Knowledge of Cambrian animal anatomy is limited by preservational processes that result in compaction, size bias, and incompleteness. We documented pristine three-dimensional (3D) anatomy of trilobites fossilized through rapid ash burial from a pyroclastic flow entering a shallow marine environment. Cambrian ellipsocephaloid trilobites from Morocco are articulated and undistorted, revealing exquisite details of the appendages and digestive system. Previously unknown anatomy includes a soft-tissue labrum attached to the hypostome, a slit-like mouth, and distinctive cephalic feeding appendages. Our findings resolve controversy over whether the trilobite hypostome is the labrum or incorporates it and establish crown-group euarthropod homologies in trilobites. This occurrence of moldic fossils with 3D soft parts highlights volcanic ash deposits in marine settings as an underexplored source for exceptionally preserved organisms.

Understanding paleoenvironmental dynamics linked to biological evolution in Earth history is a major goal within the geological community. The difficulty of achieving this goal, at least in part, lies with the secondary transformations experienced by a majority of ancient rocks, especially through metamorphism and hydrothermal activity. The Mesoproterozoic (∼1.1 Ga) shallow-marine deposits from the Taoudeni Basin, Mauritania, have suffered a complex, multiphase tectonic, and thermal evolutionary history. Representative samples from two drill cores (a background site [S2] and a dolerite intrusion-bearing drill core [S1]) from the El Mreiti Group were evaluated for transformations and overprints of original mineralogies and geochemical compositions. Our results show that the drill core hosting the dolerite intrusion (S1) is characterized by a suite of minerals (that is, pyroxene, graphite, pyrrhotite, garnet, zeolite, and authigenic clay minerals) resulting from contact metamorphism and associated hydrothermal activity. However, compared to the S1, the S2 core shows no evidence of post-depositional transformation. The geochemical data obtained from S1 reveal a striking elevation of iron contents likely delivered from the hydrothermal fluids. Moreover, concentrations of redox-sensitive trace elements (molybdenum, uranium, and vanadium) increased dramatically during hydrothermal and metamorphic activity. This study demonstrates that need for caution when assessing paleoenvironmental conditions in ancient sedimentary rocks, particularly for iron and trace metal approaches commonly used in reconstructions of paleo-redox.

Understanding and reconstruction of the paleo-condition dynamics linked to biological evolution in Earth history remain a big challenge because a majority of the ancient rocks have been affected by secondary modification processes, including tectonic, metamorphic, and hydrothermal activities. This study examines the influence of magmatic intrusion on sediment composition and paleo-environmental reconstruction from two drill cores (S1 and S2) drilled into the shallow-marine Mesoproterozoic (~1.1 Ga) El Mreiti Group of northeast Taoudeni Basin, Mauritania. Petrographic and mineralogical data show that the S1 drill core, intruded by dolerite sill, consists of a series of metamorphic minerals, including pyroxene, graphite, pyrrhotite, garnet, zeolite, talc, and saponite in sediments within the contact aureoles of the dolerite sill, indicating the influence of contact metamorphism and associated hydrothermal activities. The dominance of low-temperature minerals and the absence of metamorphic minerals in the S2 drill core sediments demonstrate that they are largely preserved and were only affected by high-grade diagenetic modifications. The anomalous enrichments of the Fe and redox-sensitive trace elements (RSTEs) in sediments within the vicinity of the dolerite sill coincide with increasing pyrrhotite contents, suggesting the transfer and remobilization of the RSTEs via thermal decomposition of pyrite to pyrrhotite during metamorphism and hydrothermal processes at elevated temperatures. This is supported by the absence of hematite, low Th/U ratios, and increasing Eu anomaly values in the dolerite sill and contact aureoles. This study reinforces the importance of screening and assessment of samples for post-depositional alteration effects before being used for the reconstruction of paleo-redox conditions in modern and ancient sedimentary rocks.

Les chapeaux de fer (ou gossans) développés à l’aplomb d’une minéralisation sulfurée et/ou oxydée primaire restent un métallotecte majeur pour l’exploration minière. L’identification de gossans au sein de régions arides à régolithe ferrugineux de différentes origines n’est pas aisée et suppose une bonne connaissance de ces formations superficielles indurées rouges à l’échelle régionale. En s’inscrivant dans cette démarche, nous avons étudié certaines cuirasses ferrugineuses de la région de Khat Oummat Elbid (KOB) d’Inchiri en Mauritanie, région activement prospectée pour l’or et le cuivre en particulier. La zone de KOB appartient au segment orogénique des Mauritanides septentrionales. Deux collines à socle de quartzite et coiffées de cuirasses ferrugineuses ont été sélectionnées pour cette étude. Les campagnes de terrain ont permis d’étudier le socle, de lever des logs dans les cuirasses et d’échantillonner pour des analyses pétrographiques, minéralogiques (microscopie optique, MEB, DRX) et géochimiques (multi-élémentaire roches totales). Il apparaît que les cuirasses étudiées sont des ferricrètes au sens strict sans lien direct avec le développement de profils latéritiques. Le socle quartzitique contribue uniquement en produisant une fraction clastique grossière à la base des ferricrètes. Le ciment des ferricrètes est goethitique à la base entre les clastes de quartzite tandis que le sommet est marqué par des granulométries très fines et des processus authigéniques dominants (nodules et pisolithes à cortex de goethite et hématite). De la base au sommet des ferricrètes, des grains d’ilménite arrondis accompagnés de grains de quartz de petite taille suggèrent un transport éolien « saupoudrant » ce matériel depuis des sources externes. Ces ferricrètes sont donc caractérisés par des allochtonies mécaniques et chimiques importantes, la source du Fe en solution comme des grains d’ilménite étant difficilement traçable avec certitude dans cette région. La comparaison avec des teneurs moyennes en éléments majeurs et traces de gossans, cuirasses latéritiques et minerai de Fe des tourbières (« bog ore ») montre que la géochimie en roche totale est utilisable pour discriminer les ferricrètes de KOB des gossans au premier ordre, mais qu’il convient d’implémenter les bases de données avec des analyses de gossans se développant au-dessus de gisements de type iron-oxides-copper-gold deposits comme ceux exploités en Inchiri (e.g. gisement de Guelb-Moghrein, près d’Akjoujt).

Recently, two independent studies suggest that the emergence of putative fossilized macro-eukaryotes in the Paleoproterozoic Francevillian Basin, ~2.1 billion years ago, may be related to a rise in seawater Zn bioavailability. This explanation is reliant on their extraordinary high Zn content and association with light Zn isotopes characteristic of eukaryotic enrichment. However, the trigger and origin of rising seawater Zn supply to the basin remains unknown. This study unravels a transient episode of intense submarine hydrothermal activity that triggered the weathering of a nutrient-rich oceanic crust reservoir, related to the collision of the Congo-São Francisco cratons during the Eburnean-Transamazonian orogeny, as the source of abundant seawater dissolved Zn, together with a suite of essential trace metals and phosphate to the continental margin waters. Surprisingly, the initiation of hydrothermal weathering coincided with the rapid onset of a rare Paleoproterozoic seawater eutrophication event. This transition is marked by basin-wide redox stratification, high sediment loading with organic carbon (C_org) and nitrogen, elevated C/N ratios, a steep negative C_org and positive bulk N isotope excursion, positive Ce anomalies, and low Mn/Fe ratios. Importantly, the transient eutrophication event ended with a reversal to lower seawater phosphate levels that coincided with rapid seawater ventilation and the appearance of macrofossil bearing sediments in Franceville. We suggest that these unexpected, localized conditions, set the stage for the emergence of the Francevillian biota.