Abstract--The isotopic composition of some modern gastropods, bivalves, and brachiopods from the Lacepede Shelf of South Australia were investigated to determine the isotopic equilibrium and disequilibrium states of biogenic and non-biogenic (inorganic) carbonates and to understand the kinetic and metabolic isotopic effects on precipitating carbonates and the isotopic fractionation between these different carbonates and dissolved bicarbonate. Seawater and high Mg-calcite (HMC) marine cements from different depths were also analyzed to determine equilibrium isotopic composition. This study shows that the shells of some cool-water gastropods and bivalves precipitate in isotopic equilibrium with their ambient waters while brachiopod shells show ~80 values heavier than equilibrium values and coralline algae, and ahermatypic corals exhibit significant metabolic and kinetic effects. The isotopic equilibrium precipitation seen in these biogenic carbonates results from lower precipitation rates that are, in turn, caused by the lower bottom temperatures in this area.

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