Behaviour of calcium carbonate in sea water

Geochimica et Cosmochimica Acta



Anomalies in the behaviour of calcium carbonate in natural solutions diminish when considered in context. Best values found by traditional oceanographie methods for the apparent solubility product constant K'CaCO3 in sea water at atmospheric pressure are consistent mineralogically—at 36 parts per thousand salinity and T-25°C, K'aragonlte is estimated as 1.12 × 10−6 and K'calcite as 0.61 × 10−6. At 30°C the corresponding values are 0.98 × 10−6 for aragonite and 0.53 × 10−6 for calcite. Because the K' computations do not compensate for ionic activity, however, they cannot give thermodynamically satisfactory results. It is of interest, therefore, that approximate methods and information now available permit the estimation from the same basic data of an activity product constant KCaCO3 close to that found in solutions to which Debye-Hückel theory applies. Such methods indicate approximate Karagonite 7.8 × 10−9 for surface sea water at 29°C; Kcalcite would be proportionately lower.

Field data and experimental results indicate that the mineralogy of precipitated CaCO3 depends primarily on degree of supersaturation, thus also on kinetic or biologic factors that facilitate or inhibit a high degree of supersaturation. The shallow, generally hypersaline bank waters west of Andros Island yield aragonitic sediments with O18O16 ratios that imply precipitation mainly during the warmer months, when the combination of a high rate of evaporation, increasing salinity (and ionic strength), maximal temperatures and photosynthetic removal of CO2 result in high apparent supersaturation. The usual precipitate from solutions of low ionic strength is calcite, except where the aragonite level of supersaturation is reached as a result of diffusion phenomena (e.g. dripstones), gradual and marked evaporation, or biologic intervention. Published data also suggest the possibility of distinct chemical milieus for crystallographic variations in skeletal calcium carbonate. It appears that in nature aragonite precipitates from solutions that are supersaturated with respect to both calcite and aragonite and calcite between saturation levels for the two species. Such a relation is consistent with Ostwald's rule of successive reactions.

Aragonitc of marine origin persists in contact with supersaturated interstitial solutions at ordinary temperature and pressure. Conversion to calcite follows transfer to solutions undersaturated with respect to aragonite or upon exposure to the moist atmosphere.

    Publication type Article
    Publication Subtype Journal Article
    Title Behaviour of calcium carbonate in sea water
    Series title Geochimica et Cosmochimica Acta
    DOI 10.1016/0016-7037(62)90117-5
    Volume 26
    Issue 8
    Year Published 1962
    Language English
    Publisher Elsevier
    Description 19 p.
    First page 867
    Last page 884
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