Mooring ST-3, recovered from the Polar Duke on 20 February while Hole 1095B was being drilled, was the third deployed within the Sediment Drifts of the Antarctic Offshore (SEDANO) program, funded by the Programma Nazionale di Ricerche in Antartide with the participation of the British Antarctic Survey and aimed at describing the modern deep oceanographic circulation in the vicinity of Drift 7. Results of two previous moorings have been described by Camerlenghi et al. (1997a) and are summarized (and a related CTD section shown) in "Regional Oceanography," in Barker and Camerlenghi (Chap. 2, this volume).
The two previous moorings, located along slopes of the drift, had shown slow, steady current flow, parallel to seabed contours. The mooring recovered during operations of Leg 178 was composed of two rotor-type current meters, 8 and 60 m above the seafloor. It was positioned in a more distal location than the previous moorings but along the same bathymetric contour (~3550 m [Fig. F46]), with the aim of observing seafloor current dynamics where the relief of the drift was minimal and obtaining information higher above the seafloor to constrain the thickness of the Ekman boundary layer.
Figure F47 shows a progressive vector diagram of the unfiltered time series (11.5 months from March 1997 to February 1998) provided by the two meters, produced by initial processing on board the JOIDES Resolution. Close to the seabed (8 m) the mean current flow direction is westward, tangential to the bathymetric contour; 60 m from the seafloor, however, the mean flow direction is to the south-southwest, angled ~80° counterclockwise from the direction at the lower mooring. The upper mean flow is faster (5.2 cm/s) than the lower (4.0 cm/s). It can be inferred that the lower meter lies within the Ekman layer, whereas the upper one lies above it.
Despite the overall difference in flow direction, there is some coherence between flow variabilities of the two time series. Highest flow velocities are found in both series in June and August 1997, concomitant with variations in flow direction, probably reflecting the passage of mesoscale eddies with strong barotropic properties that induce recirculation down to the seabed. The typical length scale of the June eddy is smaller than the August one, but both eddies have the same duration at depth. These barotropic perturbations lead to maximal current intensity at the seafloor, so that fine-grained sediment suspensions can be transported southwestward. The lowest mean current velocity along this contour is found in the most distal part of the drift (at ST-03) where the topographic control on flow direction and intensity is expected to be lowest. The generally low mean flow velocities are in agreement with preliminary results of the optical characteristics of the bottom-water masses, which suggests a virtual absence of a permanent nepheloid layer overlying Drift 7. The regional oceanography is therefore compatible with modern and ancient interglacial sedimentation composed almost exclusively of biogenic material, with minor clay particles of either bottom current or eolian origin. Textural analysis on late Pleistocene sediments from Drift 7 (Pudsey and Camerlenghi, 1998) implied that small differences in grain size between glacial and interglacial sediments do not reflect dramatic glacial-interglacial changes in bottom-water flow. Therefore, the main factor controlling glacial-interglacial changes in sediment composition appears to be the sediment source.
3Contributed by R. Laterza, A. Giorgetti, and A. Crise, Osservatorio Geofisico Sperimentale, P.O. Box 2011, Trieste Opicina 34016, Italy.