New diagenetic constraints for the interpretation of Neoarchean stromatolitic dolostone: implication for the study of Great Oxidation Event onset in Southern Africa

The stromatolitic dolostones of the Ramonnedi Formation, in the Lower Transvaal Supergroup of Botswana, formed along a shallow carbonate platform during the transition between the Archean and Proterozoic Eons. They are bound to reveal more details on the timing and modes of the Earth’s oxygenation process. Nevertheless, their reliability as archives for Neoarchean paleoenvironmental condition depends on the degree of overprinting induced by diagenetic processes and metasomatism. Here, the focus is on the C-O isotope composition in the laminated stromatolitic dolostones and on S-O isotope compositions in pyrite grains and chert laminations found within the Ramonnedi Formation. The δ13C and δ18O compositions were obtained from powders micro-drilled from laminated dolostone; while the δ34S and δ18O compositions were obtained in-situ from pyrite and chert, respectively, using a virtual SIMS facility. These novel results complement a long list of analyses performed in the past and reveal that i) δ13C values between -0.52 and -1.41‰, and δ18O values between -8.43 and -14.4‰ are comparable with Archaean values from literature; ii) The carbon isotopes indicate anoxic conditions during the deposition of the Ramonnedi Formation; iii) the pyrite grains are not common in the dolostones and when present have sulphur isotope compositions that point toward abiotic formation, and iii) the stable oxygen isotope composition of the stromatolitic dolostone and from the chert laminae in the upper Ramonnedi Formation indicate diagenetic modification of the carbonate platform. These results have important implications in the interpretation of the geochemical proxies coming from the lower Transvaal Supergroup carbonate platform and for the interpretation of the onset of the Great Oxidation Event.