C. Prasada Rao

Geochemical differences between subtropical (Ordovician), cool temperate (Recent and Pleistocene) and subpolar (Permian) carbonates, tasmania, australia

Subtropical (10°N) Ordovician carbonates are similar to modern warm-water ones and contain aChlorozoan Biota, diverse non-skeletal grains, abundant micrite, isopachous cements and early diagenetic dolomites and rare evaporites. Sr/Na ratios are around ≥ 3 as in modern warm-water carbonates. Covariance between Mn content and that of Sr and Na indicates stabilization of aragonite to calcite in a semi-closed diagenetic system. Covariance of δ18O and δ13C with Mn, Sr and Na of brachiopods and isopachous cements indicate Late Ordovician marine calcite values were around −5‰ ± 1.0 in δ18O and 1‰ ± 1.0 in δ13C. A small δ18O difference (2‰) between marine calcite and meteoric calcite indicates low latitude meteoric diagenesis. Sr/Mn ratios decrease within increasingly lighter δ13C due to dissolution of calcite in meteoric waters.Modern and Pleistocene cool temperate (40 to 44°S) carbonates are composed mainly ofBryomol fauna with marine calcite cements and non-skeletal grains are rare. Sr, Na and Sr/Na ratios (∼1) indicate mainly calcitic mineralogy with some aragonite. Covariance between Mn content and that of Sr, Na and Sr/Na indicates marine diagenesis. The δ18O and δ13C field is characterized by heavy δ18O and moderate δ13C values and is distinctly different from the warm-water field. The δ18O and δ13C values are uniform throughout the range of Sr, Na, Mn and Sr/Mn ratios as they are unaffected by meteoric diagenesis.Subpolar (80°S) Permian carbonates contain abundant glacial dropstones. Fauna is less diverse than in warm-water carbonates butEurydesma, brachiopods, bryozoa, pelecypods and crinoids is abundant. Marine to mixing-zone low Mg-calcite cements formed in water temperatures <3°C. Sr/Na ratios are low }1 due to calcitic mineralogy of fauna and low Mg-calcite cement. Covariance between Mn content and that of Sr and Na indicates open flow diagenesis. The δ18O and δ13C values of skeletal material are similar to those of warm Permian brachiopods and marine cements from other regions because the Tasmanian fauna either equilibrated with melt-water or melt-water diluted low salinity seawater (down to 28 ‰). The δ18O-δ13C field of whole rocks and cements falls in the mixing zone due to extensive melt-water influx. The covariance of δ18O and δ13C values with those of Sr, Na and Mn indicate a reduction of Sr and Na and an increase of Mn due to melt-water dilution. The covariance of Sr/Mn with δ13C reveals appreciable water/rock interaction with melt-waters.

Faunal relationship to grain-size, mineralogy and geochemistry in recent temperate shelf carbonates, western Tasmania, Australia

In western Tasmania cool temperate shelf carbonates predominate over siliciclastics and contain mainly bryozoan-molluscaforaminifera assemblages with minor algae, echinoderms, worm tubes, sponge spicules and ostracodes. Skeletons are mainly in gravel to sand fractions and minor in silt-clay fractions. Bryozoans are the main constituent in sand to gravel-size, foraminifera are the main constituent in fine sand-size and molluscans are mainly in the gravel-size fraction. Echinoderms and algae are in sand fraction, whereas sponge spicules occur in fine to very fine sand fractions.X-ray analysis of Tasmanian bulk sediments indicate that calcite (high-Mg to low-Mg calcite; mean 69%) and quartz (mean 22%) are the major minerals with minor aragonite content (mean 9%). Mg, Sr, and Na contents in bulk sediments are positively related to high-Mg calcite bryozoans. Sr and Na contents exceed abiotic calcite values due to biotic source of these elements. Compared to tropical bryozoans, the higher Sr contents in Tasmanian bryozoans indicate a higher rate of bryozoan skeletal formation in temperate waters. Mn and Fe contents of bulk sediments are closely correlated with r2 value of 0.85. These elements are derived mainly from terrigenous source and were incorporated into calcite in a dysaerobic marine environment.Tasmanian temperate bryozoan faunal assemblages differ from tropical chlorozoan assemblages due to variation in seawater temperatures. Bryozoans break down into fragments and are redistributed mainly as gravel to sand-size grains by currents. Normal salinity of seawater (34–35%) and nutrients in temperate waters allow abundant growth of fauna. Mixing of water masses maintain sufficent saturation of CaCO3 and thus preserve temperate carbonates.

Geochemical characteristics of cool-temperate carbonates, Tasmania, Australia

Modern carbonates predominate on the cool-temperate Tasmanian shelf. Bryozoa are the dominant fauna found in all samples with some mollusca, foraminifera, echinoderms, sponge spicules, algae and coccoliths. These are often encrusted and show borings, with some being ironstained. Individual grains of bryozoan sand consist predominantly of high Mg-calcites with variable amounts of low Mg-calcite and aragonite. Cementation of calcite and aragonite ranges up to 90% of the bryozoan sand.The Mg content ranges from 0.1 to 2.7% (using AAS) or 0 to 3.5% (based on X-ray speaks) which indicates <3 to 11° or 15°C ambient water temperatures. Sr-Mg data points lie both above and below the bryozoa field and extend up to aragonite and calcite fields because of appreciable calcite and aragonite cementation. Na-Mg values are mostly below the bryozoa field, due to cementation, with a few above the bryozoa field which indicates higher than normal biochemical fractionation or Na entrapment. Mn and Fe are positively correlated with Mg because of the dominance of marine diagenesis.Mn and Sr are randomly distributed, unlike the products of meteoric diagenesis. Mn and Na are positively correlated because of the marine origin of Mn. Sr and Na are positively correlated and their values are much higher than those in meteorically altered limestones. The Sr-Na data points are widely scattered away from the average bryozoa values because of cementation and biochemical fractionation of Na or due to entrapment of Na. Sr/Na ratios are ∼1, much lower than the ratios observed warm-water carbonates.Theδ18O and δ13C field is distinctly different from the warm-marine field and cool-temperate meteoric diagenetic calcite values. The sea-floor diagenesis line that corresponds to positive correlation of δ18O and δ13C passes through the Tasmanian carbonate isotopic field. The Sr and Na values are uniform throughout the range of δ18O. The Sr is positively correlated to δ13C and Na varies uniformly with δ13C. Both Fe and Mn are randomly related to δ18O and to δ13C. All these trends are indicative of marine diagenesis and are unlike meteoric diagenetic trends.The above chemical characteristics present cool-temperate marine diagenetic models, which might be used to differentiate similar ancient temperate carbonates from warm-water carbonates, and to understand meteoric and burial diagenesis common in ancient temperate carbonates.

Marine to mixing zone dolomitization in peritidal carbonates: The Gordon Group (Ordovician), Mole Creek, Tasmania, Australia

Dolomite is a common mineral in the Gordon Group, occurs in most of the stratigraphic sequences, is abundant in intertidal and supratidal carbonates and extends into some subtidal carbonates. Three major types of dolomitization common are: a) dolomitized burrows; b) mottled or dispersed dolomite; and c) laminar dolomite. Dolomite is predominantly subhedral, equigranular, fine-grained (submicron to 150 microns) and coarser than associated micrite. It is randomly distributed and replaces micrite extensively, and some oolites, peloids, intraclasts and rarely fossils. Sparry calcite cement and spar in veins are not replaced by dolomite. These features confirm that dolomite formed mainly during early diagenesis before and during spar cementation but prior to development of spar in veins. The former presence of evaporites is indicated in a few samples. Where dolomite is abundant, evidence of former evaporites is lacking, indicating that dolomites formed in normal marine to mixed-marine waters.The ranges of Sr and Na concentrations are similar to those of marine to mixing zone dolomites. The Mn and Fe concentrations in the dolomite indicate oxidizing to reducing conditions and influence of continental water during dolomitization. The decrease of Sr and increase of Mn with increasingly lighter values of both δ18O and δ13C in dolomite and associated micrite indicate meteoric diagenesis during their formation.Mole Creek dolomites are enriched in both δ18O (≈+2%∞) and δ13C (≈+0.5o/∞) relative to coexisting calcites. The δ13C values of dolomites and micrites are mostly parallel to each other in the stratigraphic sequence as a result of inheritance of δ13C from the micrite replacement. The δ18O values of dolomites and micrites are generally opposed to each other because δ18O of dolomite is derived mainly from the dolomitizing fluids.The Mole Creek dolomite isotopic field falls at the edge of the mixing zone dolomite isotope fields and overlaps that of the Ordovician-Silurian dolomite of Nevada because of the light δ18O of seawater and related meteoric water. The dolomitization is characterized by variable isotopic composition of marine and meteorically altered sediment and variable water composition. For this reason the dolomite isotopic field ranging from marine to mixing zone, overlaps marine calcite fields, extends toward meteoric calcite fields and is far removed from the burial calcite field. Dolomitization occurred simultaneously with or slightly after the transformation of metastable CaCO3 to calcite during early diagenesis. The major mechanisms of dolomitization are tidal pumping of seawater mixing with continental waters and mixing of seawater by torrential rains, reflux and capillary movements.

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 σw=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.

Shelf, coastal and subglacial polar carbonates, East Antarctica

Modern and Pleistocene polar carbonates occur in East Antarctica in shelf, coast, lakes and marginal to underneath glaciers, associated mainly with glacigene muds, boulder tills and diamictites. Shelf carbonates (in Prydz Bay) are calcitic and unlithified, and consist mainly of sponges, bryozoans, echinoderms, bivalves and diatoms. Coastal carbonates (in the Vestfold Hills) are calcitic and contain faunal assemblages similar to those on the shelf, with calcareous algae, microbial mats, minor peloids and cements. Lake carbonates are aragonitic micrites and peloids. Carbonates close to glaciers (the Løken Moraines) are aragonitic and contain abundant ooids with intragranular fibrous cements. Subglacial carbonates are aragonitic micrites and peloids. Carbonate mineralogy changes from mainly low-Mg calcite in marine shelf to aragonite in brackish to freshwater dominated inland regions.Antarctic carbonate δ18O values (4.5 to −47‰ PDB) vary markedly due to frigid temperatures (0 to −2°C) and salinity (0 to 35‰) changes, as a result of meltwater dilution from adjacent glaciers. Their δ13C values (−9 to 8‰ PDB) also vary markedly due to exposure to atmospheric CO2, the circulation of water masses and reaction of carbonate with CO2 trapped in glacial ice.The regional distribution of carbonate sediments and their sedimentology, mineralogy, and δ18O and δ13C compositions indicate three types of glacial environments of formation. The first corresponds to a glacial stage and the formation of subglacial and bank carbonates, when the Antarctic ice sheet expanded onto the inner shelves. The second corresponds to interglacial stages and the formation of ice-marginal carbonates, during the retreat of the ice sheet from the inner shelf grounding line and accompanying the discharge of appreciable meltwater. The third corresponds to an interglacial oasis and the formation of coastal carbonates, proximal to distal lacustrine carbonates, and distal subglacial carbonates.

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

In cold shallow seas undersaturated with CaCO3, 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 CaCO3 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 δ18O values that range from −0.8 to −1.7‰ SMOW. The δ13C 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 CaCO3 cements. The δ18O and δ13C 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 δ18O values of −1 to 0‰ and δ13C 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 CaCO3 and warmer shelf waters.

Implications of isotopic fractionation and temperature on rate of formation of temperate shelf carbonates, eastern Tasmania, Australia

On the eastern Tasmanian shelf, cool temperate carbonates predominate over terrigenous clastic sediments. They consist mainly of bryozoa and foraminifera with lesser mollusca and echinoderms, minor brachiopods and variable amounts of intragranular cements. Encrustations of fresh skeletons over old skeletons suggest a stable substrate. Borings and encrustations on cemented skeletal grains indicate slow rate of sedimentation. Lack of etchings, pittings and solution cavities suggest that seawater never became undersaturated with CaCO3. As the rate of carbonate formation is greater than rates of bioerosion and dissolution, extensive ancient temperate carbonates can form in regions of mixing water masses with low input of terrigenous clastics.Σ18O and Σ13C values of skeletons and whole carbonates are positively correlated. Water temperatures based on Σ18O thermometry give almost similar ranges from bryozoa, foraminifera, brachiopods and whole carbonates with brachiopods giving the maximum range of temperatures from 4 to 15°C. These temperatures are within the range of measured temperatures because biochemical fractionation is minor in these fauna. Kinetic effects on Σ18O and Σ13C values do not dominate in Tasmanian carbonates.Application of experimental inverse relationship of Σ13C values and rate of precipitation to temperate skeletons and whole carbonates indicates slow rate of formation (<40 K (L mole min−1)). As Σ13C of fauna is not significantly affected by biochemical fractionation, the differences in Σ13C values between skeletons in the same sample represent variable growth rates of skeletons within the experimental slow rate of formation with brachiopods forming at the slowest rate, bryozoa at moderate rate and forams at fastest rate. As Σ18O and Σ13C values are positively correlated with a slope of 1 in brachiopods, Σ13C values of brachiopods vary with water temperature.

Mineralogy and geochemistry of modern temperate carbonates from King Island, Tasmania, Australia

Shelf sediments around King Island range from siliciclastics to mixed carbonates and to pure carbonates. Carbonates consist mainly reworked calcitic fauna, such as bryozoans, foraminifera, echinoderms and red algae with minor intragranular CaCO3 cements. Gastropods are the main aragonitic fauna and these are rare. Bulk sediments are analyzed by both X-ray diffraction, atomic absorption spectrophotometry, X-ray fluorescence and mass spectrometry. Minerals detected by XRD are mainly high-Mg calcite, quartz and aragonite with minor low-Mg calcite. The Ca, Mg and SiO2 contents confirm the occurrence of siliciclastics to mixed carbonates and to pure carbonates. The concentrations of Sr and Na vary with carbonate mineralogy and skeletal content. The high Fe and Mn contents in calcite are due to sedimentation in reducing marine environments.The δ18O and δ13C field of bulk sediments overlaps isotope fields of bryozoans, foraminifera and brachiopods. All these isotope fields are dissected by both trendlines of seafloor diagenesis and upwelling water because sediments and fauna are in equilibrium with marine waters. The ambient water temperatures determined from δ18O values range from about 10 to 15°C, which are about 3°C lower than measured surface water temperatures.Originally calcitic ancient carbonates are abundant in stratigraphic sequences and their geochemistry can be better understood by comparison with baseline geochemical data of modern temperate calcitic carbonates rather than with modern tropical aragonitic carbonates. Many of these ancient originally calcitic bryozoan, foraminifera and echinoderm carbonates are of nontropical origin as coeval tropical aragonitic carbonates occur elsewhere.

Depth and latitudinal characteristics of sedimentological and geochemical variables in temperate shelf carbonates, Eastern Tasmania, Australia

In eastern Tasmania temperate shelf carbonates occur in latitudes between 40o30′ and 44oS in water depths from approximately 14 to 250 m. Increasing water depths correspond to decreasing water temperatures and salinities. Bryozoans (total, not species) increase with increasing water depth, bivalves are high in shallow-depths, foraminifera are high in mid-depth and gastropods are mostly located around 130 m. The amount of calcite relative to aragonite increases with increasing water depth due to decreasing water temperatures. The Mg, Sr and Na values increase with increasing water depth due to changes in carbonate mineralogy, the type of biota and the temperature. Mn and Fe values in bulk carbonates decrease with increasing water depth, due to the decreasing of terrigenous content. The δ18O values of bulk sediments, bryozoans, benthic foraminifera and brachiopods increase with increasing water depth, due to decreasing seawater temperatures and salinity, and the changes in carbonate mineralogy. The δ13C values of most of these carbonates increase with increasing water depth, mainly due to mixing of water masses and decreasing seawater temperatures. Latitudinal variations in sedimentology, carbonate elemental and isotopic compositions and mineralogy caused by seawater temperatures and salinities are small when compared to changes caused by increasing water depth. Combining present oceanographic features with those deduced from sedimentological and geochemical enables better understanding of the paleoceanography off Tasmania since the Last Glacial Maximum, related to seawater temperatures, salinity, mixing of water masses, sea-level changes, sedimentation and diagenesis.