Calcium carbonate dissolution kinetic

Two experimental approaches have been used to determine calcium carbonate dissolution kinetics in seawater. The first is suspension of different carbonates in the ocean at various depths. After a given period of time, the samples are recovered and the rate of dissolution determined by weight loss. The second experimental approach is the determination of dissolution kinetics in the laboratory at different undersaturations. A detailed discussion of the findings of these studies is presented in this section. 

The accumulation of calcium carbonate in deep ocean sediments is a complex process. It is primarily governed by the interplay between biological production of calcium carbonate in the nearsurface ocean and the chemistry of deep ocean waters. After over 100 years of study, the major problem of determining the saturation state of deep ocean water remains largely unresolved. It is currently possible, using recent laboratory measurements, to arrive at saturation states that differ by as much as a factor of 2. Both laboratory and water column experiments indicate that calcium carbonate dissolution kinetics are not simply related to saturation state. It is our opinion that the saturation state problem must be resolved and considerably more detail added to our present knowledge of calcium carbonate dissolution kinetics and accumulation patterns before attempts to model the accumulation of calcium carbonate in deep ocean sediments can be truly successful. 

Carbonate minerals are among the most chemically reactive common minerals under Earth surface conditions. Many important features of carbonate mineral behavior in sediments and during diagenesis are a result of their unique kinetics of dissolution and precipitation. Although the reaction kinetics of several carbonate minerals have been investigated, the vast majority of studies have focused on calcite and aragonite. Before examining data and models for calcium carbonate dissolution and precipitation reactions in aqueous solutions, a brief summary of the major concepts involved will be presented. Here we will not deal with the details of proposed reaction mechanisms and the associated complex rate equations. These have been examined in extensive review articles (e.g., Plummer et al., 1979 Morse, 1983) and where appropriate will be developed in later chapters. 

A major portion of the studies on calcium carbonate reaction kinetics has been done in seawater because of the many significant geochemical problems related to this system. Morse and Berner (1979) summarized the work on carbonate dissolution kinetics in seawater and their application to the oceanic carbonate system. The only major seawater component in addition to Mg2+ that has been identified as a dissolution inhibitor is SO42- (Sjoberg, 1978 Mucci et al., 1989). Sjoberg s studies of other major and minor components (Sr2+, H3BO3, F-) showed no measurable influence on dissolution rates. Morse and Berner (1979) and Sjoberg (1978) found that for near-equilibrium dissolution in phosphate-free seawater, the dissolution rate could be described as ... 

Morse J.W. (1983) The kinetics of calcium carbonate dissolution and precipitation. In Reviews in Mineralogy Carbonates - Mineralogy and Chemistry (ed. R. J. Reeder), pp. 227-264. Mineralogical Society of America. Bookcrafters, Inc., Chelse, MI. 

Morse J. W. (1979) The kinetics of calcium carbonate dissolution and precipitation, 227. 

In most of recently published studies, the calcium carbonate dissolution in seawater and in pore water of surface sediments is assumed to follow a kinetic process that can be described by the eqnation (Morse 1978 Keir 1980) ... 

Juwekar and Sharma [1] described the kinetics of the above reactions. The formation of calcium carbonate is non elementary reaction which involves the number of elementary steps as shown in Scheme 7.2, steps (iv) and (v) assumed to be instantaneous. Absorption of C02 gas and dissolution of Ca(OH)2 affects the nucleation step, both are considered as rate controlling steps. 

Berner, R.A. Morse, J.M. 1974. Dissolution kinetics of calcium carbonate in seawater. IV. Theory of calcite dissolution, American Journal of Science, 274,108-134. 

Liu Z, Dreybrodt W. Dissolution kinetics of calcium carbonate minerals in H20-C02 solutions in turbulent flow—the role of the diffusion boundary layer and the slow reaction H20 + CO2 <-> H+ + HC03. Geochim Cosmochim Acta 1997 61(14) 2879-2889. 

On the opposite end of the spectrum, thermodynamics cannot explain why some PIC can sink through undersaturated waters without dissolving to accumulate on the seafloor. This is a widespread phenomenon as evidenced by the spatial %CaCOj gradients seen in the surface sediments (Figure 15.5). If the saturation horizon dictated the survival of sinking and accumulating PIC, a sharp depth cutoff should exist below which calcium carbonate is absent from the surface sediment. The importance of this kinetic barrier to dissolution is also seen in the relatively high fraction of surfece-water PIC (20 to 25%) that accumulates in the sediments as compared to the low fraction of surfece-water POC (1%). 

The kinetics of calcium carbonate precipitation in simple solutions have received less attention than those of dissolution reactions. This perhaps reflects the fact that most sedimentary carbonates are initially formed biogenically and that the primary interest in carbonate precipitation reactions has been directed at reaction... 

Keir R.S. (1980) The dissolution kinetics of biogenic calcium carbonates in seawater. Geochim. Cosmochim. Acta 44, 241-252. 

Morse J.W. (1978) Dissolution kinetics of calcium carbonate in seawater. VI The near-equilibrium dissolution kinetics of calcium carbonate-rich deep sea sediments. Amer. J. Sci. 278, 344-355. 

The second example is for the precipitation of calcium carbonate from Ca(HC03)2 solutions. The mechanistic model was derived by Plummer et al. [40] from a kinetic study of the dissolution of Iceland spar crystals. The approach has more recently been extended to the interpretation of crystal growth [41]. The model is based upon the reaction between a partially dehydrated [Ca-HC03]+ complex or ion-pair and possible anionic reaction... 

Calcium carbonate is accumulating in deep ocean sediments, in which the overlying water is undersaturated with respect to both aragonite and calcite, and sediment marker levels closely correspond to unique saturation states. This indicates that dissolution kinetics play an important role in determining the relation between seawater chemistry and calcium carbonate accumulation in deep ocean basins. It is, therefore, necessary to have knowledge of the dissolution kinetics of calcium carbonate in seawater if the accumulation of calcium carbonate is to be understood. 

Edmond, J.M. An interpretation of the calcite spheres experiment [abst.], Amer. Geophys. Union 52, 256 (1971). Morse, J.W. and Berner, R.A. Dissolution kinetics of calcium carbonate in sea water II. A kinetic origin for the lysocline, Amer. Jour. Sci. 272, 840-851 (1972). ... 

Morse, J.W., de Kanel, J., and Harris, K. The dissolution kinetics of calcium carbonate in seawater VII. The dissolution kinetics of synthetic aragonite and pteropods, Amer. Jour. Sci. (in press). 

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