The impact of internal waves on upper continental slopes: insights from the Mozambican margin (southwest Indian Ocean)

Abstract

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. However, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

The erosive power of the Malvinas Current: Influence of bottom currents on morpho-sedimentary features along the northern Argentine margin (SW Atlantic Ocean)

“…We suggest that contourite terraces may have been initiated by erosion on the slope generated by the (paleo) Malvinas Current that progressively cut the slope landwards, widening the contourite terrace with time. The fact that the Argentine contourite terraces are much wider and flatter than other terraces observed for instance in the Mediterranean Sea (Ercilla et al, 2016;Miramontes et al, 2019), along the Mozambican margin (Thiéblemont et al, 2019;Miramontes et al, 2020) and in the Makassar Strait (Brackenridge et al, 2020) could be related to the higher speed of near-bottom currents as part of the Malvinas Current. The particular flat morphology with an abrupt edge of the terraces along the Argentine margin may favour the formation of internal waves at the terrace edge, in a similar way as at the shelf break (Jackson et al, 2012) that could also favour sediment transport and erosion along the terrace.…”

“…Thus, other processes have to be taken into consideration. Previous research reported that internal waves can form sediment waves and dunes (Hand, 1974;Reeder et al, 2011;Droghei et al, 2016;Ribó et al, 2016;Reiche et al, 2018;Yin et al, 2019;Miramontes et al, 2020). Internal waves can propagate at density discontinuities and have been previously proposed as a cause for the development of the Ewing Terrace and the La Plata Terrace (Hernández-Molina et al, 2009;Preu et al, 2013).…”

“…The initiation of the formation of the contouritic terraces was suggested by some authors to be related to internal waves propagating at water mass interfaces (Hernández-Molina et al, 2009;2016b;Preu et al, 2013;Ercilla et al, 2016;Yin et al, 2019;Llave et al, 2020). Other authors proposed that internal waves may be secondary processes that can form channels and dune fields on contourite terraces, but are not responsible for the original formation of the contourite terrace, which they argued is probably related to strong currents (Miramontes et al, 2019;Miramontes et al, 2020). The model derived 25-year average bottom currents and the near-bottom water mass distribution derived from CTD measurements confirm that the contouritic terraces in the northern Argentine margin are roughly located at water mass interfaces (Figs.…”

The erosive power of the Malvinas Current: Influence of bottom currents on morpho-sedimentary features along the northern Argentine margin (SW Atlantic Ocean)

“…We suggest that contourite terraces may have been initiated by erosion on the slope generated by the action of two water masses and their interphase trough time, being the (paleo) Malvinas Current one of the key factors that progressively cut the slope landwards, widening the contourite terrace with time. The fact that the Argentine contourite terraces are much wider and flatter than other terraces observed for instance in the Mediterranean Sea (Ercilla et al, 2016;Miramontes et al, 2019), along the Mozambican margin (Thiéblemont et al, 2019;Miramontes et al, 2020) and in the Makassar Strait (Brackenridge et al, 2020) could be related to the higher speed of near-bottom currents as part of the Malvinas Current and the long time period of erosion (since the opening of the Drake Passage). The particular flat morphology with an abrupt edge of the terraces along the Argentine margin may favour the formation of internal waves at the terrace edge, similar to those observed at the shelf break (Jackson et al, 2012), that could also favour sediment transport and erosion along the terrace.…”

Siliciclastic and bioclastic contouritic sands: Textural and geochemical characterisation

“…The upper continental slope of the Mozambican margin consists laterally of: a) a convex slope with a plastered drift and a contourite terrace to the southwest, offshore the Zambezi delta (Figs. 1B and 2A;Thiéblemont et al, 2019); and b) a concave slope with a plastered drift and a dune field, 50 km further northeast or west offshore the Zambezi delta (Miramontes et al, 2020b). This work focuses on the sector between the distal part of that contourite terrace and the deeper adjacent plastered drift (core MOZ4-CSF19; Fig.…”