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.
Figures & Tables
This volume represents some of the papers presented at the SEPM Research Symposium GeologicHistory of the Oceans at the Annual Meeting, March 1971, in Houston, Texas. Knowledge of oceanic sediments has been acquired in two ways: 1) directly by sampling and observation, and 2) indirectly through seismic investigations. Until the past decade, direct sampling and observation techniques could only provide information on the surficial materials of the ocean floor. The development of the piston corer has permitted oceanographic vessels to sample the upper 20 meters, and more recently the upper 30 meters, of the ocean floor, but such cores rarely penetrate the Pleistocene and enter older sediments. Until recently, most knowledge of the deeper sedimentary materials in the ocean basins was obtained through seismic reflection studies. The purpose of this volume is to present a number of observations, ideas, interpretations, and speculations which will be of value in considering the meaning of the increasing volume of data from older deep sea deposits.