Chemical History of the Oceans Deduced from Postdepositional Changes in Sedimentary Rocks
The materials entering the ocean today are similar in their heterogeneity, chemical and mineralogic composition, and rates of addition to those of the past two billion years of Earth history. Major differences between modern sediments and those of the past are largely the result of post-depositional changes.
Trends in the chemistry and mineralogy of shales with geologic age are shown to be similar to well-documented, short-term, post-depositional changes in Gulf Coast shales undergoing burial diagenesis. Mixed-layer clays are converted to illite, and Ca, Mg, Na, and Si are lost from the sediment system but K is fixed in both cases. The Ca, Na, and Si enter the ground water system and eventually return to the dissolved load of streams whereas Mg is transferred to carbonate rocks.
Sediments initially contain a variety of phases, many of which are lost during diagenesis as internal equilibrium is approached in the rocks. The rate of diagenetic change increases with increasing temperatures and pressures of burial; on the average 200-250 million years are required for compositional stabilization.
Because the particulate and dissolved feed of streams to the ocean has remained practically constant when on a long-term basis, it is likely that seawater composition of today is much like that of at least one-half of geologic time. Owing to their long residence times and significant oceanic masses, chloride, sulfate, sodium, and miagnesium, may have varied in oceanic concentration in the past by 10% or more from their present values. No, data, however, are available to prove such changes have occurred.