Calcite biogenic

Much of the geographic variability in sedimentary ( Paxs/ °Thxs) observed in modern sediments may be explained by variability in the composition of biogenic particles arising from variability in the structure of the planktonic ecosystem. This can be inferred from the composition-dependence of F(Th/Pa) (Fig. 8), and is shown explicitly by the relationship between sediment trap ( Paxs/ °Thxs) and the opal/calcite ratio of the trapped particles (Fig. 9). Sediment trap ( Paxs/ °Thxs) also exhibits a positive relationship with the mass flux of particles, but the correlation is poorer than that with particle composition (Fig. 9). Indeed, the relationship between particulate ( Paxs/ °Thxs)... 

Walter and Morse (1984) were able to document the relative importance of microstructure for the dissolution of biogenic carbonates. Biogenic magnesian calcites are structurally disordered and chemically heterogeneous. Both these factors play a role in the reactivity of these minerals in natural systems. 

Solubilities of the Mg-caldte as a function of MaCOn constant. The solubility is expressed in line with Eq. (8.11) as lAP g-calcite = (Ca2 1 ) (Mg2+) CO 2). The solid curves represent the general trend of results on dissolution of biogenic and synthetic Mg-calcites. The curve fitting the data of Plummer and Mackenzie (1974) is dashed. The various points refer to the results of different researches. (For the origin of the data see Morse and Mackenzie, 1990.) (IAP = ion activity product.)... 

It is doubtful that formation and dissolution of any mineral in low temperature aqueous solutions has been more fully investigated than the magnesian caicite. This mineral is a preponderant carbonate phase, mostly of biogenic origin, in seawater. Fig. 8.8 gives some data on the solubilities of Mg-calcites as a function of MgC03 content. 

Bischoff, W. D., F. T. Mackenzie, and F. C. Bishop (1987), "Stabilities of Synthetic Magnesian Calcites in Aqueous Solution Comparison with Biogenic Materials," Geochim. Cosmochim. Acfa51, 1413-1423. 

The clay minerals of aeolian origin comprise 25 to 75% of the mass of pelagic sediments. The large range in composition reflects the latitudinal nature of the dust belt as well as dilution by other locally important particle types such as clay minerals of volcanogenic origin and biogenic hard parts (calcite and opaline silica). 

The degree of satiuation decreases with increasing depth for two reasons. First, the solubility of biogenic calcite and aragonite increases with depth due to increasing... 

All surface seawater is presently supersaturated with respect to biogenic calcite and aragonite with Cl ranging from 2.5 at high latitudes and 6.0 at low latitudes. The elevated supersaturations at low latitude reflect higher [COj ] due to (1) the effect of temperature on CO2 solubility and the for HCO3, and (2) density stratification. At low latitudes, enhanced stratification prevents the upwelling of C02-rich deep waters. 

The sulfete and chloride minerals in evaporites (gypsum, anhydrite, halite) undergo congruent dissolution to produce Ca (aq), S04 (aq), Na (aq), and Cl (aq). The dissolution of evaporite and biogenic carbonates (limestone, dolomite, and calcite) generates... 

Prior to the advent of the coccolithophorids and planktonic foraminferans, 200 to 250 miUion years ago, all biogenic calcite precipitation must have been restricted to the shallow waters of the neritic zone. Thus, the evolution of the pelagic calcifiers ushered... 

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