Role of gravity, temperature gradients, and ion exchange media in the formation of fossil brines

Calculations show that gravitational settling of ions in an isothermal sediment column could produce increases of equilibrium concentrations in pore waters ranging from 1 percent per 100 m depth for chloride to 4 percent per 100 m depth for strontium.

The migration of ions in a thermal gradient (Soret effect) would cause minor salt enrichment upward toward the colder pole, but the presence of cation-exchanging particles such as clays would reverse this tendency and cause pumping of salt downward. A model calculation using literature data for the thermal potentials suggests that about 5-percent enrichment in Cl per 100 m depth may occur under steady-state conditions.

These mechanisms do not explain the greater enrichments commonly found in subsurface brines, but may modify salt distributions due to other phenomena.