Carbon and sediment fluxes inhibited in the submarine Congo Canyon by landslide-damming

Landslide-dams, which are often transient, can strongly affect the geomorphology, and sediment and geochemical fluxes, within subaerial fluvial systems. The potential occurrence and impact of analogous landslide-dams in submarine canyons has, however, been difficult to determine due to a scarcity of sufficiently time-resolved observations. Here we present repeat bathymetric surveys of a major submarine canyon, the Congo Canyon, offshore West Africa, from 2005 and 2019. We show how an ~0.09 km3 canyon-flank landslide dammed the canyon, causing temporary storage of a further ~0.4 km3 of sediment, containing ~5 Mt of primarily terrestrial organic carbon. The trapped sediment was up to 150 m thick and extended >26 km up-canyon of the landslide-dam. This sediment has been transported by turbidity currents whose sediment load is trapped by the landslide-dam. Our results suggest canyon-flank collapses can be important controls on canyon morphology as they can generate or contribute to the formation of meander cut-offs, knickpoints and terraces. Flank collapses have the potential to modulate sediment and geochemical fluxes to the deep sea and may impact efficiency of major submarine canyons as transport conduits and locations of organic carbon sequestration. This has potential consequences for deep-sea ecosystems that rely on organic carbon transported through submarine canyons. Bathymetric surveys of the submarine Congo Canyon show damming by canyon-flank landslides led to the temporary storage of substantial masses of sediment and organic carbon, interrupting their transport to the deep sea.

T he largest canyons on our planet occur on the seabed 1 , and the submarine flows that flush these canyons form the largest sediment accumulations on Earth 2,3 . Only rivers carry comparable amounts of sediment and organic carbon (OC) across such large areas 4,5 . However, unlike rivers, there are exceptionally few time-lapse surveys of submarine canyons. Only ~4 of 9,477 submarine canyons documented globally 1 have time-lapse surveys, and this is the first time-lapse survey of a submarine canyon feeding a large (>150,000 km 2 ) submarine fan in the deep (2-5 km) sea. There are also few direct measurements of the flows that flush submarine canyons [6][7][8][9] . This ensures that rates of geomorphic evolution, and the associated sediment and geochemical fluxes, including OC, are far more poorly understood than those for rivers. Previous time-lapse surveys of the upper reaches of much smaller submarine canyons have identified channel migration 10 , bedform movement 6,9 and landslides 11 . We use repeat mapping of the Congo Canyon to show how canyon-flank landslides can affect canyon geomorphology and fluxes of sediment and carbon to the deep sea.
Landslides are important geomorphic agents in terrestrial environments. Where landslides flow into river channels they can lead to full blockages (landslide-dams) 12,13 , channel constrictions, diversions or avulsions of the river channel itself 12,14,15 . Once emplaced, the landslide-dam itself can fail instantaneously, leading to erosion and transport of sediment upstream and downstream of the dam 16 . Over longer timescales, and depending on their longevity, landslide-dams can result in the accumulation of sediments and organic material upstream of the dam 13,17,18 . Upstream aggradation and downstream incision can lead to knickpoint generation in river longitudinal profiles 19 . Frequent landslide-dams or large-magnitude events can therefore notably influence river profiles over long timescales 19 . From an ecological perspective, landslide-dams can impact water quality and aquatic habitats in rivers 12,13 and are a key phenomenon in fluvial landscape evolution.
In this Article, we show how landslide-dams can impact submarine canyons. Submarine canyons are carved by slope failures and sediment flows called turbidity currents that can run out for hundreds of kilometres 11,20 . Indeed, landslide scars are ubiquitous on submarine canyon walls in many locations worldwide 11,21,22 , suggesting that landslide occurrence is common (see Supplementary  Table 1). However, a nearly complete lack of repeat bathymetric surveys ensures their geomorphic effects and exact timing are poorly constrained. As a consequence, an a priori assumption often persists that these landslides commonly evolve from low-mobility slumps to higher-mobility flows and eventually turbulent turbidity currents 11,20 . Using repeat bathymetric mapping, we show how submarine landslides can impact canyon morphology and interact with turbidity currents in submarine canyons.
We then show how landslide-dams have the potential to affect the tempo of sediment and carbon transfer to the deep sea. Submarine canyons act as the primary conduits for sediment, oxygen, nutrient, pollutants and organic-matter transport from rivers and coasts to the deep sea 1,23-25 . They are commonly envisaged as highly efficient   studies have shown that the canyon is highly active, with multiple long-duration (up to a week) turbidity currents traversing the canyon annually 8,[32][33][34] .
Bathymetric surveys of the upper canyon were conducted in 2005 and 2019 using Kongsberg EM120 and EM122 instruments. Survey design and water depths resulted in optimum gridding of these datasets at 25 m and 5 m, respectively (Methods). The con-  of 1 km and height of 390 m. The length of the associated deposit is ~800 m. Overall, the landslide is 1.7 km long, with an area of ~0.64 km 2 . The minimum excavated volume was ~0.09 ± 0.01 km 3 . A smaller landslide (0.006 ± 0.001 km 3 ) also occurred ~2 km up-canyon (Fig. 2b).
The canyon-flank landslide deposit caused a meander-bend cut-off, which shortened the thalweg length by ~3 km. Up-canyon of the flank collapse, canyon infilling occurred. The deposited infill is up to 150 m thick closest to the landslide deposit ( Fig. 2c). At the eastern edge of the survey area, 26 km up-canyon, the newly deposited infill is up to 40 m thick. The total sediment infill volume, excluding the landslide deposit, is ~0.4 ± 0.1 km 3 . A new channel has been incised into the infill (Fig. 2b). This channel is up to 20 m deep and 100 m wide and runs 6.2 km up-canyon of the landslide deposit. A series of knickpoints are associated with the landslide deposit and sediment infill. A 30-m-high knickpoint has formed where the new channel descends over the northwest end of the landslide deposit (Figs. 2b and 3). Two smaller knickpoints are also present where channels have incised into the sediment infill (Fig. 3). Down-canyon of the landslide deposit, a wedge of sediment with a volume of 0.02 ± 0.01 km 3 has been deposited.

Geomorphic change driven by canyon-flank landslides
Our data provide an opportunity to (1) test whether submarine landslide-dams may have comparable (or different) impacts to their subaerial counterparts; (2) derive models of canyon-flank landslide impacts on submarine canyon evolution and geomorphology ( Fig.  4) and (3) identify the sedimentary signature of such events.
On land, where landslides block river channels, meander cut-offs have been observed where the river is forced to incise a new channel through the deposited material 14,18,35 . Sediment aggradation behind the landslide-dam can protect the channel bed, leading to slower bedrock incision rates, thereby impacting long-term longitudinal river profile evolution 17,19 . This disequilibrium can also lead to knickpoint formation as the channel adjusts to the perturbation 14,36 .
Our data document a similar process in submarine canyons (Figs. 4 and 5). Where canyon-flank landslides are sufficiently large, landslide-dams can be emplaced across the canyon floor ( Fig.  4b (top)). Once emplaced, this material can lead to rapid canyon infilling as sediment carried by perturbed and decelerating turbidity currents is trapped behind the dam, starving down-canyon areas of sediment ( Fig. 4b (middle)). Our data show that deposition also occurs, immediately, down-canyon of the dam. This is probably a consequence of over-spilling turbidity currents that no longer have sufficient velocities to maintain sediment in suspension (Fig. 5). Once infilling has occurred, the observed new channel shows subsequent turbidity current activity can begin to erode through the landslide-dam and the infill (Figs. 2b and 4b (bottom)). We envisage that this channel was formed by erosion from the landslide-dam knickpoint once the accommodation space behind the landslide-dam was sufficiently full, such that changes in seafloor gradient did not impact the flows and allowed turbidity currents to maintain their driving force (sediment load). The rate of future channel incision and knickpoint migration will depend on turbidity current magnitude-frequency and infill-consolidation rate. The long-term existence of the observed knickpoints will depend on the incision rate through the landslide deposit and infill. Incision of a new canyon thalweg can lead to terrace formation on the canyon floor ( Fig. 4b (bottom)). Where canyon-flank landslides occur on a meander bend, incision of a new thalweg channel through the infill can result in meander-bend cut-off (Fig. 3). Indeed, our observations show the first documented meander cut-off caused by a submarine canyon-flank landslide. Sediment trapped behind the dam may be gradually released by smaller flows or eroded en masse by a larger canyon-flushing event 20,37,38 . On active margins, large earthquakes may trigger a canyon-flushing event 37,38 . However, on passive margins, an alternative trigger will probably be required. In the Congo Canyon, this may be an exceptional river flood.
Where landslides are not sufficiently large or mobile enough to block terrestrial rivers or submarine canyons, we also see parallels 14 . Partial infilling of river channels is known to cause increased erosion and meander migration 14,36 . In submarine canyons (Extended Data Fig. 1b), three possible processes are envisaged (Fig. 4a,c). First, canyon-floor constriction can increase turbidity-current confinement ( Fig. 4c (top)). This confinement can increase the flow's erosive potential, allowing the generation of scours and eventually the formation of a new incised channel 39 . This channel can further concentrate flows, resulting in up-canyon and down-canyon channel growth (Fig. 4c (bottom)). Second, the thalweg may be deflected. In this case, a more sinuous course would persist until the landslide deposit was eroded. Third, the landslide deposit may be too small to impact flow characteristics in the canyon thalweg. In this case, the deposit provides sediment that can be eroded by turbidity currents 21 . These smaller failures may be particularly prevalent in certain settings, such as seismically active regions where earthquakes may trigger frequent smaller-magnitude slope failures, thus reducing the frequency of canyon-damming events 40 .
Landslide-dams and the resulting infill should be recognizable in submarine settings. The landslide deposit itself will be resolvable in seismic cross section as a zone of chaotic reflectors/transparency that may also have evidence of folding, normal faulting or compression 41 (Fig. 4b). Where a new thalweg has been cut post-infill, these features will probably be preserved as part of a terrace and buried beneath layered sediments (Fig. 4b (bottom)). Landslide-dams and their infill may also be recognizable from the presence of stepped changes in terrace height along a canyon, representative of the up-canyon extent of infilling resulting from the landslide-dam (Fig. 5).

Deep-sea sediment transport and submarine landslide-dams
Fluvial landslide-dams can result in the storage of large volumes of water and sediment 14 . Similarly, the observed ≥0.09 km 3 (~120 ± 10 Mt assuming a wet density of 1,800 kg m −3 ) canyon-flank landslide-dam in the Congo Canyon has resulted in the trapping and storage of a large sediment volume (≥0.4 km 3 , equivalent to ~170 ± 40 Mt assuming a wet density of 1,300 kg m −3 ). Indeed, the volume of stored sediment could be much larger as it is still 40 m thick at the edge of the 2019 survey (Fig. 3). The annual sediment flux from the Congo River is estimated to be ~43 Mt (ref. 4 ), while ref. 33 calculates ~40 Mt is transported annually by turbidity currents in the canyon. The landslide occurred at some point between 2005    Fluvial landslide-dam traps water and sediment. Trapped water forms a lake behind the dam, and deltaic sediment is deposited when river water enters the lake or within the body of the lake. Once accommodation space has been filled, river water is able to overtop the dam and continue downstream. b, Submarine landslide-dam. Landslide-dam blocks the submarine canyon, interrupting the passage of turbidity currents. c, Partial flow reflection and ponding. Landslide-dam results in trapping, ponding and reflection of turbidity currents. d, Turbidity current deposits backfill accommodation space behind landslide-dam. As turbidity currents depend on their velocity to keep sediment entrained, this sediment is rapidly deposited. Parts of the turbidity current that are able to overtop the landslide-dam quickly dissipate as their sediment load is now greatly reduced. e, Gradual sediment release. Once the accommodation space is filled, more-erosive, faster-moving parts of turbidity currents are able to overtop the dam. These are able to erode the dam, leading to knickpoint formation as a new channel is incised into the infill. Turbidity current in panel b is a scaled representation of the flows described in ref. 8  The key differentiator between subaerial and submarine landslide-dams is how sediment is stored. Initially, fluvial landslide-dams will trap water and sediment. Once filled, sediment may continue to be deposited as the river enters the dammed lake, but water is still able to overtop the dam and continue to erode and transport sediment downstream [13][14][15][16][17][18] . By contrast, submarine landslide-dams will result in rapid infilling of sediment, and starve down-canyon reaches of sediment, due to the trapping of turbidity currents by the dam. Unlike rivers, turbidity currents in submarine channels are driven by their sediment load, and their ability to keep sediment entrained depends on their velocity 8 . Turbidity currents that deposit sediment lose driving force, potentially leading to further deceleration and sediment deposition; this positive feedback does not occur in rivers. Recent monitoring studies 7,8,33 have suggested that turbidity currents are driven by dense, near-bed frontal cells (Fig. 5b). These dense frontal cells will probably be retained behind the dam, perturbing the flow and potentially resulting in flow ponding and reflection, leading to rapid deposition 42 . Only when the accommodation space is sufficiently filled will parts of the turbidity current pass over the landslide-dam 42 (Fig. 5c,d). However, the loss of sediment still leads to further deposition immediately down-canyon, reducing turbidity-current run-out. Once filled, further turbidity-current activity will rework the infill (Figs. 2b and 5e).

Submarine landslide-dams and oC sequestration
Burial of young OC, produced by photosynthesis in the biosphere, in marine sediments removes CO 2 from the atmosphere, regulating the climate over geological timescales 26 . Indeed, submarine canyons are considered efficient sequesters of OC in the ocean 23,26,27 . However, few measurements of OC sequestration exist in these environments, and we are therefore yet to constrain long-term fluxes or burial efficiencies despite common submarine canyon occurrence on all continental margins.
Measurements of sediment from the Congo Fan averaged over multiple sampling locations 29 suggest a total OC content estimated at 2.75-3.00% by weight 29 . Assuming such values, the canyon-flank landslide and trapped infill contained ~3.2-3.5 and ~4.6-5.0 Mt of OC, respectively. The landslide will re-exhume already buried OC, but the infill is likely to contain younger OC 43 . The Congo River annually supplies ~2 Mt terrestrial OC (Supplementary Table 2). The buried OC therefore represents a minimum of 3-4 years of terrestrial OC supply from the river and ~4% of the global total of OC delivered annually by rivers to the oceans 5 . Thus, although worldwide frequency-magnitude of landslide-dams in submarine canyons is uncertain, this single example buried 4% of the terrestrial OC flux to the global ocean (Supplementary Table 2).
The rapid burial (<14 years) of this mass of OC should prevent oxidation and thus preserve the OC. This supports the previous assertions that submarine canyons are effective carbon sinks 23,26,27 . However, subsequent reworking by turbidity currents of OC stored in the dammed wedge of sediment will probably expose much of this carbon to future oxidation. The OC drawdown, therefore, depends on the preservation of terraces after a new channel has been incised into the infill. A similar process is envisaged for the re-exhumed OC contained in the landslide deposit. Erosion and re-exhumation are intrinsic to a meandering channel, but recurrent mass sliding, as suggested by the identified landslide, and concave canyon walls ( Figs. 1 and 2) clearly add to the amount of re-exhumation. This suggests that landslides and landslide-dams probably reduce the efficiency of OC burial in canyons and their perceived role as efficient carbon sinks.
The mass of OC buried in the Congo Canyon infill is comparable to that remobilized by the Tohoku-oki and Kaikōura earthquakes 11,44 . These earthquakes resulted in ~1.7 and ~7.0 Mt OC being remobilized, respectively. Delivery of OC to the deep sea by these and other earthquake-triggered events is believed to exert a strong control over regional deep-sea productivity and carbon sequestration 11,44 . By contrast, landslide-dams will temporally lead to reduced OC fluxes that nourish down-system benthic ecosystems [45][46][47] . Submarine canyon-damming therefore has the potential to modulate marine carbon transport and cycling. These impacts will probably be greater where canyons are directly connected to terrestrial sediment sources. However, landslide-dams in canyons that are starved of terrestrial inputs may also exert impacts on OC cycling due to the impacts on ecosystems that rely on down-canyon sediment transport 1 .
We identified a submarine landslide-dam that is comparable in scale to the largest landslide-dams of the past 120 years. It shows how submarine landslide-dams may affect sediment and OC transfer to the deep sea and submarine canyon geomorphic evolution. This single example had a comparable impact on sediment and OC fluxes to the deep sea as major earthquakes and trapped a sediment mass four times the annual flux from the Congo River, equivalent to ~4% of the annual supply of terrestrial OC to the global oceans. Similar landslide scars are commonplace on the flanks of other submarine canyons (Supplementary Table 1), including those connecting to very large rivers 48,49 . This suggests submarine landslide-dams could potentially be common phenomena, even away from active margins. Indeed, at least three other landslide-dams have been identified in modern bathymetry (Supplementary Table 1). However, time-lapse bathymetric surveys are currently available for only ~4 of the ~9,477 submarine canyons known globally. This study shows the value of repeat high-resolution bathymetric mapping of large deep-sea systems. Further work should now determine the frequency of submarine landslide-damming to constrain their impact on deep-sea ecosystems, sediment transport and OC burial on different margin types.

online content
Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/ s41561-022-01017-x.

Methods
The results presented in this paper are based on analysis of bathymetry data of the upper Congo Canyon collected during cruises JC187 of the RRS James Cook (2019) and OPTIC-CONGO2 in 2005 on board the FS Beautemps-Beaupré. JC187 used a Kongsberg EM122 system, and OPTIC-CONGO2 used a Kongsberg EM120. Both operated at 12 kHz with a vertical sounding accuracy of 0.2% of water depth. The beam aperture was 0.5° × 0.5° for JC187 and 1° × 1° for OPTIC-CONGO2. Data were processed to correct for differences in sound velocity in the water column (using data from sound-velocity profilers) and the ship's motion. JC187 data were processed in CARIS HIPS and SIPS software and gridded with a resolution of 5 m. JC187 data were able to be gridded at 5 m due to the high number of soundings that were retrieved due to the survey design. The canyon was surveyed using along-canyon lines crossed by traverse lines perpendicular to the along-canyon lines to minimize potential shadowing or reflection due to steep canyon walls. The survey paths were also designed to ensure the entire survey area was covered by the central third of the swath width to maximize the number of soundings. The OPTIC-CONGO2 survey used along-canyon lines. OPTIC-CONGO2 data were processed and gridded at 25 m in CARAIBES. Inter-survey positioning was validated by matching features in common areas of relatively stable seafloor.
Erosion and deposition of sediment between 2005 and 2019 were calculated by first resampling the JC187 5 m data to 25 m so as to be comparable with the OPTIC-CONGO2 data. A bathymetric difference map produced in ArcGIS was subsequently used to determine volumes of eroded and deposited sediment. Where canyon-flank landslides were observed, their volumes were calculated using depth changes associated with their headwalls. This was necessary because the emplaced deposit was often buried, and no seismic data penetrating the landslide deposits were available, preventing direct assessment of the deposit volume. The calculated landslide volumes are therefore probably only minimum volumes.
The bathymetric surveys were used to construct along-canyon profiles. The position of submarine canyon and channel thalwegs shift as they evolve, so down-canyon profiles were measured along the position of the thalweg in each survey. These profiles were subsequently normalized for comparison in the section of the canyon impacted by the canyon-flank landslide and infill. Normalized distances along canyon were calculated by taking a starting point ( Fig. 3; furthest point west) where the thalweg was in the same location for both surveys and dividing each measurement of distance by the distance travelled along the thalweg to the end point of the profiles (Fig. 3; furthest point east).
The morphology of the Congo Submarine Canyon observed in the JC187 and OPTIC-CONGO2 bathymetry data was also compared with high-resolution bathymetric data that has been previously published for a range of submarine canyons around the world 6,11, (Supplementary Table 1). The volumes of sediment and OC stored in-canyon and up-canyon of the observed landslide-dam were also compared with a range of known events in marine and fluvial environments in terms of displaced masses and OC transport 5,11,29,44,[86][87][88][89][90][91][92][93][94][95][96][97][98] (Supplementary Table 2).

Data availability
We do not own the rights to make the JC187 or OPTIC-CONGO2 datasets available. However, access to each dataset can be requested following these instructions. Requests for the multibeam data from JC187 should be made to the British Oceanographic Data Centre (BODC; https://www.bodc.ac.uk/). Access or requests for the multibeam data compiled on the Congo Fan, including the OPTIC-CONGO2 cruise, should be made through the Sextant Portal at Ifremer (https://doi.org/10.12770/d1076601-82a9-42a2-9d30-d0be8b2481a7).