Short-term sediment resuspension on the continental slope and geochemical implications: the Faeroe-Shetland Channel

Abstract

Continental slopes are important and complex regions that connect shallow shelf seas with the deep ocean. Since a large part of the global primary productivity takes place on the continental margins, knowledge on sediment resuspension and associated processes on the continental slope is crucial to understand the fate of particles, including biogenic debris. The objectives of the study described in this thesis were to better understand the interactions between near-bottom currents and surface sediments of the continental slope, and to assess the impact of sediment resuspension on the cross-slope redistribution of biogenic and non-biogenic particles. While most of the studies conducted on sediment resuspension on continental margins have investigated the role of seasonal to inter-annual variations of the near-bottom currents, this thesis particularly focuses on short-term (tidal to sub-tidal) mechanisms. High-resolution studies have the potential to provide valuable information on the short-term processes involved in sediment resuspension, which, due to their difficulty to be resolved, have not received much attention, yet. The novel aspect of this work lies in the combination of fast sampling near-bottom sediment traps, turbidity sensors and current meters moored on a high-energy continental slope.

Our data show that massive and abrupt sediment resuspension occurred at mid-slope and is associated with sudden drop of the temperature and upslope surge of the near-bottom currents. A more detailed study reveals that such massive resuspension was facilitated by strong vertical velocities generated at the leading edge of internal non-linear waves travelling up the slope. This finding suggests the importance of the interaction of such a phenomenon with the seabed can provide a potentially dominant mechanism for upward transport of sediment over continental slopes that may counteract downward avalanching of material by gravity. The results presented in this thesis show the importance of the short-term processes in the resuspension of sediment on the slope and in the transfer of biogenic materials from the shelf to the deep ocean. This transfer may have significant implications for organic matter recycling along the ocean margins. This work represents the first step towards a better understanding of these potentially very active processes. Clearly, further investigations are needed, particularly for the highly productive ocean margins such as areas with high continental input or coastal upwelling regions, which are also receiving much attention in terms of their paleo-climatological significance. Furthermore, high-energy processes (see Chapter 3) and the associated massive sediment transport and organic matter recycling (see Chapters 2, 4 and 5) may create major perturbations of the sedimentary signal and lead to erroneous interpretation of proxy data.