Temperate shelf carbonates reflect mixing of distinct water masses, eastern Tasmania, Australia

In cold shallow seas undersaturated with CaCO 3, carbonates disintegrate and dissolve away within a short period of time. Understanding the mixing of water masses from oceanographic and isotope point of view is important because these water masses provide nutrients and maintain CaCO 3 in cold shallow seawater. Temperature and salinity variations in surface seawater off the coast of eastern Tasmania are caused by influxes of different waters. Water from Coral Sea water provided by the East Australian Current prevails in the summer, whereas Subantarctic water dominates during the winter. Throughout the year the Tasman Sea water is mixed with low salinity and low temperature deep Antarctic Intermediate water. The Antarctic Intermediate water and Subantarctic water contain an admixture of about 4% glacial melt water, resulting in δ 18O values that range from -0.8 to -1.7‰ SMOW. The δ 13C values are ∼0‰ in Antarctic Intermediate water and they are ∼1‰ in Subantarctic water. The Tasmanian carbonates consist mainly of reworked calcitic fauna, such as bryozoans, foraminifera, echinoderms and red algae with variable intragranular CaCO 3 cements. The δ 18O and δ 13C isotope fields of eastern Tasmanian bulk carbonates, bryozoans, benthic foraminifera and brachiopods overlap and all grade into the field typical of deep-sea carbonates. The trend lines of seafloor diagenesis and upwelling water pass through fields of temperate skeletons and bulk carbonates because they are in equilibrium with mixed seawaters having δ 18O values of -1 to 0‰ and δ 13C values of 0 to 1‰. They are forming at a slower rate than tropical water carbonates. Temperate carbonates form in zones of mixing of nutrient rich cold waters saturated with CaCO 3 and warmer shelf waters. © 1995 Springer.