Early Evolution of the Oceans—A Weathering Model
The long-term chemical composition of sea water is controlled by the generalized reaction: primary igneous rock minerals + water + acid volatiles = sediments + oceans + atmosphere. Unstable crustal minerals are weathered by water and acid volatiles, and local equilibrium between the products of the reaction—oceanic sediments; sea water and the atmosphere—is closely approached.
To obtain a better picture of the evolution of the oceans as this reaction proceeds (minerals formed, mass transfers involved, changes in sea water, composition), we simulated with a model calculation on a high-speed computer the irreversible attack of “average igneous rock” by water and acid volatiles. We assumed a single-stage degassing process under reducing conditions at 25°C and 1 atm. The predicted final solid products ranked according to decreasing mass are clays and amorphous silica (= chert in the geologic record), then feldspars and carbonates. The predicted composition of the early ocean resembles that of present sea water except that (1) the dissolved sulfur is in reduced form, (2) the solution is saturated with amorphous silica, and (3) the salinity is about twice that of today because of non-removal of NaCl in evaporites.
Extension of these results to more realistic systems can at best be semiquantitative because of lack of sufficient thermochemical data. Furthermore, the recycling of sediments makes it very difficult to estimate early environmental conditions from present remnants of Precambrian sediments. Some generalizations can nevertheless be made with confidence.
A more basic initial crustal material such as oceanic basalt would lead to larger amounts of clays and carbonates in the sediments at the expense of chert and to a large concentration of dissolved ferrous iron in the ocean. Degassing of water preferentially to other volatiles would not affect the outcome of the weathering process unless the escape rates of the volatiles differed by several orders of magnitude. Although our model clearly represents an extreme case, rapid degassing, the available geologic evidence does not preclude its having taken place.