Paleosalinity reconstruction of the Francevillian Basin (2.1 Ga): geochemical insights into the depositional environment of early macroscopic life

The Francevillian Basin of Gabon (2.1 Ga) hosts one of the earliest known macroscopic organisms preserved within the fossiliferous FB2b subunit. Understanding the depositional conditions of this interval is critical for reconstructing the paleoenvironmental context of early complex life. In this study, multiple geochemical proxies (S/TOC, B/Ga, Sr/Ba, and Y/Ho ratios) are applied to evaluate paleosalinity across key stratigraphic units of the basin. Results indicate pronounced values variability. The FB2b interval is characterized by consistently low salinity, reflecting a freshwater-influenced depositional setting, while the FC and FB1c units show more marine-like signatures, in agreement with evidence of hydrothermal inputs. The FB2a and FB1b subunits display intermediate values, pointing to fluctuating freshwater–brackish conditions. These paleosalinity trends are consistent with previously recognized sea-level fluctuations, facies, and oxygenation patterns in the basin. The findings highlight the heterogeneity of hydrological conditions in the Paleoproterozoic Francevillian Basin and identify FB2b as a unique freshwater influenced setting that may have favored the development and preservation of early macroscopic life.