Oxygen Production in Mesoarchean Terrestrial Environments: Implications for a Muted MIF-S and Climate Dynamics

Archean paleosols preserve rare direct evidence of interactions among the early biosphere, atmosphere, and continental surfaces before the Great Oxidation Event (GOE), but their redox significance in the Mesoarchean record remains highly debated. Here we reassess the ca. 3.0 Ga Denny Dalton paleosol of the Pongola Supergroup, South Africa, using an expanded parent-rock dataset, Ti-normalized redistribution patterns of redox-sensitive elements, thermodynamic constraints on Fe redox speciation in clay minerals, and a quantitative model of the minimum O2 flux required for weathering. A key uncertainty in previous studies has been the composition of the unweathered basement used as the geochemical reference frame. We address this by defining the parent rock from six chemically fresh basaltic-andesitic basement samples, rather than relying on a single previously used sample that likely represents altered or transitional material. Relative to this revised baseline, the paleosol records systematic redistribution of Fe, U, Cr, and V. Iron is enriched throughout most of the weathering profile, reaching up to ~3× the parent-rock level, indicating substantial internal mobilization and reprecipitation rather than simple net loss. Uranium is enriched upward, consistent with oxidative mobilization of relatively immobile U(IV) to soluble U(VI) followed by fixation in near-surface horizons through adsorption onto neoformed oxides and/or clay minerals. In contrast, Cr and V are progressively depleted upward, consistent with oxidation to mobile Cr(VI) and V(V) species and subsequent leaching. Together, these complementary patterns indicate oxidative weathering and redox-controlled redistribution within the profile. Independent mineral-scale constraints come from thermodynamic modeling of chlorite compositions, which yields Fe3+/ΣFe values of ~0.23–0.28, with an average of ~0.26. The preservation of a substantial ferric iron component within chlorite indicates the persistence of an oxidized iron reservoir and supports an interpretation in which Fe2+ released during weathering was effectively completely oxidized, followed by partial reduction within the soil profile. Using this redox framework, we applied a paleosol mass-balance model to estimate the minimum oxygen flux needed to oxidize Fe2+ released from parent basaltic andesite during weathering. The model yields values equivalent to ~1.6 × 10–3 to 3.8 × 10–2 PAL, with a best minimum estimate of ~1.2 × 10–2 PAL. Such oxygen fluxes most plausibly reflect production by oxygenic photosynthesis involving water-splitting metabolisms, although they do not necessarily imply sustained atmospheric accumulation at that level. We interpret this finding in the context of the Mesoarchean sulfur isotope paradox. Muted but persistent mass-independent fractionation of sulfur (MIF-S) indicates that oxidative processes were active, while the atmosphere lacked sustained oxygen accumulation. A plausible resolution is that significant O2 production by early microbial ecosystems was buffered by abundant CH4 in a hazy Archean atmosphere, creating a dynamic feedback between biological oxygen production and atmospheric redox conditions. This balance limited net O2 accumulation while allowing MIF-S to persist at a relatively small magnitude. Future Fe isotope analyses will further test this redox framework and better constrain the coupling between microbial oxygen production and surface redox evolution.