Pore waters calcites

Thus, larger solid/water ratios such as are encountered in pore waters of sediments lead to smaller MgC(>3 contents in the equilibrium magnesian calcites although in either case the magnesium content of the solid increases. Wollast and Reinhard-Derie presented data to support the theory from the standpoint of dissolution and some of our results for the precipitation case... 

Martin, W.R., A.P. McNichol, and D.C. McCorkle. 2000. The radiocarbon age of calcite dissolving at the seafloor Estimates from pore water data. Geochimica et Cosmochimica Acta 64(8) 1391-1404. 

Croal LR, Johnson CM, Beard BL, Newman DK (2004) Iron isotope fractionation by anoxygenic Fe(II)-phototrophic bacteria. Geochim Cosmochim Acta 68 1227-1242 Curtis CD, Coleman ML, Love LG (1986) Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions. Geochim Cosmochim Acta 50 2321-2334... 

Fig. 1. Replacement of calcite by Si02 in a fossil. Diagrammatic relationship between solution films and pore waters. Direction of movement of chemical species indicated by arrows (Schmitt Boyd, 1981) 10>...

Temperature and pressure variations in natural systems exert major influences on carbonate mineral solubility and the distribution of carbonic acid chemical species. For example, the solubility of calcite decreases with increasing temperature, as does the solubility of CO2 gas in water. These two effects on solubilities can lead to precipitation of calcite as a cement in a marine sediment-pore water system that undergoes moderate burial. 

In addition to calcite, aragonite and occasionally magnesian calcites may also reach the ocean floor. They may play an important role even in sediments where they do not accumulate, by contributing carbonate ion to pore waters because of their dissolution, thus causing less calcite dissolution to occur than would be the case in their absence. This process is enhanced by the benthic process of bioturbation, whereby organisms cause a physical mixing of sediments down to... 

A detailed study of the chemistry of pore waters near the sediment-water interface of sediments from the equatorial Atlantic was conducted by Archer et al. (1989) using microelectrodes that were slowly lowered into the sediment. By modeling the resulting data they were able to confirm that calcite was dissolving above the saturation depth as a result of benthic oxidation of organic matter. The estimated in situ rate constant for calcite dissoluton was 1-100% day1. This rate constant is 10 to 100 times slower than the one used in previous models, which was based on experimental data. If the slower rate constant proves to be correct, then dissolution of calcite by benthic metabolic processes will be of major importance. 

Manganese appears to be capable of altering the apparent solubility behavior of calcite in deep sea sediments. This element has long been noted to be associated with calcium carbonate in deep sea sediments (e.g., Wangersky and Joensuu, 1964), and extensive experimental evidence exists for coprecipitation of Mn2+ with calcite (see Chapter 3). Both Pedersen and Price (1982) and Boyle (1983) have noted the close association of Mn with carbonate material in Panama Basin sediments. In fact, some of the pore waters approach equilibrium with MnC03,and mixed carbonate... 

Both field and laboratory observations are consistent with the idea that dissolution can proceed faster than precipitation in carbonate sediments (also see Pytkowicz, 1971 Berner et al., 1978 Moulin et al 1985 Burton and Walter, 1987), and that the pore waters reach steady-state ion activity products close to those of the most unstable phase (dissolution processes will be discussed later in this chapter). Carbonate ion may be "pumped" down to values at saturation with less soluble phases, as dissolution of the more soluble material eventually causes its removal. However, the persistence of high magnesian calcites in sediments for long periods of time indicates that this process does not involve a large amount of mass transfer under normal marine conditions. 

The precipitation of calcium carbonate (usually, but not exclusively, from petrographic evidence in the form of high-magnesian calcite, e.g., Alexandersson and Milliman, 1981) is generally inferred from a decrease in dissolved pore water calcium (e.g., Thorstenson and Mackenzie, 1974 Aller et al., 1986 Gaillard et al., 1986). Most studies of the impact of chemical diagenesis on the carbonate... 

Figure 6.11. A. Saturation state of seawater with respect to aragonite as a function of sulfate reduction. Based on general model of Ben-Yaakov (1973), updated by using pK a values and total ion activity coefficients of Millero (1982). B. Observed saturation state of Mangrove Lake, Bermuda pore waters with respect to calcite.

Although cementation is a process that can occur throughout the life of a sedimentary carbonate body, the dominant processes and types of cements produced generally differ substantially between those formed in the shallow-meteoric and deep-burial environments. Mineralogic stabilization (i.e., dissolution of magnesian calcites and aragonite, see Chapter 7) commonly drives cement formation during the early shallow-burial period, whereas the previously discussed processes of pressure solution and neomorphism are more important in the deep-burial environment. The pore waters in which cementation takes place also tend to differ substantially between the two environments. In shallow subsurface environments, cementation usually takes place in dilute meteoric waters that are oxic to only... 

The depth distribution of the Sr/Ca ratios of the recrystallized calcites in these sediments was calculated from the Sr2+/Ca2+ concentrations in the pore waters and appropriate values of the strontium distribution coefficient as a function of temperature from 5-25°C, where... 

Budd D.A. (1988) Aragonite-to-calcite transformation during fresh-water diagenesis of carbonates Insights from pore-water chemistry. Geol. Soc. Amer. Bull. 100, 1260-1270. 

Morse J.W. and Mucci A. (1984) Composition of carbonate overgrowths produced on calcite crystals in Bahamian pore waters. Sediment. Geol. 40, 287-291. 

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