Keith Chave

Observations on the Solubility of Skeletal Carbonates in Aqueous Solutions

Carbonate skeletal materials of marine organisms exhibit a wide range of solubilities in aqueous solutions. In most cases, the dissolution of the carbonate mineral is irreversible and therefore the material can have no true equilibrium solubility. Relative solubilities have been measured in distilled water and in sea water. The least soluble mineral appears to be calcite with low magnesium content; the most soluble is calcite containing 20 to 30 percent MgCO3 in solid solution. Aragonite has an intermediate solubility.

Carbonates: Association with Organic Matter in Surface Seawater.

Carbonate mineral particles suspended in surface seawater in tropical and subtropical areas do not interact measurably with the water. Identical complex mineral assemblages occur in waters in various states of saturation. Drastic alterations of temperature and pH in the laboratory do not affect the composition of the suspended carbonates. It appears that the mineral grains are protected from the water by resistant organic coatings.

Carbonate-seawater interactions

Abstract Interactions between skeletal carbonates and seawater are influenced by mineralogy, particle size and the history of the particle surfaces. Experiments using pH-sensing techniques indicate that the activity of magnesium calcites can be more than four tunes that of pure calcite. Particles of calcite 10−6 cm in diameter have activities more than 8 times greater than 1 cm particles. Grinding in a mortar produces activities as much as 7 times greater than unground material of the same composition.

Mineralogic Changes during Growth in the Red Alga, Clathromorphum compactum

The amount of magnesium in the skeletal calcite of the encrusting marine red alga Clathromorphum compactum varies seasonally in response to changes in water temperature. X-ray diffraction analyses of serial samples of this alga collected in the Gulf of Maine indicate more than a 40-percent change in composition during a year and demonstrate a more rapid calcification during warmer periods.

Calcium Carbonate: Factors Affecting Saturation in Ocean Waters off Bermuda.

Bermuda marine waters are not in equilibrium with the bulk carbonate sediments with which they are in contact, and they are supersaturated with respect to pure calcite. This apparent supersaturation seems to result from a metastable equilibrium between sea water and the most soluble solid phase available locally in excess.

Distribution of calcium carbonate in pelagic sediments

Abstract Statistical analysis of 1,350 pelagic sediment samples indicates that on a worldwide basis water depth correlates poorly with CaCO 3 percentage of sediments. When the data are grouped into 1,000 m intervals, correlations between percent CaCO 3 and water depth are not greatly improved. Grouping the data first by latitudinal ranges and then by geographic areas only improves correlations sporadically. When nearshore data are excluded to remove the effects of clastic influx, correlations between percent CaCO 3 and depth improve. This exclusion raises the correlation between percent CaCO 3 and water depth to—0.620, meaning that water depth, predicts about 40% of the variability in CaCO 3 percentage of pelagic sediments. Neither variations in CO 2 concentration nor supposed rates of inorganic CaCO 3 solution appear to be satisfactory explanations for this moderate correlation between percent CaCO 3 in pelagic sediments and water depth. We suggest that adsorbtion of organic molecules to carbonate particles may prevent or inhibit carbonate-sea water interactions.