Gas Hydrates stability evolution in Black Sea offshore Romania since the Last Glacial Maximum and its impact on seafloor stability

Understanding and quantifying the migration of free-gas in hydrate-bearing sediments through time is particularly compulsive along continental margins, where gas hydrate dissociation could have triggered some of the largest submarine landslides observed on Earth. Offshore Romania, high-resolution seismic profiles reveal low reflective or low-velocity zones, which are indicative of free gas, beneath vertical stacked Bottom Simulating Reflectors (BSRs). To further understand the occurrence of double BSRs in the area and the possible effect of gas hydrate dynamics on slope instability and free gas releases, we performed a numerical 2D transient modelling of the evolution of the thermodynamic stability of gas hydrates, integrating in-situ measured physical data and indirect assessments of sea-bottom temperature, thermal conductivity, salinity and sea-level variations. We found that the shallowest BSR matches well with the current Base of the Gas Hydrate Stability Zone (BGHSZ) and the deeper one with the Last Glacial Maximum (LGM) base of GHSZ. The reduction of the GHSZ extension subsequently led to widespread gas hydrate dissociation associated with warming conditions and an increase in Black Sea salinity. However, this dissociation is only responsible of some very superficial submarine landslides (< 30 mbsf and 3 m thick in average) that occurred during this same period. These new constraints improve our understanding of the sliding mechanisms on the Romanian slope that have been ongoing since the LGM and support less catastrophic scenarios than those suggested previously in the case of active gas hydrate dissociation. These results also allow solving the mystery of the double BSR, which here corresponds to a relic of the LGM BGHSZ.