Oxygen and carbon isotope composition of skeletons from temperate shelf carbonates, eastern Tasmania, Australia

Eastern Tasmanian shelf carbonates contain abundant skeletons of bryozoa, foraminifera and bivalve mollusca and minor brachiopods. The δ18O and δ13C isotope fields of Tasmanian bryozoa, benthic foraminifera, bivalve mollusca and brachiopods overlap other temperate brachiopods from North Atlantic and South Pacific shallow seas. The temperate skeleton isotope fields differ from isotope fields of similar types of skeletons from tropical shallow seas in having higher δ18O values. The δ18O and δ13C isotopes of temperate skeletons are least affected by metabolic effects and kinetic fractionation, in contrast to strong metabolic and kinetic effects in many tropical skeletons. The δ18O values of skeletons, taking δw=0 in δ18O‰ SMOW, give range of temperatures similar to those of measured values. The δ18O values of Tasmanian benthic foraminifera and brachiopods become heavier with increasing water depth due to the decrease in water temperature. Temperate carbonates are in equilibrium with δ13C in seawater and not with that in atmospheric CO2 The differences in δ13C and δ18O values between skeletons in the same sample represent variable growth rates of skeletons with brachiopods forming at the slowest rate, bryozoans at moderate rate and foraminifera at fast rate. The depth and latitudinal variation of δ18O and δ13C values of skeletons are due to differences in water temperatures, carbonate mineralogy, the rate of formation of these skeletons and mixing of water masses.