Freshwater lake to salt-water sea causing widespread hydrate dissociation in the Black Sea

Abstract

Gas hydrates, a solid established by water and gas molecules, are widespread along the continental margins of the world. Their dynamics have mainly been regarded through the lens of temperature-pressure conditions. A fluctuation in one of these parameters may cause destabilization of gas hydrate-bearing sediments below the seafloor with implications in ocean acidification and eventually in global warming. Here we show throughout an example of the Black Sea, the world’s most isolated sea, evidence that extensive gas hydrate dissociation may occur in the future due to recent salinity changes of the sea water. Recent and forthcoming salt diffusion within the sediment will destabilize gas hydrates by reducing the extension and thickness of their thermodynamic stability zone in a region covering at least 2800 square kilometers which focus seepages at the observed sites. We suspect this process to occur in other world regions (e.g., Caspian Sea, Sea of Marmara).

Anomalously Deep BSR Related to a Transient State of the Gas Hydrate System in the Western Black Sea

“…For the seismic profile HR31a, the range of variation is narrower (17.5–21.5 °C/km) with an occurrence peak at 20.5 °C/km. These inversion results indicate that a geothermal gradient value of ~20 °C/km could enable a better fit with the current depth of the observed BSR compared to the one of 24.5 ± 0.2 °C/km used for the modeling and derived from in situ measurements (Riboulot et al, ). However, a geothermal gradient of 20 °C/km would make the observed BSR above the predicted steady state BGHS in numerous locations on both seismic profiles.…”

“…During the GHASS cruise, in situ measurements and cores were collected. Geochemical measurements performed on gas hydrate samples, which were collected for the first time in the Romanian sector of the Black Sea, confirmed that the gas entrapped is biogenic methane with a concentration of 99.6% (Riboulot et al, ), and the remaining 0.4% is mainly N 2 . Pore water analyses performed on eight cores revealed a gradual fall in salinity from 22 psu at the seafloor level to near 2 psu at around 25 mbsf.…”

“…In Figure , we propose a schematic hydrate system view to represent the main observed features offshore the western Black Sea and the mechanisms controlling its evolution. The hydrate system is characterized by a dynamic transient regime at the top of the GHSZ related to change in salinity of sediment pore water since the reconnection between the Mediterranean and Black Seas, which generates a migration of the landward termination of the GHSZ (Riboulot et al, ). The GHSZ also migrates upward in response to past temperature and sea level changes since the LGM (Figure ), a mechanism highlighted by Ruppel and Kessler ().…”

“…Geochemistry, Geophysics, Geosystems measurements (Riboulot et al, 2018). However, a geothermal gradient of 20°C/km would make the observed BSR above the predicted steady state BGHS in numerous locations on both seismic profiles.…”

“…Consequently, the presence of hydrate seems to prevent any free gas migration into the sediments and the water column. It has to be highlighted that a large number of seeps observed by Riboulot et al (2018) are located close to the predicted landward termination of the gas hydrate stability zone (GHSZ) and where hydrates could be currently under destabilization related to the salt diffusion within the sediment (Riboulot et al, 2018).…”