Dissolution aragonite

Figure 7.19. Comparison of calcite and aragonite dissolution and precipitation rates at a Pc02 = 0-96 atm. (After Busenberg and Plummer, 1986b.)...

Acker J.G., Byrne R.H., Ben-Yaakov S., Feely R.A. and Betzer P.R. (1987) The effect of pressure on aragonite dissolution rates in seawater. Geochim. Gosmochim. Acta 51, 2171-2175. 

Berner R.A., Berner E.K. and Keir R. (1976) Aragonite dissolution on the Bermuda Pedastal Its depth and geochemical significance. Earth Planet. Sci. Lett. 30, 169-178. 

Figure 10. Rate of calcite and aragonite dissolution as a function of depth as determined by Milliman (AO) in water column experiments in the Sargasso sea...

Figure 19. Change in the rate of synthetic aragonite dissolution, relative to dissolution in very low phosphate seawater, as a function of time (57)...

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

We can use our results to predict the conditions favorable for the transformation of gaylussite to aragonite. The porous nature of the pseudomorphs and the small amounts of calcium available in the lake water (Table 24.2) suggest that the replacement occurs by the incongruent dissolution of gaylussite, according to the... 

Busenberg, E. Plummer, L.N. 1986. A comparative study of the dissolution and crystal growth kinetics of calcite and aragonite. U S. Geological Survey Bulletin, 1578, 139-168. 

The dissolution rate for calcite and aragonite have been described in terms of the following rate law (Plummer et al., 1978 Busenberg and Plummer, 1986 Chou and Wollast, 1989. ... 

Busenberg, E., and L. N. Plummer (1986b), "A Comparative Study of the Dissolution and Crystal Growth Kinetics of Calcite and Aragonite", in F. A. Mumpton, Ed., Studies in Diagenesis, U.S. Geol. Surv. Bull. 1578, pp. 139-168. 

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. 

Recently, Chou et al. (1989) studied the dissolution kinetics of various carbonate minerals in aqueous solution. Figure 2.10 illustrates the experimental results for aragonite, calcite, witherite, dolomite, and magnesite. These data can be fit by rate equations, an example of which is shown in equation 2.28 for calcite. 

Chave et al. (1962) limited their measurements to changes of pH during the dissolution reaction, and estimated the pH at infinite time by extrapolation of plots of pH versus the reciprocal of the square root of time. These plots were chosen empirically, because they usually yield linear plots as infinite time is approached when calcite, aragonite, or other simple carbonate minerals are used (Garrets et al., 1960). Chave et al. claimed that in the case of magnesian calcites, however, only... 

Bertram (1989) showed, in dissolution studies of magnesian calcites, that their solubilities decrease with increasing temperature. This trend and its relationship to solubility vs. temperature trends for calcite and aragonite are discussed in Chapter 7. 

More recent studies by Byrne et al. (1984) and Betzer et al. (1984, 1986) have contributed substantially to our understanding of aragonite sedimentation in the pelagic environment. In their work, short term deployments of large sediment traps were utilized to minimize the problem of dissolution of material within the trap. Byrne et al. (1984) carried out an elegant examination of pteropod dissolution in the water column in the North Pacific. Considerable variation of the expected extent of dissolution for different species was observed. Some of the pteropods. 

Betzer et al. (1984, 1986) studied the sedimentation of pteropods and foraminifera in the North Pacific. Their sediment trap results confirmed that considerable dissolution of pteropods was taking place in the water column. They calculated that approximately 90% of the aragonite flux was remineralized in the upper 2.2 km of the water column. Dissolution was estimated to be almost enough to balance the alkalinity budget for the intermediate water maximum of the Pacific Ocean. It should be noted that the depth for total dissolution in the water column is considerably deeper than the aragonite compensation depth. This is probably due to the short residence time of pteropods in the water column because of their rapid rates of sinking. 

A reason that there has been so much controversy associated with the relation between the extent of carbonate dissolution occurring in deep sea sediments and the saturation state of the overlying water is that models for the processes controlling carbonate deposition depend strongly on this relation. Hypotheses have ranged from a nearly "thermodynamic" ocean where the CCD and ACD are close to coincident with calcite and aragonite saturation levels (e.g., Turekian, 1964 Li et... 

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