Elemental composition of marine calcite from modern temperate shelf brachiopods, bryozoans and bulk carbonates, eastern Tasmania, Australia

Abstract

Marine calcite is the predominant mineral in Tasmanian cool temperate shelf carbonates: brachiopods are low-Mg calcite, bryozoans are low-Mg to high-Mg calcite with appreciable aragonite and buld carbonates are mainly high-Mg calcite with minor aragonite. δ18O and δ13C of these fauna and bulk carbonates are in equilibrium with seawater temperatures and are not affected by meteoric diagenesis. Bryozoans and bulk carbonates contain CaCO3 cements. Therefore, these fauna and bulk carbonates provide baseline elemental composition of marine biotic and abiotic low-Mg calcite, low-Mg to high-Mg calcite and calcite and aragonite. Originally calcitic ancient carbonates are abundant and these can be better understood by comparison with modern marine calcite elemental composition. Ca values vary by about 3% between brachiopods, bryozoans and bulk carbonates mainly due to high concentrations of Mg and trace elements in bryozoans and bulk carbonates. Mg values are the lowest in brachiopods (0 to 1%), highly variable in bryozoans (0.1 to 2.1%) and are the highest in bulk carbonates (1.1 to 2.2%) due to differences in their mineralogy. The percentile distributions of Sr, Na, Mn and Fe in Tasmanian brachiopods, bryozoans and bulk carbonates indicate low concentrations of Mn, moderate Fe, high Sr and very high Na. The Sr/Na ratio is 30. In each sample the concentrations of Mg, Sr, Na, Mn and Fe are high in bryozoans and bulk carbonates compared to brachiopods due to temperature fractionation of these elements in a dysaerobic environment. The Na, Fe and Mn concentrations in both fauna and bulk carbonates are much higher than in abiotic aragonite due to calcite and vaterite mineralogy. From experimental data the Mg values of pure calcite correspond to seawater temperatures <16°C. These temperatures are close to measured temperatures of 2 to 18°C from deep water (≈500m) to surface summer seawater temperatures respectively. The positive correlation of Sr, Mn and Fe with Mg in Tasmanian bryozoans and brachiopods is due to a positive relationship of these elements with seawater temperatures. The covariation of Sr and Mg in brachiopods and bryozoans is due to higher rates of formation of temperate brachiopod and bryozoan skeletons compared to their tropical counterparts and higher aragonite content. Thus bryozoans and brachiopods are more abundant in cool to cold shelf waters than in tropical waters. Similar slopes of regression lines for Sr and Mg in brachiopods and bryozoans are due to the uniform composition of seawater. The positive correlations between Mn and Mg and Fe and δ18O indicate that these elements were derived from seawater and were incorporated into calcite in dysaerobic environment. The source of Mn and Fe is the terrigenous clastic sediments present in shallow Tasmanian shelf. Mg concentrations in Tasmanian calcite bryozoans, bulk carbonates and brachiopods are lower than those associated with eastern Tasmanian surface seawater temperatures (12 to 18°C) due to higher pCO2 levels in shelf water and mixing with upwelling cooler deep water.