ELEMENTAL COMPOSITION OF MARINE CALCITE FROM MODERN TEMPERATE SHELF BRACHIOPODS, BRYOZOANS AND BULK CARBONATES, EASTERN TASMANIA, AUSTRALIA

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 bulk carbonates are mainly high-Mg calcite with minor aragonite. delta(18)O and delta(13) C Of these fauna and bulk carbonates are in equilibrium with seawater temperatures and are not affected by meteoric diagenesis. Bryozoans an d bulk carbonates contain CaCO3 cements. Therefore, these fauna and bu lk 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 the se can be better understood by comparison with modern marine calcite e lemental 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 dif ferences in their mineralogy. The percentile distributions of Sr, Na, Mn and Fe in Tasmanian brachiopods, bryozoans and bulk carbonates indi cate low concentrations of Mn,moderate Fe, high Sr and very high Na. T he Sr/Na ratio is <1, whereas Fe/Mn ratio is >30. In each sample the c oncentrations 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 concent rations in both fauna and bulk carbonates are much higher than in abio tic aragonite due to calcite and vaterite mineralogy. From experimenta l data the Mg values of pure calcite correspond to seawater temperatur es <16 degrees C. These temperatures are close to measured temperature s of 2 to 18 degrees C from deep water (similar to 500m) to surface su mmer seawater temperatures respectively. The positive correlation of S r, 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 hi gher 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 shel f waters than in tropical waters. Similar slopes of regression Lines f or Sr and Mg in brachiopods and bryozoans are due to the uniform compo sition of seawater. The positive correlations between Mn and Mg and Fe and delta(18)O indicate that these elements were derived from seawate r and were incorporated into calcite in dysaerobic environment. The so urce of Mn and Fe is the terrigenous elastic sediments present in shal low 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 degrees C) d ue to higher pCO(2) levels in shelf water and mixing with upwelling co oler deep water.