Contourite development: Analysing the interaction between bottom currents and sedimentary systems

Abstract

Oceanic currents flowing near the seafloor erode, transport and deposit sediments, organic carbon, nutrients, and pollutants in deep-water sedimentary systems. Sediment deposits, which have mainly formed under the influence of these bottom currents (contourites) are high-resolution archives for reconstructing past ocean conditions. However, the interaction of sedimentary systems with oceanographic processes in deep-water environments is not well understood. The main objective of this thesis is to better understand the connection between contourites and the hydrodynamics that generate them in order to improve reconstructions of paleocurrents and sediment transport pathways. To achieve this objective a multidisciplinary study combining multibeam bathymetry, seismo-acoustic data, sediment samples, vessel-mounted Acoustic Doppler Current Profiler data, numerical modelling of ocean currents and three-dimensional flume tank experiments has been conducted.

Elongated depressions (moats) and their associated drifts form at the northern Argentine margin on top of relatively flat seafloor surfaces (terraces) next to a steep slope. The main current direction is along-slope, and the speed is higher near the slope and decreases on the terrace basinwards. Flume tank experiments show that a moat-drift system forms if there is a secondary basinward flow near the seafloor. The secondary flow increases with higher speeds and steeper slopes, leading to steeper adjacent drifts. Once the moat-drift system has developed, the secondary circulation is confined by the drift into a helix structure. Simultaneously, the primary along-slope velocity is increased. The measured current speed over the moats from the Argentine continental margin and the Bahamas area is high (up to 63 cm/s at 150–200 m) and decreases by 5-48% over the associated drift. Measurements from 185 cross-sections of moat-drift systems distributed at 39 different locations worldwide show that moats at steeper slopes have a steeper drift and that the angle of the slope side is on average 1.6 times higher than the angle of the drift side. However, no statistical relation is found between latitude and moat-drift morphology or stratigraphy. The flume tank experiments show that moat-drift systems can form solely because of along-slope currents without any additional oceanographic processes as eddies, internal waves or ocean current surface fronts. However, Acoustic Doppler Current Profiler data and the hydrodynamic model from the Argentine continental margin show that eddies near the seafloor can form on a contourite terrace. These eddies might lead to the small erosion surfaces on the Ewing Terrace, even though it is mainly a depositional environment and currents are relatively weak (below 30 cm/s). Experiments show that the migration of the moat-drift system and the formation of internal stratigraphic architecture is a function of current strength in combination with sediment supply. The different stratigraphic types of more erosive and more depositional moat-drift systems have been observed in seismic data. A new sub-classification of moat-drift systems based on their stratigraphy is proposed.

In summary, this study provides new insights into the interaction between ocean currents and sedimentary systems. It shows the importance of current strength, current variability (in time and space) and sediment supply for the formation of contourite systems. The combined data suggest that higher speeds and steeper slopes intensify the secondary flow, leading to steeper adjacent drifts. Thus, the morphology and internal architecture of the moat-drift systems can be used as a paleo-velocimeter. Furthermore, this study identifies the need for more in situ measurements near the seafloor.