Meteoric diagenesis

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

Figure 7.15. Major settings and principal controls of meteoric diagenesis, and a schematic drawing of the meteoric environment. (After James and Choquette, 1984.)...

From the discussion in the previous two sections, it can be seen that the rate of alteration of carbonate sediments during meteoric diagenesis is strongly influenced by the original mineralogy of the sediments and the temperature and... 

Figure 7.42. Comparison between (A) an idealized plot of variation in 8180 and 813C for carbonates subjected to vadose and phreatic meteoric diagenesis (after Lohmann, 1988) with (B) the meteoric alteration trend observed for the Key Largo Limestone, Florida, U.S.A. (after Martin et al., 1986). The critical trend in isotopic composition is termed the meteoric calcite line. This trend may be modified at the water recharge surface where evaporation is an important process, caliche is formed and the diagenetic phases are depleted in 13C derived from soil-gas CO2. Another modification can occur distally to the recharge area where precipitating carbonate cements may have isotopic ratios nearly equivalent to dissolving phases.

Under the modest temperature and pressure conditions characteristic of the environments in which meteoric diagenesis typically takes place, many of the most important reactions are slow. This has severely constrained the study of the chemical mechanisms and kinetics involved in such fundamental processes as the aragonite to calcite transformation and dolomite formation. Information on these processes obtained under conditions not typical of the meteoric realm (e.g., elevated temperatures) are of questionable applicability to "real world" carbonate diagenesis. 

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