François Gauthier-Lafaye

Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago

The evidence for macroscopic life during the Palaeoproterozoic era (2.5–1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr–old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6–1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45–2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.

Natural fission reactors in the Franceville basin, Gabon: A review of the conditions and results of a “critical event” in a geologic system

Abstract Natural nuclear fission reactors are only known in two uranium deposits in the world, the Oklo and Bangombedeposits of the Franceville basin: Gabon. Since 1982, five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative European program. New geological, mineralogical, and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2.0 Ga years. The Franceville basin and the uranium deposits remained geologically stable over a long period of time. Therefore, the sites of Oklo and Bangombeare well preserved. For the reactors, two main periods of actinide and radionuclides migration have been observed: during the criticality, under P-T conditions of 300 bars and 400–500°C, respectively, and during a distention event which affected the Franceville basin 800 to 900 Ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors. New isotopic analyses on uranium dioxides, clays, and phosphates allow us to determine their respective importance for the retention of fission products. The UO 2 matrix appears to be efficient at retaining most actinides and fission products such as REEs, Y, and Zr but not the volatile fission products (Cd, Cs, Xe, and Kr) nor Rb, Sr, and Ba. Some fissiogenic elements such as Mo, Tc, Ru, Rh, Pd, and Te could have formed metallic and oxide inclusion in the UO 2 matrix which are similar to those observed in artificial spent fuel. Clays and phosphate minerals also appear to have played a role in the retention of fissiogenic REEs and also of Pu.

δ13C pattern of dissolved inorganic carbon in a small granitic catchment: the Strengbach case study (Vosges mountains, France)

Abstract The transfers and origins of dissolved inorganic carbon (DIC) were studied for a year in a soil–spring–stream system in the Strengbach catchment, Vosges mountains, France. This 80 ha experimental research basin is located on the eastern side of the mountains, at an altitude ranging from 883 to 1146 m.a.s.l. and is mainly covered by spruce (80%). Brown acid and podzolic soils developed on a granitic basement, and, as a result, the DIC originates solely from CO2 generated by oxidation of soil organic matter. The ( δ 13 C DIC ) in catchment waters is highly variable, from about −22‰ in the springs and piezometers to about −12‰ in the stream at the outlet of the catchment. In the springs, pronounced seasonal variations of δ 13 C DIC exist, with the DIC in isotopic equilibrium with the soil CO2 that has estimated δ 13 C of about −24‰ in winter and −20‰ in summer. These seasonal variations reflect an isotopic fractionation that seems only induced by molecular diffusion of soil CO2 in summer. In stream water, seasonal variations are small and the relatively heavy DIC (−12‰ on average) is a result of isotopic equilibration of the aqueous CO2 with atmospheric CO2.

Sm-Nd isotopic dating of Proterozoic clay material: An example from the Francevillian sedimentary series, Gabon

Clay fractions of < 0.4 μm and < 0.2 μm and acid leachates of these clay fractions were separated from two different kerogen-rich black shale samples from the Francevillian sedimentary series in Gabon. They yielded identical Sm-Nd isochron ages of2099 ± 115 Ma and2036 ± 79 Ma, with initialδNd values of−6.5 ± 2.0 and−9.3 ± 1.5, respectively. These ages define multi-episode illitization during early diagenesis, as suggested by the differentSm/Nd ratios of the clay fractions. The Sm-Nd isotope data of the kerogen source rock (up to 13% organic carbon) plot between the two isochrons but close to the clay fractions, suggesting that this organic matter reached Nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids. Analyses of fracture-filling bitumen showed that it had very elevated143Nd/144Nd and147Sm/144Nd ratios. These values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons. This indicates that the bitumen reached Nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals. Secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between Sm and Nd with an attendant increase in theSm/Nd ratio. The results suggest that the Sm-Nd isotope method has potential for both dating diagenetically related illitization using leachate-residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil-producing sedimentary basins.

Characterization and migration of atmospheric REE in soils and surface waters

Abstract Rainwater and snow collected from three different sites in France (Vosges Mountains, French Alps and Strasbourg) show more or less similar shapes of their REE distribution patterns. Rainwater from Strasbourg is the most REE enriched sample, whereas precipitations from the two mountainous, less polluted catchments are less REE enriched and have concentrations close to seawater. They are all strongly LREE depleted. Different water samples from an Alpine watershed comprising snow, interstitial, puddle and streamwater show similar REE distributions with LREE enrichment (rainwater normalized) but MREE and HREE depletion. In this environment, where water transfer from the soil to the river is very quick due to the low thickness of the soils, it appears that REE in streamwater mainly originate from atmospheric inputs. Different is the behaviour of the REE in the spring- and streamwaters from the Vosges Mountains. These waters of long residence time in the deep soil horizons react with soil and bedrock REE carrying minerals and show especially significant negative Eu anomalies compared to atmospheric inputs. Their Sr and Nd isotopic data suggest that most of the Sr and Nd originate from apatite leaching or dissolution. Soil solutions and soil leachates from the upper soil horizons due to alteration processes strongly depleted in REE carrying minerals, have REE distribution patterns close to those of lichens and throughfall. Throughfall is slightly more enriched especially in light REE than filtered rainwater probably due to leaching of atmospheric particles deposited on the foliage and also to leaf excretion. Data suggest that Sr and Nd isotopes of the soil solutions in the upper soil horizons originate from two different sources: 1) An atmospheric source with fertilizer, dust and seawater components and 2) A source mainly determined by mineral dissolution in the soil. These two different sources are also recognizable in the Sr and Nd isotopic composition of the tree’s throughfall solution. The atmospheric contributions of Sr and Nd to throughfall and soil solution are of 20 to 70 and 20%, respectively. In springwater, however, the atmospheric Sr and REE contribution is not detectable.

Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo–Okélobondo and Bangombé natural nuclear reaction zones (Franceville basin, Gabon)

Abstract Large-scale light rare earth element (LREE), uranium, lead and phosphorus migration has been evidenced in the FA Lower Proterozoic sandstones of the Franceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) in relation with extensive accessory mineral alteration by highly saline diagenetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl2 eq.) at about 140°C and 1 kbar. Monazite is the most severely altered accessory mineral in the coarse-grained sandstones of the basal FA formation. Detrital monazite crystals are altered to Th–OH silicate microcrystalline phase with very low concentrations of U and LREE. The Th/La ratio increase from non-altered (Th/La∼0.27) to altered sandstones (Th/La∼1.14) shows that about 76% of the LREE was leached. This corresponds to the leaching of 2.01×109 metric tons at the scale of the FA formation in the Franceville basin. Similarly, the Th/U increase from monazite (Th/U=18.6) to the Th-silicate phase (Th/U=88.7) is interpreted as a result of an alteration by oxidizing brines with leaching of U together with LREE and P. It corresponds to the leaching of 9.06×106 metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville basin. In zircon crystals, the cores are generally homogeneous, weakly fractured and well preserved as attested by the Archean ages (2867±24 and 2865±51 Ma) obtained by ionic microprobe analysis on zircon of the FA Formation, respectively, from the marginal and central parts of the basin. Their composition corresponds to the pure end-member (Zr,Hf)(SiO4), poor in Th and U (Th/U∼1). At the contrary, their rims, which present several growth zones with cracks fillings, are enriched in REE, P, Th and U with higher Th/U ratios (5–10). Both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings. For example, in zircon, the Pb of galena crystals (3–23 wt.%) largely exceeds the amount of Pb (maximum 0.1 wt.%) that would have been produced in situ by radioactive decay in this mineral. Nearly all the lead were introduced into altered zones of accessories. Dissolution of accessory minerals occurred at 2000 Ma, producing a porous and distorted crystal structure which has allowed a later incorporation of Pb. Galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions. Altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non-radiogenic Pb isotopic compositions, confirming the external origin of lead. Pb isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizations and reactor zones, which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms, between 1000 and 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride-rich fluids during the diagenesis of a sedimentary basin.

Natural nuclear fission reactors: time constraints for occurrence, and their relation to uranium and manganese deposits and to the evolution of the atmosphere

Abstract Knowledge of the formation conditions of Francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 Ma ago. The model explaining the formation of the Oklo uranium deposits suggests that at the time of sediment deposition in the Franceville basin 2150 million years ago, the oxygen deficient atmosphere would have inhibited uranium dissolution. Dissolution of uranium was only possible during later diagenesis, approximately 1950 Ma. Reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to U4+, forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions. Such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a ‘transition phase’ some 2450–2100 Ma ago. The manganese deposits were formed before the uranium deposits, during the deposition of the black shales and very early diagenesis, and thus at a time when oxygen content in atmosphere was very low. Carbon isotopes data of organic matter show decrease of δ13C upward in the Francevillian series (−20 to −46% PDB) reflecting the high CH4 and low O2 contents in the atmosphere during sediment deposition. This favoured anoxic conditions during deposition of the basinal FB black shales and likewise the migration of Mn over long distances. The manganese precipitated first as Mn-oxides at the shallow edges of the Franceville basin, in photic zones, where photosynthetic organisms flourished. Mn-oxides were then reduced in the black shales forming Mn-carbonates when conditions became more reducing during transgression episodes and/or the first stages of burial. In the black shales, reducing conditions prevailed until recent weathering, allowing the good preservation of organic matter and the Mn deposits. The present-day alteration is responsible for the dissolution of Mn-carbonates and precipitation of Mn-oxides at the water table to form the high grade Mn ore (45–50% Mn). Development of photosynthesizing organisms, a volcanic source of the Mn, and favourable palaeogeography of the Francevillian basins are all important parameters for the formation of the Mn deposits. For the occurrence of the natural nuclear reactors, the age of 2.0 Ga is the main parameter that controls the abundance of fissile 235U and the critical mass. Before 2.0 Ga the 235U/238U ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved. Then, after 2.0 Ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235U/238U ratio was too low to support criticality.

Oxygen dynamics in the aftermath of the Great Oxidation of Earth’s atmosphere

Significance The Great Oxidation of Earth’s atmosphere about 2.3 billion years ago began a series of geochemical events leading to elevated oxygen levels for the next 200 million years, with a collapse to much lower levels as these events played their course. This sequence of events is represented in rocks from the Republic of Gabon. We show oxygenation of the deep oceans when oxygen levels were likely their highest. By 2.08 billion years ago, however, oxygen dropped to levels possibly as low as any time in the last 2.3 billion years. These fluctuations can be explained as a direct consequence of the initial oxygenation of the atmosphere during the Great Oxidation Event. The oxygen content of Earth’s atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth’s oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth’s oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest–lived positive δ13C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earths surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

REE mobility in groundwater proximate to the natural fission reactor at Bangombé (Gabon)

The natural nuclear fission reactor at Bangombe is located at a depth of approximately 12 m and has undergone supergene weathering and chemical exchange with groundwater under moderately oxidizing conditions. This reactor has been studied as an analogue for the migration of radionuclides in a geologic repository. Five water samples were taken from a drill hole in the reactor zone and from drill holes that are situated along the direction of the groundwater flow. Dissolution of phosphates in the weathering profile provided an important mechanism for the mobilization of phosphorous and REE. This phosphorous allowed the formation of secondary minerals, e.g., phosphatian coffinites and Fe-uranyl phosphate hydroxide hydrates. The filtered groundwaters (dissolved phase) obtained directly from the reactor zone had anomalous 143Nd/146Nd, 145Nd/146Nd and 149Sm/147Sm isotope ratios of 0.7235, 0.4933 and 0.843, respectively, confirming mixing between a fissiogenic and normal REE component. In this dissoved phase, a small fraction of the Nd (2.3 at.%) is of fissiogenic origin. The suspended loads (filtered particulates) of the same groundwater from within the reactor show similar isotopic anomalies with a fissiogenic Nd contribution of 3.2 at.%. Similar fissiogenic Nd contributions are observable for groundwater samples from outside the reactor zone (3 m distance). Both dissolved and suspended load samples from the more distant well (25 m) have normal isotopic ratios. Thus, the prevailing physico-chemical conditions within this groundwater system allow the migration of the fissiogenic REE over at least 3 m but less than 25 m. Although REE phases are abundant in the reactor zone, the REE concentrations of the water in contact with the sediments of this zone are very low (<17 ppt). The most important lanthanide- and actinide-bearing phases are uraninite, coffinites and Fe-uranyl phosphate hydroxide hydrate. The presence of all of these phases retards the mobility of the fissiogenic lanthanides and actinides.