Pascal Philippot

Early planetary degassing and atmospheric escape are two major, yet unconstrained, processes that shaped early Earth. Modeling predicts that the atmospheric ²⁰ Ne/ ²² Ne ratio is sensitive to solar-Ne mantle degassing over geological time. Until the Great Oxidation Event, atmospheric escape progressively depletes the atmosphere in Xe, leaving an isotopic imprint. However, the quantity of xenon in the ancient atmosphere remains largely unknown. In this study, we analyzed noble gases in Archean hydrothermal quartz fluid inclusions and show that a modern atmospheric ²⁰ Ne/ ²² Ne ratio was almost reached 2.7 Ga ago, implying intense mantle degassing during the first 1.7 billion years of Earth’s history, three orders of magnitude higher than today. Furthermore, we determine an Archean atmospheric Xe/Kr ratio, 2.3 times higher than today, consistent with models of Xe depletion over time through atmospheric escape.

Sedimentological evidence and detrital zircon provenance analysis show that the Amazonian Craton, Brazil, was subjected to large-scale subaerial erosion by c. 2.7 Ga. To assess weathering conditions attending the emergence of the Amazonian Craton, we used the chemical composition of terrigenous sediments deposited in the Carajás Basin, SE Amazonian Craton, between c. 2.7 and 2.6 Ga. Results from 41 samples collected in two drill cores along 150 and 250 m thick intervals show that emerged lands have been affected by strong chemical weathering conditions, similar in intensity to those occurring in the present-day inter-tropical belt. These intense chemical weathering conditions were probably due to the conjunction of warm temperatures (potentially as high as 22 ± 3°C), abundant precipitation (mean annual precipitation estimated at 1540 ± 150 mm) and high atmospheric CO ₂ content ( p CO ₂ ≥ 10 times the present atmospheric level) that enhanced the chemical alteration of labile minerals. These intense weathering conditions contrast with those inferred in other cratons, which suggests that regionally contrasted climates existed during the Neoarchean.

Banded iron formations (BIFs) are chemical sedimentary rocks commonly utilized for exploring the chemistry and redox state of the Precambrian ocean. Despite their significance, many aspects regarding the crystallization pathways of iron oxides in BIFs remain loosely constrained. In this study, we combine magnetic properties characterization with high‐resolution optical and electron imaging of finely laminated BIFs from the 2.7 Ga Carajás Formation, Brazil, to investigate their nature and potential for preserving ancient environmental conditions. Our findings reveal that magnetite, in the form of large 0.1–0.5 mm crystals, is the main iron oxide, with an overall averaged saturation magnetization (M_s) of 25 Am²/kg (corresponding to ~27 wt% of magnetite) over the studied 230 m of the sequence. Nevertheless, the non‐negligible contribution of minerals with higher coercivity suggests variable proportions of hematite along the core. Additionally, we observe non‐uniform behavior in magnetite grains, with distinct populations identified through low‐temperature measurements of the Verwey transition. Petrographic observations indicate that the original sediment was an Fe–Si mud consisting of a ferrihydrite–silica mixture formed in the water column. This assemblage was rapidly transformed into nano‐scale hematite embedded in silica as indicated by a honeycomb structure composed of Si‐spherules distributed in a microscale hematite matrix. Textural relationships show that the nucleation of magnetite started during or soon after the formation of hematite, as indicated by the preservation of the Si‐spherules within magnetite cores. Further magnetite overgrowth stages are characterized by inclusion‐free rims, associated with continuous Si supply during the evolving diagenetic or early metamorphic stages. These findings, combined with existing literature, suggest that ferrihydrite precipitated alongside Si and organic material, later crystallizing as hematite on the seafloor. Anaerobic respiration by Fe(III)‐reducing microorganisms likely contributed to early magnetite formation in a fluid‐saturated, unconsolidated sediment. Subsequent low‐grade metamorphism and Si mobilization led to palisade quartz precipitation and a second stage of magnetite growth likely formed at the expense of matrix hematite through thermochemical Fe(III) reduction. Low‐temperature magnetic analyses revealed that the two generations of magnetite core and rim are associated with specific stoichiometry.

The nitrogen isotopic composition of sedimentary rocks (δ¹⁵N) can trace redox-dependent biological pathways and early Earth oxygenation^1,2. However, there is no substantial change in the sedimentary δ¹⁵N record across the Great Oxidation Event about 2.45 billion years ago (Ga)³, a prominent redox change. This argues for a temporal decoupling between the emergence of the first oxygen-based oxidative pathways of the nitrogen cycle and the accumulation of atmospheric oxygen after 2.45 Ga (ref. ³). The transition between both states shows strongly positive δ¹⁵N values (10–50‰) in rocks deposited between 2.8 Ga and 2.6 Ga, but their origin and spatial extent remain uncertain^4,5. Here we report strongly positive δ¹⁵N values (>30‰) in the 2.68-Gyr-old shallow to deep marine sedimentary deposit of the Serra Sul Formation⁶, Amazonian Craton, Brazil. Our findings are best explained by regionally variable extents of ammonium oxidation to N₂ or N₂O tied to a cryptic oxygen cycle, implying that oxygenic photosynthesis was operating at 2.7 Ga. Molecular oxygen production probably shifted the redox potential so that an intermediate N cycle based on ammonium oxidation developed before nitrate accumulation in surface waters. We propose to name this period, when strongly positive nitrogen isotopic compositions are superimposed on the usual range of Precambrian δ¹⁵N values, the Nitrogen Isotope Event. We suggest that it marks the earliest steps of the biogeochemical reorganizations that led to the Great Oxidation Event.

The chemical exchange between the atmosphere, crust and mantle depends on sediment recycling via subduction. However, it remains unclear how atmospherically modified sediment may affect mantle oxygen fugacity through time. The Great Oxidation Event, among the most important atmospheric changes on Earth, offers an opportunity to investigate changes in magmatism related to surface–mantle interactions. Here we use sulfur K-edge micro X-ray absorption near-edge structure spectroscopy to measure the relative abundances of S⁶⁺, S⁴⁺ and S²⁻ state in apatite inclusions hosted in 2.4–2.1-billion-year-old igneous zircons from the Mineiro Belt, Brazil. The host magmas record intracrustal melting of juvenile crust and the involvement of recycled sediments in the sub-arc mantle wedge. Unaltered apatite inclusions reveal a change from reduced to more oxidized magmas from pre- to post-Great Oxidation Event during the early Proterozoic. We argue that this change is a direct result of deep subduction of oxidized sediments and thus evidence of mantle–atmosphere interaction across the Great Oxidation Event. This suggests that the onset of sediment recycling in the Archaean provided atmospheric access to the mantle, and early ‘whiffs’ of oxygen may have already contributed to a localized increase of calc-alkaline magmatism and related ore deposits on Earth.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The earliest evidence of life captured in lithified microbial mats (microbialites) predates the onset of oxygen production and yet, modern oxygenic mats are often studied as analogs based on their morphological similarity and their sedimentological and biogeochemical context. Despite their structural similarity to fossil microbialites, the presence of oxygen in most modern microbial mats disqualifies them as appropriate models for understanding early Earth conditions. Here we describe the geochemistry, element cycling and lithification potential of microbial mats that thrive under permanently anoxic conditions in arsenic laden, sulfidic waters feeding Laguna La Brava, a hypersaline lake in the Salar de Atacama of northern Chile. We propose that these anoxygenic, arsenosulfidic, phototrophic mats are a link to the Archean because of their distinctive metabolic adaptations to a reducing environment with extreme conditions of high UV, vast temperature fluctuations, and alkaline water inputs from combined meteoric and volcanic origin, reminiscent of early Earth.