Calcite-carbonate-equilibrium

The Calcite-Carbonate-Equilibrium in Marine Aquatic Systems... 

Primary Reactions of the Calcite-Carbonate-Equilibrium with Atmospheric Contact in Infinitely Diluted Solutions... 

Another essential condition for describing the calcite-carbonate-equilibrium consists in the fulfillment of the neutral charge rule. This holds that charges may be neither lost nor gained on reaching the equilibrium state. 

A system which is closed to CO exists wherever the final calcite-carbonate-equilibrium is reached without any concurrent uptake or release of atmospheric CO in its process. This implies that the Equations 9.1 and 9.2 are no longer valid. In their stead, a balance of various C-species is related to calcium. This balance maintains that the sum of C-species must equal the calcium concentration in solution, since both can enter the solution only by dissolution of calcite or aragonite ... 

In seawater, the differences between activities and concentrations must always be considered (cf. Sect. 15.1.1). The activity coefficients for monovalent ions in seawater assume a value around 0.75, for divalent ions this value usually lies around 0.2. In most cases of practical importance, the activity coefficients can be regarded with sufficient exactness as constants, since they are, over the whole range of ionic strengths in solution, predominately bound to the concentrations of sodium, chloride, and sulfate which are not directly involved in the calcite-carbonate-equilibrium. The proportion of ionic complexes in the overall calcium or carbonate content can mostly be considered with sufficient exactness as constant in the free water column of the ocean. Yet, this cannot be applied to pore water which frequently contains totally different concentrations and distributions of complex species due to diage-netic reactions. 

Examples for Calculation of the Calcite-Carbonate-Equilibrium in Ocean Waters... 

The CO2 concentration in the earth s atmosphere is ultimately governed by the calcium carbonate equilibrium in the ocean (e.g., Berner et al. 1983). If the oceans are in equilibrium with calcite, which is usually the case, then to a reasonable approximation, the PCO2 of the atmosphere is defined by the equilibrium ... 

Figure 15.5. Buffer intensity versus pH for some heterogeneous systems and for the homogeneous dissolved carbonate system. Buffer intensities 0ct (dissolved carbonate, Cr = 10 M) /Scacoj (carbonate solution in equilibrium with calcite), (carbonate solution in equilibrium with pco2 atm), /3an kaoi (solution in equilibrium with...

Investigations of the state of carbonate equilibrium of water which is transiting the vadose zone have not been numerous. The equilibrium COj (carbon dioxide partial pressure) and state of saturation with respect to calcite was studied by Holland et al. (1964) in Indian Echo Cave, Pennsylvania, and Luray Caverns, Virginia, and by Thrailkill (1971) in Carlsbad Caverns, New Mexico. These investigations showed that seepage water entering the cave was in equilibrium with a f cO2 much higher than that of the normal atmosphere and was often supersaturated with respect to calcite. 

If nucleation of calcite cement is controlled by the presence of certain types of biogenic carbonate which act as favourable nucleation substrates, then calcite cement nuclei will tend to be concentrated where most biogenic carbonate is present, i.e. within biogenic carbonate-rich layers. If nucleation is determined by calcite supersaturations, nuclei will still tend to be concentrated in biogenic carbonate-rich layers, as these contain the dominant source of dissolved calcite, and the supersaturations necessary for calcite cement nucleation will therefore normally be first achieved within these layers. The nucleation of calcite cement may be a result of the difference in solubility between calcite cement and carbonate fossils, i.e. the concentration of dissolved calcite in equilibrium with biogenic carbonate may exceed the supersaturation necessary for nucleating the less soluble calcite cement. 

We determined the shell C and O isotopic composition of the cultured foraminifera, and compared these isotopic values with the water chemistry of the culture chambers, and also with the shell chemistry of field specimens collected from sites on the North Carolina and South Carolina (USA) continental margin. The culmred foraminifera showed substantial offsets from the 8 C of system water dissolved inorganic carbon (—0.5 to —2.5%c, depending on species) and smaller offsets (0 to — 0.5%o) from the predicted 8 0 of calcite in equilibrium with the culture system water at the growth temperature. These offsets reflect at least three factors species-dependent vital effects ontogenetic variations in shell chemistry and the aqueous carbonate chemistry ([COJ] or pH) of the experimental system. 

Fig. 9. Continued) and Ab O values (this study, and McCorkle et al 1997). The carbon isotopic offsets are calculated relative to bottom water DIC, and the oxygen isotopic offsets are calculated relative to calcite in equilibrium with bottom water using the expressions of McCorkle et al (1997) (solid diamonds) and Shackleton (1974) (open circles). The vertical line shows the trend in isotopic offsets predicted by a pore water b C effect only. The observed b C depletions of pore water DIC in the 0-0.5 cm depth interval range from — 1 to — 2.2%o at the nearby sites discussed by McCorkle et al (1997) (white hexagons, dotted line). Because pore water carbonate ion concentration is likely to be lower than bottom water values at these relatively shallow continental margin sites, a carbonate ion influence on shell composition would tend to cause Ab O values greater than zero (ellipse).

Solid carbonate precipitation/dissolution are included as kinetic processes to the overall DOC biodegradation reactive model depending on the actual saturation. Further TIC (total inorganic Carbon) and the Calcite solubility equilibrium (dissolved Calcium and solid Calcite) are considered as main pH buffering factors. The actual... 

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