Physical Chemistry of Carbonates in Oceans: ABSTRACT

The common carbonate minerals found in oceanic sediments are aragonite and high and low magnesium varieties of calcite. A large proportion of deep seawater is undersaturated with respect to all three. This conclusion is based on laboratory studies of the effects of temperature and pressure on carbonate equilibria in seawater combined with shipboard measurements of water properties as well as upon actual measurements of dissolution rates of calcium carbonate samples held at various depth. Undersaturation is caused by biologic production of CO2 at shallower depths and by the effect of increased pressure at greater depths on the solubility of CaCO3. The so-called below which CaCO3 disappears from deep sea sediments does n t represent simply a downward change from supersaturated to undersaturated water. This is proved by the presence of undersaturated water above the compensation depth and by the fact that the compensation depth may be thousands of meters below the depth where CaCO3 begins to disappear from the sediments; i.e., there is a zone of disappearance and not a single sharply defined depth. The rate of dissolution of CaCO3 in undersaturated seawater is slowed by dissolved Mg and by dissolved organic matter and this helps account for the lack of dissolution where it is expected to occur. Surface seawater, 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++ and organic matter as dissolved species and as surface coatings on mineral grains. End_of_Article - Last_Page 331------------