Canyon Head Research Articles

Sediment dynamics on the narrow, canyon-incised and current-swept shelf of the northern KwaZulu-Natal continental shelf, South Africa

Side-scan sonar, multibeam bathymetry, ShipekTM grab, and high- to moderate-resolution sub-bottom data for the northern KwaZulu-Natal continental shelf reveal further insights into the interactions between sediment dynamics, strong western boundary currents and submarine canyon topography. Unlike previously recognised mechanisms for bedload parting on current-swept shelves, bedload partings here are the result of complex interactions between the western boundary poleward-flowing Agulhas Current and submarine canyon topography. This has resulted in bedforms orientated orthogonally to the canyon axis, with sediments entrained equator-wards into the canyon heads before resuming their dominant southerly migration. It is in these zones of parting where the most prominent bedforms occur; these bedform fields are formed by positive feedback in the boundary layer between an increasingly undulatory Agulhas Current and a seafloor incised by regularly spaced submarine canyons. Bedform morphometrics such as wavelength–height, depth–height and distance from thalweg–height relationships show no distinct patterns, indicating that the bedforms are heavily reworked and appear to be out of equilibrium with the inherent oceanographic conditions.

Celtic Sea linear tidal sand ridges, the Irish Sea Ice Stream and the Fleuve Manche: Palaeotidal modelling of a transitional passive margin depositional system

The linear tidal sand ridges (LTSR) of the Celtic Sea constitute the largest examples of their bedform type on Earth. Previous sedimentological and seismic stratigraphic interpretation suggests that the LTSR are moribund tidally remobilised sediments representing the transgressive systems tract. This interpretation is supported by two-dimensional finite-difference model reconstructions of the M 2 tide, forced using the output from a glacial isostatic adjustment model to derive palaeotopography, and a global ocean model to derive the tides on the ocean boundary, used here to reconstruct peak bed stress vectors for the Celtic Sea for timesteps covering the transgression since the Last Glacial Maximum (last 21 ka). These data are coupled with interpretation of recent published observations on the outer shelf depocentres of the Fleuve Manche and the Irish Sea Ice Stream (ISIS) to confirm that the LTSR distribution is consistent with modelled sand transport paths during transgression. The main phase of LTSR growth was between 20 cal ka and 12 cal ka. Ridge axis orientations reflect the final phase of LTSR construction around 12 cal ka, with some later growth of the most southerly LTSR as late as 10 cal ka. LTSR growth was from the SW across the shelf towards the NE. Strong tidal pumping of sediments into slope canyon heads on the outer shelf occurred between 20 cal ka and 12 cal ka, contributing to turbidite activity and the growth of the Celtic and Armorican deep sea fans. It is proposed that 1. the Fleuve Manche shelfal estuary-delta, and the Irish Sea Ice Stream (ISIS) shelf fan, were the main sediment depocentres supplying the growing LTSR, 2. the lack of sediments available in the western Channel limited LTSR growth in this area, and that the easterly termination of the LTSR in this sector results from sediment starvation, and 3. the northeasterly termination in the Celtic Sea sector is a function of declining peak bed stresses rather than sediment starvation. The Celtic Sea margin represents a passive margin depositional system transitional between true glacial ice stream-trough mouth fan systems (to the north) and the fluvial canyon systems characteristic of the margin to the south. This interpretation therefore complements studies of macroscale sedimentation linked to glaciation on the continental margins of the North Atlantic.

Calcareous nannoplankton and benthic foraminiferal assemblages from the Nazaré Canyon (Portuguese continental margin): Preliminary results

Submarine canyons are assumed to play an important role in oceanic/neritic circulation, marine productivity and sedimentary processes, acting as preferential conduits between the littoral and deep oceanic domain. Here we present first results of a comparative micropalaeontological study on calcareous nannoplankton and benthic foraminifera from surface sediments from the surroundings of the upper Nazaré Canyon (Portuguese continental margin) and from the shelf north of the canyon. Regardless of the difficulty to distinguish taphonomical from (palaeo)ecological effects in such a complex and still poorly known marine system, the first results suggest that the canyon's hydro-sedimentary dynamic regime act as a prolongation of the shore/inner shelf hydrodynamic conditions towards west, preventing deposition and/or preservation of the smaller and fragile species of calcareous nannoplankton (e.g. E. huxleyi and G. ericsonii) and enhancing the record of the larger and more opportunistic ones (e.g. G. oceanica); and disturbing benthic foraminiferal productivity and/or diversity, or their preservation in the fossil record. Both calcareous nannoplankton and benthic foraminifera are more abundant off the canyon's domain, suggesting that its highly energetic thalweg conditions are probably filtering the fossil record in the sediment. Still, preliminary results suggest that the occurrence of persistent physical phenomena related with the canyon's morphology and proximity to the coast (e.g. solitary internal waves) may be locally promoting favourable conditions for calcareous nannoplankton, as shown by high values of nannoliths, chlorophyll a and 19' hexanoyloxyfucoxantine (unpublished data) north of the canyon's head. It is our goal to test this hypothesis in the near future by (a) studying multicore and surficial sediments from more recent surveys, and (b) calibrating the sediment results with water column data presently in process at the Institute of Oceanography (IO).

Sedimentary processes in a submarine canyon excavated into a temperate‐carbonate ramp (Granada Basin, southern Spain)

AbstractDuring the Late Tortonian, shallow‐water temperate carbonates were deposited in a small bay on a gentle ramp linked to a small island (Alhama de Granada area, Granada Basin, southern Spain). A submarine canyon (the ‘Alhama Submarine Canyon’) developed close to the shoreline, cross‐cutting the temperate‐carbonate ramp. The Alhama Submarine Canyon had an irregular profile and steep slopes (10° to 30°). It was excavated in two phases reflected by two major erosion surfaces, the lowermost of which was incised at least 50 m into the ramp. Wedge‐shaped and trough‐shaped, concave‐up beds of calcareous (terrigenous) deposits overlie these erosional surfaces and filled the canyon. A combination of processes connected to sea‐level changes is proposed to explain the evolution of the Alhama Submarine Canyon. During sea‐level fall, part of the carbonate ramp became exposed and a river valley was excavated. As sea‐level rose, river flows continued along the submerged, former river‐channel, eroding and deepening the valley and creating a submarine canyon. At this stage, only some of the transported conglomerates were deposited locally. As sea‐level continued to rise, the river mouth became detached from the canyon head; littoral sediments, transported by longshore and storm currents, were now captured inside the canyon, generating erosive flows that contributed to its excavation. Most of the canyon infilling took place later, during sea‐level highstand. Longshore‐transported well‐sorted calcarenites/fine‐grained calcirudites derived from longshore‐drift sandwaves poured into and fed the canyon from the south. Coarse‐grained, bioclastic calcirudites derived from a poorly sorted, bioclastic ‘factory facies’ cascaded into the canyon from the north during storms.

Estimates of PAHs burdens in a population of ampeliscid amphipods at the head of the Mississippi Canyon (N. Gulf of Mexico)

Abstract The bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) by a dense population of ampeliscid amphipods (12,094±2499 ind m −2 ) in the head of the Mississippi Canyon at a depth of 480 m is examined and compared with estimation for amphipods from a shallow-water location, the Houston Ship Channel. The distribution of individual PAHs in sediments varies from the distribution in the organisms both in deep and shallow water, suggesting preferential uptake/depuration or uptake from pore or bottom waters. The average bioaccumulation factor (4.36±2.55) and the biota sediment accumulation factor (0.24±0.13) of total PAHs by the ampeliscid amphipods at the canyon's head are within the range reported for other benthic invertebrates. The average bioaccumulation factors are highest for dibenzothiophenes (up to 132 for C1-dibenzothiophenes) and alkylated PAHs and lowest for parent high molecular weight PAHs (HPAHs). Shallow-water amphipods are similar to deep-water amphipods in showing high preference to bioaccumulate alkylated PAHs and low preference to bioaccumulate parent HPAHs.

Tectonically active sediment dispersal system in SW Taiwan margin with emphasis on the Gaoping (Kaoping) Submarine Canyon

The sediment dispersal system in southwestern Taiwan margin consists of two main parts: the subaerial drainage basin and the offshore receiving marine basin. In plan view, this sediment dispersal system can be further divided into five geomorphic units: (1) the Gaoping (formerly spelled Kaoping) River drainage basin, (2) the Gaoping (Kaoping) Shelf, (3) the Gaoping (Kaoping) Slope, (4) the Gaoping (Kaoping) Submarine Canyon and (5) the Manila Trench in the northernmost South China Sea. The Gaoping River drainage basin is a small (3250 km 2), tectonically active and overfilled foreland basin, receiving sediments derived from the uprising Central Range of Taiwan with a maximum elevation of 3952 m. The Gaoping Submarine Canyon begins at the mouth of the Gaoping River, crosses the narrow Gaoping Shelf (~ 10 km) and the Gaoping Slope, and finally merges into the northern termination of the Manila Trench over a distance of ~ 260 km. The SW Taiwan margin dispersal system is characterized by a direct river-canyon connection with a narrow shelf and frequent episodic sediment discharge events in the canyon head. In a regional source to sink scheme, the Gaoping River drainage basin is the primary source area, the Gaoping Shelf being the sediment bypass zone and the Gaoping Slope being the temporary sink and the Manila Trench being the ultimate sink of the sediment from the Taiwan orogen. It is inferred from seismic data that the outer shelf and upper slope region can be considered as a line source for mass wasting deposits delivered to the lower Gaoping Slope where small depressions between diapiric ridges are partially filled with sediment or are empty. At present, recurrent hyperpycnal flows during the flood seasons are temporarily depositing sediments mainly derived from the Gaoping River in the head of the Gaoping Submarine Canyon. On the decadal and century timescales, sediments temporarily stored in the upper reach are removed over longer timescales probably by downslope-eroding sediment flows within the canyon. Presently, the Gaoping Submarine Canyon serves as the major conduit for transporting terrestrial sediment from the Taiwan orogen to the marine sink of the Manila Trench. Seismic data indicate that the Gaoping Submarine Canyon has been eroding the Gaoping Slope intensely by presumed hyperpycnal flows and transporting sediments from the canyon head to the middle and lower reaches of the canyon. The middle reach is a sediment bypass zone whereas the lower reach serves as either a temporary sediment sink or a sediment conduit, depending on relative prevalence to deposition or erosion during canyon evolution. Contrast differences in channel gradient and travel length between the Gaoping and Amazon sediment dispersal systems suggest that the Gaoping (Kaoping) River-Canyon system is an active sediment dispersal system for transporting terrestrial materials to the deep sea. The fate of the Gaoping River sediment is the northern Manila Trench.

Turbulent mixing and internal tides in Gaoping (Kaoping) Submarine Canyon, Taiwan

Turbulent overturning on scales greater than 10 m is observed near the bottom and in mid-depth layers within the Gaoping (formerly spelled Kaoping) Submarine Canyon (KPSC) in southern Taiwan. Bursts of strong turbulence coexist with bursts of strong sediment concentrations in mid-depth layers. The turbulence kinetic energy dissipation rate in some turbulence bursts exceeds 10 − 4 W kg − 1 , and the eddy diffusivity exceeds 10 − 1 m 2 s − 1 . Within the canyon, the depth averaged turbulence kinetic energy dissipation rate is ~ 7 × 10 − 6 W kg − 1 , and the depth averaged eddy diffusivity is ~ 10 − 2 m 2 s − 1 . These are more than two orders of magnitude greater than typical values in the open ocean, and are much larger than those found in the Monterey Canyon where the strong turbulent mixing has also been. The interaction of tidal currents with the complex topography in Gaoping Submarine Canyon is presumably responsible for the observed turbulent overturning via shear instability and the breaking of internal tides and internal waves at critical frequencies. Strong 1st-mode internal tides exist in KPSC. The depth averaged internal tidal energy near the canyon mouth is ~ 0.17 m 2 s − 2 . The depth integrated internal tidal energy flux at the mouth of the canyon is ~ 14 kW m − 1 , propagating along the axis of the canyon toward the canyon head. The internal tidal energy flux in the canyon is 3–7 times greater than that found in Monterey Canyon, presumably due to the more than 10 times larger barotropic tide in the canyon. Simple energy budget calculations conclude that internal tides alone may provide energy sufficient to explain the turbulent mixing estimated within the canyon. Further experiments are needed in order to quantify the seasonal and geographical distributions of internal tides in Gaoping Submarine Canyon and their effects on the sediment flux in the canyon.

Glacial, periglacial and glacio-volcanic structures on the Echus Plateau, upper Kasei Valles

An extensive region of low, sinuous ridges occupies the Hesperian plateau above Echus Chasma in the upper Kasei Valles, Mars. The ridges have lengths of up to 270 km, heights of 100 m and widths of 10 km. The total volume of the ridge material is 6×10 11 m 3. In this paper, volcanic flows, depositional and erosional features are discussed using Mars Observer Laser Altimeter (MOLA), THEMIS and Mars Orbiter Camera (MOC) imagery and a chronology that places the ridge formation in the Late Hesperian is developed. The plateau is bounded to the north and west by more recent Late Hesperian and Amazonian lava flows. The plateau floor suddenly changes from being relatively smooth, to elevated, rough, hummocky terrain that extends eastwards to Echus Chasma. This rough terrain is penetrated by 2 km broad, shallow entrant channels that join with the canyons of Echus Chasma. The sinuous ridges appear to control the surface drainage associated with the entrant channels. The sinuous ridges’ size and morphology are similar to those associated with volcanic ridge eruptions. Their degraded structure is reminiscent of Moberg ridges. The rough, hummocky terrain is interpreted as glacial outwash, subsequently eroded by short-lived floods associated with ridge eruptions. The presence of both volcanic and glacial structures on the Echus Plateau raises the possibility that the ridge system arose from subglacial, volcanic events. The resulting jokulhlaups eroded the broad, entrant channels. As surface flow declined, groundwater flows dominated and canyon heads eroded back along the entrant channels, by sapping.

Geomorphic study of fluvial landforms on the northern Valles Marineris plateau, Mars

Fluvial landforms are observed on the plateau near Echus Chasma and other locations on the Valles Marineris plateau using high resolution (10 to 50 m/pixel) Mars Express images and topography. Branching valleys have a 20‐ to 100‐m deep V‐shape profile typical of fluvial processes. Their incision occurred in a thin (<150 m) and weak, dark unit that overlies the plateau basement. The valleys are distributed close to watershed boundaries as expected for overland flows, and different from pure glacial or hydrothermal processes. The 2D geometry of valley networks show drainage densities reaching 1.3 km−1that indicate a strong 2D extension stage that usually requires a minimum of thousands of years, as established from terrestrial examples. However, the 3D valley geometry shows a limited incision, and a lack of concavity, suggesting a limited development in time (millions of years of evolution are unlikely). Many outlets connect to the heads of canyons of Echus Chasma that might involve sapping processes. These canyons might have formed coevally with shallow valleys: their difference in geometry is a consequence of the difference in lithology which induced a difference in the erosion capacity, and an enhanced infiltration conducing to sapping. Valleys are submitted to modification by aeolian processes, sometimes leading to the formation of inverted channels as observed more broadly through the region. These landforms formed late in the Early Mars history and might be considered as examples of episodic fluvial events due to short‐term climate changes and/or regional fluvial activity after the Noachian.

Observation of internal tidal currents in the Kaoping Canyon off southwestern Taiwan

Internal tidal currents and associated water-mass displacements were investigated during multiple cruises in the Kaoping Submarine Canyon off southwestern Taiwan. Observations from both moored and shipboard Acoustic Doppler Current Profilers and hydrographic casts were conducted along the canyon. The velocity data showed that in the lower layer the major axis of the tidal currents aligned with the orientation of the canyon, and currents moved up-canyon during flood and down-canyon during ebb. The vertical-phase shift and amplitude of the currents indicated that the semidiurnal internal tide dominated with intensity increasing with depth toward the canyon head. Tidal energy was channeled from the shelf landward with a beamlike internal wave, guided by bottom topography. The estimated phase velocity was 1.4–1.7 m s −1, based on normal mode analysis and the phase lag between sampling stations. Empirical orthogonal function analysis of hydrographic profiles confirmed that the first mode (M 2 internal tide) explained 70% of the total variance. The strong convergence of internal tidal currents near the canyon head during flood may play an important role in the daily migration of cherry shrimps, which burrow along the canyon wall.

Geomorphology and sedimentary features in the Central Portuguese submarine canyons, Western Iberian margin

The Central Portuguese submarine canyons (Nazaré, Cascais and Setúbal–Lisbon canyons) dissect the Western Iberian margin in an east–west direction from the continental shelf, at water depths shallower than 50 m, down to the Tagus and Iberian abyssal plains, at water depths exceeding 5000 m. We present an analysis of the geomorphology of the canyons and of the sedimentary processes that can be inferred from the observed morphology of the three canyons, based on a compilation of swath bathymetry data and TOBI deep-towed side-scan sonar imagery. This first complete detailed mapping of the Central Portuguese canyons reveals substantial differences in their morphologies and downslope evolution. The canyons are divided into three sections: 1) canyon head and upper reach, 2) middle canyon, and 3) canyon mouth and distal part. The canyon heads and upper reaches are severely indented into the continental shelf, and they are characterised, in the Nazaré and Setúbal–Lisbon canyons, by sinuous V-shaped valleys entrenched within high canyon walls occupied by rock outcrops dissected by gullies. The Cascais upper canyon is complex, with multiple branches with high axial gradients and signs of mass wasting. Middle canyon sections, indented in the slope, display axial incisions with perched, stacked terraces, and are affected by debris avalanches originating from the canyon walls. At the base of slope, the distal Cascais and Setúbal–Lisbon canyons show many characteristics of channel-lobe transition zones: erosional features such as isolated to amalgamated chevron scours, and depositional bedforms such as mud to gravel waves. Pervasive scouring occurs up to 95 km beyond the canyon mouths. By contrast, the Nazaré canyon opens into a 27 km wide and 94 km long channel, whose flat-bottomed thalweg is occupied by sediment waves, irregular, comet-shaped and crescentic scours, and a second-order channel. Transverse, kilometre-scale sediment waves occupy the overbank area of the southern channel margin. The present morphology of the Central Portuguese canyons is the result of erosive processes, subsequent sediment transport and deposition, and sediment instability, whereas inherited tectonic fabric controls their location. Morphological differences between the canyons are explained by the main mechanisms driving their activity. Overall, these morphological features suggest that these canyons have acted as an efficient conduit of sediment to the deep basin, transporting large quantities of material to the deep sea during high-energy events.

Comparison of horizontal and downward particle fluxes across canyons of the Gulf of Lions (NW Mediterranean): Meteorological and hydrodynamical forcing

In the framework of the European project E urostrataform, an array of six near-bottom mooring lines was deployed in the heads of the major submarine canyons incising the continental slope of the Gulf of Lions (NW Mediterranean). All moorings were equipped with sediment traps, current meters and turbidity Optical Backscatters Sensors (OBS) situated at few meters above the bottom. Particulate mass fluxes were recorded weekly by the sediment traps between November 2003 and May 2004 and compared with horizontal export fluxes obtained from the turbidity and current meters. Results show that near-bottom vertical particulate mass fluxes generally increase along slope between the northeastern-most and the southwestern region of the Gulf of Lions. Horizontal export fluxes also show some increase southwestward with flux in the southwestern-most canyon up to two-orders of magnitude higher than in northeast canyons. Furthermore, most of the export flux is driven by energetic and short events such as marine storms and dense shelf water cascading. Although the consequence of storms and cascading was obvious from traps and OBS measurements, substantial differences between vertical and horizontal fluxes appeared. While sediment traps recorded the largest vertical flux during the autumn storm (4 December) with contributions from river discharge and resuspended material, horizontal flux as obtained from OBS data was highest in winter (21–24 February). We believe that such discrepancies are partly due to measurement artifacts originating from the particle size effects and/or the different height above the bottom occupied by the traps and OBS which complicate comparisons between vertical and horizontal fluxes. However, different water-column conditions (stratified versus unstratified) together with the hydrodynamic regime during the two periods appear as a likely explanation for the flux differences between fall and winter storms.

Submarine landsliding and canyon evolution on the northern KwaZulu-Natal continental shelf, South Africa, SW Indian Ocean

The morphometric and geomorphological analysis of landslides found within several submarine canyons of varying sizes and morphologies from the northern KwaZulu-Natal continental shelf, South Africa, provides insight into submarine canyon evolution. Six large shelf indenting canyons are recognised (Leven, North and South Leadsman, Diepgat, Wright, and White Sands) interspersed with smaller canyons that occur prominently in the northernmost Mabibi area. Landslides and their surficial attributes were determined from high resolution multibeam bathymetry data. Landslide area, volume, runout length, the slope of the runout, the headscarp slope and height, scar slope, adjacent local unfailed slope gradient, and the depth of each failure are measured and the relationships statistically analysed for each individual submarine canyon. Landslide position and the type of landslide are also documented. The ratio between headscarp height to runout length ( D/L) is assumed as a good proxy for the dynamic rheology of each failure. The largest failures occur in Diepgat, Wright, Leven and Leadsman Canyons. The presence of steeper and larger failure headscarps indicates that the failure rheology was competent in Diepgat and Wright Canyons, whereas failures, particularly in the heads of Leven and Leadsman Canyons are assumed to possess fluidised rheologies based on their small D/L values. A landslide vs. a fluid sapping trigger is postulated for the Diepgat/Wright and Leven/Leadsman landslide sets respectively. The smallest landslides are found in the Mabibi area, and are limited to single retrogressive slumps in the heads of each small canyon. This style of failure is also evident in other smaller canyons that occur between the larger shelf indenting varieties. Headward erosion by small retrogressive failure is assumed to occur, before the canyon thalwegs become oversteepened relative to their walls. Once oversteepened, lateral canyon extension may occur, coupled with increased downslope erosion by sediment flow. The presence of 1) incised inner gorges; 2) hanging or perched slides in the canyon headwalls; 3) terraced slides; and 4) benches in the slide material indicate that axial incision and catastrophic slope clearances during this phase of submarine canyon evolution were prominent. The presence of sea-level notches within the failure scarps indicates that canyons are currently inactive and are unlikely to enlarge further pending relative sea-level fall during the next hypothermal (glacial) period.

Seasonal and event-controlled export of organic matter from the shelf towards the Gulf of Lions continental slope

To investigate the role of coastal canyons in the transfer of organic matter from the shelf to the slope and basin, we deployed sediment trap/current meter pairs at the head of five canyons in the Gulf of Lions (GoL) between November 2003 and May 2004. Analysis of organic carbon, biogenic silica, Corg isotopic composition, Corg/total nitrogen, chloropigments, and amino acids clearly shows the seasonal influence and effect of extreme meteorological events on the composition of collected particles. The sampling period was divided into three “scenarios”. The first corresponded to a large easterly storm and flood of the Rhone river during stratified water column conditions; the composition of material collected during this event was influenced by increased transfer of riverine and coastal particulate matter, with a lower Corg content. During the second “fall-winter” scenario, northern and northwestern winds blowing over the shelf caused cooling and homogenization of the shelf water column; particles collected at this time reflected the homogeneous source of particulate matter transported through canyons; particles sitting in the vicinity of canyon heads are most likely swept downslope by the general south-westward circulation. Organic tracers indicate a degraded origin for organic matter transported during this period. A third “spring” scenario corresponded to northern winds alternating with eastward windstorms that triggered and/or enhanced the cascading of dense waters accumulated on the bottom of the shelf due to previous cooling. These conditions occurred in conjunction with increased phytoplankton productivity in shelf surface waters. Organic matter advected mainly by dense shelf water cascading was fresher due to the transport of newly produced particles and a variable terrestrial fraction; this fraction depended on the proportion of resuspended material accumulated during previous high discharge periods that was involved in each transport pulse. The tight link shown between meteorological conditions and organic matter transport is important for continental margin geochemical studies as future changes in climatic conditions may lead to dramatic changes in carbon sequestration capability and in the ecosystems of deep margin environments.

Storm-driven shelf-to-canyon suspended sediment transport at the southwestern Gulf of Lions

Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves H s>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max H s: 8.4 m) and the second on 20–22 February 2004 (max H s: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events.

Channel-mouth lobe complex of the recent Amazon Fan: The missing piece

The Amazon Fan is one of the largest river-fed submarine fan systems in the world. It is located in the Western Equatorial Atlantic and extends 1100 km northward from the Amazon canyon head located at the shelf break off northern Brazil. Numerous previous investigations on the Amazon Fan mostly focused on the upper and middle parts of the fan. Two recent cruises have provided a new dataset based on bathymetric, acoustic imagery and 3.5 kHz sub-bottom profiler data. It is the first detailed exploration of the youngest Amazon channel-mouth lobe complex that remained the “missing piece” of one of the best studied modern deep-sea fans. The stratigraphic framework of the surveyed channel-mouth lobe complex of the Amazon Fan consists of eight channel-levee systems and their associated channel-mouth lobe systems (from Brown to Amazon). These depositional systems record a complex depositional history in a short time period between the Last Glacial Maximum (∼ 20,000 cal yr BP) and the rise in sea-level until 10,100 cal yr BP, when sediment supply to the fan ceased. The main body of the lobes is characterised by a general low to average backscatter facies (from white to light grey) scattered with higher backscatter fingers (dark grey). Two types of lobes are distinguished based on the planform shape of their backscatter facies: (1) elongated and narrow lobes with branching patterns like the veins of a leaf and (2) radial lobes with crescent-shaped features opened in the downstream direction, that do not have a clear organisation within the lobe. The eight channel-levee systems and associated lobes were built in a time span of ∼ 10 ka and with an average time between successive avulsions of 1250 years. The life span of lobes in the Amazon system is around 600 years which is very short compared to the life span of the fan as a whole. The observed wide range in channel lengths and lobe sizes suggests that each system had a variable time duration and/or sediment supply rate. The variability of the channel lengths could be controlled by internal and/or external forcings linked to processes occurring at different stages in the system in the Amazon drainage basin, on the Amazon shelf, at the canyon head or at the channels bifurcation point. Numerous channel-levee systems and lobes are concentrated in a relatively narrow area. Locally, the pre-existing morphology of the distal depositional area influences the shape and the location of lobes, providing an argument for strong topographical control where local gradient and degree of topographic confinement are key control parameters.

Sediment delivery, resuspension, and transport in two contrasting canyon environments in the southwest Gulf of Lions

Multiple canyons incise the continental slope at the seaward edge of the continental shelf in the Gulf of Lions and are actively involved in the transfer of sediment from shelf to deep sea. Two canyons in the southwest region of the Gulf of Lions, Lacaze-Duthiers Canyon and Cap de Creus Canyon, were instrumented with bottom-boundary-layer tripods in their heads to evaluate the processes involved in sediment delivery, resuspension and transport. In both canyons, intense cold, dense-water flows carry sediment across the slope. In the Lacaze-Duthiers canyon head (located ∼35 km from the shoreline), dense-water cascading into the canyon was episodic. Currents were highly variable in the canyon head, and responded to interactions between the along-slope Northern Current and the sharp walls of the canyon. Inertial and other high-frequency fluctuations were associated with suspended-sediment concentrations of ∼5 mg/l. In Cap de Creus canyon head (located ∼14 km from the shoreline), downslope currents were higher in magnitude and more persistent than in Lacaze-Duthiers canyon head. Greater suspended-sediment concentrations (peaks up to 20 mg/l) were observed in Cap de Creus Canyon due to resuspension of the canyon seabed during dense-water cascading events. The similarities and contrasts between processes in these two canyon heads emphasize the importance of the interaction of currents with sharp canyon bathymetry. The data also suggest that cold, dense-water flows have more potential to carry sediment to the slope on narrow shelves, and may more efficiently transfer that sediment to the deep sea where a smooth transition between shelf and slope exists.

Sediment accumulation in the western Gulf of Lions, France: The role of Cap de Creus Canyon in linking shelf and slope sediment dispersal systems

Previous work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River, ∼160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon. To understand the pattern of off-shelf sediment export from the western Gulf of Lions, and more specifically, the role of Cap de Creus Canyon in this transport, box cores were collected within the canyon and on the adjacent shelf during five cruises from November 2003 to April 2005. Geochronology ( 210Pb-derived accumulation rates), grain-size distributions, and sedimentary structures (X-radiography) were analyzed to assess temporal and spatial sedimentation patterns. Results indicate two mid-shelf depocenters (30–90 m water depth) in the northern and southern portions of the study area, separated by a zone of bypassing due to current acceleration around a headland (Cap Bear). Estimates of a sediment budget indicate that ∼6–8% of the sediment input to the Gulf is sequestered on the shelf region. Within the Cap de Creus Canyon, there is a significant spatial asymmetry in both grain size and accumulation rates. The northern flank is a modern depocenter of fine-grained sediments, while the southern flank is primarily non-depositional for mud and includes locations of apparent erosion. This suggests the influence of multiple oceanographic processes supplying sediment to the canyon: advection of nepheloid layers from the northern rim that provide a relatively continuous sediment supply (over decadal timescales), and episodic strong currents affecting the southern rim, which can scour sediment from the southern flank. The mid-depth thalweg has an ephemeral mud layer, overlying sand and consolidated mud. The mud layer appears to be flushed down canyon periodically. The canyon head contains coarse material, suggesting reworked sands may be entering. The 100-year sediment budget, based on accumulation rates for the fine-grained fraction in the upper canyon, indicates that ∼1% of the total sediment input to the Gulf is accumulating in upper Cap de Creus Canyon. However, this number may significantly underestimate the total sediment entering the canyon because water-column measurements show that sediment is likely moving through the upper canyon during major dense-water cascading events from the shelf and being deposited deeper in the canyon system. The ephemeral mud layer also indicates rapid deposition and frequent flushing of sediment through the upper canyon. Overall, this study shows that Cap de Creus Canyon is an active conduit of sediment past the shelf break, despite its location distal to the primary sediment source to the Gulf.

Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California

We performed a multibeam survey of Eel Canyon, offshore northern California. The survey revealed a significant bend in the canyon that appears to be due to the oblique compressional tectonics of the region. A series of steps within a linear depression, similar to 280 m above the canyon floor, extends from the canyon rim at this bend to the subduction zone and a distinct fan-like topographic rise. We hypothesize that the linear depression is a distributary channel and the steps are cyclic-step bedforms created by turbidity currents. Our interpretation indicates that turbidity currents are able to run up and overspill the 280-m-high canyon wall, resulting in a partial avulsion of the canyon and the construction of a fan lobe that is offset from the canyon mouth. Simple hydraulic calculations show that turbidity currents generated in the canyon head from failure of 2-3 m of material would be capable of partially overflowing the canyon at this bend, assuming steady-uniform flow, full conversion of the failed mass into a turbidity current, and a range of friction coefficients. These estimates are consistent with analyses of sediment cores collected in the head of Eel Canyon, which suggest that 2-3 m of material fails on decadal time scales. Our calculations show that the overflowing parts of the currents would have large shear velocities (>10 cm/s) and supercritical Froude numbers, consistent with erosion of the distributary channel and formation of cyclic steps by turbidity currents.

Sedimentary features and processes in the Nazaré and Setúbal submarine canyons, west Iberian margin

Here we present part of the first complete sidescan sonar dataset of the Nazaré and Setúbal Canyons, west Iberian margin, which, in combination with multibeam bathymetry, shallow seismic profiles and precise piston coring of intra-canyon targets, are used to characterise the sedimentary dynamics of these deep-sea settings. The results show that Nazaré and Setúbal Canyons are highly complex environments. They display a range of sedimentary features and processes that reflect changes in downslope canyon geometry and a transition from erosive proximal to more depositional distal sections. The proximal (upper) sections of both canyons are characterised by a deeply incised, narrow, V-shaped thalweg, flanked by small gullies and terraces. Numerous small and localised intra-canyon landslides and rock avalanches occur in this section, triggered by instability processes that are preconditioned by the steep topography. Sequences of stacked thin-bedded, fine-grained turbidites occur locally on intra-canyon terraces, and are interpreted to be the result of small-volume, possibly river flood-generated turbidity currents that do not appear to reach the lower canyon. In Nazaré Canyon these turbidites are associated with abundant coalified organic fragments. Part of the upper section in Nazaré Canyon has acted as an apparent depocenter through the Holocene, with very high sedimentation rates related to enhanced nepheloid layer activity. Much larger slope failures are sourced from the shelf break and canyon head and are the result of occasional releases of large volumes of sediment, likely related to earthquake activity. These failures rapidly evolve into large-volume, high-energy, sand-rich turbidity currents that flush the entire canyon and dominantly deposit in the lower canyon and distal abyssal plains. This episodic turbidity current activity results in highly heterogeneous sediment distribution across the distal (lower) U-shaped floor of both canyons, with abundant erosional scours and depositional bedforms such as sediment waves. Our results highlight the complex interplay of sedimentary processes operating within major canyon systems, and have potential implications for efficient characterisation of hydrocarbon reservoirs, accurate spatial and temporal prediction of geohazards, and the distribution of benthic ecosystems in canyon environments.