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

Abstract

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change.

Turbidity currents generating lobes: flow rate influence on 3D experiments without slope break

“…Recent studies have shown that the frequency of occurrence of turbidity currents is higher than previously proposed (Stevenson et al 2018, Hage et al 2019, Heijnen et al 2022 and without the catastrophic characteristics reported for high turbulent turbidity currents (e.g., Mulder et al 1997).…”

“…Much less is known about the flow properties and velocities when turbidity currents become unconfined. As also shown by recent field measurements, largescale turbidity currents can travel long distances, for example > 100 km before sedimentation in unconfined settings takes place (Stevenson et al 2018, Talling et al 2022. As a consequence, turbidity currents form preferential paths, building submarine channels or canyons along areas with high slopes.…”

“…These currents can become highly erosive in their way (Shepard 1981), re-entraining or reworking previously deposited sediments that then can become part of the flow, increasing their driving force. Turbidity currents can flow for minutes to weeks (Azpiroz-Zabala et al 2017) and commonly display velocities ranging from 0.5 to 6 m/s (Cooper et al 2013, Xu et al 2014, Talling et al 2022. Much less is known about the flow properties and velocities when turbidity currents become unconfined.…”

Predicting turbidity current activity offshore from meltwater-fed river deltas

“…Recent field-scale flow monitoring offshore from river mouths has recognised the combined effect of elevated river discharge and tidal drawdown on flow generation (Ayranci et al, 2012;Clare et al, 2016;Hage et al, 2019;Hill and Lintern, 2021;Hughes Clarke et al, 2012;Lintern et al, 2016;Talling et al, 2022). For example, elevated river discharge may rapidly deposit sediment on the delta-lip, thus preconditioning the delta slope to failure.…”

Editorial: Source or sink? Erosional and depositional signatures of tectonic activity in deep-sea sedimentary systems

“…Whilst the continental slope was conventionally perceived to primarily experience sediment bypass in source-to-sink studies, the net bypass character of the slope is the result of alternation of deposition and erosion over various magnitudes and time-scales (e.g., Prather et al, 2017). Although the basin-plain is typically considered as a sink, the same concept applies, with varying degrees of erosion and deposition occurring in the deep-ocean (e.g., Talling et al, 2022). Hence, the deep-sea has a diverse capacity for sediment storage but also great potential for redistribution (e.g., Shumaker et al, 2018).…”