Carbon and oxygen isotope fractionations between aragonite and calcite of shells from modern molluscs

Carbon and oxygen isotope fractionations between calcite and aragonite were investigated by analyzing marine gastropods and bivalves that lived under temperate to tropical climates. Species that secrete both aragonite and calcite layers were studied to ensure similitude of changes in the isotopic composition of water, in diet, and in metabolic activity during shell growth. Aragonite and calcite layers from the adult parts of the shell were identified and mapped by using Raman spectroscopy. Powder samples were obtained by micro‐drilling under a stereo microscope. Thirty‐six pairs of aragonite–calcite samples were obtained from ten gastropod and five bivalve species. Biogenic aragonite is 13C‐enriched by 0.95 ± 0.81‰ and 18O‐enriched by 0.37 ± 0.65‰ relative to co-existing biogenic calcite. Direction and magnitude of the carbon isotope fractionation are compatible with those already determined by using low‐temperature experimental approaches. The observed oxygen isotope difference between biogenic aragonite and calcite is assigned to the difference (≈ 0.4‰) in the acid fractionation factor values that must be taken into account during digestion of carbonate polymorphs at 90 °C. It is concluded that biogenic calcium carbonate polymorphs precipitate close to, but not in isotopic equilibrium with seawater. Therefore, empirical oxygen isotope fractionation equations that were established on the basis of modern mollusc shells and ambient waters should be preferred for the calculation of aquatic paleotemperatures.