Anthony Chappaz

The formation of pyrite (FeS₂) is a critical process in sedimentary environments, central to the global iron and sulfur cycles and sedimentary redox cycling. However, the mechanisms governing pyrite formation under natural conditions remain poorly constrained. Using Mössbauer spectroscopy and X-ray photoelectron spectroscopy (XPS), we present direct evidence of polysulfides facilitating the transformation of FeS _x phases into pyrite in natural sediments from Saanich Inlet, British Columbia. We identify two nanoparticulate FeS _1+x phases: a mackinawite-like phase (np-FeS _1+x −1) and a greigite-like phase (np-FeS _1+x −2). Under persistently sulfidic conditions, the greigite-like phase can compete with pyrite as the prevailing Fe-S mineral. We also demonstrate that sequential chemical extractions underestimate highly reactive nanoparticulate Fe phases owing, in part, to pyrite framboids encapsulated within diatom frustules. Notably, these findings provide direct spectroscopic evidence consistent with polysulfide-mediated Fe-S transformations in low-temperature natural sediments, bridging a long-standing gap between experimental models and environmental observations and refining our understanding of early diagenetic pyrite formation.

Li is a critical element for the global transition toward low-carbon energy technologies. Thus, assessing Li in geological materials is timely relevant. Particularly in soils, Li has gained attention due to its effects on plant growth and as potential contaminant due to improper disposal of batteries and mining. This study aimed to develop a calibration for portable laser-induced breakdown spectroscopy (pLIBS) to assess low Li concentrations in tropical soils from Brazil. Li pLIBS-derived concentrations of 200 georeferenced soil samples were compared with those obtained by laboratory-based method. Twenty-seven certified reference materials with low Li concentrations (3 to 130 mg kg⁻¹) were used for calibration and validated using the results after total digestion and ICP determination. Li-related elements (K and Rb) were also considered. The calibration achieved R² values of 0.98, 0.78, and 0.85 and RMSE values of 6.51, 4470, and 17.87 mg kg⁻¹ for Li, K, and Rb, respectively. Independent validation showed strong correlation with reference methods (R² of 0.95, 0.87, and 0.95 for Li, K, and Rb, respectively). Expected Li–K–Rb geochemical relationships were observed, including positive correlations of Li with K and Rb, and a negative correlation with the K/Rb ratio. The Li-pLIBS distribution showed spatial similarity with the distribution obtained using the reference method. The results demonstrate the potential of pLIBS combined with a matrix-representative calibration for assessing low Li concentrations and identifying anomalous areas relevant to environmental monitoring and mineral exploration in highly weathered tropical soils.

The origin of terrestrial life and ecosystems fundamentally changed the biosphere. Lichens, symbiotic fungi-algae partnerships, are crucial to nutrient cycling and carbon fixation today, yet their evolutionary history during the evolution of terrestrial ecosystems remains unclear due to a scarce fossil record. We demonstrate that the enigmatic Devonian fossil Spongiophyton from Brazil captures one of the earliest and most widespread records of lichens. The presence of internal hyphae networks, algal cells, possible reproductive structures, calcium oxalate pseudomorphs, abundant nitrogenous compounds, and fossil lipid composition confirms that it was among the first widespread representatives of lichenized fungi in Earth’s history. Spongiophyton abundance and wide paleogeographic distribution in Devonian successions reveal an ecologically prominent presence of lichens during the late stages of terrestrial colonization, just before the evolution of complex forest ecosystems.

Identification of U( iv ), U( v ) and U( vi ) oxidation states in rocks can be achieved with HERFD-XRF imaging at the uranium M ₄ edge.

This paper is the first comprehensive synthesis of what is currently known about the different natural and anthropogenic fluxes of rhenium (Re) on Earth's surface. We highlight the significant role of anthropogenic mobilization of Re, which is an important consideration in utilizing Re in the context of a biogeochemical tracer or proxy. The largest natural flux of Re derives from chemical weathering and riverine transport to the ocean (dissolved = 62 × 10⁶ g yr⁻¹ and particulate = 5 × 10⁶ g yr⁻¹). This review reports a new global average [Re] of 16 ± 2 pmol L⁻¹, or 10 ± 1 pmol L⁻¹ for the inferred pre‐anthropogenic concentration without human impact, for rivers draining to the ocean. Human activity via mining (including secondary mobilization), coal combustion, and petroleum combustion mobilize approximately 560 × 10⁶ g yr⁻¹ Re, which is more than any natural flux of Re. There are several poorly constrained fluxes of Re that merit further research, including: submarine groundwater discharge, precipitation (terrestrial and oceanic), magma degassing, and hydrothermal activity. The mechanisms and the main host phases responsible for releasing (sources) or sequestrating (sinks) these fluxes remain poorly understood. This study also highlights the use of dissolved [Re] concentrations as a tracer of oxidation of petrogenic organic carbon, and stable Re isotopes as proxies for changes in global redox conditions.

A promising superparamagnetic nanomagnetite dipped with Gd was synthesized for possible medical applications. Its size and morphology are independent of Gd content ranging from 1 to 5%. Gadolinium (III) replaced Fe(III) in the lattice. The sizes of Gd-doped nanoparticles ranged from 5 to 50 nm and exhibited a pure magnetite mineralogical phase.

The Cambro‐Ordovician interval marks a significant transition from extinction to bio‐diversification in deep time. However, the relationship of bio‐transition to volcanism, commonly characterized by mercury (Hg) systematics in sedimentary records, has not been examined. We present the first Cambro‐Ordovician Hg systematics from the Scandinavian Alum Shale. Our results show pronounced Furongian Hg enrichments, coupled with positive Δ ¹⁹⁹ Hg, Δ ²⁰⁰ Hg, and Δ ²⁰¹ Hg values and negative Δ ²⁰⁴ Hg values that we ascribe to atmospheric Hg transport over long‐distances, while Early Ordovician Hg anomalies, characterized by near‐zero mass‐independent isotope values, indicative of submarine source. Our findings are supported by two new proxies: molybdenum‐Hg and vanadium‐δ ²⁰² Hg co‐variations, demonstrating Hg systematics were strongly influenced by changes in source and depositional conditions. Constrained by a synchronous atmospheric‐tectonic‐oceanic model, we hypothesize Furongian subaerial volcanism contributed to global extinction and oceanic anoxia, whereas Early Ordovician submarine volcanism concurrent with ocean water upwelling promoted the nascent bio‐diversification.