Armelle Riboulleau

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.

Pyrolytic and spectroscopic study of a sulphur-rich kerogen from the ''Kashpir oil shales'' (Upper Jurassic, Russian platform)

Abstract The kerogen of an organic-rich sample, termed f top, from the Gorodische section (Russian platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods so as to examine its chemical structure, source organisms and formation pathway(s). This kerogen, which is mainly composed of orange gel-like, nanoscopically amorphous organic matter, exhibits a relatively high aliphatic character; organic sulphur is mainly present as di(poly)sulphides and alkylsulphides. The f top kerogen was chiefly formed via intermolecular incorporation of sulphur in algal or cyanobacterial lipids and carbohydrates. However, its formation also involved oxidative condensation via ether linkages. Comparison of f top sample with other S-rich kerogens points to a closer similarity with Monterey kerogens rather than with a kerogen from the bituminous laminites of Orbagnoux.

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.

Évolution de la paléotempérature des eaux de la plate-forme russe au cours du Jurassique supérieur

The evolution of the paleotemperatures of the Russian Platform over the Late Jurassic are inferred from stable isotope measurements on belemnite rostra. The δ 18O data indicate temperature variations ranging from 7 to 18 °C during the Oxfordian and a stable temperature cal 17 °C over Kimmeridgian and Volgian. The results are compared to data from the literature in the frame of the paleogeographic reconstruction of Europe for the Late Jurassic.

Controls on organic accumulation in late Jurassic shales of northwestern Europe as inferred from trace-metal geochemistry

In the Kimmeridge Clay Formation of the Wessex-Weald Basin, five organic-matter-rich intervals (or ORIs), dated from Kimmeridgian-Tithonian times, can be correlated from distal depositional environments in Dorset and Yorkshire (UK) to the proximal environments in Boulonnais, northern France. The ORIs are superimposed on a meter-scale cyclic distribution of organic matter (OM), referred to as primary cyclicity, which is commonly interpreted to result from Milankovitch climate forcing. The present work addresses the distribution of redox-sensitive and/or sulfide-forming trace metals and selected major elements (Si, Al and Fe) in Kimmeridge Clay shales from the Cleveland Basin (Yorkshire) and the Boulonnais cliffs with two objectives: 1) to determine whether the ORIs formed in similar paleoenvironments, and 2) to identify the mechanism(s) of OM accumulation. High-resolution geochemical data from primary cycles in the Yorkshire boreholes (Marton and Ebberstone boreholes), were studied and the results are then applied with lower resolution sampling at the ORI scale in the Flixton borehole and Boulonnais cliff. Good correlations are found between total organic carbon (TOC) vs Cu/Al and Ni/Al, but relationships between TOC and Mo/Al, V/Al and U/Al are more complex. Cu and Ni enrichment is interpreted to have resulted from passive accumulation with OM in an oxygen-deficient basinal setting, which prevented the subsequent loss of Cu and Ni from the sediment. Mo and V were significantly enriched only in sediments where considerable amounts of OM (TOC>7 %) accumulated, the result of strongly reducing conditions and OM burial. At the scale of the Flixton ORIs, the samples with the highest Mo and V concentrations also show relative Fe enrichment, suggesting pyrite formation in the water column (combination of euxinic conditions and presumably low sedimentation rates). Samples from all ORIs were slightly enriched in Si relative to Al, interpreted as reflecting decreased sediment flux during transgressive and early-highstand systems tracts. The data show that in some ORIs, OM accumulation proceeded while productivity was not particularly high and sediments were not experiencing strong anoxia. In other ORIs, OM accumulation was accompanied by widespread anoxia and possibly euxinic conditions in distal settings. Though somewhat different from each other, the ORIs have all developed during episodes of reduced terrigenous supply (transgressive episodes). The common feature linking these contrasted episodes of enhanced OM storage (ORIs) must be the conjunction of productivity coupled with a decrease in the dilution effect by the land-derived supply, in a depositional environment prone to water stratification and, therefore, favorable to OM preservation and accumulation.

Origin of contrasting features and preservation pathways in kerogens from the Kashpir oil shales (Upper Jurassic, Russian Platform)

Abstract A kerogen, termed aBS base , from the Gorodische section (Russian Platform) was studied using a combination of microscopic, spectroscopic and pyrolytic methods in order to determine its chemical structure, source organisms and formation pathway(s). This kerogen was mainly formed via degradation-recondensation of phytoplanktonic material. Selective preservation and natural sulphurisation pathways only played a minor role, whereas a substantial contribution of ether linked lipids was noticed, revealing large oxygen cross-linking. Such observations allowed us to put forward, for the first time to the best of our knowledge, a substantial role for the oxidative reticulation pathway in the formation of a kerogen. Comparision with a previously studied sample from the same outcrop revealed contrasting features which reflect differences in preservation pathways triggered by different depositional conditions.

Occurrence of proteinaceous moieties in S- and O-rich Late Tithonian kerogen (Kashpir oil Shales, Russia)

The polar fraction, isolated from the off-line pyrolysate at 400°C of a Late Tithonian, sulphur- and oxygen-rich, kerogen was examined via Raney Nickel desulphurization and TMAH thermochemolysis. Important information on this kerogen, not accessible via conventional pyrolysis, was thus obtained: (i) its structure is not simply based on alkyl skeletons cross-linked by ether and (poly)sulphide bridges, (ii) TMAH thermochemolysis afforded direct evidence of the survival of proteinaceous moieties in this 140 million years old kerogen and (iii) encapsulation within an aliphatic organic matrix was probably the main pathway reponsible for such a conspicuous preservation, also possibly favoured by the presence of numerous sulphur links.

Depositional conditions and organic matter preservation pathways in an epicontinental environment: the Upper Jurassic Kashpir Oil Shales (Volga Basin, Russia)

Abstract The Middle Volgian Kashpir Oil Shales Formation, located on the Russian Platform, is a lateral equivalent of the North Sea and West Siberian petroleum source rocks. In the Volga Basin, this formation is 6 m thick and shows alternations of marls and black shales. The organic carbon content is often higher than 1%, although bioturbation and benthos are abundant, even in the black shales. In the marls, highly degraded organic matter (OM) dominates, while aliphatic, sulphur-rich OM is dominant in the black shales. The combination of sedimentological and geochemical studies allowed to determine that the redox conditions of the sediment regularly fluctuated from oxic to anoxic, under relatively productive waters, while climatic conditions were increasingly arid. It is proposed that OM deposition occurred in relation to the increasing aridity, through the combination of recurrent disruption of salinity stratification and aeolian supply of iron, which both promoted phytoplankton productivity.

Factors controlling the survival of proteinaceous material in Late Tithonian kerogens (Kashpir Oil Shales, Russia)

Thermochemolysis experiments with tetramethylammonium hydroxide (TMAH), on three Late Jurassic kerogens with contrasting features, confirmed the major role of encapsulation into aliphatic structures for the survival of proteinaceous moieties in kerogens, probably via lipid sulphurisation. It also appeared that (i) some amino acid moieties survived even in the kerogen of a sediment deposited under oxic conditions, although in lower relative abundance compared to the other two kerogens, (ii) the survival of amino acids on geological time scales, within an organic matrix, is probably rather common in kerogens, especially for glycine and alanine, and (iii) thermochemolysis of kerogen sub-units with a lower degree of cross-linking rather than direct thermochemolysis of whole kerogens is recommended to test the presence of amino acid moieties, so as to avoid dilution problems and to increase the accessibility of the TMAH.

Chapter 8Productivity and bottom water redox conditions at the Frasnian-Famennian boundary on both sides of the Eovariscan Belt: constraints from trace-element geochemistry

Abtract The Late Devonian Frasnian-Famennian (F-F) mass extinction event coincides in many places with the depositionof C org -rich “Kellwasser” facies. Four F-F boundary sections representative of platform and basin environments from widely separated locations (Morocco, Germany, and France) were analysed for inorganic geochemistry, especially trace elements (redox and productivity proxies), in order to describe paleodepositional environments for the Kellwasser horizons. Ni/Co, V/Cr, U/Th, and V/(V+Ni) ratios, as well as redox trace metal concentrations indicate that oxygen-depleted conditions existed during the times of Kellwasser facies deposition. In platform settings, dysoxic conditions seem to be limited to the Late Frasnian. In basinal settings, oxygen depletion was stronger and persisted into the Early Famennian. Enrichments of Ba, Cu, Ni, that are limited to the Late Frasnian, show that surface productivity was relatively high and organic matter could accumulate, especially in the deeper environments. The stratigraphical distribution of several geochemical markers are linked with two positive excursions of the δ 13 C carb signal that result from enhanced organic matter burial. Reducing conditions likely resulted from high productivity of Late Devonian marine ecosystems. Intense nutrient supply resulted probably from the biogeochemical recycling of nutrients, and/or runoff from emerged lands. Coupled with other factors, such as rapid sea-level fluctuations and climatic changes, oxygen-depleted conditions and eutrophication would have modified Late Devonian environments and could be possible factors in the F-F mass mortality.