New diagenetic constraints for the interpretation of Neoarchaean stromatolitic dolostone geochemical composition and their palaeoenvironmental significance

Abstract

Abstract 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 Archaean and Proterozoic eons. These dolostones are bound to reveal more details of the mechanisms and timing of Earth oxygenation and particularly the role of microbial communities in the formation of carbonate platforms in the Archaean; but only if we are able to discriminate what signature is pristine and what is due to diagenetic overprinting. Here, the focus is on the isotopic composition of stromatolitic dolostone, pyrite grains and chert laminations found within the Ramonnedi Formation. Oxygen and carbon isotopes were investigated from the Ramonnedi Formation stromatolitic dolostones in order to constrain chemo-physical conditions at the time of deposition and, eventually, evaluate to what extent the system was re-set during diagenetic processes. In situ δ34S and δ18O compositions were determined by secondary ionisation mass spectrometry (SIMS) from pyrite and chert, respectively. These novel results complement previous analyses and reveal that: (i) the pyrite grains are not common in the dolostones and when present have sulphur isotope compositions that point toward abiotic formation; (ii) dolostone δ13CVPDB values between -1.2 and -0.7‰ are close to marine values and might reflect pristine Archaean signature; (iii) the dolostone δ18OVPDB values, between -9 and -11‰, on the other hand, might reflect diagenetic resetting or very warm Archaean seawater, in line with the δ18O values from the chert laminae in the upper Ramonnedi Formation. (iv) The correct interpretation of these results has 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 (GOE). Aim of this work is to complement the existing data on the Lower Transvaal Supergroup of Botswana and assess to what extent the geochemistry of these dolostone was overprinted during diagenesis/metasomatism. These results indicate that a multi-analytical approach is required to ensure the correct interpretation of geochemical proxy data related to the onset of the GOE.