Climate and sea level controlled sedimentation processes in two submarine canyons off NW-Africa

Abstract

This study focuses on the trigger mechanisms of gravity-driven sediment transport in two submarine canyons at the passive continental margin off NW-Africa during the past 240 kyr. The sedimentary records allow to determine the turbidite emplacement times based on high resolution age models. The sediment textures of the turbidites were studied by using X-ray radiographies. The sedimentary properties like the terrigenous silt size distribution and XRF-core scanning element data allow to identify the variability of the aeolian dust input in the hemipelagic sediments. These variations can be used to reconstruct the climatic conditions in the hinterland which strongly influence the sediment supply on the shelf. In addition a clay mineral assemblage was used to reconstruct the fluvial input of the West-African rivers.The trigger mechanisms of gravity-driven sediment transport in submarine canyons during sea level changes has been reported from many regions. However, the relationship of sea level changes and short-term climatic events with turbidite deposition is poorly documented. The turbidite history of the Dakar Canyon during the late Quaternary was reconstructed using gravity cores directly recovered from the canyon axis. The highest frequency of turbidite deposition is restricted to the last two major climatic terminations when remobilisation of sediments from the shelf was triggered by eustatic sea level rise. Coarse terrigenous silt size data and high Ti/Ca ratios reflect an overall increased dust supply during the last two peak glacials resulting in the formation of extensive sand seas covering the exposed shelf. The interglacials were characterised by less intensive wind stress. However, sporadic turbidite events coincide with the timing of Heinrich events in the North Atlantic. During these times continental climate has changed rapidly towards increased aridity and enhanced wind strength. This in turn led to a higher dust supply which has fed turbidity currents.The turbidite being deposited during the two main periods of activity consist of sandy to silty sediments. Detailed grain size analyses were used to reconstruct the sedimentary characteristics of these turbidity currents. With increasing distance to the source area a downslope sorting trend in the grain size distribution towards finer grain sizes was observed. A much higher frequency in turbidite activity is recorded during the previous peak glacial in contrast to the last peak glacial. This suggests a higher sediment budget in the source area. The correlation of turbidites is based on their stratigraphic position. A single turbidite can exactly be dated at all core locations in the thalweg. Based on the sedimentological analysis of the turbidites this study provides a schematic model for the sedimentation processes in the Dakar Canyon.The turbidite activity of the Dakar Canyon was then compared to the neighbouring submarine Diola Canyon. Both canyon systems are affected by the seasonal shifts of the Intertropical Convergence Zone. However, the southern Diola Canyon is further influenced by the discharge of the West African rivers. During the late Quaternary a mixture of fluvial derived material and aeolian dust was deposited at the exposed southern Senegalese shelf. Frequent turbidite deposition in the Diola Canyon is recorded during minor sea level rises which forced the remobilisation of this sediment mixture. Furthermore, the post glacial sea level rise removed the remaining sediment deposits from the now flooded shelf region.The results of this PhD study highlight the influence of sea level rise and the subsequent remobilisation of shelf sediments as well as short-lasting climatic changes which act as main factors controlling the sediment flux in the investigated canyon systems.