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The Franceville Basin of Gabon contains one of the best-preserved records of Earth's Paleoproterozoic surface environments that began with the oxygenation of the atmosphere and its association with the largest known positive carbonate carbon isotope (δ 13 C carb ) excursion – the Lomagundi-Jatuli Event (LJE). In this study, we conducted a detailed petrographic, mineralogical, and geochemical characterization of carbonate rocks to assess the influence of authigenic and diagenetic processes affecting the primary δ 13 C carb record preserved in core LST12 obtained in the Lastoursville sub-basin. Previous studies reported an up-section trend in δ 13 C carb from positive values (5–9 ‰) to near-zero, and further to negative values (−5 to −17 ‰). This δ 13 C carb shift marking the termination of the typical LJE isotopic signature coincides with changing depositional facies settings. Our new results show that this isotopic shift parallels changes in carbonate mineral composition from stoichiometric dolomite with minor early diagenetic Fe-rich dolomite overgrowths to increasingly Fe- and Mn-rich carbonates formed under sediment-buffered, closed-system diagenetic conditions. The formation of complex diagenetic Ca-Mn-Fe-rich carbonate phases in the topmost manganiferous black shales of core LST12, however, was driven by open-system diagenesis. This involved microbial remineralization of organic matter during progressive basin restriction and dynamic hydrothermal influx. The declining δ 13 C carb trend in carbonate lithologies of LST12 core is therefore interpreted to record syn-depositional variations in the dissolved inorganic carbon pool, reflecting changing environmental conditions in the Paleoproterozoic Francevillian basin.
Nzamba et al. (Sat,) studied this question.