New insights into submarine channel evolution revealed by repeat seafloor mapping

Submarine channel systems are one of the main conduits for the transport of land-derived material, such as sediment, organic carbon, nutrients, and pollutants to the deep sea. The seafloor-hugging flows, called turbidity currents, that transfer this material through these channels can be destructive and can affect the shape and stability of the seafloor. As a result, these flows can damage important seafloor infrastructure such as telecommunication cables that underpin the internet. Despite their importance, these submarine systems are less well understood than rivers. This is because submarine channels are difficult to monitor, since satellite imagery has limited penetration beyond shallow water depths, and because of the destructive nature of turbidity currents. The paucity of direct observations of turbidity currents and how they shape submarine channels means that we lack a complete understanding of how these systems work. This thesis presents two unique sets of repeat seafloor surveys of active submarine channel systems. First, ten repeat surveys of the submarine channel in Bute Inlet, British Columbia, Canada, are presented; each of which covers the system from source to sink. Second, a set of 20-year spanning surveys of the Congo Channel, offshore West Africa, is presented, which show for the first time how a major margin-scale submarine channel system evolves. These high resolution repeat surveys are analysed to document the nature and rate of erosion and deposition that occurs along submarine channel systems from source to sink for the first time. The overarching aim of this thesis is to use these observations to determine which processes control the evolution of submarine channel systems and how they modulate the transport of sediment to the deep sea. This aim is addressed through three sub aims. First, this thesis investigates which processes control the evolution of submarine channels. Previous studies have shown that processes such as meandering, the growth of levees, and the migration of bedforms can control channel evolution. Based on repeat surveys in Bute Inlet, the rapid upstream-migration of steep waterfall-like features, called knickpoints, are instead shown to play the most important role in submarine channel evolution than these other features. Similar knickpoints have been documented from seafloor surveys in many other submarine channels worldwide; hence this mechanism may be globally significant. Second, this thesis investigates how sediment is transported through the submarine channel in Bute Inlet and onto the terminal lobe, that is located at the distal end of the channel. Some sediment transport models suggest that sediment is transported directly from the head of a system to the lobe by a single turbidity current, while others suggest that submarine channel systems undergo cycles of filling and flushing. This thesis demonstrates that sediment is most likely to go through several steps of deposition and re-excavation before reaching the lobe. These steps of reworking are controlled by knickpoints and can generate zones of sediment bypass on longer timescales. The delivery of sediment to the lobe is discontinuous and does not directly correlate with signals in sediment input into the system; hence, terminal lobes are not necessarily strong recorders of external environmental signals, as was previously often assumed. The third aim is to investigate how different processes control channel evolution along the length of a much longer margin-scale submarine channel system. This thesis shows that the Congo Fan system can be divided in three distinct zones based on the different processes observed: i) a proximal canyon dominated by canyon-flank collapses; ii) a meandering intermediate channel; and iii) a distal knickpoint-dominated channel. The transitions between these zones are controlled by channel maturity and basin structure. Although different zones are present, sediment can be temporarily stored and re-excavated along the entire margin-scale system. This thesis presents new insights into the processes that control submarine channel evolution, recognising that the role played by knickpoints may have been previously underestimated or entirely overlooked in many systems. Furthermore, this thesis shows how the nature of evolution in large submarine channels may act to both temporarily inhibit or promote sediment transfer to the deep sea. Dynamic seascape modification by recurrent flows of variable power ensures that sediment transfer to the system terminus is temporally staggered and discontinous, rather than continuous. In general, the repeat surveys reveal the complexities and importance of different internal processes in sediment transport, storage, re-excavation, and its ultimate fate. Submarine channel systems can be remarkably dynamic, with pronounced reworking along the entire length of active systems. New marine technology has and will continue to provide important new insights in understanding these dynamic systems that play a globally important role in sediment transfer.

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