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.
The Dnepr palaeo-delta on the north-western continental slope of the Black Sea is a prolific seepage zone characterised by a highly variable heat flow ranging from 24 to 88 mW/m(2). New thermal data were collected at 33 closely spaced stations in order to better understand the apparent relation between heat flow variability and seepage features. Strong relief gradients and associated landslide shaping may explain first-order heat flow variability but, locally, thermal parameters appear to be controlled by fluid and gas migrations. High heat flow anomalies are found at sites where faults and diapirs offer pathways for warm fluid flow from deeper sedimentary levels. Low heat flow is most strongly expressed at ridge crests near seepage sites but is also found where seeps are absent. Reduced heat flow and nonlinear temperature-depth variations are interpreted to result from natural or induced gas ebullition of saturated shallow gas covered by a thin, relatively impermeable sedimentary seal. The presence of sealed gas pockets, in particular at ridge crests, is supported by methane pore-water analysis and a shallow gas front widely observed in the study area
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