Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals

Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, ~720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval

Diagenetic transformations and redox fluids invasion in siliciclastic FA formation, Franceville basin (2.2-2.0 Ga), Gabon

Dans le bassin de Franceville (Gabon), les sédiments détritiques non-métamorphosés d’âge Paléoprotérozoïque (2,15 Ga) des formations FA et FB ont fait l’objet d’une étude faciologique, pétrographique et géochimique. L’objectif était de déterminer l’origine de ces matériaux clastiques et des minéralisations uranifères associées, mais aussi d’en retracer l’histoire diagénétique à travers l’évolution des paléoconditions rédox et des fluides qui les ont percolés.La pétrographie des faciès montre que la minéralogie et la texture initiales de ces sédiments ont été fortement modifiées au cours de la diagenèse précoce. Au toît de la formation FA, les quartz-arénites faiblement granoclassées ont été très tôt cimentées par du quartz, s’opposant ainsi à l’action ultérieure de la diagenèse d’enfouissement. Dans les arènes arkosiques, la séquence paragénétique liée aux ciments authigènes suggère que les interactions fluides-roches ont été polyphasées et que les éléments chimiques nécessaires à leur précipitation trouvent leur origine dans l’altération de minéraux détritiques. Pour un ensemble lithologique donné, les effets de la diagenèse varient peu d’un faciès à l’autre, ce qui indique un fort contrôle des paramètres initiaux tels que la minéralogie, la texture et la composition du fluide poral. Par conséquent, ces faciès sédimentaires nous informent directement sur les propriétés hydrologiques qui régnaient dans le bassin sédimentaire, et sur la nature de la diagenèse précoce qui les a affectés.Les données pétrographiques et géochimiques montrent que la précipitation de l’hématite en lits ferrugineux a débuté juste après le dépôt des sédiments, dont l’altération des minéraux primaires a libéré le fer, plus tard redistribué durant la diagenèse. Ainsi, la mise en évidence d’une corrélation positive entre les valeurs de "delta" 56Fe et les rapports Fe/Mg mesurés sur échantillon total suggère que le fer se répartit entre deux pôles que sont l’hématite authigène et les silicates porteur du fer. En revanche, l’absence de relation entre les rapports Fe3+/FeT et les compositions isotopiques du fer démontre que des oxydes riches en isotopes lourds du fer préexistaient dans les sédiments lorsque la diagenèse précoce a débuté. Le fer présent initialement à l’état réduit dans des faciès sédimentaires verts s’est progressivement oxydé au cours de la diagenèse, processus à l’origine de la formation des lits ferrugineux.Dans la formation FA du Bassin de Franceville, les gisements uranifères résultent de la libération de l’uranium contenu à l’origine dans les grès oxydés des dépôts fluviatiles inférieurs puis de sa migration et de son piégeage dans les grès réduits et les mudstones silteux des formations de deltas de marée supérieures, où il s’est concentré par un mécanisme de type roll-front.L’analyse des éléments chimiques majeurs et traces des formations FA et FB indique que leurs matériaux constitutifs trouvent leur source dans des roches ignées felsiques. Tandis que les diagrammes de terres rares normalisés aux chondrites, les anomalies négatives en Eu et les rapports GdN/YbN plaident plus précisément en faveur des roches archéennes et paléoprotérozoïques. Enfin, des diagrammes discrimants et des rapports en éléments majeurs et traces montrent que, si la plupart des grès de la formation FA se sont déposés sur une marge passive, les mudstones et les grès fins des formations FA et FB ont sédimenté sur une marge continentale active en relation avec la tectonique paléoprotérozoïque de la Ceinture orogénique du Centre-Ouest de l’Afrique.The FA and FB Formations clastic sediments have been subjected to detailed facies, petrographic, and geochemical analyses in relation to diagenesis, fluid flow, paleo-redox conditions, provenance, and uranium mineralization during the evolution of the unmetamorphosed Paleoproterozoic (ca 2.15 Ga) Franceville Basin, Gabon. Lithofacies analyses in combination with petrographic studies indicate that the original mineralogical and textural properties of the sediments have been greatly modified during diagenesis. The moderately sorted quartz arenite at the top of FA underwent early quartz cementation; thus preventing it from subsequent burial diagenetic processes. The inferred paragenetic sequence of authigenic cements in the arkosic arenites suggest a multiphase of fluid-rock interactions with most of the ions needed for their precipitations likely sourced during alteration of detrital precursors. The observed slight variations in the diagenetic pathways in different lithofacies associations are closely related to primary mineralogy, texture, and nature of pore fluid. This relationship suggests that depositional facies can provide an insight into the diagenetic pathways and hydrologic properties of sediments in sedimentary basins. Petrographic features coupled with whole rock geochemical and iron isotope analyses suggest that hematite precipitation in the red beds started after sediment deposition with the iron internally derived by alteration of iron-bearing minerals and redistributed during late diagenesis. Positive correlation between Fe/Mg ratio and "delta" 56Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates. The lack of relationship between Fe3+/FeT ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis or burial. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a roll-front type in the FA Formation of the Franceville Basin. Major and trace element geochemical data for the FA and FB formation are consistent with sediments derived exclusively from felsic igneous source. Chondrite-normalized patterns with high LREE/HREE ratios, negative Eu anomalies, and GdN/YbN ratios favour mixture of Archean and Post-Archean felsic sources. Discriminant diagrams and elemental ratios of major and trace elements suggest deposition of most of the FA sandstone in a passive margin, while the mudstones and fine-grained sandstones of FA and FB were deposited in an active continental margin during Paleoproterozoic tectonic regimes of the West Central African Belt

Elemental geochemistry and Nd isotope constraints on the provenance of the basal siliciclastic succession of the middle Paleoproterozoic Francevillian Group, Gabon

International audienceWhole-rock elemental geochemistry and Nd isotope composition of siliciclastic rocks from the basal part of the unmetamorphosed middle Paleoproterozoic Francevillian Group, Gabon (FA Formation and FB1 Member) were examined to provide insights into their provenance and its crustal evolution. Chemical weathering index diagrams suggest moderate degree of weathering in the provenance and secondary K-addition during illitization. Trace element geochemical proxies including the REEs provide evidence for sediment derivation from felsic-dominated upper continental crust (UCC) similar to Archean trondhjemite-tonalite-granodiorite (TTG) with minor contribution of mafic components. The low 143Nd/144Nd and negative εNd values (-10.83 to -5.76) provide further evidence for old and evolved upper continental crust components in detrital sources. The depleted mantle model ages for the provenance protolith (2.73-3.05 Ga) are consistent with sediments sourced from the Mesoarchean granitoids of East Gabonian block exposed in the northern and southern parts of Gabon. We infer that the evolved, Mesoarchean felsic TTG-like granitoids were the main source for the sediments of the basal Francevillian Group, while minor mafic inputs from the contemporaneous greenstone belt with the Belinga Group cannot be completely discounted. The predominantly old crustal sources for the sediments suggest crustal addition and recycling of differentiated granitoid plutons instead of formation of juvenile crust during deposition in the Francevillian basin

Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon)

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Textural and paleo-fluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon

International audienceThe Paleoproterozoic (similar to 2.15 Ga) Franceville Basin, South Eastern Gabon, is a continental sedimentary basin that host unmetamorphosed sediments. This study involve detailed mineralogy, sedimentology, and petrography of the basal sedimentary units of FA and lower FB (FB1) Formations, from the basin margin to centre in relation to mineral paragenesis and fluid flow. The FA Formation conglomerate, sandstone, and mudstone consists lithofacies of mixed fluvial and fluvio-deltaic transitional origin, while the overlying FB1 Formation includes alternating organic rich black shale and sandstone of marine deposit. The medium- to coarse grained fluvio-deltaic quartz arenite in upper part of FA Formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history. This provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis. In contrast, the clay and unstable detrital grains rich fluvial arkosic to sub-arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts. These arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities. From the proximal to distal basin and within sample suites, there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore-fluid. The predominance of 1M(t) illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid/rock ratio. Dolomite, anhydrite, barite, and Fe-oxides are the main crystallized authigenic minerals aside illite and Fe-rich chlorite clay minerals. The mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the FA Formation in the Franceville Basin are mainly controlled by depositional fades. (C) 2015 Elsevier B.V. All rights reserved

Origin of red beds in the Paleoproterozoic Franceville Basin, Gabon, and implications for sandstone-hosted uranium mineralization

Red beds are extensively used as evidence for an oxygenated atmosphere in the Paleoproterozoic. Red beds in the unmetamorphosed, ca. 2.15 Ga FA Formation of the Franceville Basin, Gabon were geochemically, petrographically, and mineralogically characterized to constrain the process of their formation and their relationship to atmospheric oxygenation in the early Paleoproterozoic. Petrographic observations indicate that ferric oxides are dispersed in clay filling intergranular pores and along platy cleavage in altered phyllosilicates. Grain-coating hematite is generally rare to absent in most samples suggesting hematite precipitation after sediment deposition and during burial diagenesis. Textures and geochemical data suggest that iron was likely sourced internally by alteration of iron-bearing minerals such as biotite and chlorite in sediments and redistributed in rocks during late diagenesis. Positive correlation between Fe/Mg ratio and δ56Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates (biotite and chlorite). The lack of relationship between Fe3+/FeT ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis and burial. Large range of δ56Fe values extending towards positive values, up to +1 permil, is similar to that observed in Archean and Paleoproterozoic iron formations or modern groundwater-derived Fe-oxyhydroxides, suggesting partial oxidation of Fe under mildly oxidizing conditions during early diagenesis. In addition, positive correlation between Cr/Fe ratios and iron isotope values, especially in the fine-grained sandstones and mudstones, is evidence for authigenic Cr enrichment under locally mildly-reducing conditions in a fluvial setting and strongly oxidizing conditions during weathering and riverine transport. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a sandstone-type in the FA Formation of the Franceville Basin

Hydrothermal seawater eutrophication triggered local macrobiological experimentation in the 2100 Ma Paleoproterozoic Francevillian sub-basin

It is thought that the global predominance of small-size unicellular prokaryotic life in the oceans until the emergence of large-size multicellular organisms to ecological dominance in the Ediacaran Ocean after 635 million years ago (Ma), was partly constrained by paleo-dynamic nutrient limitation, with phosphorus (P) being the principal limiting resource. Here we couple an episode of intense submarine hydrothermal alteration of a nutrient-rich seafloor reservoir to the collision of the Congo-São Francisco cratons at ∼2100 Ma, to unravel a paleo-geodynamic incident of seawater P enrichment in the Paleoproterozoic Francevillian sub-basin. We propose that this previously unrecognized local pulse in dissolved seawater P concentration, of comparable magnitude to Ediacaran seawater levels, set the stage for Earth’s earliest biospheric experimentation towards macrobiological complexity ∼2100 million years ago

Emergence of the 2.1 Ga Francevillian biota was preceded by unprecedented hydrothermally driven seawater eutrophication

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 (Corg) and nitrogen, elevated C/N ratios, a steep negative Corg 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

EARTH’S OLDEST PRESERVED K-BENTONITES IN THE CA. 2.1 Ga FRANCEVILLIAN BASIN, GABON

International audienceBentonites are the alteration product of volcanic tephra typicallypreserved in low-energy, sedimentary environments below baseline. Although volcanictuffs occur throughout the Earth’s history, bentonites older than ca. 1.5 Ga have notbeen described. We present the mineralogy, geochemistry, and age data for Kbentonitebeds within the FB Formation in the unmetamorphosed PaleoproterozoicFrancevillian Basin, Gabon. The clay mineralogy of the K-bentonites consists predominantlyof illite with substantial amounts of kaolinite and trace amounts of long-orderedillite/smectite (R3) mixed layer. The kaolinite content and co-existing 1M and 2M1illites are indicative of diagenetic smectite illitization over a prolonged period of timewith minimal burial temperature. Their chemical characteristics suggest derivationfrom calc-alkaline intermediate to felsic magma, related to continental volcanic arcmagmatism in a subduction setting. The zircon grains are relatively small, rounded tosub-rounded, and yield 207Pb/206Pb dates that have a narrow range with a weightedmean of 2971 13 Ma, consistent with the age of the underlying crystalline calcalkalineArchean basement granitoids. This age indicates incorporation of zirconsfrom the Archean basement granitoids into the magma during magmatic activity.Considering that the FB Formation bentonites were derived from a volcanic arcdeveloped along the margin of the West Gabonian block and are preserved in the lowerpart of the Francevillian Basin, we infer that this basin reflects high-rate, but short-livedsedimentation in a pro-foreland basin setting. Paleogeographically, these K-bentonitescould thus serve as a potential correlation marker for the Paleoproterozoic Gabonianand adjacent cratons at ca. 2.1 Ga. Based on the current records, these are the world’soldest bentonite beds so far reported