The geologic history of sea water—an attempt to solve the problem

Abstract

Thermodynamics and kinetics are both important in the operation of the ocean-atmosphere system. The carbonate chemistry of the oceans can be treated in terms of perturbations of a well-defined equilibrium state. The equilibrium state for silicates in sea water is less well defined, and thermodynamic arguments are much less persuasive. Kinetics are of overwhelming importance in determining—among other parameters—the nitrate concentration of sea water and the pressure of oxygen and nitrogen in the atmosphere. It is likely that a similar mix of attainment and non-attainment of equilibrium prevailed during the entire history of the oceans, and that the sedimentary record must be interpreted accordingly. Marine evaporites are among the most helpful sediments in reconstructing the geologic history of sea water. The relative abundance of minerals in marine evaporites is a complex function of the history of the enclosing basins, but their sequence, and the absence of many minerals typical of non-marine evaporites set rather interesting limits on possible excursions of the composition of sea water since late Precambrian time. It is likely that the concentration of the major constituents of sea water has rarely, if ever, been more than twice or less than half their present concentration during the past 700 million years. The similarity of the mineralogy of sedimentary rocks during the past 3500 million years suggests that the chemistry of sea water has been highly conservative. However, the extensive bedded cherts of the Precambrian, and the frequently associated banded iron formations do indicate a real evolution of the ocean-atmosphere system. They are explained most readily in terms of a higher silica content in Precambrian sea water and a lower oxygen pressure in the atmosphere. Fossil fuel burning will probably introduce some ten times the present content of CO 2 into the atmosphere during the next two centuries. The effects of the additional CO 2 on climate and on the shallow marine biota are presently very difficult to predict. Other man-made chemicals are produced in much smaller quantities than CO 2. However a number of these are so toxic, that their integrated effects are potentially much more damaging than those of fossil fuel burning.