A large proportion of deep sea water is undersaturated with respect to both calcite and aragonite. This conclusion is based upon laboratory studies of the. effects of temperature and pressure on carbonate equilibria in sea water combined with shipboard measurements of water properties as well as upon actual measurements of dissolution rates of calcium carbonate samples held at various depths. Undersaturation is caused by biological production of CO2 at shallower depths and by the effect of increased pressure on the solubility of CaCO, at greater depths. The so-called compensation depth below which CaCO3 disappears from deep sea sediments does not simply represent a downward change from supersaturated to undersaturated water. This is proven by the presence of undersaturated water above the compensation depth and by the fact that the compensation depth may be located thousands of meters below depths where CaCO<sub>3</sub> shows definite signs of dissolution.
About 1000 metres above the compensation depth, the rate of dissolution shows an abrupt downward increase. This zone, called the lysocline, has been predicted from laboratory measurements of dissolution rate as a function of the degree of undersaturation of sea water. The sudden rate increase can be explained by a change in the mechanism of dissolution at the surface of calcite. Dissolution at all depths is too slow to be controlled by transport of material from particle surfaces by either diffusion, turbulence, or changes in water velocity. This is due to the inhibiting effects of ions and/or organic molecules adsorbed onto the carbonate surfaces from the surrounding sea water.
Surface sea water, in contrast to deep water, is generally supersaturated with respect to both calcite and aragonite. However, inorganic precipitation rarely occurs due to the inhibiting effects of Mg++, phosphate, organic matter and possibly other substances both as dissolved species and as adsorbed coatings on mineral grains.
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.