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5th International Network for Submarine Canyon Investigation and Scientific Exchange International Symposium (INCISE), 14-18 June 2021, Bottom trawling is one of the most widespread fishing practices in the world’s oceans. It involves towing of nets to harvest benthic and demersal living resources. The dragging of trawling gears along the seafloor results in scraping and ploughing the seabed, which leads to the formation of turbid plumes of resuspended sediments, changes in the sediment erosion/accumulation rates and modifications of their fluxes and budgets, which results in measurable alterations of the submarine geomorphology. As submarine canyons are increasingly targeted by trawlers, there is a growing need to quantify, monitor and mitigate the impacts of bottom trawling in these environments. The TrawledSeas Project aims to quantitatively characterise the contribution of bottom trawling on the geomorphic evolution of submarine canyons, over a range of spatial scales, from fine (m–dam) to mesoscale (5–100 km). To address this objective, a new automated marine landscape mapping technique is being developed to quantify the morphological signature of bottom trawling, based on the analysis of high-resolution multibeam data implemented in a Geographic Information System (GIS). The proposed methodology integrates standard general (e.g. curvature, rugosity, roughness or fractal dimension) and specific (e.g. object-based image methods) geomorphic techniques with new ones developed in this project in a multiscale approach. It combines GIS open source tools with bathymetric dataset at different resolutions, from hull-mounted multibeam data to compare the large-scale morphology of trawled and untrawled areas, to Remotely Operated Vehicles (ROV) and autonomous underwater vehicle (AUV) bathymetric data to identify and quantify trawl marks at small spatial scale. Additionally, data from repeated surveys will be used to assess potential temporal changes in the seafloor morphology of new fishing grounds. The implementation of these geomorphological tools in different study sites incised by submarine canyons (e.g. Catalan, Malta-Sicilian, Norwegian, Canterbury, Patagonian and W Canadian continental margins), will allow to characterise the differential impact of bottom trawling on the canyons’ seafloor, in terms of extent, rates and volume change in different geologic and climatic settings

Submarine landslides are often characterized by a basal surface of rupture parallel to the stratigraphy, in which downslope movement is initiated. However, little is known about the sedimentology and physical properties of the sediments within these surfaces. In this study, we present a multiproxy analysis of the sediments collected from a giant piston core penetrating a shallow submarine mass transport deposit, in combination with high-resolution seismoacoustic data to identify and characterize the basal glide plane and the weaker sediments in which movement was initiated. The initial phase of instability consists of a single fracture that formed due to the downslope movement of a mostly intact slab of sediments. The 16m long core, comprising mostly undisturbed massive and laminated ice-rafted debris-rich clay penetrated this slab. The base of the slab is characterized by a high-amplitude semicontinuous reflection visible on the subbottom profiler data at about 12.5m depth, interpreted to originate from the glide plane on top of a plumite deposit. This plumite has dilative behavior with pore pressure decrease with increasing shear strain and high undrained shear strength. Movement probably started within contouritic sediments immediately above the glide plane, characterized by higher sensitivities and higher water contents. The occurrence of the mass movements documented in this study are likely affected by the presence of a submarine landslide complex directly downslope. The slide scar of this landslide complex promoted retrogressive movement farther upslope and progressive spreading of strain softening along the slide base and in the slide mass. Numerical models (infinite slope, BING, and retrogressive slope models) illustrate that the present-day continental slope is essentially stable and allow reconstruction of the failure processes when initiated by an external trigger.

Submarine tailings placements (STPs) is an alternative practice of handling mine waste, which involves deposition of tailings into submarine environments. However, the environmental impacts of STPs remain poorly understood. This thesis is a contribution to improve the understanding of how submarine tailings placements interact with natural sediments. It is based on multi-proxy analyses of 10 sediment cores from three northern Norwegian coastal areas with submarine tailings histories: the fjords Bøkfjorden and Ranfjorden, as well as the sound Stjernsundet. The tailings in Bøkfjorden and Ranfjorden are associated with extraction of iron ore, whereas Stjernsundet is associated with extraction of nepheline syenite. The applied proxies included physical properties (e.g. magnetic susceptibility), geochemical properties (qualitative and quantitative element geochemistry), bulk mineral assemblages and lithological analyses. Three depositional environments are distinguished: a natural depositional environment (endmember 1), tailings depositional environment (endmember 2) and transitional depositional environment (mix of endmember 1 & 2). Endmember 1 is defined by alternating grainsize, low magnetic susceptibility and stable geochemistry. Endmember 2 is defined by fine-grained sediments, high magnetic susceptibility and elevated levels of tailings associated elements. Transitions are defined by gradual increase (tailings-dominance) or decrease (natural dominance) in grainsize, magnetic susceptibility and tailings associated elements. The relation and composition of the identified depositional environments reflect either gradual or abrupt changes in sediment supply, sedimentary and depositional processes. Ranfjorden and Bøkfjorden show a large lateral extent of tailings (> 15 km) due to a high sediment input of fine-grained tailings, bed load transportation and associated submarine meandering channel systems. Stjernsundet shows a limited lateral extent of tailings (< 2 km) due to a low sediment input of relatively coarse-grained tailings. The analyzed material from Ranfjorden and Stjernsundet reflects a gradual evolving dominance of tailings, whereas the material from Bøkfjorden reflect an abrupt evolving dominance of tailings.