Seafloor morphology in the Mozambique Channel: Evidence for long-term persistent bottom-current flow and deep-reaching eddy activity

Abstract

New high-resolution bathymetric and sub-bottom profiler data collected in the Southern Mozambique Channel along a grid of 16 parallel, non-overlapping lines show a large variety of bedforms which were formed by strong bottom currents. They are classified into four main microtopographic zones and several sub-zones and divide the study area into regions with (1) smooth seafloor, (2) undulating bedforms, (3) seamounts and islands, and (4) the Zambezi Channel. A smooth seafloor occurs on the Mozambican continental slope together with downslope mass-wasting processes, north and south of Bassas da India, on the eastern levee of the Zambezi Channel and in the Zambezi cone. Undulating bedforms of some kilometres wavelength and several tens of metres height cover most of the southern, central and northeastern study area. The most spectacular bedforms are numerous, closely spaced, giant erosional scours of up to ~450 m depth, more than ~20 km length and ~3 - 7 km width in the southwestern part of the study area. Here, northward flowing Antarctic Bottom Water (AABW) is topographically blocked to the north and deflected towards the east due to the shallowing bathymetry of the Mozambique Channel. SW-NE trending undulating bedforms aligned parallel to the deflected AABW and interpreted as small contourite mounds allow to trace the AABW flow path eastwards. An ~100 km long W-E trending channel indicates the northernmost extension of the AABW. NW-SE oriented undulating bedforms in the west, hummocky bedforms in the east and arcuate, cross-cutting features in-between reflect a completely different current regime in the central study area. Comparisons with LADCP sections show, that the western part lies in the range of deep-reaching anticyclonic Mozambique Channel eddies (MCEs), so that the undulating bedforms are again considered to be small contourite mounds aligned parallel to a part of the swirl. The cross-cutting features in the middle mark the eastern boundary of the MCE, where a northbound flow direction prevails. The hummocky bedforms in the east may have developed under the influence of seasonally variable cyclonic East Madagascar Current eddies pretending at least two different flow directions. The origin of arcuate bedforms, sediment ridges and circular or elongate depressions in the northeastern study area is not clear. Bottom currents which interact with the topography of the Bassas da India complex and the Zambezi Channel may contribute to their formation. All morphological features are draped with sediments indicating that the present-day current velocities are not strong enough to erode sediments. This agrees with published LADCP bottom-current velocities of 0.1 m/s. Hence, the microtopography must originate from a time when bottom-current velocities were stronger. Assuming a published sedimentation rate of 20 m/Myrs and a drape of at least 50 m thickness the microtopography may have developed during Pliocene times or earlier