Variations in Benthic Boundary Layer Phenomena: Nepheloid Layer in the North American Basin

Abstract

Observations of phenomena associated with a benthic boundary layer were made over a 19-day period in two different regimes in the western North American Basin. Repeated measurements of temperature, in situ and in vitro turbidity, suspended particulate concentrations and excess radon versus depth, as well as bottom photographs, were made as a function of time at two locations on the lower slope of the Blake-Bahama Outer Ridge (BBOR) and at one location on the Ratteras Abyssal Plain (HAP). At the BBOR sites the benthic boundary layer was manifest by high concentrations of suspended particulates, high turbidity, and intense vertical mixing indicated by excess radon. Vertical distributions of particulate matter and radon were related to the thermal structure of the water mass and, although the strongest manifestations of frictional interaction between the water and bottom were seen below 150–300 m, some influence was seen as high as 1500 m above bottom.

The 150–300-m particulate and radon boundary also coincided with the top of an adiabatic layer. Large, regular, temporal variations in these parameters with a period of about one week were in phase at the two stations, located 110 km apart. This benthic boundary layer regime was associated with current velocities from 10 to 30 cm/sec and with a current direction and temperature and salinity characteristics indicative of the Western Boundary Undercurrent (WBUC). At the RAP site, manifestations of the benthic boundary layer were less intense, less variable and restricted to a thinner layer. Turbidity and concentration of particulate matter were much lower than on the BBOR and vertical mixing measured by excess radon was an order of magnitude lower. Depth profiles of these parameters showed the strongest evidence of the benthic boundary layer to be restricted to 80–100 m above the bottom (cf. 150–300 m on the BBOR) but with some evidence of mixing of particulate matter and cold bottom waters to 900 m above the bottom (cf. 1500 m on the BBOR). Again, the top of the 80–100-m zone coincided with the top of the adiabatic bottom layer. Bottom photographs and water mass characteristics indicate that this benthic boundary layer regime is associated with the rather slowly northward moving Antarctic Bottom Water (AABW). An hypothesis to explain the variations observed on the BBOR is presented in which eddies of clear AABW are injected into the more turbid, rapidly southward flowing WBUC north of the study area. These are seen as temporal variations in the intensity of the nepheloid layer along the BBOR.