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International audience; Based on swath bathymetry, two-dimensional, high-resolution seismic reflection profiles, and Ocean Drilling Program/Deep Sea Drilling Project (ODP/DSDP) data, we describe a seafloor honeycomb pattern and propose a model for its formation in Pliocene–Miocene carbonate deposited on the uneven oceanic basement of the Carnegie Ridge (offshore Ecuador). Hydrothermal fluids derived from the basement aquifer fractured and dissolved carbonate sediment, creating seafloor pits above basements highs. Fluids expelled along polygonal faults may have assisted the nucleation of seafloor depressions. At the Pliocene-Pleistocene boundary, strong bottom currents scoured previously damaged sediments, enlarging the initial depressions and producing the seafloor honeycomb pattern. This regional erosive episode was contemporaneous with the final closing of the Isthmus of Panama and the clogging of the Ecuador Trench by the subduction of the Carnegie Ridge, so that the honeycomb pattern may be viewed as a regional marker of these two geodynamic events.

International audience; Time-series bathymetric data acquired from 1967 to 2011 are used to evaluate the morphological evolution of the continental shelf and upper continental slope off the city of Nice (SE France, Ligurian Sea). Mapping in water depths of 0–300 m was undertaken to identify the changing morphology of landslide scars and their erosive chutes. Quantitative Digital Terrain Model (DTM) comparisons reveal areas of erosion and deposition over intervals of 5–8 years. Sediment remobilization events on the upper slope (above depths of 200 m) are frequent and significant; landslide scars with volumes > 25,000 m3 can occur with frequencies of < 8 years. Shelf break retreat toward the coastline can reach 60 m over 7–8 years where the continental shelf is wider (> 200 m). Periods of quiescence (1980–1990 and 2006–2011) are seen to alternate with periods when rapid retrogressive failure increase sediment volumes eroded from the upper slope-shelf transition by an order of magnitude (1999–2006). Temporal variations in landslide activity were correlated to several potential triggering factors that individually would not induce failures, including earthquake activity, rapid deposition of fine-grained sediments on a steep slope, and rainfall leading to fresh groundwater circulation below the shelf. This 4D bathymetric study suggests that over the last 50 years the most important factor triggering landslides offshore Nice is overpressure due to freshwater outflows.

Voluminous mass-transport deposits (MTD) have been identified on seismic profiles across the central Nile Deep-Sea Fan (NDSF). The youngest MTDs are buried under 30–100 m of well-stratified slope deposits that, in water depths of 1,800–2,600 m, are characterized by undulating reflectors correlated with slope-parallel seabed ridges and troughs. Seabed imagery shows that, in the western part of the central NDSF, short, arcuate undulations are associated with fluid venting (carbonate pavements, gas flares), while to the east, long, linear undulations have erosional furrows on their downslope flanks and fluid seeps are less common. Sub-bottom profiles suggest that the western undulations correspond to rotated fault-blocks above the buried MTDs, while those in the east are sediment waves generated by gravity flows. We suggest that fluids coming from dewatering of MTDs and/or from deeper layers generate overpressures along the boundary between MTDs and overlying fine-grained sediment, resulting in a slow downslope movement of the sediment cover and formation of tilted blocks separated by faults. Fluids can migrate to the seafloor, leading to the construction of carbonate pavements. Where the sediment cover stabilizes, sediment deposition by gravity flows may continue building sediment waves. These results suggest that complex processes may follow the emplacement of large MTDs, significantly impacting continental-slope evolution.