Irregular BSR: evidence of an on-going reequilibrium of a gas hydrate system

Gas hydrate (GH) systems constitute methane sinks sensitive to environmental changes such as pressure, temperature or salinity. It remains a matter of debate whether the large GH system of the Black Sea has achieved a steady state since the last glacial maximum (LGM). We report on an irregular free gas distribution occurring in specific sediment layers marking an irregular bottom-simulating reflector (BSR). This anomalous free gas distribution revealed by very high resolution seismic images, acquired by a deep-towed multichannel seismic system, might be an evidence of an on-going migration of the base of the GH stability zone (GHSZ). We show that the reequilibrium is not occurring homogeneously as overpressure from hydrates dissociation slows their decomposition in specific sedimentary layers. The Black Sea example highlights that dissociation and the associated methane released in the water column or even in the atmosphere could be largely delayed by overpressure accumulation.