The GHASS zone was chosen because (1) the Black Sea is considered as the world's most isolated sea (432 000 km²), the largest anoxic water body on the planet and a unique energy-rich sea, (2) the Danube deep-sea fan is one of the largest sediment depositional systems in the world, (3) the continental slope exhibits numerous sedimentary instabilities, (4) the fluid system dynamics is dominated by the occurrence of methane at different physical states (free gas, hydrate and dissolved gas). The analysis of GHASS dataset changed our comprehensive view of the hydrate systems and their impact on the seafloor and the water column.

1- First of all, the acquisition of new seismic and coring data greatly improve our knowledge of the Danube channel/levee system (Martinez-Lamas et al., 2020). The Romanian Margin is composed by a well-developed continental shelf at the Danube mouth (160 km) and a continental slope, incised by several submarine canyons including the Danube Canyon. This last one results from erosive sediment flows which fed the Danube deep-sea fan, mainly during lowstand periods (Constantinescu et al., 2015). At the foot of the continental slope at the end of this canyon, the deposit system, one of the most developed in the world, has a large channel/levee complex. The functioning of this system during the last glacial maximum shows a vertical and horizontal stack of 5 channel/levee complex (approximately 150 m thick each). The timing and forcing mechanism for the initiation and evolution of the upper Danube Deep-Sea Fan, ~32-14.7 ka (Fig. 5; Martinez-Lamas et al., to be submitted). This specific work about the understanding of the establishment of Quaternary sedimentary deposits associated with the activity of the Danube will be used to identify the period of landsliding (work in progress, Fabre et al., in prep.).

Figure 5: Interpretation of a random seismic profile (GHASS and BLASON seismic profiles) showing the basal surface of the recent Danube Channel-Levee complex which is sampled by the core Gas-CS01 studied in Martinez-Lamas et al. (2020).

2- The continental slope morphology of the Romanian area is incised by several landslides inside and outside canyons. It is a complex study area characterized by several sedimentary and fluid related processes such as seafloor erosion and instability, mass wasting, formation of gas hydrates, fluid migration, gas escape, where the imprint of geomorphology seems to dictate the location where gas seep occurs and conversely. We have detected 1409 active seeps within the 1200 km2 of the shelf and slope north-east of the Danube Canyon (Riboulot et al., 2017). The vast majority of the gas seeps (96%) are not randomly distributed in this area (Fig. 6). They occur along canyon flanks, scarps, crest lines, faults and in association with pockmarks and mounds. Moreover, the water depth limit for 98% of the gas seeps coincides with the predicted landward termination of GHSZ. This suggests gas hydrates formed at the base of the GHSZ act as an effective seal preventing gas from reaching the seafloor and the water column (Fig. 7). The extent and the dynamics of GHs have a probable impact on the sedimentary destabilization observed at the seafloor (Fabre et al., in prep).

Figure 6: Bathymetric, dip and geomorphological maps of a sector of the Romanian Margin of the Black Sea, illustrating the presence at the seafloor of sedimentary ripples, scars, pockmarks, and canyons. Are also reported, free gas and BSR areas provided by literature and the landward termination of gas hydrate stability zone.

Figure 7: The 3D view of the seafloor and water column (GLOBE software © Ifremer), with processed polar echograms, shows the crest line zone inside the GHSZ (only 2% of the whole seeps in the study area). The acoustic anomalies recorded in the water column are echoes caused by escaping gas bubbles through the sea?oor. The acoustic imprint of the plumes almost reaches a height of 400 m above the seafloor.

3- The comparison between the hydrate-related seismic reflector called the Bottom Simulating Reflector (BSR) and the numerical simulation of the base of the Gas Hydrate Stability Zone (GHSZ) shows an anomalously deep BSR. By combining velocity geophysical characterization, in-situ measurement dataset and numerical calculation, we demonstrate the way in which the dissociation of gas hydrates since the LGM may modify the local stability conditions. The gas hydrate melting increasing the pore pressure delays the hydrate system to reach a new equilibrium associated with present conditions. The BSR depth, in these regions, is commonly used to assess the geothermal gradient but through the example of the Western Black Sea, we show for the first time that a transient state of the hydrate system prevents assessing the geothermal gradient but can be used instead to determine the excess pore pressure generated by hydrate dissociation (Fig. 8; Ker et al., 2019).

Figure 8: 3D view of a part of the gas hydrate system offshore Romania. The characterization of the gas hydrate system is performed by using an integrated analysis of geophysical data, multibeam seafloor and water column data, 2D high-resolution and very high-resolution pre-stack depth migrated seismic reflection profiles, high-resolution P-wave velocity fields and modelling of the base of hydrate stability. The jointed analysis of high resolution velocity analysis and pre-stack depth migration seismic data is an effective tool to characterize hydrate systems.

4- The SYSIF deep-towed seismic data make it possible to image with an unprecedented resolution the stability zone of the gas hydrates (Marsset et al., 2018, Colin et al., 2020a). The interpretation of this dataset confirms the conclusion of Ker et al., 2019 and report for the first time an irregular free gas distribution in specific sediment layers marking an irregular bottom simulating reflector (Fig. 9). This anomalous free gas distribution might be evidence of an ongoing migration of the base of the GH stability zone (GHSZ). The reequilibrium is not occurring homogeneously as overpressure from hydrate dissociation slows their decomposition in specific sedimentary layers. The Black Sea example highlights that dissociation and the associated methane release in the water column or even in the atmosphere could be largely delayed by overpressure accumulation. Our results show that the destabilization of a hydrate system is actually a slow process, spanning several millennia. As such, a catastrophic destabilization of a gas hydrate system is unlikely (Colin et al., 2020b).

Figure 9: The irregular BSR. a) VHR seismic reflection profile PL01PR12. b) The interpreted seismic profile shows the free gas-GH boundary. We observe several free gas related enhanced reflections (ERs) above the regional trend of the BSR (numbered 1 to 6). c, d) We identify a high-amplitude isolated reflector within the assumed GHOZ (orange reflector). e, f) Close-up views showing the free gas related ERs composed of many diffractions and presenting a patchy aspect.

5- The gas hydrate system is affected by another very major factor. During the Late Quaternary, the Black Sea and the Mediterranean Sea experienced several phases of connection and disconnection with relevant impact on the salinity of the Black Sea oscillating between freshwater lake and salt-water sea conditions. Since the last phase (9000 years ago), the Black Sea communicates with the Mediterranean Sea through the Bosphorus strait. So, the Black Sea which was a freshwater lake, has become a salt-water sea. The salinity of the Black Sea reached its current value of 22 psu at ~2000 yr cal BP (Soulet et al., 2010). The Black Sea salinity is significant enough to appraise the extent of the Gas Hydrate Stability Zone (GHSZ). The salinity changes and its diffusion into the sediment are still active and are today causing the dissociation of gas hydrates over a large area of the Black Sea (Fig. 10; Riboulot et al., 2018).

Figure 10: Simulation of the evolution of the GHSZ over time. The results of the simulation using the two-dimensional numerical model are presented through 4 steps of the calculation: a t = 0 kyr, b t = 5 kyrs, c t = 20 kyrs, and d t > 200 kyrs. The landward termination of the GHSZ evolve over time. Hydrate deposit, the gray area, named "Gas Hydrate Destabilization Zone" (GHDZ) is currently undergoing dissociation caused by salt diffusion through sediments.

6- The results of analyzes of the different datasets also show that:

- Early diagenesis affects shallow sedimentary layers increasing their resistance to undrained shear inducing sediment discontinuity; predisposing factor to slope instabilities (Ballas et al., 2018);

- Diagenesis processes occurring in the sediment are revealed by geochemical analysis on 12 cores. Intense methane seepages and organic matter degradation contribute to the sulfate reduction. In response to seawater intrusion, ion exchange, dissolution and reverse weathering reactions change the composition of clay constituting the sediment and iron is associated with clay minerals (Ruffine et al., 2021).

- Gas hydrates, investigated by gas chromatography and Raman spectroscopy, are sructure sI and contains mainly CH4 and small amounts of N2 and CO2. The Raman analysis reveals the spatial variability in guest's distribution, cage occupancy, and hydrate composition. These results are important in the total budget estimation of methane in the system (Chazallon et al., 2021).

These last results will be integrated in an ongoing geotechnical study to understand their impact on the slope stability.