Isotopic partitioning between scallop shell calcite and seawater: Effect of shell growth rate

The relationship between molluscan shell growth rate and skeletal delta(18)O and delta(13)C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell delta(18)O and delta(13)C. At low shell growth rates, shell delta(18)O were more positive (of the order +0.4parts per thousand) and delta(13)C more negative (up to -2parts per thousand) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in delta(18)O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell delta(13)C were consistent with incorporation of isotopically depleted respiratory C-13 (i.e., a metabolic effect). A trend toward more depleted shell delta(18)O and delta(13)C values occurred at higher shell growth rates, with negative delta(18)O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal delta(18)O and delta(13)C Were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in fight of a model of isotopic behavior based on the findings of the study. Copyright (C) 2002 Elsevier Science Ltd.