Physiographic influences on dense shelf-water cascading down the Antarctic continental slope

Predicting the source areas for Antarctic Bottom Water (AABW) requires knowledge of how cold, dense water masses form and move from the Antarctic shelves to the continental slope. Here we use a review of nearly 50 years of direct hydrographic observations to infer the main broad-scale influences on the distribution of dense shelf-water (DSW) overflows that cascade down the continental slope around Antarctica. The dynamics and distribution of large ice shelves, coastal polynyas and the physiography of the Antarctic continental shelves are each considered. The catalogue we present increases the number of DSW observations to 27, adds 20 additional stations where this process is likely to have occurred, and identifies 41 areas where DSW appears to be absent. Our pan-Antarctic, multi-decadal review enhances the understanding of the formation and export of DSW and highlights the variability and complexity of ice-ocean systems on high-latitude continental margins. The study also provides a context for understanding recent episodes of Antarctic ice-shelf instability, and how the relationship between DSW and AABW may evolve with climatic and oceanographic changes

An experimental approach to submarine canyon evolution

We present results from a sandbox experiment designed to investigate how sediment gravity flows form and shape submarine canyons. In the experiment, unconfined saline gravity flows were released onto an inclined sand bed bounded on the downstream end by a movable floor that was used to increase relief during the experiment. In areas unaffected by the flows, we observed featureless, angle-of-repose submarine slopes formed by retrogressive breaching processes. In contrast, areas influenced by gravity flows cascading across the shelf break were deeply incised by submarine canyons with well-developed channel networks. Normalized canyon long profiles extracted from successive high-resolution digital elevation models collapse to a single profile when referenced to the migrating shelf-slope break, indicating self-similar growth in the relief defined by the canyon and intercanyon profiles. Although our experimental approach is simple, the resulting canyon morphology and behavior appear similar in several important respects to that observed in the field

Imaging the seascapes of the Mediterranean

The Mediterranean Sea is a relative newcomer to Earth"s landscape. Due to its complex tectonic history, this mid-latitude sea is composed of a cluster of basins. Their seascape is in most cases dominated by geologically young structures, but also by sedimentary processes. Among the latter, sedimentary processes related to the dynamics of the largest rivers in the Mediterranean (Ebro, Rhône, Po, Danube, and Nile) stand out. This overview article illustrates the main sedimentary processes and their products contributing to shape the Mediterranean seascape within a source-tosink approach. To highlight this approach, this article mainly focuses on one of the EUROSTRATAFORM project study areas: the northwestern Mediterranean

Bathymetric extent of recent trawl damage to the seabed captured by an ROV transect in the Alboran Sea

Bottom trawl fishing is among the most destructive anthropogenic pressures acting on benthic ecosystems, but the full extent of the damage is undocumented because of the limited number of deep-sea observations of impacted regions (e.g., Brennan et al., 2012, 2016). As part of its continuing ocean exploration mission, in 2011, E/V Nautilus conducted a remotely operated vehicle (ROV) survey along a transect in a submarine canyon in the Mediterranean's Alboran Sea off southern Spain at depths ranging from 1,200 m to <300 m (Coleman et al., 2012). This exploration along the South Alboran Ridge offered the opportunity to directly observe with video the bathymetric extent and intensity of recent trawling damage to the seafloor in this area. This dive revealed large furrows running in multiple directions caused by trawl doors scraping across the seabed. Little biological activity was evident in the depth ranges where these scars were observed. The destructive nature of bottom trawl fishing should be viewed with the same public affront as subaerial clear-cutting of forests and strip-mining. The only difference is that the ocean hides trawl damage from the public eye. The more we explore the deep sea, repeatedly map the seafloor with sonar, and observe the seabed and its ecosystems with video captured by ROVs, the greater we can understand the full impacts of trawling

Evolution of submarine canyon-fan systems in fault-controlled margins: Insights from physical experiments

Different fault settings make the morphology of submarine canyon-fan systems on active margins complex and diverse. In this study we explore the continuum of erosion, transport and sedimentation processes taking place in fault-controlled canyon-fan systems by using physical experiments and a morphodynamic model. Based on morphometric analyses we show how Hack's scaling relationships exist in submarine canyons and fans. The DEM of differences (DoDs) demonstrate the growth patterns and allow to establish relevant relationships between volumes of canyons and their corresponding fans. We reveal strong self-similarities on canyon-fan long profiles and, through a new morphodynamic model, we capture their evolution over time, including the trajectory of internal moving boundaries. We observe that fault slip rate controls the merging speed of coalescent submarine canyon-fan systems and, when coupling fault slip rate with inflow discharge, a competitive influence arises. In this study we also uncover scaling relationships spanned from laboratory to field-scale. Overall, our findings are inspiring and valuable for field investigators and modelers to better interpret and predict the morphological evolution and sedimentary processes of submarine canyon-fan systems in active fault settings

High-Performance Compression of Multibeam Echosounders Water Column Data

Over the last few decades, multibeam echosounders (MBES) have become the dominant technique to efficiently and accurately map the seafloor. They now allow to collect water column acoustic images along with the bathymetry, which is providing a wealth of new possibilities in oceans exploration. However, water column imagery generates vast amounts of data that poses obvious logistic, economic, and technical challenges. Surprisingly, very few studies have addressed this problem by providing efficient lossless or lossy data compression solutions. Currently, the available options are only lossless, providing low compression ratios at low speeds. In this paper, we adapt a data compression algorithm, the Fully Adaptive Prediction Error Coder (FAPEC), which was created to offer outstanding performance under the strong requirements of space data transmission. We have added to this entropy coder a specific pre-processing stage tailored to theKongsbergMaritime water column file formats. Here, we test it on data acquired with Kongsberg MBES models EM302, EM710, andEM2040.With this bespoke pre-processing, FAPEC provides good lossless compression ratios at high speeds, whereas lossy ratios reach water column file sizes even smaller than bathymetry raw files still with good image quality. We show the advantages over other lossless compression solutions, both in terms of compression ratios and speed.We illustrate the quality of water column images after lossy FAPEC compression, as well as its resilience to datagram errors and its potential for automatic detection of water column targets. We also show the successful integration in ARM microprocessors (like those used by smartphones and also by autonomous underwater vehicles), which provides a real-time solution for MBES water column data compression

Transient erosion in the Valencia Trough turbidite systems, NW Mediterranean Basin

Submarine canyons can efficiently drain sediments from continental margins just as river systems do in subaerial catchments. Like in river systems, submarine canyons are often arranged as complex drainage networks that evolve from patterns of erosion and deposition. In the present paper we use a morphometric analysis of submarine canyon-channel long-profiles to study the recent sedimentary history of the Valencia Trough turbidite system (VTTS) in the NW Mediterranean Sea. The VTTS is unique in that it drains sediment from margins with contrasting morphologies through a single "trunk" conduit, the Valencia Channel. The Valencia Channel has been active since the late Miocene, evolving in response to Plio-Quaternary episodes of erosion and deposition. The integrated analysis of long-profiles obtained from high-resolution bathymetric data across the entire turbidite system shows evidence for transient canyon incision in the form of knickpoints and hanging tributaries. Multiple factors appear to have triggered these periods of incision. These include a large debris flow at 11,500 yr BP that disrupted the upper reaches of the VTTS and glacio-eustatic lowstands that forced shifting of sediment input to the VTTS. Based on these inferences, long-term time-averaged incision rates for the Valencia Channel have been estimated. The evidence we present strongly suggests that Foix Canyon has played a key role in the drainage dynamics of the VTTS in the past. This study builds conceptually on a recent modeling study that provides a morphodynamic explanation for the long-term evolution of submarine canyon thalweg profiles. The procedure and results from this work are of potential application to other submarine sediment drainage systems, past and present, including those containing mid-ocean type valleys like the Valencia Channel

On the termination of deep-sea fan channels: Examples from the Rhone Fan (Gulf of Lion, Western Mediterranean Sea)

The termination of a deep-sea turbiditic channel represents the ultimate sink of terrigenous sediment in the oceans or lakes. Such environment is characterized by rapid slope decrease and by loss of confinement of turbidity currents. It results in the deposition of Channel-Mouth-Lobes that can be separated from the channel mouth by an erosional (scoured) or by-pass dominated Channel-Lobe Transition Zone. Several factors can control the occurrence, extent and morphologic expression of the area such as the slope break angle, the upslope and downslope angle and the mud/sand ratio in flows. Disentangling these factors remains challenging due to the scarcity of outcrops and to the usual faint morphologies and low thickness of deposits. With bathymetric and seismic data we calculated the morphometric parameters of 8 channel-levees and their Channel-Mouth Lobes from the deepest area of the Rhone fan, a mud-sand rich system, and among which the youngest one (called neofan) was deposited at the end of the Last Glacial Maximum between 21.5 and 18.3 ka cal. BP. Emplacement and shape (finger-shaped or pear-shaped bulges) of Channel-Mouth Lobes is controlled by the seabed morphology (adjacent channel-levees and salt diapirs). A less prominent morphology of the neofan is attributed to premature quiescence related to the post sea-level rise sediment starvation. We show that the occurrence and expression of a Channel-Lobe Transition Zone is controlled by the gradient upstream of the channel mouth slope break. The extended Channel-Lobe Transition Zone and detached lobe of the neofan are attributed to the high upslope gradient (0.26°) while the less detached or attached lobes of other channel-levees is attributed to lower upslope gradient (0.13°). We show that scouring and scours concatenation into flutes at the Channel-Lobe Transition Zone is a major driver for the inception of channels and further confinement of turbidity current. For the first time we show that concatenation of scours in shingled disposition developed an incipient channel sinuosity at this very early stage of channel development. The channel-levee can extend downslope nearly instantaneously by tens of kilometers when isolated nascent channels connect to the channel mouth

A review of undulated sediment features on Mediterranean prodeltas: distinguishing sediment transport structures from sediment deformation

Most Mediterranean prodeltas show undulated sediment features on the foresets of their Holocene wedges. These features have been described all along the Mediterranean for the last 30 years and interpreted as either soft sediment deformation and incipient landsliding, and more recently, as sediment transport structures. We perform a review and detailed analysis of these undulated sediment features using ultrahigh-resolution seismic and bathymetric data as well as geotechnical information and hydrodynamic time series and hydrographic transects. In this study we show that the characteristics of the sediment undulations (configuration of the reflections down section and between adjacent undulations and overall morphologic characteristics) are incompatible with a genesis by sediment deformation alone and do not show evidence of sediment deformation in most cases. Various processes in the benthic boundary layer can be invoked to explain the variety of features observed in the numerous areas displaying sediment undulations

Ploughing the deep sea floor

Bottom trawling is a non-selective commercial fishing technique whereby heavy nets and gear are pulled along the sea floor. The direct impact of this technique on fish populations1,2 and benthic communities3,4 has received much attention, but trawling can also modify the physical properties of seafloor sediments, water-sediment chemical exchanges and sediment fluxes5,6. Most of the studies addressing the physical disturbances of trawl gear on the seabed have been undertaken in coastal and shelf environments7,8, however, where the capacity of trawling to modify the seafloor morphology coexists with high-energy natural processes driving sediment erosion, transport and deposition9. Here we show that on upper continental slopes, the reworking of the deep sea floor by trawling gradually modifies the shape of the submarine landscape over large spatial scales. We found that trawling-induced sediment displacement and removal from fishing grounds causes the morphology of the deep sea floor to become smoother over time, reducing its original complexity as shown by high-resolution seafloor relief maps. Our results suggest that in recent decades, following the industrialization of fishing fleets, bottom trawling has become an important driver of deep seascape evolution. Given the global dimension of this type of fishery, we anticipate that the morphology of the upper continental slope in many parts of the world's oceans could be altered by intensive bottom trawling, producing comparable effects on the deep sea floor to those generated by agricultural ploughing on land