STABLE-ISOTOPE ASPECTS OF MODERN MOLLUSKS, BRACHIOPODS, AND MARINE CEMENTS FROM COOL-WATER CARBONATES, LACEPEDE SHELF, SOUTH AUSTRALIA

The isotopic composition of some modem gastropods, bivalves, and brach iopods from the Lacepede Shelf of South Australia were investigated to determine the isotopic equilibrium and disequilibrium states of bioge nic and non-biogenic (inorganic) carbonates and to understand the kine tic and metabolic isotopic effects on precipitating carbonates and the isotopic fractionation between these different carbonates and dissolv ed 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 gast ropods and bivalves precipitate in isotopic equilibrium with their amb ient waters while brachiopod shells show O-18 values heavier than equi librium values and coralline algae, and ahermatypic corals exhibit sig nificant metabolic and kinetic effects. The isotopic equilibrium preci pitation seen in these biogenic carbonates results from lower precipit ation rates that are, in turn, caused by the lower bottom temperatures in this area. Copyright (C) 1997 Elsevier Science Ltd