POSSIBLE RANGE OF OCEAN SALINITIES IN THE PHANEROZOIC

A series of models using different assumptions all support the idea that the salinity of the ocean has decreased during the Phanerozoic. The most conservative component of ocean salinity is chlorine, which, expressed in moles, accounts for 94.5% of the anionic content of seawater and occurs only rarely in minerals. The other significant anionic component is sulfate, which makes up 4.9% of the molar content today but is difficult to trace because it can leave the ocean in a variety of forms. The models take into account the amounts of existing salt deposits, the saline water trapped in pore space in sediments, and sediment recycling. Recyling rates for different tectonic settings are used to reconstruct the original size of salt deposits and pore space volumes, and to account for the return of salts to the ocean. The greatest uncertainties lie in estimations of the amounts of salt deposited in the Gulf of Mexico and in the North and South Atlantic during the breakup of Pangaea. The lowest plausible estimate suggests ocean salinities were about 43 (g/kg) for most of the Paleozoic, declining to the modern value of 34.7 g/kg during the Mesozoic and Cenozoic. The highest estimates suggest salinities as high as 60 g/kg in the Cambrian. The most probable values are betwee 48 and 52 g/kg in the Early Paleozoic, with step-wise sharp declines in the Mesozoic. At these elevated salinities the temperature-salinity-density relations for seawater were different from today. In particular, temperature chages near the freezing point would have had a much greater effect on density than they do in the modern ocean.