Relationship between exchangeable cations and palaeosalinity

In Carboniferous shales a relationship has been found between exchangeable cations and palaeosalinity. In the marine shales the concentration of exchangeable Mg is higher than in shales deposited in fresh and brackish water, and the concentrations of exchangeable Ca, Na and K are lower. The same relationships have been observed in modern sediments. When the cation concentrations in the whole rock are compared with the exchangeable cation concentrations, incorporation into the sediment of K and possibly Mg in the marine environment is indicated. Furthermore, the loss of Ca and Na from the sediment can be accounted for by the loss from exchange sites. Reactions taking place during halmyrolysis would appear to be more important than those taking place during diagenesis.