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• W2601223255 abstract "Abstract Chemical sediments, such as carbonate rocks, iron formations and cherts, record physico-chemical conditions of the hydrosphere, atmosphere, and biosphere at the time of their formation. In order to reconstruct the depositional environment, chemical composition of ancient seawater, and atmosphere-biosphere evolution, sedimentological, mineralogical, isotope and trace-element geochemical studies of the c. 3-billion-years-old microbial carbonates of the Pongola Supergroup were undertaken. Sedimentary carbonates are extremely rare in the Palaeo- to Mesoarchaean Eras as compared to Neoarchaean and younger rock records. A brief revi5ew of the occurrences and characteristics of Archaean carbonate rocks revealed that they display specific characteristics in terms of thickness, mineralogy, texture and diversity in microbial structures, which are unique in each Era. Stromatolitic carbonate rocks of the Chobeni Formation of the Nsuze Group, Pongola Supergroup, were studied in the White Mfolozi River valley of KwaZulu-Natal in South Africa. The 3.0-2.9 Ga old Pongola Supergroup was deposited on continental crust of the southeastern Kaapvaal Craton and is one of the oldest well-preserved successions of epicratonic volcanic and sedimentary rocks. Four carbonate-bearing stratigraphic horizons ranging from 2 to 28 m in thickness were encountered in the siliciclastic-dominated Chobeni Formation, and three of them were studied in detail. Sedimentological and facies analyses of carbonate and associated siliciclastic rocks suggest deposition in a tide-dominated, shallow-marine environment. In total, fourteen facies were identified and grouped into three facies assemblages, which formed in subtidal, intertidal and supratidal sub-environments under the influence of cyclic variations of Mesoarchaean sea level. Various types of microbialites were observed in rocks of different sedimentary sub-environments. Field observations, petrographic studies and mineralogical analyses revealed that microorganisms were responsible for microbialite formation by inducing carbonate precipitation via metabolic processes, in- situ carbonate precipitation and, to a lesser extent, by trapping and binding of sediments. Well-preserved ooids with a variety of fabrics (concentric, radial, radial-concentric and micritic) and preserving carbonaceous matter were discovered that merited detailed petrographic and mineralogical-geochemical analyses. NanoSIMS ion mapping and Raman spectroscopic imaging confirmed the biogenicity of carbonaceous matter, and along with scanning electron microscopy, the distribution of mineral and elemental phases within ooids was determined. Based on major and trace-element data obtained by electron microprobe and LA-ICP-MS, the absence/ presence of carbonaceous matter, the level of fabric preservation in ooids with different fabrics, and the similarity of Chobeni ooids with their modern counterparts, a combination of both biogenic and non-biogenic processes (environmental conditions) are suggested responsible for ooid formation. For better constraining the degree of alteration of the rocks and investigating their suitability as a geochemical proxy for the chemical characteristics of the depositional environment and Mesoarchaean seawater, carbon (C), oxygen (O) and Strontium (Sr) isotopic analyses were undertaken on selected carbonate samples. These data were compared and interpreted together with their major and trace-element geochemical compositions. The Sr, Fe and Mn contents of the carbonate samples from the different sections of the main, middle and upper carbonate are variable. The main section carbonate is characterized by relatively low FeO and MnO concentrations, higher SrO content, and higher C and O isotopic values as compared to those of the upper and middle sections, which indicates a less pronounced diagenetic and metamorphic overprint. The lower δ18O values in the upper and middle carbonate sections are probably a result of meteoric diagenesis, alteration by hydrothermal/ metamorphic fluids and isotopic exchange with clastic material. The highest δ13C value of these carbonates (2.4 ‰PDB) is interpreted to represent the value closest to that of coeval seawater. 87Sr/86Sr ratios correlate positively with the concentration of siliciclastic detritus. The least radiogenic 87Sr/86Sr ratio of ca. 0.708 is higher than the ratio proposed for a depleted mantle-buffered Archaean ocean. The elevated ratio in the sample may be due to overprinting by fluids with highly radiogenic Sr from intercalated shales during diagenetic/ metamorphic processes, yet, it could well be a primary signal recording radiogenic continental Sr derived from riverine input into the shallow epicontinental Pongola Sea. Shale-normalized rare earth element and yttrium (REE+Y) distribution patterns and positive shale-normalised La, Gd and Y anomalies are similar to Archaean marine chemical precipitates. Relatively subdued marine signals along with a convex upward pattern of the middle REE may be a result of organic matter complexation, and fractionation during estuarine mixing, suggesting that the Pongola carbonates were deposited in a restricted marine setting with riverine input and variable exchange to the open-ocean as also indicated by their Sr isotope composition. The presence of a slightly negative Ce anomaly and Th/U > 2 in some samples may indicate mildly oxygenated conditions in line with the likely presence of photosynthetic microorganisms responsible for microbialite formation." @default.
• W2601223255 created "2017-04-07" @default.
• W2601223255 date "2017-02-01" @default.
• W2601223255 modified "2023-09-27" @default.
• W2601223255 title "A sedimentological and geochemical study of stromatolitic carbonate rocks of the Mesoarchaean Pongola Supergroup, South Africa, and inferred environmental conditions" @default.
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