No abstract

Geological methane, generated by microbial decay and the thermogenic breakdown of organic matter, migrates towards the surface (seabed) to be trapped in reservoirs, sequestered by gas hydrates or escape through natural gas seeps or mud volcanoes (via ebullition). The total annual geological contribution to the atmosphere is estimated as 16-40 Terragrammes (Tg) methane; much of this natural flux is 'fossil' in origin. Emissions are affected by surface conditions (particularly the extent of ice sheets and permafrost), eustatic sea-level and ocean bottom-water temperatures. However, the different reservoirs and pathways are affected in different ways. Consequently, geological sources provide both positive and negative feedback to global warming and global cooling. Gas hydrates are not the only geological contributors to feedback. It is suggested that, together, these geological sources and reservoirs influence the direction and speed of global climate change, and constrain the extremes of climate.

No abstract

After the Last Glacial Maximum, the semi-land-locked Black Sea basin was flooded by warm water from the Mediterranean Sea. This major sea level rise and change of physical water properties had a large impact on the gas hydrate reservoir in the sediments below. Modelling of the regional response of the gas hydrate stability zone (GHSZ) to the Black Sea flooding 7100 years ago shows that a strong effect of near-bottom temperature increase pushes the gas hydrate reservoir to a large shrinking of 15-62% that may release up to 1.1-4.6 Gt of methane. This catastrophic scenario is, however, delayed because of the transient nature of the heat wave propagation. The large-scale reduction of the GHSZ is only to take place within the next thousand years. At present, widespread hydrate dissociation is only expected to occur where there is a minimum water depth for hydrate stability.Postglacial flooding changes in gas hydrate reservoir • J. Poort et al.