Carbonate minerals, major and minor elements and oxygen and carbon isotopes and their variation with water depth in cool, temperate carbonates, western Tasmania, Australia

Abstract

Cool temperature bryomol carbonates predominate on shallow shelves around western Tasmania. Many samples are bored and encrusted due to the occurrence of submarine hardgrounds and to slow rates of sedimentation. Intragranular CaCo 3 cementation is common. Tasmanian whole carbonates are mainly mixtures of high-Mg calcite (HMC) and low-Mg calcite (LMC) with minor amounts of aragonite. HMC and aragonite contents decrease while LMC content increases with increasing water due to a decrease in water temperature. Mg concentrations in calcite correspond to temperatures of < 10°C. Sr and Na concentrations in bryozoa decrease due to calcite cementation, whereas their concentrations increase with aragonite content. Mn and Fe concentrations are higher than in warm shallow marine counterparts and increase with increasing aragonite percentage. Sr and Na concentrations in calcite slightly increase with increasing LMC percentage due to decreasing water temperatures; Mn and Fe values in calcite are independent of water temperature. HMC percentage and HMC/LMC ratios decrease while LMC percentage increases with increasingly heavier δ 18O values due to decreasing temperatures. LMC is a marine mineral and not a product of meteoric diagenesis. Unusually high concentrations of Sr, Na, Mn and Fe in aragonite are due either to the tendency of aragonite formed in cold seas to incorporate more of these elements or to aragonite formation by transformation of metastable cold water CaCo 3 minerals such as vaterite or hydrous calcite. Sr contents decrease with increasing water depth due to a decrease in aragonite and increase in low-Mg calcite. Na values increase with increasing water depth due to increasing salinity. Mn and Fe concentrations decrease with increasing water depth due to decreasing continental terrigenous input. Depth related variations in Mg, Sr, Na, Mn and Fe are due to marine diagenesis and are unrelated to meteoric diagenesis. δ 18O and δ 13C values are positively correlated and this isotopic field is bisected by the diagenesis trend line for upwelling water that passes through the point −1‰ δ 18O, 0‰ δ 13C. The δ 18O values become heavier and Mg values decrease with increasing water depth due to decreasing temperatures, from about 17 to 6°C for normal seawater and from 13 to 2°C for upwelling water. These temperatures are lower than measured summer values because of the strong seasonal effect of surface waters, influx of subantarctic water in winter, and mixing of cold upwelling waters in the bottom water. As carbonates in upwelling regions are in equilibrium with upwelling water having δ 18O values of −1‰, in paleotemperature determinations seawater values that are lower by 1‰ than those postulated for different geological periods should be considered for those carbonates suspected of being non-tropical in origin on the basis of geological and geochemical evidence. Originally calcitic limestonesare abundant in ancient sequences and their temperatures of formation and diagenesis can be better understood by using the data base provided by this study.