Relationship between water-soluble cations and palaeosalinity

Finely ground shale samples from a Carboniferous borehole sequence were shaken in water and the concentration of the water-soluble Na, K, Ca and Mg determined. The marine shales in the sequence were characterized by low concentrations of Na and K, and high concentrations of Ca and Mg. The reverse situation was found for the non-marine and brackish shales. The electrical conductivity of the water extract was higher for the marine shales than for the non-marine/brackish shales. It is suggested that the water-soluble cations were present in the sediment at the time of deposition as exchangeable cations which were released into the pore water during diagenesis as some of the exchange sites were eliminated.