Aragonite carbon enrichment

Concerning calcium carbonate polymorphs calcite and aragonite, the experiments of Romanek et al. (1992) at low T (10 to 40 °C) seem to indicate that the enrichment factor of in the solid phase for CaC03-C02 partitioning is reduced by about two units in the aragonite polymorph with respect to the calcite counterpart, r being equal (e.g., at T = 10 °C, 1000 In a 13 for calcite-C02 and 11 for... 

Note that the curve for this relation falls between the aragonite-bicarbonate and calcite-bicarbonate enrichment factors of Rubinson and Clayton, consistent with the fact that the calcium carbonate was a mixture of aragonite and calcite. 

Fig. 3.17. Carbon isotopic enrichment factors for aragonitic (dots) and calcitic (circles) foraminifera and inorganic precipitates as a function of temperature. Shown are the inorganic precipitate data of Baertschi (1957)- circle with dot Emrich et al. (1970)- large dots and Rubinson and Clayton (1969) revised - see text aragonite - solid diamond, calcite - hollow diamond. (After Grossman, 1984.)...

Fig. 3.18. Bicarbonate-calcium carbonate 13C enrichment factors vs. precipitation rate constant (k, L mole min 1). Precipitates that are partly aragonite are marked with a star. (After Turner, 1982.)...

Carbonate minerals form from dissolved HCO3 with only a minor fractionation that is relatively insensitive to temperature. The two most important calcium carbonate minerals—calcite and aragonite— typically have values that are 0.9%o and 2.7%o enriched in 5 C relative to seawater bicarbonate at 25°C (Rubinson and Clayton 1969). The assumption that fractionation factors between seawater HCO3 and calcium carbonate minerals are invariant through time permits the use of stratigraphic variations in carbonate S C values as a proxy for secular changes in seawater 5 X values. 

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