Calcium carbonate dissolution

A big concern in swimming pools is prevention of etching and scaling (ie, precipitation of CaCO ) which can be controlled by maintenance of a proper degree of saturation with respect to calcium carbonate. The calcium carbonate dissolution-precipitation equiUbrium is represented by ... 

At equilibrimn, the rate of calcium carbonate dissolution is equal to the rate of its precipitation. The concentrations of the reactants and products remain constant over time, so no further net dissolution occms. Since the solution can dissolve no more calcium carbonate, it is said to be saturated. The for this reaction is given by... 

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. 

It is important to keep in mind in the following discussions that it is difficult to observe signs of calcium carbonate dissolution in deep sea sediments unless major dissolution has occurred. 

Early models for the process of calcium carbonate dissolution from deep sea sediments (e.g., Takahashi and Broecker, 1977) were based on simple diagenetic models in which calcium carbonate dissolved into the pore waters of the sediments. 

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. 

Reaves C.M. (1986) Organic matter metabolizability and calcium carbonate dissolution in nearshore marine muds. J. Sediment. Petrol. 56, 486-494. 

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. 

Open Ocean Dissolution Experiments. The first direct studies of calcium carbonate dissolution in deep seawater were made by Peterson (41) and Berger (42). Peterson suspended spheres of Iceland spar calcite, held in pronged plastic containers, at various depths in the Central Pacific Ocean for four months. The amount of dissolution was determined by weight loss, which was small relative to the total weight of the spheres. On the same mooring Berger suspended sample chambers, which consisted of... 

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. 

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

Berelson W. M., Hammond D., and Cutter G. A. (1990) In situ measurements of calcium carbonate dissolution rates in deep-sea sediments. Geochim. Cosmochim. Acta 54, 3013-3020. 

Jahnke R. J. and Jahnke D. B. (2002) Calcium carbonate dissolution in deep-sea sediments implications of bottom water saturation state and sediment composition. Geochim. Cosmochim. Acta (in press). 

Stephens M. and Kadko D. (1997) Glacial-Holocene calcium carbonate dissolution at the central equatorial Pacific seafloor. Paleoceanography 12, 797-804. 

The carbonate compensation depth (CCD) occurs where the rate of calcium carbonate dissolution is balanced by the rate of infall, and the calcium carbonate content of surface sediments is close to Owt.% (e.g., Bramlette, 1961). The CCD has been confused with the calcium carbonate critical depth (sometimes used interchangeably with the lysocline discussed next), where the carbonate content of the surface sediment drops below 10 wt.%. A similar marker level in deep-sea sediments is the ACD, below... 

The fundamental parameter in all models for calcium carbonate dissolution in the deep sea is the saturation state of pore waters. In order to determine the samration state, not only must the composition of the pore waters be known, but also the solubility of the calcium carbonate. Therefore, many studies of carbonate chemistry in deep-sea sediment pore waters have focused on the apparent solubility behavior of carbonates in these sediments. 

A puzzling observation has recently been made by R. A. Jahnke and D. B. Jahnke (in press). They found that in sediments above the saturation depth that contain high concentrations of calcium carbonate, the ratio of the calcium carbonate dissolution rate to the organic matter remineralization rate was substantially less than at other types of sites. They have suggested that this may be the result of exchange on carbonate particle surfaces coupled with particle mixing, but this process has yet to be clearly substantiated. 

The change in DIG and At of the solution during GaGOs dissolution is veiy different from that resulting from OM degradation and oxidation. One mole of calcium carbonate dissolution... 

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