Abyssal hydrography, nephelometry, currents, and benthic boundary layer structure in the Vema Channel

Abstract

New data from closely spaced hydrocasts, thermograd profiles, vertical nephelometer profiles, and direct bottom current observations within the Vema Channel (near 30°13′S) allow an interpretation of the flow regime and the structure of the benthic boundary layer. A sharp gradient in potential temperature, light scattering, concentration of suspended particulates, and excess radon is present in the transition zone between northward flowing antarctic bottom water (AABW) and the overlying north Atlantic deep water. This transition zone, however, exhibits pronounced eastwest asymmetry across the channel axis. To the east the benthic thermocline is relatively deep (∼4250 m) and sharp (0.8°/100 m to 1.7°/100 m). In the channel axis adjacent to the western wall the thermocline is several hundred meters shallower and more gentle (∼0.3°/100 m). Gradients in light scattering exhibit a corresponding asymmetry. Coldest bottom water temperatures (θ < −0.18°C) and highest values of near-bottom turbidity are present on the eastern side of the channel, even though the bathymetric and structural asymmetry of the channel suggests preferential erosion and perhaps strongest current velocities on the western side, where the channel is deepest. Near-bottom current velocities measured in both the main branch and the western branch of the channel exhibit a significant cross-contour component in the mean flow. This deviation is approximately 40° in the main branch and 10°–20° in the western branch. The direction of deviation from contour-following flow is toward the east or in the same direction as would be predicted for Ekman layer transport in a bottom boundary layer. The temperature and light scattering data are consistent with a model of an asymmetrical flow regime such that (1) strongest northward current velocities are adjacent to the western wall of the channel, (2) fractional effects due to the presence of the wall induce strong turbulence in the flow, (3) turbulent mixing within the AABW results in a ‘blurring’ of the benthic thermocline and slight elevation of near-bottom temperatures in the channel axis, and (4) upslope (eastward) advection in a bottom boundary Ekman layer results in a veering of the mean velocity vector and the transport of coldest bottom water (θ = −0.18°C) to the eastern margin of the channel. Uncertainties in estimating AABW transport through the channel are introduced by difficulties in identifying an appropriate level of no motion and by the unknown extent to which frictional effects in the benthic boundary layer invalidate the geostrophic approximation. If a level of no motion corresponding to σ4 = 46.00 is assumed, the geostrophic transport of AABW through the main branch is of the order of 1.4×106 m3/s. Intervals of abrupt increase in bottom water turbidity within the channel axis show no obvious correlation with variations in current velocity observed simultaneously. Consequently, the time variability in the nepheloid layer may reflect varying rates of sediment entrainment upstream in the Argentine Basin rather than resuspension of sediment within the channel itself. The total weight of suspended particulates per unit area within the channel appears to be comparable to that in the basin to the south. The principal effect of the channel on the bottom nepheloid layer is to increase its thickness from a few tens of meters to several hundred meters.