First source-to-sink monitoring shows dense head controls sediment flux and runout in turbidity currents

Abstract

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.

Longest sediment flows yet measured show how major rivers connect efficiently to deep sea

“…35 ) in which faster turbidity current fronts comprise a dense (>20-40% volume) near-bed layer, in which grains do not settle individually, and which is weakly turbulent. Field evidence from Congo Canyon and elsewhere suggests faster turbidity currents contain such a dense near-bed layer at their front, while slower moving flows lack a dense layer 33,34,59 . Behaviour of this dense layer may depend on variations in excess pore pressures, dense layer thickness, substrate properties and erosion rates 60,61 , rather than settling velocity of individual grains.…”

“…Changes in the front speed of turbidity currents with distance have only been measured in detail at five sites 4,24,25,34,35,58,59 . However, three key observations emerge from four locations where flows were confined within canyons-channels (Figs.…”

“…These flow fronts either sustain speeds of 5-8 m/s (autosuspend), or accelerate from ~5 to 8 m/s (ignite). It is these flows that carry the largest amounts of sediment and organic carbon 59 , travel furthest, and pose the greatest hazard. Conversely, flows whose fronts travel at <4 m/s tend to decelerate and dissipate.…”

“…The threshold flow-front velocity (4-5 m/s) needed for ignition or autosuspension (grey rectangle) is similar in each system, despite major differences in grain sizes, sediment input, triggers and other parameters. These field sites are A the Congo Canyon-Channel (this study), B Monterey Canyon offshore California 34,35 , C Gaoping Canyon offshore Taiwan 24,25 , and D Bute Inlet in British Columbia, Canada 58,59 . Data from individual flows are shown by different coloured lines and dots.…”

Turbidity Currents Can Dictate Organic Carbon Fluxes Across River‐Fed Fjords: An Example From Bute Inlet (BC, Canada)

“…Based on these examples, it is plausible that rare and extreme events may also affect the OC burial efficiency and distribution in Bute Inlet, potentially increasing our burial efficiency estimate. It was recently shown that most turbidity currents (∼90%) dissipate within the shallowest‐water (<200 m depth and <12 km along channel from the Homathko Delta) part of Bute Inlet, whereas less frequent (∼10%) events rework this material and progressively shuffle it downstream to the lobe (Heijnen et al., 2022; Pope et al., 2022). We further suspect that rare long runout events can flush material to the distal flat basin, as evidenced by thick sandy accumulations found at >3 m depth in 8‐m‐long piston cores collected in the distal basin at the location of core 15 (Figure 3, Heerema, 2021).…”