Morphology of pockmark-like features relative to the methane hydrate stability zone on the central Nile deep-sea fan

The expulsion of gas-rich fluids from submarine sedimentary accumulations may result in the formation of seafloor depressions, or pockmarks, of metric to kilometric size. Methane flux drives biogeochemical processes favouring the precipitation of authigenic carbonates, which over time can form seafloor pavements of high acoustic reflectivity. In deep waters, it has been proposed that seafloor morphology may be influenced by gas hydrate formation and dissolution to form depressions of complex internal relief, referred to as ‘gas hydrate pockmarks’. In contrast, seafloor vents of positive relief are typically assumed to record sediment expulsion as mud volcanoes. The central province of the Nile fan, which contains evidence of a gas hydrate system, provides an interesting setting to study the morphology of seafloor fluid vents : in addition to a dozen mud volcanoes (kilometric widths), it contains hundreds of smaller (decametric widths) sub-circular high-backscatter features that have been shown to correspond to fractured carbonate pavements. Originally referred to as pockmarks, many of these features have been found to be of metric-scale positive relief. Here we present a morphometric analysis of pockmark-like features across the central Nile fan using available multibeam sonar and 3D seismic seafloor datasets. Seafloor morphologies were captured for analysis using a semi-automated training approach adapted to data types : multibeam data (20-25 m grids of bathymetry and backscatter) were used to capture high backscatter patches across an 1135 km² area of the mid- to lower slope (water depths 1525-2395 m); 3D seismic seafloor data (8 m grid) were used to capture sub-circular features (of +ve or -ve relief) across a 3275 km² area of the upper slope (water depths 137-1655 m). Water column data indicate the upper limit of the methane hydrate stability zone (MHSZ) to lie in depths of 1230 ± 25 m. We identify a total of 1309 pockmark-like features in water depths of 189-2382 m, comprising three main morphotypes : negative relief (depressions, 70%), mixed relief (complex or flat, 18%) and positive relief (domes, 12%). Their depth distribution shows a striking relationship with the MHSZ limit : of 971 features above the MHSZ, almost all (93%) are depressions, with widths of 58-408 m and depths up to 20 m; in contrast, of 338 features within the MHSZ, almost all are of positive or mixed relief (43% and 50% respectively), with widths of 54-790 m and relief up to 20 m, while only 7% are depressions. We suggest pockmark-like features within the MHSZ to be carbonate pavements formed above gas hydrate pockmarks, their domal or mixed relief and fractured character reflecting the evolution of near-surface gas hydrate lenses. Depressions above the MHSZ are pockmarks uninfluenced by gas hydrate dynamics. Interestingly, almost all features within the MHSZ lie outwith an area of bottom simulating reflection (BSR) patches indicating the presence of gas and/or gas hydrates at depth. Our findings suggest a key role of gas hydrate dynamics on the morphology of deep-water fluid vents. This study is a contribution of the MEGA project (ANR-22-CE01-0031).