Paleosalinity is the measure of salt in an ancient aquatic system.  It is recorded in terms of Å (parts per thousand, or per mil) (Corfield and Norris, 1998).  Paleosalinity can be calculated from measurements of strontium isotope ratios within an environment and comparison with the ocean ratio   This is possible because the oceans have a homogeneous strontium isotope ratio. Ocean strontium ratios are relatively well known for the Phanerozoic (Schmitz et al., 1997).  Any variation in measured values from ocean values are the result of mixing ocean and freshwater (Reinhardt et al., 1998).

The Phanerozoic marine Strontium curve. Plotted on the y-axis is ⁸⁷Sr/⁸⁶Sr, and time is plotted on the x-axis in millions of years.    Diagram from Dicken, (1997).

This freshwater system has a much lower strontium concentration than ocean it will eventually empty into.

     Freshwater ratios reflect the continental crust from which the strontium has been released, (Ingram and Sloan, 1992) and can be higher or lower than oceanic values (Schmitz et al., 1997.)  One problem with this is that strontium concentrations are higher in ocean waters than in freshwater systems; therefore a considerable amount of freshwater must be added to an ocean system to affect the measured isotope ratio (Reinhardt et al., 1998).

   Strontium isotope values are typically measured from biogenic calcite and apatite.  For these values to be useful the organism must incorporate the strontium  ratio of the water in which it precipitated its hard parts without altering it (Koch et al., 1992; Schmitz et al., 1997; Reinhardt, 1997).   This value must survive through diagenesis unaltered (Schmitz et al., 1997), and should not be altered by the metabolic processes of the organism (Reinhardt et al., 1998). 

These brachiopods will have a strontium ratio incorporated into their shells, whether their ratio is indicative of their depositional environment is questionable.  Photo from Thompson, (1997).