How is a Brine Pool/Lake Maintained at Seafloor?

Highly saline brine pools or lakes have been found at seafloor, mostly associated with a sharp interface of fluid density at their top and often accompanied by high gas concentration of overlain water column and mud/gas venting from below. The occurrence of such brine layer between the seafloor and the overlain seawater may be the result of a vigorous double-diffusive convection within it. This double-diffusive convection is both promoted by a heating from below and suppressed by an upward-decreasing salinity gradient across it. The stability of a hydrostatic brine layer can be analyzed using the Rayleigh number modified by including the salinity effect. The maximum salinity difference is limited by the temperature difference across the brine layer, which supports the occurrence of multiple double-diffusive convection layers in larger brine lakes. Many researchers have suggested that the source of the brine is the salt dissolved from underlain salt dome and carried upwards by convection fluids. However, it is more difficult for fluid convection to occur below than above the seafloor due to both the resistance to fluid flow by and the enhanced salinity stabilization in porous medium. The maximum salinity gradient associated with such convection is further limited to a small number, which makes such convection almost impossible to occur near a salt dome, contrary to what most people believe. Consequently, the salt accumulated in a brine pool or lake must be carried up from below by a different mechanism, most likely an upward gas flow. This explains the fact that most seafloor brine pools/lakes are associated with overlain water column with a high gas (mostly methane) concentration. A possible source of the gas may be related to dissociating natural gas hydrates in depth. Another question is how the sharp interface between a brine pool/lake and the overlain water column is maintained. Without such a mechanism the interface, even if existed earlier, will be smoothed out gradually. A possible mechanism is that the gas-bearing fluid produces gas hydrate as it approaches the seafloor and enters into brine pool/lake. The less dense gas hydrate keeps moving upwards and eventually dissociates near the pool/lake surface. This gas hydrate formation/dissociation process acts to continuously extract fresh water from the brine pool/lake then release it at the surface and thus to maintain the sharp interface. This mechanism may also contribute to or be solely responsible for the accumulation of salt in a brine pool/lake.