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Seafloor currents, including episodic sediment-laden turbidity flows and more sustained thermohaline-driven bottom currents, control the ultimate fate of sediment, carbon and pollutants in the deep-sea. Collectively, these currents form the largest sediment accumulations on Earth, from which past climate records are reconstructed. A long-standing view has been that thermohaline-driven bottom currents are persistent and stable; however, a paucity of direct near-seabed monitoring means this remains untested at field-scale. Here, using the most spatially-extensive and long-duration (4-yrs) monitoring of bottom currents at 34 deep-sea instrumented moorings, we show that such currents are far from steady and continuous. Bottom currents instead show dynamic, spatial variability in velocity and direction over tidal and seasonal timescales, being strongly influenced by local seabed morphology. Prior lower resolution monitoring has underestimated this spatio-temporal variability of bottom currents; hence our results highlight a need for near-bed measurements to more robustly quantify deep-sea sediment, carbon and pollutant fluxes.

Fine-grained coastal and marine sediments represent the largest permanent sink for carbon on our planet. They harbor large stocks of particulate organic carbon (POC) derived from marine plankton, land plants and coastal vegetation that build POC via photosynthesis and in this way take up CO2 from the atmosphere. The Helgoland Mud Area (HMA) represents the main depocenter for fine-grained and organic-rich sediments in the German Bight, North Sea. It covers a seafloor area of approximately 500 km2 off the island of Helgoland with a sediment thickness of up to 21 m and water depths between 11 and 27 m. In order to be able to quantify the long-term storage of carbon and its turnover during mineralization in the sediments of the HMA, we reconstruct sedimentation rates for the past decades and determine POC contents and origin. These results on the natural POC dynamics in the HMA are fundamental in order to understand how on-going anthropogenic activities (e.g., bottom trawling, dredging, offshore infrastructure) may affect the POC storage capacity of the HMA. Complementing existing estimates of sedimentation rates, we have determined 210Pbxs and 137Cs activities from the upper ~ 30 cm of the sediments in a high spatial and vertical resolution across the HMA. Based on this new dataset, we establish age models and describe the variation of depositional conditions across the HMA. The source of POC is determined by using the pore-water dissolved inorganic carbon concentrations (DIC) and respective stable carbon isotopic compositions with Keeling and Miller-Tans plot analysis as well as thermal gravimetric analysis (TGA). Published sedimentation rates for the HMA range from 0.5 to 11 mm yr-1 with strong lateral variations. The highest rates have been reported for the NW and central HMA. Our first results on sedimentation rates show ~ 2 mm yr-1 for these high accumulation areas within the HMA. POC contents in these areas are at ~ 1 wt% throughout the upper ~ 30 cm of the sediments. The source isotopic composition of POC ranges from -2.6 to -28.2 ‰ throughout the study area and needs to be complemented by on-going TGA analyses in order to reliably determine the origin of the POC. Once these analyses are completed, we aim to better understand the sources of organic carbon in this depocenter and to assess how the variation in depositional processes controls the mineralization and long-term burial of POC in the sediments of the HMA.

Bottom currents are key processes that contribute to the shaping of submarine slopes, with the redistribution of sediments in contourite systems. Despite numerous recent studies on contourite systems, the complexity and diversity of these sedimentary systems are still not fully understood and often underestimated. Their understanding requires comprehensive works integrating all scales from seismic architecture to microfacies. This paper focuses on a contouritic ridge located between 2000 m and 2500 m water depth on the Mozambique margin. Bathymetry, seismic data and piston cores collected during the PAMELA-MOZ3 cruise allow a multi-scale study from large depositional geometries to sedimentary facies. At the seismic scale, the contouritic ridge show three stages of evolution with: (i) initiation and development of the drift/moat system; (ii) an intermediate stage with successive incisions and aggradations; and (iii) moat infill and drift erosion during the Plio-Quaternary. Plio-Quaternary deposits are composed of hemipelagic, turbiditic and contouritic facies filling the moat. Coarse-grained contouritic facies, dominated by planktonic foraminifera, are identified on the western flank of the ridge between the moat infill and the erosional area at the top of the ridge. They consist of condensed deposits, with sedimentation rate about 0.3 cm/ka, indicating a strong and stable bottom current that winnows away the fine-grained component. This facies could be present more generally in intermediate position between erosion and depositional areas in contourite systems. At present, the contourite system is located at the transition depth between North Atlantic Deep Water and Antarctic Intermediate Water. Trajectories of bottom currents are complex and interact with sporadic turbidity currents and anti-clockwise eddies that participate in reshaping the sea floor morphology. Although Plio-Quaternary depositional geometries indicate the end of drift/moat development, the moat filling and drift erosion are also related to bottom currents and constitute atypical contouritic sedimentation.

The interaction of sedimentary systems with oceanographic processes in deep-water environments is not well understood yet, despite its importance for palaeoenvironmental reconstructions, and for a full understanding of source-to-sink sediment transport. The aim of this study is to improve the understanding of how contourite moats, elongated depressions formed by bottom currents associated with contourite drifts, develop and of the link between moat-drift system morphology and bottom current dynamics. This study provides a systematic comparison of 185 cross-sections of moat-drift systems distributed at 39 different locations worldwide, and a detailed analysis of the morphology of six moats that cover a wide range of typical geological and hydrodynamic settings. Additionally, in situ measured current data were analysed to better link hydrodynamics to moat morphology. The median of all profiles across all moat-drift systems reveals a 50 m relief, a width of 2.3 km, a relief to width ratio of 0.022, a slope angle of 6°, a drift angle of 3° and a concave-up shaped morphology. Moats can be over 100 km long. Some moats are driven by sediment erosion while others are depositional and primarily exist due to differential sedimentation inside the moat compared to the drift alongside the moat. A new sub-classification of moat-drift systems based on their stratigraphy is proposed. This classification distinguishes moats depending on the degree of erosion versus deposition. No relation is found between latitude and moat-drift morphology or stratigraphy in the analysed examples. The combined data indicate that a steeper slope focuses the current more than a gentle slope, resulting in an increase of the relief-width ratio and drift angle. Thus, this study provides new insides into the interaction of ocean currents with sedimentary morphology, which thereby affects the evolution of a poorly understood deepwater sedimentary system.

We investigated sediments from three different depositional environments along the northern Argentine continental margin to assess the main processes controlling sediment deposition since the last glacial period. Further, we evaluated how different depositional conditions affect (bio)geochemical processes within sediments. Sediment cores were collected during expedition SO260 in 2018[1]. Two sites are located at ~1100 m water depth north and south of the Mar del Plata Canyon (N- and S-Middle Slope Site). Another site is situated at the lower continental slope at 3600 m water depth (Lower Slope Site). Reliable age constraints of sediments deposited during the last glaciation at the Argentine margin are difficult to obtain due limited amounts of carbonate. We overcame this issue by combining radio-isotope analyses (14C,230Thex) with sedimentological, geochemical and magnetic data demonstrating that all sites experienced distinct changes over time.Both, N- and S-Middle Slope Sites, record at least the last 30 ka. The S-Middle Slope Site is dominated by continuously organic carbon-starved and winnowed sandy deposits, which according to geochemical and magnetic data leads to insignificant sulfate reduction and sulfidation of iron (oxyhydr)oxides. Glacial sedimentation rates at the Middle Slope increase northwards suggesting a decrease in bottom-current strength. The N-Middle Slope Site records a transition from the last glacial period, dominated by organic carbon-starved sands, to the early deglacial period when mainly silty and organic carbon-rich sediments were deposited between 14-15 ka BP. Concurrently, glacial sedimentation rates of ~50 cm/ka significantly increased to 120 cm/ka. We propose that this high sedimentation rate relates to lateral sediment re-deposition by current-driven focusing as response to sea level rise. Towards the Holocene, sedimentation rates strongly decreased to 8 cm/ka. We propose that the distinct decrease in sedimentation rates and change in organic carbon contents observed at the N-Middle Slope Site caused the nonsteady-state pore-water conditions and deep sulfate-methane-transition (SMT) at 750 cm core depth. The Lower Slope Site records the last 19 ka. Continuously high terrigenous sediment input (~100 cm/ka) prevailed during the Deglacial, while sedimentation rates distinctly decreased to ~13 cm/ka in the Holocene. Here, pore-water data suggest current steady-state conditions with a pronounced SMT at 510 cm core depth. Our study confirms previous geochemical-modelling studies at the lower slope, which implied that the observed SMT fixation for ~9 ka at specific depth relates to a strong decrease in sedimentation rates at the Pleistocene/Holocene transition[2].During the Holocene, total organic and inorganic carbon contents, inorganic carbon mass accumulation rates and XRF Si/Al ratios (preserved diatom flux) increase at our sites. We relate this to increased primary production in surface waters and less terrigenous input along the continental margin. Our multidisciplinary approach presents improved age constraints at the northern Argentine Margin and demonstrates that lateral/vertical sediment transport and deposition was strongly linked to Glacial/Interglacial variations in bottom currents, seafloor morphology, sea level and sediment supply. The dynamic depositional histories at the three sites still exert a significant control on modern sedimentary (bio)geochemical processes. [1]Kasten et al. (2019).Cruise No. SO260. Sonne-Berichte.[2]Riedinger et al. (2005).Geochim. Cosmochim. Acta. 69.

The genesis of submarine channels is often controlled by gravity flows, but channels can also be \udformed by oceanographic processes. Using multibeam bathymetry and two-dimensional seismic data from the \udwestern South China Sea, this study reveals how pockmarks can ultimately form channels under the effect of \udbottom currents and gravity-driven sedimentary processes. We demonstrate that alongslope and across-slope \udchannels were initiated by pockmark trains on the seafloor. Discrete pockmarks were elongated due to the \uderosion of gravity-driven sedimentary processes and bottom currents, and later coalesced to form immature \udchannels with irregular thalwegs. These gradually evolved into mature channels with continuous overbanks and \udsmooth thalwegs. Submarine channel evolution was significantly influenced by seafloor topography since the \udLate Miocene. The evolutionary model documented here is a key to understanding how channels are formed in \uddeep-water environments.

International audience; The morphology and present-day sediment distribution of the Zambezi turbidite system was investigated using new bathymetric and sub-bottom profiler data as part of the PAMELA research project. The Zambezi turbidite system is composed of two depositional systems: a channelized fan (the Zambezi Fan) and a semi-confined fan (in the Intermediate Basin). The Zambezi Fan includes the Zambezi Valley, which is deeply incised in the Mozambique Channel and is more than three times as large and deep as the great Tanzanian and North Atlantic Mid-Ocean channels. The erosion in the Zambezi Valley is evidenced by its V-shaped morphology and the existence of a U-shaped thalweg affected by several generations of incisions. Based on echo facies and cores from literature, sediments of the Zambezi Fan are dominantly coarse-grained and fine-grained overbank deposits are infrequent. The distal portion of the Zambezi Fan is a main depositional area where typical transparent wedged-shape seismic bodies are interpreted as terminal lobes. Seismic facies in the Intermediate Basin are thought to represent mostly fine-grained turbidites intercalated with infrequent coarse-grained sheet-like turbidites. Hydrodynamic circulation (from surface eddies to the deep circulation of NADW) appears to have a great impact on the Mozambique Channel sedimentation and is suggested (1) to be involved in the delivery of the Zambezi River sediments along the Mozambique margin, (2) to entrain the upper suspended load of turbidity currents, contributing to the absence of fine-grained sedimentation and (3) to contribute to the erosion of the valley flanks leading to the exceptionally great dimensions of the valley

International audience; Contourites are common morphological features along continental margins where currents encounter the seafloor. They can provide long-term archives of palaeoceanography, may be prone to sediment instability, and can have a great potential for hydrocarbon exploration. Despite their importance and increasingly recognised ubiquitous occurrence worldwide, the link between oceanographic processes and contourite features is poorly constrained. In particular, it is unclear under which specific conditions sediments are mobilised, modified and deposited by bottom currents. Here, we aim to determine key bottom current characteristics (velocity and bottom shear stress) affecting contourite deposition, by assuming that recent oceanographic regimes may be extended back in time over the past glacial-interglacial cycles, with strong winter circulation assumed similar to glacial conditions and weak summer circulation to interglacials. We present an integrated study from the NW Mediterranean Sea that couples results of the MARS3D hydrodynamic model with high-resolution sedimentological and geophysical data (piston cores, multibeam bathymetry and high resolution seismic data). Near bottom circulation was modelled during winter and summer 2013 as representative of past periods of high and low current intensity, respectively. Model results match well with the extent of contourite depositional systems and their different localised morphologic elements. We deduce that higher intensity events control the formation of erosional features such as moats and abraded surfaces. The heterogeneous distribution of bottom-current intensity on slopes explains the development of different types of contourite drifts. Plastered drifts form in zones of low bottom-current velocities constrained upslope and downslope by higher current velocities. Separated elongated mounded drifts develop where fast bottom-currents decelerate at foot of the slope. In contrast, no mounded contourite morphologies develop when the current velocity is homogeneous across the slope, especially in margins prone to downslope sediment transport processes. In confined basins, gyres may transport sediment in suspension from a margin with a high sediment supply to an adjacent starved margin, favouring the development of fine-grained contourites in the latter. Our results provide new insights into how detailed bottom-circulation modelling and seafloor geomorphological analyses can improve the understanding of palaeoflow-regimes, at least over time spans when the overall paleogeography and the distribution of contourite drifts is comparable to present-day conditions. The approach of coupled hydrodynamic models and geomorphological interpretations proposed here for depositional, erosional and mixed contourite features may be used to understand other areas affected by bottom currents, and for a better conceptual understanding of bottom-current processes and their interactions with the seafloor. © 2019 Elsevier B.V.

The purpose of this study is to differentiate and characterise contouritic sands in two different locations with variable sediment compositions (siliciclastic and bioclastic) based on a multiproxy approach that includes the analysis of sedimentary texture, semi-quantitative geochemistry, microfacies and ichnological information, as well as a Principal Component Analysis (PCA) applied to geochemical and sedimentary data on sediment cores. The integration of sedimentological analyses and the PCA permits also to the differentiation between fine-grained deep-water deposits such as hemipelagites, muddy contourites and hyperpycnites. A depositional model is proposed here, based on geochemical and sedimentological characteristics of the Holocene-highstand Mozambique Channel upper slope sands, and glacial-lowstand Corsica Contourite Depositional System middle slope sands. The upper continental slope of the Mozambican margin is characterised by siliciclastic sandy contourites, muddy contourites and muddy hyperpycnites. Mozambique siliciclastic sandy contourites constitute large accumulations of well-sorted very fine to coarse sand with evidences of strong winnowing and reworking under high-energy conditions. The sedimentary facies represents highstand contourite sands and shows a reversely-graded trend. The contourite drift on the Pianosa ridge (eastern flank of the Corsica Trough) consists of bioclastic sandy and muddy contourites and hemipelagites. Bioclastic sandy contourites are made up of shallow-marine winnowed bioclasts with a reversely- and normally-graded trend and represent lowstand contourite sands. The PCA in the two environments —showing a distinctive geochemical signal— allows for differentiation of the contourite deposits. In siliciclastic sands, reworking is marked by an accumulation of Si, Zr, and Sr in fine- to medium-grained sands. In bioclastic sands, reworking is less evident but it is characterised by accumulations of Ca and Sr. The reworking and winnowing bottom current effects are also observed at the microfacies scale. Both types of contourite deposits show evidences of intermittent depositional conditions depending on the ichnodiversity, distribution and abundance of trace fossils. This work is useful to discriminate similar fine-grained deposits in different continental margins which would contribute to a better understanding of sedimentary deposits and processes in deep-marine environments.

The Argentina Continental Margin represents a unique geologic setting to study interactions between bottom currents and sediment deposition as well as their impact on (bio)geochemical processes, particularly the cycling of iron (Fe). Our aim was to determine (1) how different depositional conditions control post-depositional (bio)geochemical processes and (2) how stable Fe isotopes (δ56Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of δ56Fe of solid Fe pools as a proxy to trace past diagenetic alteration of Fe, which might be decoupled from current redox conditions. Sediments from two different depositional environments were sampled during RV SONNE expedition SO260: a site dominated by contouritic deposition on a terrace (Contourite Site) and the lower continental slope (Slope Site) dominated by hemipelagic sedimentation. Sequentially extracted sedimentary Fe [1] and δ56Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary δ56Fe dataset at the Argentina Continental Margin.The depositional conditions differed between and within both sites as evidenced by variable grain sizes, organic carbon contents and sedimentation rates. At the Contourite Site, non-steady state pore-water conditions and diagenetic overprint occurs in the post-oxic zone and the sulfate-methane transition (SMT). In contrast, pore-water profiles at the Slope Site suggest that currently steady-state conditions prevail, leading to a strong diagenetic overprint of Fe oxides at the SMT. Pore-water δ56Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water δ56Fe values are mostly positive and range between -0.35‰ to 1.82‰. Positive δ56Fe values are related to high sulfate reduction rates that dominate over Fe reduction in the post-oxic zone. The HS- liberated during organoclastic sulfate reduction or sulfate-mediated anaerobic oxidation of methane (AOM) reacts with Fe2+ to form Fe sulfides. Hereby, light Fe isotopes are preferentially removed from the dissolved pool. The isotopically light Fe sulfides drive the acetate-leached Fe pool towards negative values. Isotopic trends were absent in other extracted Fe pools, partly due to unintended dissolution of silicate Fe masking the composition of targeted Fe oxides. Significant amounts of reactive Fe phases are preserved below the SMT and are possibly available for reduction processes, such as Fe-mediated AOM [4]. Fe2+ in the methanic zone is isotopically light at both sites, which is indicative for a microbial Fe reduction process.Our results demonstrate that depositional conditions exert a significant control on geochemical conditions and dominant (bio)geochemical processes in the sediments of both contrasting sites. We conclude that the applicability of sedimentary δ56Fe signatures as a proxy to trace diagenetic Fe overprint is limited to distinct Fe pools. The development into a useful tool depends on the refining of extraction methods or other means to analyse δ56Fe in specific sedimentary Fe phases. References:[1]Poulton and Canfield, 2005. Chemical Geology 214: 209-221.[2]Henkel et al., 2016. Chemical Geology 421: 93-102.[3]Homoky et al., 2013. Nature Communications 4: 1-10.[4]Riedinger et al., 2014. Geobiology 12: 172-181.

International audience; The NW Madagascar continental margin receives high loads of terrigenous particulate organic matter during the wet season and especially linked to extreme events, originating from two major rivers, the Betsiboka and the Mahavavy Rivers. This particulate matter contains a high content of iron minerals from the weathering of red ferruginous/ferralitic soils of Madagascar. The presence of pockmarks, i.e. gas or fluid expulsion features on the continental slope, testifies to past/present methane migration through the sedimentary column, associated with early diagenetic processes. This study globally aims at deciphering the interactions between episodic sedimentation and geochemical processes influenced by fluids upward migration, using a sediment trap mooring and interface sediment cores at two sites on the continental slope. The present-day sedimentation along this margin undergoes two patterns. During the wet season, high continental fluxes generally result in an increase in particle fluxes on the slope. Longshore currents may deflect river plumes alongslope resulting in some periods of low particle flux on the slope during the wet season. During the dry season, the particles collected in the water column are probably originated from sediment remobilization from the shelf and slope. The observed progradation of the Mahavavy River delta and prodelta between 1984 and 2016 argues for an increase in temporary connections of the river with the canyon head during extreme events, inducing pulsed sedimentation offshore. This pulsed sedimentation could be responsible for enhancing pyritization on surface sediments, due to higher inputs of terrigenous organic matter and iron oxides. Finally, methane upward migration also influences the pyritization process through anaerobic oxidation of methane using sulfate as an electron acceptor and methanotrophs associated with this reaction significantly impact the δ13C of organic carbon towards more negative values.

Not just settling What controls the distribution of microplastics on the deep seafloor? Kane et al. show that the answer to that question is more complicated than particles simply settling from where they are found on the sea surface (see the Perspective by Mohrig). Using data that they collected off the coast of Corsica, the authors show that thermohaline-driven currents can control the distribution of microplastics by creating hotspots of accumulation, analogous to their role in causing focused areas of seafloor sediment deposition. Such currents also supply oxygen and nutrients to deep-sea benthos, so deepsea biodiversity hotspots are also likely to be microplastic hotspots. Science , this issue p. 1140 ; see also p. 1055

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. On the other hand, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record.

The Argentina Continental Margin represents a unique geologic setting where fundamental interactions between bottom currents and sediment deposition as well as their impact on biogeochemical processes and element cycling, in particular iron, can be studied. The aims of this study were to investigate 1) the consequences of different depositional conditions on biogeochemical processes and 2) diagenetic cycling of Fe mineral phases in surface sediments. Furthermore, it was 3) studied how sedimentary stable Fe isotope signatures (δ56Fe) are affected during early diagenesis and finally 4) evaluated, under which conditions δ56Fe might be used as proxy for microbial Fe reduction in methanic sediments. During RV SONNE expedition SO260, carried out in the framework of the DFG-funded Cluster of Excellence “The Ocean in the Earth System”, surface sediments from two depositional environments were sampled each using gravity corer and multi corer. One study site is located on the lower continental slope at 3605 m water depth (Biogeochemistry Site), while the other site is situated in a contourite system on the Northern Ewing Terrace at 1078 m water depth (Contourite Terrace Site). Sequential Fe extractions were performed on the collected sediments to determine four operationally defined reactive Fe phases targeting Fe carbonates, (easily) reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides [1]. Purification of extracts for δ56Fe analysis of the Fe carbonates and easily reducible Fe (oxyhydr)oxide fractions followed [2]. The dataset was combined with pore-water data obtained during the cruise and complemented by concentrations and stable carbon isotope signatures of dissolved methane determined post-cruise. The extent of the redox zonation and depth of the sulfate-methane-transition (SMT) differ between the two sites. It is suggested that sedimentation rates at the Biogeochemistry Site are low and that steady state conditions prevail, leading to a strong diagenetic overprint of sedimentary Fe phases. In contrast the Contourite Terrace Site is characterized by high sedimentation rates and a lack of pronounced diagenetic overprint [3]. Reactive Fe phases are subject to reductive dissolution at the SMT. Nevertheless, significant amounts of reactive Fe phases are preserved below the SMT as evidenced by the presence of dissolved Fe in the methanic sediments, and are available for deep Fe reduction possibly through Fe-mediated anaerobic oxidation of methane [4]. In this study, δ56Fe signatures of reactive Fe phases in methanic sediments were determined for the first time. These data suggest significant microbial fractionation of Fe isotopes during deep Fe reduction at the Biogeochemistry Site, whereas at the Contourite Terrace Site the δ56Fe signatures do not indicate remarkable microbial Fe isotope fractionation. It is concluded that the applicability of δ56Fe signatures as tracer for microbial Fe reduction might be sensitive to the depositional regime, and thus may be limited in high sedimentation areas.References:Poulton, SW. and Canfield, DE., 2005. Chemical Geology 214: 209-221.Henkel, S. et al., 2016. Chemical Geology 421: 93-102.Riedinger, N. et al., 2005. Geochimica et Cosmochimica Acta 69: 4117-4126.Riedinger, N. et al., 2014. Geobiology 12: 172-181.

Recently acquired data from the Iles Eparses (southwestern Indian Ocean) reveal new information about the geomorphology, depositional processes, and sedimentary deposits on the slopes of atolls and atoll-like platforms. The deposits discussed here lie on the deepwater flanks of isolated, inactive volcanos that are capped by shallow, relatively flat carbonate platforms 45–210 km2 in area. Much of the slope geomorphology is controlled by the underlying volcanic edifice. Steep (~25–35°) upper slopes consist of outcrops of volcanic basement, smooth banks, failure scarps, and channels. Sedimentary features seen in the lower slope and proximal basin (2000–3500 m deep) consist of channels, levees, lobes, and mass transport deposits (MTDs). In places, channels terminate 13–18 km from the platform margin, ending in lobes up to 3.5 km across, a feature not often seen in modern carbonates. In the subsurface, MTDs are present near all platforms. Within MTDs, seismic character is variable, often consists of chaotic reflections indicative of sediment gravity flow processes. Subsurface units with organized (retro- or progradational) reflections are interpreted as turbidite lobes or MTDs with compressional features. Core taken within lobes and near the base of slopes reveal decimeter-scale turbidites and debrites composed primarily of graded and massive bioclastic grainstones and packstones with abundant neritic skeletal components, interbedded with hemipelagic aragonitic and clay-rich foraminiferal ooze. Slope depositional processes are therefore primarily gravity-driven and occur at different scales; i.e., bed-scale turbidites and muds may be remobilized and redeposited through slope failure and deposition of large MTDs. Dominant wind direction may also play a role in slope sedimentation: leeward slopes are generally less rugose and show increased sedimentation at the toe of the slope. This study thus provides new insight into depositional systems surrounding atoll-like carbonate platforms, and provides a new analogue for similar deposits in the geologic record.

Contourite drifts are sediment bodies formed by the action of bottom currents. They are common features found on continental slopes and are often affected by slope failure. However, processes controlling slope instability in contourite depositional systems are still not well constrained, and it is not clear whether contourites have particular properties that make them more susceptible to slope failure. In this study, we compare sedimentological and geotechnical properties of contouritic and hemipelagic sediments within the Corsica Trough (northern Tyrrhenian Sea) using geophysical data sets and sediment cores in order to get a better understanding of the controlling factors of slope stability. Geomorphological and slope stability analyses reveal that differences in sediment properties have little influence on the location of submarine landslides, in comparison with the morphology of the drifts. Hence, the steep downslope flanks of plastered drift deposits are the most susceptible zones for local failure initiation. Moreover, as erosion is common at the foot of plastered drifts, undercutting is thought to contribute to the development of large-scale failure up to the point that submarine landslides are triggered.

Sediment deposits formed mainly under the influence of bottom currents (contourites) are widely used as high-resolution archives for reconstructing past ocean conditions. However, the driving processes of Contourite Depositional Systems (CDS) are not entirely understood. The aim of this study is to establish a clearer link between contourite features and the oceanographic processes that form them. The morphosedimentary characteristics of a large CDS were analysed together with the current dynamics along the northern Argentine continental margin. This study combines multibeam bathymetry, seismo-acoustic data, sediment samples, vessel-mounted Acoustic Doppler Current Profiler (VM-ADCP) data and numerical modelling of ocean currents. The contouritic features include large contourite terraces (La Plata Terrace, Ewing Terrace) and an abraded surface connecting the terraces, as well as smaller erosional and depositional features like moats, erosion surfaces on the Ewing Terrace, sediment waves and contourite drifts. Measured and modelled near-bottom currents are vigorous (up to 63 cm/s at 150–200 m above the seafloor) where abraded surfaces and moats are present, and relatively weak (below 30 cm/s) on the La Plata Terrace and the Ewing Terrace. Generally, bottom currents follow the upper and middle slope morphology. Decreasing velocity of water masses flowing northward leads to less erosion and finer sediment deposits. ADCP data and the hydrodynamic model show the formation of eddies near the seafloor which probably lead to the small erosion surfaces on the Ewing Terrace, even though it is mainly a depositional environment. Overall, this study contributes to a better understanding of the formation of CDS and can help future reconstructions of past ocean conditions based on sedimentary structures.

International audience; Over the past ten years, a huge amount of source-to-sink studies have aimed to unravel the tectonic, climatic and other processes that shape the landscape from mountains to the deep ocean. Interestingly, these studies have been mainly dedicated to siliciclastic or mixed systems, for which the connection between drainage basins, continental shelves, slope and basin environments are often well constrained. Here we present a study focusing on a source-to-sink study dedicated to a pure carbonate system, located in the SW Indian Ocean (Glorieuses archipelago). Extensive field sampling and geophysical acquisition across the carbonate platform have allowed us to estimate the composition, the lateral variability, and volumes of neritic sands deposited on the platform top. Additional seismic and bathymetric surveys across the platform interior illustrate the presence of plurimetric sandy bodies deposited along the leeward platform edge, corresponding to the export of carbonate sediments from the platform top toward the platform edge, under the influence of dominant currents and winddriven processes. High-resolution seismic, bathymetric data and sediment cores acquired along the leeward slope and basin adjacent to the carbonate platform highlight the presence of channel-levee complexes and turbiditic lobes, which have accumulated on the seafloor on top of a 250 m-thick sedimentary basin at 2000-3400 m water depth. Our study points out that carbonate sands and aragonitic mud produced on the platform top during the Holocene have been shed to the adjacent basin. We also demonstrate that this routing system was active at least throughout the last three glacial/interglacial cycles. This study has important consequences for our understanding of carbonate sedimentation processes occurring in the vicinity of isolated carbonate platforms: 1 - it highlights the role of oceanographic conditions in the distribution of sediment on a platform top and its export along the platform edge and the adjacent basin; 2 - it contributes to quantify the productivity of a carbonate platform as well as its sediment storage capacity; 3 - a first estimate of a carbonate source-to-sink system is proposed, demonstrating that 0,57 km(3) of sediments have been produced during the Holocene, 0,3 km(3) being presently stored on the platform, and the remaining having been exported to the deep basin.

International audience; We present a combined study of the geomorphology, sedimentology, and physical oceanography of the Mozambique Channel to evaluate the role of bottom currents in shaping the Mozambican continental margin and adjacent Durban basin. Analysis of 2D multichannel seismic reflection profiles and bathymetric features revealed major contourite deposits with erosive (abraded surfaces, contourite channels, moats, furrows and scours), depositional (plastered and elongated-mounded drifts, sedimentary waves), and mixed (terraces) features, which were then used to construct a morpho-sedimentary map of the study area. Hydrographic data and hydrodynamic modelling provide new insights into the distribution of water masses, bottom current circulation and associated processes (e.g., eddies, internal waves, etc.) occurring along the Mozambican slope, base-of-slope and basin floor. Results from this work represent a novel deep-sea sedimentation model for the Mozambican continental margin and adjacent Durban basin. This model shows 1) how bottom circulation of water masses and associated sedimentary processes shape the continental margin, 2) how interface positions of water-masses with contrasting densities (i.e., internal waves) sculpt terraces along the slope at a regional scale, and 3) how morphologic obstacles (seamounts, Mozambique Ridge, etc.) play an essential role in local water mass behaviours and dynamics. Further analysis of similar areas can expand understanding of the global role of bottom currents in deep-sea sedimentation.

Highlights • Ocean current plays an essential role in shaping ocean floor. • Observed Kuroshio Current in the Kenting Plateau is up to 1.8 m/s. • Intense Kuroshio Current shaped the Kuroshio Knoll into flat topped elevated surface. • The parent rocks of the gravels were buried 2 to 4 km below the seafloor. • Decrease in grain size and sand content away from the Plateau indicates the Plateau acts as source for the sand. The Kenting Plateau is characterized by unusual low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism off southern Taiwan at 400–700 m water depth. Multibeam bathymetric data, reflection seismic data, Acoustic Doppler Current Profiler (ADCP) data, surface grab samples, and sediment cores were collected in and around the Plateau to identify evidence of erosion in the Kenting Plateau and understand how the morphological evolution has been influenced by submarine erosion over geological time scales. The most distinctive feature on the Kenting Plateau is a 3 km × 7 km bean-shaped flat elevated platform (Kuroshio Knoll) revealed by multibeam bathymetry. Seismic data show almost no reflections beneath the seafloor and erosional truncations at the seafloor, especially in the Plateau's eastern half, evidencing widespread erosion. The P-wave velocity of the gravels recovered from the top of the Plateau ranges from 2.2 to 4 km/s. After comparing the velocity with the borehole data from nearby basin the burial depth of the parent rocks was found to be around 2 to 4 km below the seafloor, indicating that the parent rocks have been uplifted and gravels were formed due to erosion of the Plateau. The truncation of the seafloor shown on seismic sections suggests significant erosion on the Plateau. Sand content of the sediment cores decreases away from the Plateau, suggesting that sediment transport is effective in this area with high energy deposition, thereby accumulating coarse sediments on the Plateau and removing fine particles away from it. The presence of a dune field migrating northward of the Plateau, parallel to the Kuroshio Current also evidences active sediment transport in the area. Flow velocity of the Kuroshio Current observed from the ADCP data is very high, reaching up to 1.8 m/s on top of the Kuroshio Knoll (SE domain). We thus interpret that the observed intense erosion is caused by the Kuroshio Current, while the uplift of the Kenting Plateau is partially due to isostatic rebound caused by sediment removal through erosion and compression of the accretionary wedge. The higher sedimentation rate and coarser in grain size during sea level lowstand (20,000–12,000 yrs. BP) suggests that the erosion was more intense during the glacial period compared to that of deglacial period (< 12,000 yrs. BP) as seen from the MD97–2145 core. Submarine erosion is predominant throughout the Plateau, and it controls the geomorphology of the Plateau, especially the Kuroshio Knoll.

The Pianosa Contourite Depositional System (CDS) is located in the Corsica Trough (Northern Tyrrhenian Sea), a confined basin dominated by mass transport and contour currents in the eastern flank and by turbidity currents in the western flank. The morphologic and stratigraphic characterisation of the Pianosa CDS is based on multibeam bathymetry, seismic reflection data (multi-channel high resolution mini GI gun, single-channel sparker and CHIRP), sediment cores and ADCP data. The Pianosa CDS is located at shallow to intermediate water depths (170 to 850 m water depth) and is formed under the influence of the Levantine Intermediate Water (LIW). It is 120 km long, has a maximum width of 10 km and is composed of different types of muddy sediment drifts: plastered drift, separated mounded drift, sigmoid drift and multicrested drift. The reduced tectonic activity in the Corsica Trough since the early Pliocene permits to recover a sedimentary record of the contourite depositional system that is only influenced by climate fluctuations. Contourites started to develop in the Middle–Late Pliocene, but their growth was enhanced since the Middle Pleistocene Transition (0.7–0.9 Ma). Although the general circulation of the LIW, flowing northwards in the Corsica Trough, remained active all along the history of the system, contourite drift formation changed, controlled by sediment influx and bottom current velocity. During periods of sea level fall, fast bottom currents often eroded the drift crest in the middle and upper slope. At that time the proximity of the coast to the shelf edge favoured the formation of bioclastic sand deposits winnowed by bottom currents. Higher sediment accumulation of mud in the drifts occurred during periods of fast bottom currents and high sediment availability (i.e. high activity of turbidity currents), coincident with periods of sea level low-stands. Condensed sections were formed during sea level high-stands, when bottom currents were more sluggish and the turbidite system was disconnected, resulting in a lower sediment influx.

International audience; Mixed turbidite-contourite systems can be found in oceans where bottom currents and turbidity currents interact. The Zambezi turbidite system, located in the Mozambique Channel (SW Indian Ocean), is one of the largest sedimentary systems in the world in length and area of the related catchments. The oceanic circulation in the Mozambique Channel is intense and complex, dominated by eddies flowing southwards and deep currents flowing northwards along the Mozambican margin. Current measurements obtained from moorings at 3400–4050 m water depth in the Zambezi and Tsiribihina valleys show periods of intense currents at the seafloor with peaks of 40–50 cm s−1 that last up to one month and are not related to turbidity currents. These strong bottom-current events are correlated with a change in current direction and an increase in temperature. The periods of current intensification may be related to eddies, since they present similar frequencies (around 7 per year). Moreover, modelling results show that during periods of intense deep circulation an anticyclonic eddy is present between the Mozambican slope and the centre of the Mozambique Channel, which may block the northward transport of the deep water mass and thus enhance the southward transport along the western slope of Madagascar. According to our hydrodynamic modelling of the circulation near the seafloor, intense currents are often present along the Zambezi Valley, especially along the valley flanks. Multi-channel seismic reflection data show that the Zambezi turbidite system does not show the typical characteristics of turbidite systems, being dominated by erosional processes, which mainly affect the valley flanks. Levees associated with the valley are absent in the main axis of the system. The effect of bottom currents on sedimentation in the basin is evidenced by the low sedimentation rates that witness winnowing in the basin, the presence of contouritic sand in the Zambezi Valley flanks and the abundance of current-related bedforms observed in multibeam bathymetry and seismic data. The intense oceanic processes observed in the Mozambique Channel may transport a large part of the fine sediment out of the basin and erode the seafloor even at great depths. Therefore, the Zambezi turbidite system could at present be considered as a mixed turbidite-contourite system, with important implications for source-to-sink studies.

International audience; Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep-marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south-west Indian Ocean). The acquired data include multibeam bathymetry, multi-channel high-resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull-mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec−1 and maximum speeds of 57 cm sec−1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high-frequency variability) and to anticyclonic eddies near the seamounts (low-frequency variability). This study contributes to a better understanding of the formation of dunes in deep-marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology. © 2018 The Authors. Sedimentology © 2018 International Association of Sedimentologists

International audience; A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s−1 in winter 2013 and calculated bottom shear stress exceeding 0.2 N m−2 in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.

International audience; The Pianosa Ridge forms the eastern flank of the Corsica Trough in the Northern Tyrrhenian Sea: it is the site of preferential accumulation of contourites and Mass Transport Deposits (MTDs). Along the Pianosa Ridge, 11 MTDs with a total volume of 6.5 km 3 were identified. These MTDs are distributed in three areas: (A) one small MTD associated to canyon flank destabilisation in the northern part of the study area; (B) six intermediate size MTDs in the central area; (C) four MTDs of larger size (up to 2.62 km 3) to the south, including the Pianosa Slump, which is the most recent MTD in this area (aged at 42–50 kyr BP) and analysed in more detail. The main factor controlling the formation of MTDs in areas A and B seems to be steep slopes associated to erosion and heterogeneous sedimentation caused by bottom currents, respectively. In contrast, multiple factors may control slope instability in the zone where the largest MTDs took place (area C): the incision generated by contour currents, the presence of coarser layers in contourite drifts that may accumulate gas and the location of normal faults near the headwall.

Summary Although contourites and mixed turbidite/contourite depositional systems are significant to the oil industry, they are still relatively poorly known by this sector, and there is no present-day accepted sedimentological model for sandy contourites. Our goal here is to better understand the possibility of occurrence of contouritic sands susceptible to form potential reservoirs for hydrocarbons, and therefore possible new plays. After the Gulf of Cadiz ( Hernandez-Molina et al., 2013 ) and the Brazilian margin ( Mutti et al., 2014 ), the discovery of coarse granulometry contourite facies in Plio-pleistocene deposits offshore Mozambique is a great step toward the proof of concept of reservoir potential for these objects. The results presented derive from the Pamela (PAssive Margin Exploration Laboratories) project, led by Ifremer and TOTAL in collaboration with Universites de Bretagne Occidentale, Rennes-1, P&M Curie, CNRS and IFPEN. First class outcome, the PAMELA-MOZ-3 cruise has revealed occurrence of coarse-grained contourite facies deposited in a condensed layer wedging on a contourite terrace. Sandy contourite facies (siliciclastic or carbonate) is now proved to occur where favorable conditions gather. Consolidated with state-of-the art techniques of subsurface data interpretation, the new model integrating geometrical and amplitude characters should give keys to help interpreters predict the reservoir potential of contourites.