Carbonate dissolution rate

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. 

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. 

Fig. 9.5 Production, accumulation and fluxes of marine inorganic carbon (in x 10 mol yr" ) in the modern ocean, summarizing production, accumulation, and fluxes of particulate inorganic carbon, as well as carbonate dissolution rates as given in Table 9.1 (modified after Milliman and Droxler 1996, Wollast 1994).

Fig. 9.7 Model results of calcium carbonate dissolution rates as a function of oxygen uptake rates and degree of saturation with different values of from Berelson et al. (1994). The boxes represent averaged benthic lander fluxes for each station (see text for explanation).

Halhach, P. and Puteanus, D., 1984. The influence of the carbonate dissolution rate on the growth and composition of Co-rich ferromanganese crusts from the central Pacific seamount areas. Earth and Planetary Science Letters, 68 73-87. 

The protocol involving NaOAc-HOAc at pH 5 was first proposed and used by Jackson (1958) to remove carbonates from calcareous soils to analyze soil cation exchange characteristics (Grossman and Millet, 1961). Other researchers used HOAc for the extraction of metals from sediments and soils (Nissenbaum, 1972 Mclaren and Crawford, 1973). Tessier et al. (1979) first used the NaOAc-HOAc solution at pH 5 to dissolve the carbonate fraction from sediments. Since then, the NaOAc-HOAc buffer has been widely used as a specific extractant for the carbonate phase in various media (Tessier et al., 1979 Hickey and Kittrick, 1984 Rapin et al., 1986 Mahan et al., 1987 Han et al., 1992 Clevenger, 1990 Banin et al., 1990). Despite its widespread use, this step is not free from difficulties, and further optimization is required in its application. Questions arise with regard to this step in the elemental extraction from noncalcareous soils, the dissolution capacity and dissolution rates imposed by the buffer at various pHs, and the possibility that different carbonate minerals may require different extraction protocols (Grossman and Millet, 1961 Tessier et al., 1979). 

Broecker and Peng, p. 59 dlsscon = 7 dissolution constant in DJSS pcpcon = / carbonate precipitation constant disfac -. 01 scaling factor in dissolution rate eole/e 3/y delcorg 10 Fractionation by photosynthetic organises dcse = 2 Delta 13C Isotope ratio for sea eater, per eil... 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

Figure 8-5 plots the carbonate parameters in the steady state as a function of depth between 0 and 400 centimeters. The figure shows that the saturation index, dissolution rate, and respiration rate all are very close to zero at 400 centimeters. The results for this simulation therefore do not depend on the total thickness of the sedimentary column, provided that this total thickness exceeds 400 centimeters, a limit that depends on the rate at which respiration decreases with increasing depth. 

The estimation of the carbonate and evaporite weathering rate requires the quantification of the elements originating from the two components. Roy et al. [10] calculated the carbonate weathering rate (TDS originating from carbonate dissolution) as follows ... 

But within the pH range of natural waters, the dissolution (and precipitation) of carbonate minerals is surface controlled i.e., the rate of dissolution is rate determined by a chemical reaction at the water-mineral interface. Fig. 8.1 gives the data on the dissolution rates of various carbonate minerals in aqueous solutions obtained in careful studies by Chou and Wollast (1989). 

Dissolution rate of carbonates as a function of pH. These experiments were carried out with a continuous flow reactor in open systems with controlled pco2-... 

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... 

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

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