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.

天然气水合物（Gas hydrate, GH）系统是对压力、温度或盐度等环境变化敏感的甲烷汇。黑海大型天然气水合物系统自末次冰盛期（Last Glacial Maximum, LGM）以来是否达到稳态，目前仍存在争议。我们报道了一种出现在特定沉积层中的不规则游离气分布，该分布对应一处不规则的海底模拟反射层（bottom-simulating reflector, BSR）。这套由深拖多道地震系统获取的超高分辨率地震图像所揭示的异常游离气分布，或可作为天然气水合物稳定带底界（Gas Hydrate Stability Zone, GHSZ）正在发生迁移的证据。研究表明，水合物分解产生的超压会减缓特定沉积层中水合物的分解过程，因此水合物再平衡并非均匀发生。黑海实例表明，水合物分解及其向水柱甚至大气中释放的甲烷，很大程度上会因超压的积累而被大幅延迟。