ABSTRACT

If present stream discharge and dissolved load are assumed to be representative of the geologic past, and if the volume of the ocean has remained essentially constant, many problems arise concerning the disposal of the dissolved constituents brought into the ocean by streams. Two of these problems relate to silica and bicarbonate.

The present rate of delivery of dissolved silica to the ocean would produce a much greater volume of sediment than is observed in the geologic column if all this silica were precipitated chemically or biochemically as "free" SiO₂. The bicarbonate ion transported to the ocean must either be recycled through the atmosphere as CO₂, or removed in carbonate sediments. Yet the precipitation of carbonate minerals, with concomitant loss of CO₂ to the atmosphere, leaves about 40 percent of the HCO₃^- unaccounted for!

These two problems can be solved by assuming that an acid--base reaction of the type

H₄SiO₄ + HCO₃^- = H₃SiO₄^- + CO₂ + H₂O,

in which HCO₃^- is converted to CO₂ and recycled through the atmosphere, can account for the excess HCO₃^- and resolve the silica balance. Furthermore, we suggest that SiO₂ and HCO₃^- react with cations and the small, but significant, X-ray amorphous-Al-silicate fraction of the suspended load of streams according to reactions of the type

X-ray amorphous-Al-silicate + HCO₃^- + SiO₂ + cations = cation Al silicate + CO₂ + H₂O.

Reactions of this type may be considered "reverse weathering" and take place before deposition and burial. Chemical synthesis of compounds of the composition of chlorite, illite, and montmorillonite implies that these minerals are in equilibrium, or nearly so, with an aqueous solution of the composition of average sea water. Therefore, the bulk of these clay minerals should not be drastically altered upon entering sea water as allogenic particles.