Oceanographic process-driven sediment transport and morphological response in the northeastern Atlantic Ocean

Deep-sea sedimentary systems archive the history of environmental change and sediment transport, and are shaped by various oceanographic processes at different spatiotemporal scales. How these processes interact and sculpt seafloor morphology and stratigraphy remains a key, yet not fully resolved, question in sediment dynamics. This doctoral thesis investigates the Rockall continental margin, northeastern Atlantic Ocean, as a natural laboratory. By integrating multidisciplinary datasets, it elucidates the roles and coupling mechanisms of different hydrodynamic processes governing sediment transport from the continental shelf to the foot of the continental slope, advancing our understanding of the formation and evolution of sedimentary features on passive continental margins.
Geophysical data reveal that distinct sedimentary features occur in the study area. On the upper slope of the NE Rockall Trough, off NW Ireland, widespread contourite terraces and associated plastered drifts develop at 500–1500 m water depths, where a pycnocline dominates the water column. Water-column acoustic and turbidity data directly capture the presence of breaking internal waves and the formation of nepheloid layers within this depth range. This indicates that internal waves are a key mechanism driving local sediment resuspension, nepheloid-layer formation and across-slope transport. Suspended sediments can be effectively differentiated from biological scatterers by using a combination of multi-frequency acoustic backscatter and optical imagery. Observations show that nepheloid layers can extend over 40 km offshore, and their spatiotemporal distribution correlates closely with mesoscale eddy activity identified by satellite altimetry, suggesting that eddies impact the long-distance across-slope transport and dispersal of suspended particles into the open ocean. Contourite moat-drift systems extending over 265 km along the western slope of the Rockall Bank are identified. The moat is dominated by older, coarser-grained deposits, whereas the adjacent drift accumulates younger, finer-grained sediments with higher organic content. Mooring records link this differentiation to dynamic oceanographic processes. Semidiurnal (M2) tides dominate the alongslope background flow, while the passage of mesoscale eddies causes bottom current intensification and pulsed high-turbidity events. Integrated analysis indicates that tides and the passing eddy drive both alongslope and across-slope transport of sediments from the erosive moat to the depositional drift, thereby controlling the long-term morphosedimentary evolution.
Through an integrated approach, this thesis establishes direct links between diverse hydrodynamic processes and their sedimentary responses. It demonstrates that the observed sedimentary features result from the interplay of multi-scale processes including internal waves (including internal tides), mesoscale eddies and background circulation. Internal waves dominate local erosion and sediment entrainment. Eddies modulate the regional transport pathways of suspended sediment, while background currents and tides provide the fundamental transport forcing. These processes jointly determine sediment fluxes, spatial distribution, and final depositional behaviours. The findings not only advance our understanding of sedimentary dynamics on the NE Atlantic slope, but also provide critical observational constraints for reconstructing past oceanographic conditions from sedimentary records and for developing robust process-to-product models.