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The delivery and burial of terrestrial particulate organic carbon (OC) in marine sediments is important to quantify, because this OC is a food resource for benthic communities, and if buried it may lower the concentrations of atmospheric CO2 over geologic timescales. Analysis of sediment cores has previously shown that fjords are hotspots for OC burial. Fjords can contain complex networks of submarine channels formed by seafloor sediment flows, called turbidity currents. However, the burial efficiency and distribution of OC by turbidity currents in river-fed fjords had not been investigated previously. Here, we determine OC distribution and burial efficiency across a turbidity current system within a fjord, in Bute Inlet (Canada). We show that 60 ± 10 % of the OC supplied by the two river sources, is buried across the fjord surficial (2 m) sediment. The sand-dominated submarine channel and its terminal lobe contain 63 ± 14 % of the annual terrestrial OC burial in the fjord. In contrast, the muddy overbank and distal flat basin settings contain the remaining 37 ± 14 %. OC in the channel, lobe and overbank exclusively comprises terrestrial OC sourced from rivers. When normalized by the fjord’s surface area, at least three times more terrestrial OC is buried in Bute Inlet, compared to the muddy parts of other fjords previously studied. Although the long-term (>100 year) preservation of this OC is still to be fully understood, turbidity currents in fjords appear to be efficient in storing OC supplied by rivers in their near-surface deposits.

Until recently, despite being one of the most important sediment transport phenomena on Earth, few direct measurements of turbidity currents existed. Consequently, their structure and evolution were poorly understood, particularly whether they are dense or dilute. Here, we analyze the largest number of turbidity currents monitored to date from source to sink. We show sediment transport and internal flow characteristic evolution as they runout. Observed frontal regions (heads) are fast (>1.5 m/s), thin (<10 m), dense (depth averaged concentrations up to 38%vol), strongly stratified, and dominated by grain-to-grain interactions, or slower (<1 m/s), dilute (<0.01%vol), and well mixed with turbulence supporting sediment. Between these end-members, a transitional flow head exists. Flow bodies are typically thick, slow, dilute, and well mixed. Flows with dense heads stretch and bulk up with dense heads transporting up to 1000 times more sediment than the dilute body. Dense heads can therefore control turbidity current sediment transport and runout into the deep sea.

Submarine channels deliver globally important volumes of sediments, nutrients, contaminants and organic carbon into the deep sea. Knickpoints are significant topographic features found within numerous submarine channels, which most likely play an important role in channel evolution and the behaviour of the submarine sediment-laden flows (turbidity currents) that traverse them. Although prior research has linked supercritical turbidity currents to the formation of both knickpoints and smaller crescentic bedforms, the relationship between flows and the dynamics of these seafloor features remains poorly constrained at field-scale. This study investigates the distribution, variation and interaction of knickpoints and crescentic bedforms along the 44 km long submarine channel system in Bute Inlet, British Columbia. Wavelet analyses on a series of repeated bathymetric surveys reveal that the floor of the submarine channel is composed of a series of knickpoints that have superimposed, higher-frequency, crescentic bedforms. Individual knickpoints are separated by hundreds to thousands of metres, with the smaller superimposed crescentic bedforms varying in wavelengths from ca 16 m to ca 128 m through the channel system. Knickpoint migration is driven by the passage of frequent turbidity currents, and acts to redistribute and reorganize the crescentic bedforms. Direct measurements of turbidity currents indicate the seafloor reorganization caused by knickpoint migration can modify the flow field and, in turn, control the location and morphometry of crescentic bedforms. A transect of sediment cores obtained across one of the knickpoints show sand–mud laminations of deposits with higher aggradation rates in regions just downstream of the knickpoint. The interactions between flows, knickpoints and bedforms that are documented here are important because they likely dominate the character of preserved submarine channel-bed deposits., Accepted Author Manuscript, Applied Geology

Submarine channels deliver globally important volumes of sediments, nutrients, contaminants and organic carbon into the deep sea. Knickpoints are significant topographic features found within numerous submarine channels, which most likely play an important role in channel evolution and the behaviour of the submarine sediment-laden flows (turbidity currents) that traverse them. Although prior research has linked supercritical turbidity currents to the formation of both knickpoints and smaller crescentic bedforms, the relationship between flows and the dynamics of these seafloor features remains poorly constrained at field-scale. This study investigates the distribution, variation and interaction of knickpoints and crescentic bedforms along the 44 km long submarine channel system in Bute Inlet, British Columbia. Wavelet analyses on a series of repeated bathymetric surveys reveal that the floor of the submarine channel is composed of a series of knickpoints that have superimposed, higher-frequency, crescentic bedforms. Individual knickpoints are separated by hundreds to thousands of metres, with the smaller superimposed crescentic bedforms varying in wavelengths from ca 16 m to ca 128 m through the channel system. Knickpoint migration is driven by the passage of frequent turbidity currents, and acts to redistribute and reorganize the crescentic bedforms. Direct measurements of turbidity currents indicate the seafloor reorganization caused by knickpoint migration can modify the flow field and, in turn, control the location and morphometry of crescentic bedforms. A transect of sediment cores obtained across one of the knickpoints show sand–mud laminations of deposits with higher aggradation rates in regions just downstream of the knickpoint. The interactions between flows, knickpoints and bedforms that are documented here are important because they likely dominate the character of preserved submarine channel-bed deposits.

The first detailed measurements from active turbidity currents have been made in the last few years, at multiple sites worldwide. These data allow us to investigate the factors that control the structure of these flows. By analyzing the temporal evolution of the maximum velocity of turbidity currents at different sites, we aim to understand whether there are distinct types of flow, or if a continuum exists between end-members; and to investigate the physical controls on the different types of observed flow. Our results show that the evolution of the maximum velocity of turbidity currents falls between two end-members. Either the events show a rapid peak in velocity followed by an exponential decay or, flows continue at a plateau-like, near constant velocity. Our analysis suggests that rather than triggers or system input type, flow structure is primarily governed by the grain size of the sediment available for incorporation into the flow.

Rivers (on land) and turbidity currents (in the ocean) are the most important sediment transport processes on Earth. Yet, how rivers generate turbidity currents as they enter the coastal ocean remains poorly understood. The current paradigm, based on laboratory experiments, is that turbidity currents are triggered when river plumes exceed a threshold sediment concentration of ~1 kg.m‐3. Here we present direct observations of an exceptionally dilute river‐plume, with sediment concentrations one order of magnitude below this threshold (0.07 kg.m‐3), which generated a fast (1.5 m.s‐1), erosive, short‐lived (6 min) turbidity current. However, no turbidity current occurred during subsequent river‐plumes. We infer that turbidity currents are generated when fine‐sediment, accumulating in a tidal turbidity maximum, is released during spring tide. This means that very dilute river‐plumes can generate turbidity currents more frequently and in a wider range of locations, than previously thought.