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The Paleoproterozoic interval is characterized by the first evidence for incontrovertibly free oxygen in the ocean/atmosphere system. In this study, carbonates from the Transvaal Group, the Silverton, Lucknow, Rooinekke and particularly the Duitschland Formations, were analyzed for isotopes of carbon, oxygen and strontium in order to constrain models dealing with the patterns of seawater evolution. Although the trace element data showed variable alteration of the carbonate samples, selected samples with a lesser degree of alteration yielded interpretable results. The “best” measured 87Sr/86Sr isotope value of 0.7031 at 2.35 Ga ago fits well with the published curve for Precambrian seawater confirming that Paleoproterozoic budget of Sr was dominated by input from continental sources. During the Paleoproterozoic times, carbonates with δ13C values of about +10‰ are known to have been present worldwide and they have been studied also during this research. The observation that in the Duitschland Formation such high isotope values and “normal marine” values were both present within a single formation, indeed within its single beds, suggested that the large spread of δ13C values reflects local or regional rather than global phenomena. The oxygen isotope data show a large spread of values, from −3‰ to −16‰, while their corresponding δ13C signals are almost unchanged over this entire range, with the Lucknow and Silverton Formations clustering around 10‰ and the Rooinekke Formation around −2‰. The Duitschland Formation has a bimodal distribution with both δ13C populations present. The last formation also contains samples with δ18O as low as −20‰, particularly in the vicinity of the Bushveld intrusive body. These highly 18O-depleted samples show homogenization of δ13C values towards +5‰. Modelling suggests that the trends could have resulted from post-depositional alteration by basinal fluids, at near-surface to about 50 °C temperatures, in a rock buffered system. The Duitschland Formation samples, particularly at Duitschland Farm, the locality closest to the Bushveld Complex, were then further altered at temperatures up to about 80 °C, in response to this intrusion.

The Vindhyan Supergroup of central India records a substantial portion of Proterozoic time and contains some of the most disputed Precambrian/Cambrian fossil discoveries. Despite the significance of these “fossil” finds, many issues, including reliable geochronology, remained unresolved. Similarly, the geochemical aspects of the sedimentary sequences and their implications to the evolution of Proterozoic oceans, have been studied only at a reconnaissance level. Here we report the results of the Pb–Pb dating for the Rohtasgarh Limestone from the Lower Vindhyan Supergroup giving an age of 1601±130 Ma, an estimate that is in accord with the recent U–Pb zircon dating of the Lower Vindhyan Supergroup by Rasmussen et al. [Geology 30 (2002) 103] and Ray et al. [Geology 30 (2002) 131]. Sr isotope stratigraphy of carbonates from the Upper Vindhyan Supergroup suggests a Mid-Neoproterozoic (750–650 Ma) age for these formations. This, in turn, argues for a long hiatus between the deposition of the Lower and Upper Vindhyan Supergroups. Our results conflict with the reports of Cambrian age small shelly fossils and Ediacaran fossils from the Upper Vindhyan rocks. At the same time, they suggest that the alleged trace fossils in the Lower Vindhyan rocks must be of Paleoproterozoic age. The most common δ 18 O values for calcitic components from limestone formations of the Supergroup range from −10 to −7‰, similar to Paleo-Neoproterozoic carbonates worldwide. The modes of δ 13 C in the Lower and Upper Vindhyan Supergroups are 0±2 and 3±2‰, respectively, again consistent with the worldwide dataset. The secular patterns of carbon isotope trends do not support the earlier assumptions that the carbonate sequences at the southern margin correlate with those at the western or northern margins of the Vindhyan Basin. C and Sr isotope data provide new calibration points for the seawater evolutionary curves, particularly for the Paleoproterozoic. For strontium isotopes, the data from the Lower Vindhyan Supergroup suggest that seawater departed probably less rapidly from mantle values than was previously believed.

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between 4 m (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The 18 Oa nd 13 C values vary from − 0.1 to − 14.3‰ (PDB) and 0.2 to − 2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, 18 O, 87 Sr/ 86 Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The 13 C values show only small variations at the base of the formation, followed by a major negative plunge ( 4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87 Sr/ 86 Sr values of 0.70614–0.70734. The lowest 87 Sr/ 86 Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative 13 C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The 34 S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment. © 2001 Elsevier Science B.V. All rights reserved.

The Una Group, Irece Basin (Bahia state) is correlative with the better known Bambui Group of the Sao Francisco Basin (Minas Gerais state); these basins are remnants of widespread carbonate platform that covers > 300 000 km2 of the Sao Francisco Craton in Brazil. The Irece area contains a well-controlled stratigraphic succession hosting economic phosphate deposits and sulfide mineralization. Una Group strata comprise a basal siliciclastic sequence, mostly composed of glacial diamictites (Bebedouro Formation), followed by a carbonate succession that is several hundred meters thick, the Salitre Formation. An angular unconformity separates the two units. The carbonates of the Salitre Formation are characterized by 87Sr/86Sr ratios ranging between 0.70745 and 0.71776, and δ13C values ranging between −4.4 and +9.4‰ PDB. The carbonates also contain beds and nodules of barite and gypsum with highly positive δ34S values (+25.2 to +32.8‰ CDT). The ‘best preserved’ carbonate samples selected on the basis of their Mn and Sr concentrations as well as their ratios, yield 87Sr/86Sr values of 0.70745–0.70765. Comparison of these values with the secular strontium isotopic curve for the Neoproterozoic seawater suggests a 600 (to 670) Ma age for the Salitre Formation and for its correlative units in the Sao Francisco Basin.

Reply to comment by Chakraborty P.P. and Sarkar A. on “C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Supergroup, India: age, diagenesis, correlations and implications for global events” Jyotiranjan S. Ray a,∗, J. Veizer b,c a Physical Research Laboratory, Navrangpura, Ahmedabad 380009, India b Department of Earth Sciences, Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa ON, Canada K1N 6N5 c Institut fur Geologie, Mineralogie und Geophysik, Ruhr Universitat, Bochum 44780, Germany

Studies on sedimentary carbonate rocks, covering ∼ 2800 Ma time range, showed that K, Rb and Na are associated with aluminosilicate phases, particularly illite and (authigenic) K-feldspars. Additional Na is carbonate bound and controlled by facies (paleosalinity). The K 2 O/Al 2 O 3 and K/Rb ratios of the rocks correlate positively with their geologic age, while no secular trends were observed for the other parameters. The Phanerozoic portion of these secular variations is consistent with published observations from the Russian Platform and North America. However, the two sets of results diverge during the Precambrian. The Russian Platform and North American data suggest a provenance control of alkali-metal concentrations. Consequently, the deviation of the present data indicates either a different evolutionary path for Australia or a non-representative nature of the trends.

Chemical and mineralogical studies of sedimentary carbonate rocks, representing ∼ 2800 Ma time span, revealed that Ca, Mg, Fe and Mn are present mostly in the lattice positions of calcite and dolomite. Si in limestones is present often as quartz and in dolostones as chert. Ti and P are associated with a coarser fraction of the insoluble residue. The chemical composition of carbonates varies with geologic age. The total FeAl2O3, Fe2+Al2O3, MnOAl2 O3, MnO/total Fe, MgOCaO and SiO2Al2 O3 ratios increase from the Cenozoic to the Archean. The MgOCaO trend is valid for the pooled carbonate population only, while the other secular trends were observed in both limestones and dolostones. The major control on chemistry was exerted by depositional1 and early diagenetic environments, with epigenetic processes playing a subordinate role. This and the observation that the described secular trends are associated mainly with (bio)chemical precipitates indicate systematic changes in chemical composition of intervening solutions (e.g., coeval sea water, its early diagenetic derivatives, fossil meteoric water, or their mixtures). The causes of such evolution are not clear, but the inference of a lower PO2 or stronger volcanic influences, or both, during the early history of the Earth would be consistent with the observations presented.