Seawater calcite precipitation from

Mucci and Morse (1983) found no statistically significant dependence of the partition coefficient for Mg in calcite precipitated from seawater on reaction rate over a range of seawater supersaturations from about one half ( 2 =3) to close to three times ( 2=17) that of typical surface seawater. However, their experiments had durations of from only a few hours to days. The compositions of magnesian calcites grown very near equilibrium ( 2=1.2) over periods of several months were determined by Mucci et al. (1985). They found excellent agreement with the results of Mucci and Morse (1983) even though different experimental techniques were used. The rate independence of the partition coefficient of Mg in calcite, therefore, has been found to be independent of reaction rate over about 12 orders of magnitude in seawater. 

Mucci A. (1988) Manganese uptake during calcite precipitation from seawater Conditions leading to the formation of a pseudokutnahorite. Geochim. Cosmochim. Acta 52, 1859-1868. 

The isotopic composition of calcium in aragonite and calcite precipitated from seawater, shows a lower content of the heavier " " Ca relative to Ca of some — 0.6%o to — 0.7%o as compared with the ratio in seawater (Gussone et al. 2003). Fewer measurements of Mg relative to Mg show a lowering of — 1.0 to — l.l%o relative to the ratio in seawater (Chang et al. 2004). This difference is consistent with the difference in absolute masses of Mg and Ca. We need to account for the negative values. 

In addition, the strontium in this sample is enriched in radiogenic Sr ( Sr Sr = 0.71417 0.00002) relative to 0.70809 0.00001 for the Eimer and Amend Sr-isotope standard. These data indicate that the calcite precipitated from glacial meltwater, that the carbon is of biogenic origin, and the strontium was derived from crustal rocks rather than from seawater (Photo by G. Fame)... 

Fig. 17.14 Range of isotopic compositions of oxygen, carbon, and strontium of the clasts of black calcite and opaline silica from the Elephant Moraine. If the calcite precipitated from glacial meltwater at 0°C, then the water must have had a value between -47.3%o and -54.9%o (SMOW) in good agreement with the value of ice in the Elephant Moraine. The carbon in the black calcite is strongly depleted in which is characteristic of photosynthetic plants and of bituminous coal in the Beacon Supergroup. Fragments of coal occur in the Elephant Moraine which is composed of clasts derived from the base of the ice sheet. The strontium in the black calcite is strongly enriched in radiogenic Sr compared to seawater (SW) of Phanerozoic age...

Sanyal A, Nugent M, Reeder RJ, Bijma J (2000) Seawater pH control on the boron isotopic composition of calcite evidence from inorganic caldte precipitation experiments. Geochim Cosmochim Acta 64 1551-1555... 

The central question was, how major is the influence of coprecipitated magnesium Berner (1976) used a clever approach to find out. His reasoning was that whereas calcite has a major coprecipitate from seawater (Mg) capable of altering its solubility, aragonite that can be precipitated from seawater is greater than 99% pure, and its solubility in seawater should be measurable. He also pointed out that if the solubility ratio of calcite to aragonite was precisely known, it would... 

It should be emphasized that an equilibrium can be established between a metastable phase(s) and its environment. Aragonite, for example, can be precipitated from seawater at 25°C, but it is unstable at Earth-surface T and P, and can persist metastably because of kinetic reasons. This statement is illustrated by the following calculation. We can use the free energies of formation of Table 6.1, and calculate the Gibbs free energy of reaction for the mass action equation representing aragonite-calcite equilibrium ... 

Mucci A. (1986) Growth kinetics and composition of magnesian calcite overgrowths precipitated from seawater Quantitative influence of orthophosphate ions. Geochim. Cosmochim. Acta 50, 2255-2265. 

Mucci A., Morse J.W. and Kaminsky M.S. (1985) Auger spectroscopy analysis of magnesian calcite overgrowths precipitated from seawater and solutions of similar composition. Amer. J. Sci. 285, 289-305. 

Zhong S. and Mucci A. (1989) Calcite and aragonite precipitation from seawater solutions of various salinities Precipitation rates and overgrowth compositions. Chem. Geol. (in press). 

The 5 0 from the calcite tests of benthic foraminifera preserved in ocean sediments can be used to estimate upper-ocean density because both the 5 0 of calcite (S Ocaicite) and density increase as a result of increasing salinity or decreasing temperature (Lynch-Stieglitz et al., 1999a). The fractionation between calcite precipitated inorganically and the water in which it forms increases by 0.2%c for every 1 °C decrease in temperature (Kim and O Neil, 1997). The relationship between S Ocaidte and salinity is more complex. The S Ocaidte reflects the 5 0 of the water in which the foraminifera grew. The 5 0 of seawater (S Owater) primarily reflects patterns of evaporation and freshwater influx to the surface of the ocean. Because salinity also reflects these same processes, salinity and... 

The boron isotopic composition of calcite precipitated by foraminifera has been used to reconstruct the pH of the ancient seawater. In appropriate oceanographic settings, pH estimates derived from boron isotope analyses of planktonic... 

Berner (1976) reviewed the problems of measuring calcite solubility in seawater, and it is these problems, in part, that have led to the use of stoichiometric solubility constants for calcite and aragonite (see Section 7.03.3). The most difficult problem is that, although the solubility of pure calcite is sought in experiments with seawater solutions, extensive magnesium co-precipitation can produce magnesian calcites. The solubility of these magnesian calcites differs from that of pure calcite. Thus, it is not possible to measure the solubility of pure calcite directly in seawater. 

Seawater in the upper regions of the ocean is strongly supersaturated with respect to calcite, aragonite, and ideal dolomite. Why these minerals fail to precipitate directly from it has been a problem of major interest. In experimental studies (e.g., Pytkowicz, 1965, 1973 Berner, 1975), the magnesium ion has been observed to be a strong inhibitor of calcite precipitation and largely responsible for its failure to precipitate... 

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