Precipitation of carbonate minerals

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

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). 

Reactions with alkali feldspars do not provide divalent cations for the precipitation of carbonate minerals and initially were thought to be of little significance for mineral trapping (Gunter et al. 1997). However, more recent work indicates that dissolution of alkali feldspars contributes to the fixing of C02 as the sodium alumino-carbonate mineral dawsonite, NaAlC03(0H)2 (Johnson et al. 2001). In this case, the Na necessary for dawsonite precipitation is available in abundance in the brine, but dissolution of alkali feldspar provides a source of aluminum and neutralizes the acidic C02 according to (Johnson et al. 2001) ... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (e.g., Berner, 1971). Although the many reactions involved are complex, this process can be schematically represented as ... 

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

As noted above, the groundwater chemistry can provide additional information about the likely composition of the aquifer sediments. This is particularly trae for the alkaline earth elements (Mg, Ca and Sr in particular) since their concentrations are often controlled by the dissolution and precipitation of carbonate minerals. 

We will next examine the kinds of natural processes that can influence the dissolution or precipitation of carbonate minerals. The following discussion revolves around Fig. 6.7, which is a simplified, schematic version of Fig. 6.4, but considers only Ca and PCO2 and their relationship to calcite precipitation or dissolution. The same arguments apply to other carbonate minerals, as well. 

Physical-mathematical models have been devised that call upon inorganic chemical precipitation of carbonate minerals at sediment-water interfaces to amplify existing cm-scale seafloor topography to produce stromatolitic structures. 

Carbon dioxide was also released to the atmosphere due to the precipitation of carbonate minerals in the oceans. The reaction is ... 

Travertine stone is a natural chemical precipitate of carbonate minerals deposited from the water of mineral springs saturated with dissolved calcium bicarbonate (Gauri Bandyopadhyay, 1999). It consists of calcite or aragonite, of low to moderate intercrystalline porosity and often high mouldic porosity (Pentecost, 2005). Precipitation results primarily through the transfer of carbon dioxide from or to a groundwater source leading to calcium carbonate (CaCOj) supersaturation. 

The introduction of CO2 into pore water as a result of oxidation of organics, sulfate reduction, or methanogenesis does not result in the precipitation of carbonate minerals. As Curtis (1987) points out, the resulting drop in pH due to CO2 dissolution and H2CO3 dissociation,... 

Thus, as a sandstone passes through early burial environments characterized by sulfate reduction or methanogenesis, Fe/Mn reduction or silicate hydrolysis, and a source of calcium, there is ample opportunity for the precipitation of carbonate minerals. Sulfate reduction and methanogenesis during shallow burial are inevitable processes if the system contains bacteria and organic material. Likewise, Fe/Mn reduction and silicate hydrolysis are common processes in early burial (Berner 1980). It would be a rare sandstone/shale system that during shallow burial would not have the necessary components for sulfate reduction or methanogenesis and for Fe/ Mn reduction or silicate hydrolysis. Thus, the key to the formation of early carbonate cements typically may be the availability of Ca" and Mg". ... 

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