on the edges of zones of cold (θ<1.78°C) water. Particle volume concentrations in the diameter range 1.26–32 μm measured by Coulter Counter on 10 cm3 samples correlate linearly with light beam attenuation coefficient. Correlation between particle volume and weight concentration indicates lower apparent particle density for low‐concentration suspensions, a feature that is attributed to aggregation during aging of the suspension. Particle size distributions show unimodal form with a peak at ∼4 μm in high concentrations near the bed changing to a bimodal form with an additional mode at ∼16 μm in low concentrations high above the bed (500 and 1000 m.a.b. (meters above bottom)). The presence of the coarse mode at 1000 m.a.b. coupled with its increase in the lower 250 m of the water column suggests that it both settles from the surface and is resuspended from the bed. Ultrasonic disaggregation experiments show that much of the fine mode is of aggregates, but the coarse mode is single solid particles. Distributions with a low volume‐concentration increase in volume when disaggregated and are interpreted as having a relatively large part of their total distribution in unmeasured sizes >32 μm. Distributions with a high concentration decrease in counted volume on disaggregation. Inversions in the light attenuation profiles are taken to indicate the presence of former bottom mixed layers, and their occurrence suggests thickening of the nepheloid layer by detachment and lateral injection of these turbid mixed layers. They occur at levels sufficiently high as to provide a nepheloid layer over 1500 m thick over the adjacent Sohm Abyssal Plain. The coarse mode probably comprises solid particles with a calculated settling velocity of ∼12 m day−1. Its absolute concentration increases toward the bed, but as a proportion of the distribution it increases upward. It is thus thought to be particles of surface origin both settling to the bottom and, in the lower 250 m of the water column, resuspended from the bed. The fine mode at 4 μm has a spatially constant form thought to be controlled by physical coagulation mechanisms. Rates of coagulation estimated from shear mechanisms suggest that the postulated change of peaked into flat size distribution by particle aggregation and settling has a time scale of a few years rather than months, which is probably long when compared with the lateral injection process lower in the nepheloid layer. For that reason, distribution with a fine mode peak are found well above the bottom mixed layer.