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Biogenicity and taphonomy of the early life fossil records are debated as most of the previous studies focussed mainly on isotopes geochemistry. The non-metamorphosed Paleoproterozoic (~2.1 Ga) sedimentary succession of the Francevillian Basin (Gabon) contains the oldest complex multicellular organisms embedded in black shale facies. Several studies have confirmed the biogenicity of these soft-bodied organisms. Here, we used multi-proxy techniques to show that the preservation of these macro-organisms happened in a close system that limits interaction with their host rocks, which leads to their good preservations. The macro-organisms are present in different shapes and sizes: lobate (L), elongate (E), tubular (T), segmented (S), and circular (C), and are often associated with bacterial mats. Except for the C form, most of the other specimens are pyritized. Sulfur isotopes data confirms that pyritization occurred by bacterial sulfato-reduction during early diagenesis. We compare the clay mineral assemblages between the pyritized specimens and the late-diagenetically formed pure pyritized concretions in the sediments because the early pyritization process could not explain the taphonomic preservation alone. Our clay mineralogical data show that the specimens are dominated mainly by randomly mixed layer Illite-smectite (IS MLMs), illite, and chlorite relative to the host rocks. The abundance of IS MLMs indicates incomplete illitization of smectite, potassium deficiency, and limited mineral reactions in a semi-close local chemical system within the fossils. In addition, the authigenic chlorites are more iron-rich and show vermicular habitus. By contrast, the pyritized concretions mainly consist of well-crystallized illite and less iron-rich chlorite, while the smectite phases are absent. These results confirmed that the diagenetic reaction is controlled by interaction with an open late diagenetic system. We concluded that taphonomic preservation of the ancient fossil record resulted from the early diagenetic growth of pyrite crystals during bacterial sulfato reduction in the fossils, which creates a semi-closed system that drastically reduced fluid-rock interactions with the host sediments.

Igneous sill intrusions into young organic-rich sedimentary basins have major impact on the carbon cycle but also on the transfer of major and trace element between deep and superficial geological reservoirs. The Guaymas Basin in the Gulf of California represents the nascent stage of an ocean characterized by siliceous organic-rich sediments (diatom ooze) deposited at high sedimentation rates. A very dense network of shallow sill intrusions recently invaded the basin. We focused on Site U1546 (Holes A and C) located at about ~51 km northwest of the axial graben of the northern Guaymas Basin spreading segment; this site recovered 540m of sediments and an 80m-thick sill located at 350-430 meters below the seafloor (mbsf). The relatively high geothermal gradient (> 200 °C/km) induces measurable diagenetic transformations in sediments, involving sulfides, carbonates and silica (and clay minerals). Based on retrieved materials from IODP Expedition 385, we present here geochemical and mineralogical characterization of the sedimentary intervals at sill contacts. Our results indicate that sulfides and silica polymorphs are the main phases impacted by contact metamorphism. The transition between opal CT-quartz and pyrite-pyrrhotite is observed in the contact aureoles. In the upper aureole, authigenic quartz and disseminated 20-50 micron pyrrhotite partly fill secondary pores and detrital feldspars are partially dissolved. Patchy carbonate also fills primary interparticle sediment pores just above the contact. In the lower contact aureole, quartz and 200-micron-size euhedral crystals of pyrrhotite are also present. Additionally, a significant metasomatism is observed in the lower contact-aureole meta-sediments with authigenic plagioclase precipitated around detrital feldspars and locally euhedral pyroxenes included in patches of carbonate cement; this suggests precipitation by late to post magmatic fluids at T>300°C. The lower contact aureole is also more enriched in CaO, Na2O, Fe2O3 and trace elements (Cu, As, Zn…). Based on these petrological investigations a new conceptual model of magma sediment fluid interactions will be proposed.

The diagenesis of biogenic silica is arguably one of the most significant early diagenetic processes in the shallow subseafloor and can also have profound implication for the routing of magma in rift basins. The transformation of glass-like amorphous silica (opal-A) in diatoms and radiolarians into crystalline forms (opal-CT) converts soft and watery biogenic oozes into harder sedimentary rocks. The silica diagenetic front is extensive (km-scale) forming prominent seismic reflectors, causes regional-scale differential compaction, subsidence, and the expulsion of pore fluids. Most importantly, it corresponds to changes in sediment rheology from ductile oozes to brittle, fracture-prone and more permeable sedimentary rocks. The relationships between silica diagenesis and subseafloor magmatism have been investigated by Expedition 385 of the Integrated Ocean Discovery Program (IODP) in the off-axis region Guaymas Basin (GB) of the Gulf of California at the neighboring Sites U1545 and U1546, the latter including a ~70 m-thick sill intrusion, and at Site U1547, where the top of a massive sill was recovered at shallower depths. The lithostratigraphic and mineralogic analyses of the cores recovered by the expedition combined with interstitial water geochemistry and physical properties unveil a new and somewhat unexpected picture of the GB’s subseafloor environment. The first discovery is that despite the super-fast sedimentation/burial (up to 1m/kyr) and the very high geothermal gradients, the transformation of opal-A to opal-CT occurs at ~75 ºC or higher in situ temperatures which is much hotter (and deeper) than expected based on previous deep-sea core studies, outcrop studies, laboratory experiments or calculated by models. We hypothesize that the apparent ‘sluggishness’ of silica transformation is the result of the current kinetic model not being able to predict this transformation when burial rates are much faster than typical biogenic sediments in open ocean conditions for which they were originally created. The second important finding relates the Opal CT-zone with magma intrusions. The massive sill at Site U1546 splits the opal-CT zone, though the latter has identical characteristics (e.g. total thickness, gradual increase in silica crystallization with depth) as the opal-CT zone at the nearby Site U1545, located just outside the extent of the sill. Moreover, the sill intrusion is much shallower at Site U1547 where the opal-CT zone is also shallower due to the higher geothermal gradient (~510 Cº/km as opposed to ~100 ºC/km at Site U1546). Not only these observations suggest that the sill formation postdates the silica phase change, but also that this diagenetic interface controls the way magma moves in the GB subseafloor whereby the opal-A/opal-CT transition zone acts as major physical anisotropy in the sedimentary column to reroute magma from vertical to lateral movement. In conclusion, this study greatly expands the range of depths/temperatures at which amorphous silica can persists in the subseafloor and establishes fascinating connections between seemingly disconnected processes in the natural world: surface water biological productivity and crustal architecture of a newborn ocean.

Organic microfossils preserved in three dimensions in transparent mineral matrices such as cherts/quartzites, phosphates, or carbonates are best studied in petrographic thin sections. Moreover, microscale mass spectrometry techniques commonly require flat, polished surfaces to minimize analytical bias. However, contamination by epoxy resin in traditional petrographic sections is problematic for the geochemical study of the kerogen in these microfossils and more generally for the in situ analysis of fossil organic matter. Here, we show that epoxy contamination has a molecular signature that is difficult to distinguish from kerogen with time-of-flight secondary ion mass spectrometry (ToF-SIMS). This contamination appears pervasive in organic microstructures embedded in micro- to nano-crystalline carbonate. To solve this problem, a new semi-thin section preparation protocol without resin medium was developed for micro- to nanoscale in situ investigation of insoluble organic matter. We show that these sections are suited for microscopic observation of Proterozoic microfossils in cherts. ToF-SIMS reveals that these sections are free of pollution after final removal of a

International audience; Widespread anoxic events affected the Tethys Ocean during the Mesozoic. The Ocean Anoxic Event 1a (Early Aptian), expressed as the Selli Level or Goguel Level (GL) in European basins. The GL was deposited in the French Vocontian Basin, a semi-enclosed basin connected to the Tethys Ocean. This study presents an integrated approach (Rock Eval, clay minerals, grain-size, inorganic geochemistry and molecular biomarkers), applied to four sections in the basin distributed along a proximal-distal transect. This study shows that the GL was perturbed by turbidites. In particular, the results demonstrate: (a) a homogeneous redox status of the basin that experienced oxic to suboxic conditions, according to trace element distributions and (b) low organic matter contents (total organic carbon ca 1 wt%) in the three sections where turbiditic deposits are observed. The distal, condensed section exhibits the highest organic matter contents (total organic carbon >3 wt%). In addition, the presence of gammacerane and isorenieratene derivatives in the distal sections suggests that the water-column was intermittently stratified, with hypoxia developing in the photic zone. This stratification did not result from strong surface productivity but more likely from: (a) limited renewal of deep water in the basin; (b) reputedly high surface-water palaeotemperatures during the Early Aptian; and (c) the influx of waters, possibly depleted in free oxygen and in some dissolved trace elements, into the basin. The turbiditic inputs, in addition to organic matter dilution in the sediments and a brief rupture of the water-column stratification in the proximal areas of the basin, ventilated the sea floor and more specifically re-oxidized the sediment-water interface as well as underlying sediments. Such episodes of benthic re-ox-ygenation could have altered the long-term palaeoredox record, even in the distal sections where reducing conditions prevailed during deposition. In the area deprived of turbiditic input, sedimentary condensation, coupled with low oxygen conditions, furthered organic matter preservation and concentration.

The Marnes Bleues Formation from the Vocontian Basin (Southeastern France) shows many dark-colored levels, some concomitant to oceanic anoxic events OAE1a (the so-called Goguel Level) and OAE1b (the Jacob, Kilian, and, above all, Paquier levels). These levels are usually referred to as being organic-rich; they are scattered through a thick, rather homogeneous, succession of marls, that is poor in organic matter (OM). Through a multi-parameter approach, the dark levels from the Aptian-Albian were characterized. Our results show that all dark levels are not OM-rich and that the dark levels exhibit variable characteristics, such as OM nature (marine vs. continental), sedimentation and accumulation rates, redox conditions, surface-water productivity and relative sea level, but they all show low to modest enrichments in OM. Furthermore, all the levels share in common the fact that they formed under conditions of normal to low productivity and oxic to suboxic conditions. Thus, our results strongly suggest that, in the absence of high productivity and anoxic bottom conditions, the other factors reputed to favor OM accumulation only led to sporadic and low enrichments in organic contents. It is as if such factors could only enhance OM accumulation but could not induce it alone. What was true for the Vocontian Basin may be extended to other settings, regardless of their time of deposition or location.

Summary The biomarker content of several dark coloured and/or organic-rich levels from the Marnes Bleues Formation (Aptian-Albian) from the Vocontian Basin (South-eastern France) was analysed in order to compare the modes of formation of such deposits in a hemipelagic, basinal setting. The studied levels include the Goguel and Paquier Levels, deposited during oceanic anoxic events (OAE) 1a and 1b, respectively. Unexpectedly, most studied samples show comparable biomarker contents, regardless of their richness in organic matter. The data indicates an organic matter of marine algal origin, deposited under oxygenated conditions. The Paquier level, however, shows an abundance of archaea-derived biomarkers and of organo-sulfur compounds in the extract, indicating deposition under reducing conditions. The Paquier level excepted, the biomarker content indicates that depositional conditions remained overall stable during deposition of the Marnes Bleues formation, including during OAE1a. Nevertheless variations of the relative proportions of steranes, hopanes, terrestrial plant biomarkers and fire-derived polyaromatic hydrocarbons, suggest that minor environmental changes occurred and occasionally favoured organic matter enrichment in the sediment. In the end, no unique mechanism explaining the deposition of these dark coloured and/or organic-rich levels can be identified.

The Mars organic molecular analyzer (MOMA) of the Rosalind Franklin rover (ExoMars project) will combine laser desorption-ionization mass spectrometry (LDI-MS) and gas-chromatography mass spectrometry (GC-MS) to assess the origin of organic matter on planet Mars. In order to further assess the type of molecular information that can be retrieved with the former technique, we applied high-resolution laser two-step mass spectrometry (L2MS) to fossil organic matter of sedimentary rock from the Jurassic deposit of Orbagnoux, France. Abundant sulfur-rich microbial organic matter has been thoroughly documented in this deposit [1]. This sample has been chosen following the detection of thiophenes at Gale Crater on Mars by the Sample analysis at Mars (SAM) instrument [2]. In our L2MS instrument [3], the samples are irradiated with a pulsed desorption laser (532 or 266 nm), which generates a plume of chemical species that can be further ionized with a second orthogonal laser beam (266 nm). A radiofrequency ion guide is used to carry the ions to an orthogonal time-of-flight mass spectrometer (oToF-MS system by Fasmatech), yielding high-resolution mass spectra (m/Δm ~10000 at 128 m/z). Focusing of the desorption laser using a reflective objective and automated micro-positioning of the sample were used to generate hyperspectral raster mappings. Subsamples included solvent-extracted molecules (bitumen and maltene fractions), insoluble macromolecular organic matter (kerogen), rock powder and a polished slice. Our analyses showed that we can extract chemical information with LDI-MS from both soluble and insoluble organic fractions of the Orbagnoux samples and that various chemical families can be distinguished even in mineralized samples. Carbon clusters, including sulfurated and hydrogenated species could be detected in all subsamples. With the exception of the rock slice, polyaromatic hydrocarbons could be detected in all samples. Oxygenated molecules and alkylbenzenes could only be detected in extracts, which generated rich and intense mass spectra. Various inorganic ions were also generated in all sample fractions. Using focused desorption beams, carbon clusters (including sulfurated clusters) and inorganic species could be detected and mapped in the polished slice with [1] Mongenot, T., Derenne, S., Largeau, C., Tribovillard, N.P., Lallier-Vergès, E., Dessort, D., Connan, J., 1999. Spectroscopic, kinetic and pyrolytic studies of kerogen from the dark parallel laminae facies of the sulphur-rich Orbagnoux deposit (Upper Kimmeridgian, Jura). Org. Geochem. 30, 39–56. [2] Eigenbrode, J.L., Summons, R.E., Steele, A., Freissinet, C., Millan, M., Navarro-González, R., Sutter, B., McAdam, A.C., Franz, H.B., Glavin, D.P., Archer, P.D., Mahaffy, P.R., Conrad, P.G., Hurowitz, J.A., Grotzinger, J.P., Gupta, S., Ming, D.W., Sumner, D.Y., Szopa, C., Malespin, C., Buch, A., Coll, P., 2018. Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars. Science (80-. ). 360, 1096–1101. [3] A. Faccinetto, P. Desgroux, M. Ziskind, E. Therssen, C. Focsa, High-sensitivitydetection of polycyclic aromatic hydrocarbons adsorbed onto soot particles using laser desorption/laser ionization/time-of-flight mass spectrometry: An approach to studying the soot inception process in low-pressure flames, Combustion and Flame 158 (2011) 227–239.

The sedimentary fabrics of Precambrian mat-related structures (MRS) represent some of the oldest convincing evidence for early life on Earth. The ca. 2.1 billion-year (Ga) old MRS in the FB2 Member of the Francevillian basin in Gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated, shallow-marine settings, but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source. Despite these insights, what remains lacking is a full characterization of the geochemical composition of the MRS to test whether the bulk composition of fossilized MRS is distinct from the host sediments (sandstones and shales). Here, we show that the trace element (TE) content of microbial textures belonging to pyritized MRS, poorly pyritized MRS, and “elephant-skin” textures (EST) is highly variable and differs from that of the host sediments. The poorly pyritized MRS contain a unique matrix with embedded Ti- and Zr-rich minerals and syngenetically enriched in TE. The EST, some of which are developed along the same stratigraphic horizon as the poorly pyritized MRS, display a distinct distribution of TE-bearing heavy minerals, suggesting a local difference in physical conditions during sedimentation. Similarly, high chalcophile-element (CE) content in pyritized MRS relative to the host sediments of the FB2 Member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis. The geochemical relationship between the MRS and the Francevillian sediments (e.g., FB, FC, and FD formations) indicates that specific biological pathways for CE enrichments (i.e., microbially controlled accumulation) are not apparent. Our findings highlight bulk-rock TE distinction between the 2.1-billion-year-old MRS and their host sediments, but also indicate that environmental conditions, such as hydrodynamic regime and water-column redox chemistry, may simply overwhelm any potential biological signal. Our data suggest that the microbial impact may have only passively influenced TE enrichment in the studied sediments, implying that TE concentrations in MRS are a poor biosignature. Importantly, this work indicates that bulk TE geochemistry does not unveil specific microbiological processes in the rock record, which is consistent with the observed patterns in modern analogues.

The Lower Devonian Rhynie chert of Scotland is an iconic geological formation that preserves the earliest known terrestrial ecosystem. This assemblage contains key evidence of earliest lineages of land plants, e.g., protracheophytes and paratracheophytes (former Rhyniaceae), together with animals, fungi, algae, and bacteria (Edwards et al., 2017). The exquisite preservation of this early biota provides an ideal scenario to explore the basal evolution of the land biosphere.The Rhynie chert has been vastly studied from multiple viewpoints, however, the biomolecular composition, i.e., “molecular signature”, of the Rhynie flora, including the early detection of fossil lignin, remains clearly unresolved. Lignin biosynthesis has been considered as one of the crucial influences behind the survival and proliferation of land plants in terrestrial ecosystems. Here, we characterize the molecular fossils to help decipher the botanical affinities of the Rhynie flora.Kerogens were isolated by the standard HF/HCl extraction procedure, and bitumens were extracted from the kerogens using organic solvents. The bitumens were studied with GC-MS, and the kerogens were analysed using Py-GC-MS and Py-GC×GC–TOFMS in the presence of TMAH reagent. The bitumens are characterized mainly by some aliphatic compounds such as a series of n-alkanes, pristane, phytane, and a series of diterpanes in very low abundance, as well as a set of aromatic compounds such as naphthalene and methylnaphthalenes, phenanthrene and methylphenanthrenes and retene. The pyrolysates, obtained using Py-GC-MS are dominated by benzene and methyl benzenes, phenol and methylphenols, Polycyclic Aromatic Hydrocarbons (PAHs) like naphthalene and methylnaphthalenes, phenanthrene and methylphenanthrenes, anthracene and methylanthracene, fluoranthene, pyrene, etc. Series of fatty acid methyl esters (FAME) and of n-alkane/alkene doublets were also detected. The thermochemolysates acquired from Py-GC×GC–TOFMS include the same compounds; additional methoxybenzene derivatives, methoxy toluene, methoxy benzaldehydes, and benzoic acid methyl esters, generated by reaction with TMAH, were also identified. Phenols and methoxybenzenes in the pyrolysates and thermochemolysates originate from lignin, and this is the first time that lignin monomers are formally identified from Rhynie chert samples.References:Edwards, D., Kenrick, P., Dolan, L., 2017. History and contemporary significance of the Rhynie cherts—our earliest preserved terrestrial ecosystem. Philosophical Transactions B Royal Society 373: 20160489.

International audience; The organic-rich deposits of Late Devonian age are the second most important petroleum source rocks in the Ghadames Basin (Libya). From available stratigraphy, the peak of organic matter deposition, corresponding to a level of radioactive shales easily recognized in gamma ray logs, occurred at the Frasnian-Famennian transition. The present study was focused on samples from borehole D1–26, located in the central part of the Ghadames Basin, belonging to the Aouinet Ouenine III and IV formations (Frasnian to Famennian). The mineral and organic content of the rocks were analyzed in order to determine the origin of organic matter accumulation and estimate if the organic matter enrichment could be related to the well-known Upper Kellwasser event. The results indicate that during the Frasnian, the planktonic primary productivity was moderate, but anoxic conditions reaching the euphotic zone were frequent and allowed efficient preservation of the organic matter. At the Frasnian-Famennian transition, the deposition of the radioactive shales level can be related to a eustatic rise, which allowed incursion of nutrient-rich water in the basin, leading to increase in primary productivity and photic zone anoxia. During the Famennian, the conditions changed, associated with a relative sea level fall and/or a progradation of detrital sediments originating from the south. Photic zone anoxia was only episodic and plankton-derived organic matter was poorly preserved. Nevertheless, the important influx of organic matter of terrestrial origin allowed significant enrichment of the sediment in organic matter. Several indications suggest the radioactive shales interval could be coeval with the Upper Kellwasser level, nevertheless further stratigraphic work is needed to confirm this hypothesis.

14 pages; International audience; The late Jurassic (Tithonian) marlstones of the Boulonnais area (English Channel, France) contains diagenetic carbonate beds and nodules. Some nodules exhibit cone-in-cone structures on their lower face. We studied such nodules using various techniques of imaging and chemical (major and trace-elements) and isotopic analyses (Ccarb, Corg, O and S stable isotopes). We interpret the cone-in-cone to be the end product of carbonate-nodule formation during early diagenesis. The diagenetic carbonate precipitation was induced by microbial activity (bacteria and(?) archeae) fueled by upward-migrating fluids. Fluid expulsion was itself triggered by synsedimentary fault movements. Under such circumstances, cone-in-cone structures can form during early diagenesis as the result of bacterially-mediated sulfate reduction, possibly coupled to sulfate-dependent anaerobic oxidation of methane. This genetical scheme may be transposed to many other settings.

The FC Formation of the Francevillian of Gabon displays the oldest Gunflint-type assemblage of microfossils that is hosted in shallow-water stromatolites. The FC Formation was deposited between 2.14 and 2.08 billion years ago (Ga), after the Great Oxygenation Event (∼2.4–2.3 Ga) and near the end of the Lomagundi event (∼2.3–2.06 Ga). Although they have been used as a benchmark for the search for older microfossils, the nature of Gunflint-type microfossils has remained elusive due to their simple shapes, their small sizes, and their alteration. Here, we report the first nanoscale study of Francevillian Gunflint-type microfossils. We used a combination of Raman spectroscopy, palynology, in situ focused ion beam sectioning, and analytical electron microscopy. In spite of the relatively high thermal maturity of organic matter (inferred peak burial temperature ∼296 ± 30 °C), spherical cell walls ( Huroniospora ) and filamentous sheaths ( Gunflintia minuta : ≤3 µm in diameter, and broader filaments) are preserved. Organic matter in/on cell walls and sheaths, is associated with nanocrystalline quartz, whereas coarser quartz crystals fill and surround the microfossils. This pattern, likely inherited from recrystallization of texturally heterogenous opal generations, could have allowed the observed preservation of organic structures and limited migrations of organic matter. Moreover, we demonstrate the preservation of thick-sheathed broad (>3 µm) filaments for the first time in a stromatolitic Gunflint-type assemblage; such thick sheaths are common in cyanobacteria, but not in other filamentous bacteria. We distinguished two types of star-shaped organic microstructures ( Eoastrion ) but found no diagnostic evidence for/against a microfossil nature at the nanoscale. Furthermore, we show that titanium is commonly associated with organic structures of microfossils, likely as a result of diagenetic mineralization. In contrast, iron-rich nanocrystals associated with microfossils occur in quartz, not in organic matter, and could possibly represent recrystallized biominerals.

International audience; The Vaca Muerta Formation is a famous source rock being largely present in Argentina. The formation was examined in the southern part of the Neuquén Basin through clay-mineral assemblage composition, as well as organic and inorganic geochemical characteristics. In total, 259 samples were analyzed on both sides of the Huincul Arch (major E-W structure that divided the southern part of the basin into two depocentres). South to this structure (Picún Leufú area), the total organic carbon content (TOC) can reach 20% but is generally ranging from 0.1 to 6%. To the north (Covunco area), average TOC does not exceed 0.5%. Clay-mineral assemblages indicate an evolution of the sources of clastic supply through time in response to evolving weathering/erosion at the regional scale. Redox-sensitive trace-element distribution shows that the particulate iron shuttle process operated, with an initially-euxinic depositional environment that evolved rapidly to oxygenated seawater and suboxic sediments. Our results suggest an episodically restricted circulation pattern that occurred at the beginning of the deposition of the Vaca Muerta Formation (highest TOC content).

The 2.1 billion-year-old sedimentary strata contain exquisitely preserved fossils that provide an ecologic snapshot of the biota inhabiting an oxygenated shallow-marine environment. Most striking are the pyritized string-shaped structures, which suggest that the producer have been a multicellular or syncytial organism able to migrate laterally and vertically to reach for food resources. A modern analogue is the aggregation of amoeboid cells into a migratory slug phase in modern cellular slime molds during time of food starvation. While it remains uncertain whether the amoeboidlike organisms represent a failed experiment or a prelude to subsequent evolutionary innovations, they add to the growing record of comparatively complex life forms that existed more than a billion years before animals emerged in the late Neoproterozoic.Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232–235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023–1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351–381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment–water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).

Illitisation requires potassium incorporation into a smectite precursor, a process akin to reverse weathering. However, it remains unclear whether microbes facilitate K+ uptake to the sediments and whether illitisation was important in the geological past. The 2.1 billion-year-old Francevillian Series of Gabon has been shown to host mat-related structures (MRS) and, in this regard, these rocks offer a unique opportunity to test whether ancient microbes induced illitisation. Here, we show high K content confined to illite particles that are abundant in the facies bearing MRS, but not in the host sandstone and black shale. This observation suggests that microbial biofilms trapped K+ from the seawater and released it into the pore-waters during respiration, resulting in illitisation. The K-rich illite developed exclusively in the fossilized MRS thus provides a new biosignature for metasediments derived from K-feldspar-depleted rocks that were abundant crustal components on ancient Earth., The transition from smectite to illite requires potassium incorporation, yet the role of microbes in facilitating K+ uptake remains debated, especially during the early Earth. Here, the authors suggest that the Paleoproterozoic microbial mats extracted potassium from sea water and induced localized illitization during early low-temperature diagenesis.

The Upper Devonian Protosalvinia phytoclasts from Ohio Shale and its host rock were analyzed using GC/MS, GC-ir/MS, in situ FTIR and pyrolysis–GC/MS, in order to determine its chemical composition and to discuss the taxonomic position of this enigmatic fossil. Based on molecular ratios, the maturity of the organic matter present in the phytoclasts and rocks is comparable to a bituminous rank coal. Diagenetic products of Chlorobiaceae carotenoids were observed in the hosting rocks of Protosalvinia, indicating that anoxia prevailed during the sedimentary deposition. The soluble organic matter of phytoclasts shows land-derived biomarkers including long chain n-alkanes with a slight odd/even predominance and bicyclic sesquiterpenes with eudesmane skeletons. The carbon isotopic composition of n-alkanes is consistent with a land plant origin. The macromolecular fraction of Protosalvinia consists of a condensed aromatic network. The pyrolysis products of the phytoclasts show polycyclic aromatic hydrocarbons commonly observed in coals, including naphthalenes, phenanthrenes and dibenzofurans. Based on what is known about the thermal evolution of organic matter, these components could derive from demethylation and condensation of phenylpropanoids or polysaccharides present in the original structure of Protosalvinia. The former components are exclusively metabolized by land plants.

International audience; The 2.1‐billion‐year‐old (Ga) Francevillian series in Gabon hosts some of the oldest reported macroscopic fossils of various sizes and shapes, stimulating new debates on the origin, evolution and organization of early complex life. Here, we document ten representative types of exceptionally well‐preserved mat‐related structures, comprising “elephant‐skin” textures, putative macro‐tufted microbial mats, domal buildups, flat pyritized structures, discoidal microbial colonies, horizontal mat growth patterns, wrinkle structures, “kinneyia” structures, linear patterns and nodule‐like structures. A combination of petrographic analyses, scanning electron microscopy, Raman spectroscopy and organic elemental analyses of carbon‐rich laminae and microtexture, indicate a biological origin for these structures. The observed microtextures encompass oriented grains, floating silt‐sized quartz grains, concentrated heavy minerals, randomly oriented clays, wavy‐crinkly laminae and pyritized structures. Based on comparisons with modern analogues, as well as an average δ13C organic matter (Corg) composition of −32.94 ± 1.17‰ (1 standard deviation, SD) with an outlier of −41.26‰, we argue that the mat‐related structures contain relicts of multiple carbon pathways including heterotrophic recycling of photosynthetically derived Corg. Moreover, the relatively close association of the macroscopic fossil assemblages to the microbial mats may imply that microbial communities acted as potential benthic O2 oases linked to oxyphototrophic cyanobacterial mats and grazing grounds. In addition, the mat's presence likely improved the preservation of the oldest large colonial organisms, as they are known to strongly biostabilize sediments. Our findings highlight the oldest community assemblage of microscopic and macroscopic biota in the aftermath of the “Great Oxidation Event,” widening our understanding of biological organization during Earth's middle age.

International audience; This study focuses on the thermal maturity assessment of Silurian-Devonian sediments from the Ghadamis Basin, North Africa, comparing optical and geochemical analyses of palynomorphs. In southern Tunisia, the investigated subsurface cored section comprises the Argiles Principales Formation of Silurian age. In Libya, the succession studied covers the Awaynat Wanin III and IV formations, assigned to the Late Devonian (Frasnian-Famennian).Geochemical approaches used to reconstruct thermal alteration of sediments necessitate advanced, relatively expensive analytical techniques. In this study, the effectiveness of the less costly, relatively simple approaches of visually assessing palynomorph colour to determine thermal alteration (i.e., SCI: Spore Colour Index, TAI: Thermal Alteration Index and PDI: Palynomorph Darkness Index) was evaluated.SCI and TAI are qualitative methods, strictly related to the operator's perception, which use ten and five point scales respectively, to characterize colour in terms of illustrated specimens and/or descriptions. In contrast, PDI is obtained from the measurement of the red, green and blue (RGB) intensities of light transmitted through palynomorphs, using standard optical microscopes and digital cameras.The palynomorph-based thermal alteration estimates were compared to Rock-Eval pyrolysis data from the same samples. This calibration showed a linear relationship between these quantitative parameters and PDI. These results show that PDI is more reliable than the SCI and TAI methods.

International audience; Neutral aldoses of the upper 50 m (representing ca. 120 kyr) of sediments from the Cariaco Basin were studied using a new technique based on water extraction followed by sequential acid hydrolysis with 2N, 4N and 6N trifluoroacetic acid (TFA) to investigate the fate of carbohydrates during diagenesis. The distribution and abundance of neutral aldoses and their evolution with depth shed light on the origin and fate of carbohydrates upon burial. The carbohydrates originated mainly from phytoplanktonic production, in particular diatoms. The porewater carbohydrate fraction was invariant and its composition attested to considerable biodegradation of the source material. Bacteria in the sediment may have also contributed to this fraction. The carbohydrates in the 2N fraction represented a decreasing fraction of the total organic carbon (TOC) with increasing depth/age. This fraction likely derived from storage polysaccharides, which were progressively degraded in the sediment. The 4N fraction was dominated by hexoses, suggesting that it comprised sugars derived from structural polysaccharides and exopolymeric substances. It showed a slight decrease with depth, over the 120 kyr. The 6N fraction comprised a relatively constant fraction of TOC with sediment depth. The aldose distribution suggested that the 6N fraction showed the imprint of the initial biological signal preserved by a proteinaceous macromolecular structure and/or a mineral matrix, but not from condensation in the sediment of more labile polysaccharides

ACL; International audience; The Tithonian-Berriasian interval in the southern part of the Neuquén Basin is represented by the Vaca Muerta and the Pic un Leuf u Formations. Facies analysis and correlation of the Vaca Muerta Formation and the lower part of the Pic un Leuf u Formation in the Pic un Leuf u Anticline allow us to characterize the evolution of successive facies belts representing siliciclastic shelf and mixed ramp environments. Shoreface and offshore facies are developed on the siliciclastic shelf in the western and southern parts of the Pic un Leuf u Anticline. The offshore transition domain is characterized by storm beds and slump features, whereas the offshore domain corresponds to black to grey shales and turbidites. The mixed siliciclastic-carbonate ramp is characterized by the development of a lagoon and high-energy shoal in the proximal part of the inner ramp, whereas the distal part comprises a tidal complex. The mid-ramp zone is characterized by storm influence and the outer ramp by fine-grained deposits. Two major transgressive-regressive sequences and five high-frequency transgressive-regressive sequences are recognized. The high-frequency transgressive-regressive sequences make up three progradational sequences, an aggradational-progradational sequence and an aggradational sequence. The geochemical characteristics and clay mineralogy of the Tithonian-Berriasian interval in the southern Neuquén Basin indicate that (1) climate played a key role in the evolution of the sedimentary environment, (2) a "normal marine" depositional environment with oxic sea water and sediment pore waters, rapidly changing to suboxic conditions at shallow depth below the seabed and (3) the occurrence of episodically restricted water-mass circulation at the onset of deposition of the Vaca Muerta Formation. Keywords: Vaca Muerta Formation / source rocks / siliciclastic shelf / mixed siliciclastic-carbonate ramp / sequence stratigraphy / oxygenation status Résumé-Évolution paléoenvironnementale de la partie sud du bassin de Neuquèn (Argentine) pendant l'intervalle tithonien-berriasien (formations Vaca Muerta et Pic un Leuf u) : approche multi-proxy. L'intervalle tithonien-berriasien de la partie méridionale du bassin Neuquén est représenté par les formations de la Vaca Muerta et de Pic un Leuf u. L'analyse de faciès et la corrélation de logs sédimentaires de la formation de la Vaca Muerta et de la partie inférieure de la formation de Pic un Leuf u dans l'anticlinal Pic un Leuf u a permis de mettre en évidence un environnement de rampe silicoclastique évoluant vers un environnement de rampe mixte. En environnement de rampe silicoclastique, les faciès de shoreface et d'offshore se localisent dans les parties ouest et sud de l'anticlinal Pic un Leuf u. La zone de transition est caractérisée par des niveaux de tempête et des niveaux déstructurés et slumpés, alors que le domaine d'offshore correspond à des marnes noires à grises ainsi qu'à des niveaux de turbidites. La rampe mixte silicoclastique-carbonatée se caractérise dans sa partie proximale par le développement d'un lagon isolé du milieu marin ouvert où se déposent des faciès sédimentaires de haute énergie soumis à l'influence de la marée alors que la zone distale est caractérisée par des faciès tidaux. La rampe médiane est caractérisée par This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. l'influence des dépôts de tempêtes et la rampe externe par des dépôts à granulométrie fine. Deux séquences transgressives-régressives majeures et cinq séquences transgressives-régressives de haute fréquence sont reconnues. Les séquences transgressives-régressives de haute fréquence forment trois séquences progradantes, une séquence mixte caractérisée par de l'aggradation et de la progradation et une séquence purement aggradante. La caractérisation géochimique et la minéralogie des argiles de l'intervalle tithonien-berriasien dans la partie méridionale du bassin de Neuquén indiquent (1) que le climat a joué un rôle clé dans l'évolution de l'environnement sédimentaire, (2) que l'environnement de dépôt initialement « normal » évolue rapidement vers des conditions suboxiques à l'interface eau-sédiment et (3) la présence d'épisodes où l'oxygénation de la masse d'eau peut être faible principalement au début du dépôt de la formation Vaca Muerta. Mots clés : formation Vaca Muerta / roches mères / plateforme silicoclastique / rampe carbonatée / stratigraphie séquentielle / degré d'oxygénation

Kerogens from the anoxic sediments of the Cariaco Basin spanning the last 130 ky were analyzed in order to determine the organic matter (OM) preservation pathways, and the evolution of these pathways with time. The kerogens were isolated using a protocol avoiding the formation of artificial melanoidins. Bulk elemental content, spectroscopic and pyrolytic features of the kerogens show that sulfurization is an important process in the insolubilisation of the OM, even in the most recent sample studied (~ 400 a). The degree of OM sulfurization on the studied interval depends on the redox conditions of the sediment during deposition and is largely correlated to the total organic carbon content of the sediment. A trend of downward increasing sulfurization of the organic matter is observed in the sediments of the studied two interglacials. From the available porewater analyses of Cariaco sediments and comparison with Black Sea sediments, the data suggest that downward progressive sulfurization of the OM is mainly due to the production of H 2 S by sulfate-dependent anaerobic oxidation of methane occurring at the sulfate/methane transition zone located a few meters below sea floor.

International audience; An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The mineralogical composition of 95 surface sediment samples from the Cariaco Basin continental shelf and Orinoco delta was investigated in order to constrain the clay-mineral main provenance and distribution within the Cariaco Basin. The spatial variability of the data set was studied using a geo-statistical approach that allows drawing representative clay-mineral distribution maps. These maps are used to identify present-day dominant sources for each clay-mineral species in agreement with the geological characteristics of the main river watersheds emptying into the basin. This approach allows (1) identifying the most distinctive clay-mineral species/ratios that determine particle provenance, (2) evaluating the respective contribution of local rivers, and (3) confirming the minimal present-day influence of the Orinoco plume on the Cariaco Basin sedimentation. The Tuy, Unare, and Neveri Rivers are the main sources of clay particles to the Cariaco Basin sedimentation. At present, the Tuy River is the main contributor of illite to the western part of the southern Cariaco Basin continental shelf. The Unare River plume, carrying smectite and kaolinite, has a wide westward propagation, whereas the Neveri River contribution is less extended, providing kaolinite and illite toward the eastern Cariaco Basin. The Manzanares, Araya, Tortuga, and Margarita areas are secondary sources of local influence. These insights shed light on the origin of present-day terrigenous sediments of the Cariaco Basin and help to propose alternative explanations for the temporal variability of clay mineralogy observed in previously published studies

International audience; Dolomite concretions from Paleoproterozoic organic-rich sediments of the Franceville Basin (Francevillian) in southeastern Gabon are studied. These concretions, belonging to one of the rare well-preserved Proterozoic sediments in the world, are mainly observed in the shaly levels of the Francevillian B1 Formation. The concretions often show a central pyrite-rich layer. The decrease in the carbonate content from the centre to the edges, and the carbon isotopic signal of carbonate within the concretions indicate a concentric growth of the concretions prior to compaction and precipitation of carbonate associated with the degradation of organic matter during early diagenesis. From its geochemical signature and texture, dolomite was not recrystallized. Moreover, the interior of the concretions shows well-preserved accumulations of microbial mats. These concretions thus appear to represent an exceptional record of the biogeochemical signature of the crucial period when the oxygen rose for the first time in Earth's atmosphere. If their rare-earth element pattern does not seem to reflect that of Paleoproterozoic seawater, they appear as potential tools for estimating the original oceanic temperature in surface sediments.

Twenty one core samples of Late Ordovician to Early Devonian age from sections in southern Tunisia, North African Platform (Gondwana) and containing marine and terrestrial organic matter with microbial input were investigated to link the aliphatic and aromatic hydrocarbon distributions with the terrestrial and marine palynomorph content (e.g. acritarchs, prasinophytes, chitinozoans, cryptospores and trilete spores). In addition to several biomarkers of algal/bacterial origin, long chain n-alkanes that might be derived from land plants, as well as the terrestrial diterpane, norabietane, were found. Several land plant-derived biomarkers, such as retene, cadalene, simonellite, tetrahydroretene and C19 isohexylalkyl naphthalene were observed in the aromatic fractions. While these terrestrial biomarkers could be clearly recognized in the middle Silurian–lower Devonian samples, their presence in the upper Ordovician–lower Silurian sediments is more doubtful, because of much lower relative abundances. The land plant biomarkers show a fairly good correlation with the occurrence and abundance of cryptospores and trilete spores, derived from bryophytes and tracheophytes, which covered the emerged areas around the Ghadamis Basin during the Silurian and Devonian. The early tracheophytes (e.g. Cooksonia, lycophytes and zosterophylls) are therefore suggested as a new terrestrial source for most of the saturated and aromatic biomarkers found in sediments of Middle Silurian to Early Devonian age.

Organic petrography and geochemical analyses have been carried out on shales, carbonaceous shales and coals of the Shemshak Group (Upper Triassic–Middle Jurassic) from 15 localities along the Alborz Range of Northern Iran. Thermal maturity of organic matter (OM) has been investigated using vitrinite reflectance, Rock-Eval pyrolysis and elemental analysis of kerogen. Reflectance of autochthonous vitrinite varies from 0.6 to 2.2% indicating thermally early-mature to over-mature OM in the Shemshak Group, in agreement with other maturity parameters used. The shales of the Shemshak Group are characterized by poor to high residual organic carbon contents (0.13 to 5.84%) and the presence of hydrogen-depleted OM, predominantly as a consequence of oxidation of OM at the time of deposition and the hydrogen loss during petroleum generation. According to light-reflected microscopy results, vitrinite/vitrinite-like macerals are dominant in the kerogen concentrates from the shaly facies. The coals and carbonaceous shales of the Shemshak Group show a wide range in organic carbon concentration (3.5 to 88.6%) and composition (inertinite- and vitrinite-rich types), and thereby different petroleum potentials. Thermal modelling results suggest that low to moderate palaeo-heat flow, ranging from 47 to 79 mW m−2 (57 mW m−2 on average), affected the Central-Eastern Alborz basin during Tertiary time, the time of maximum burial of the Shemshak Group. The maximum temperature that induced OM maturation of the Shemshak Group seems to be related to its deep burial rather than to a very strong heat flow related to an uppermost Triassic–Liassic rifting. The interval of petroleum generation in the most deeply buried part of the Shemshak Group (i.e. Tazareh section) corresponds to Middle Jurassic–Early Cretaceous times. Exhumation of the Alborz Range during Late Neogene time, especially along the axis of the Central-Eastern Alborz, where maximum vitrinite reflectance values are recorded, probably destroyed possible petroleum accumulations. However, on the northern flank of the Central-Eastern Alborz, preservation of petroleum accumulations may be expected. The northern part of the basin therefore seems the best target for petroleum exploration.

The early Miocene Pedregoso Formation is one of the numerous formations rich in organic matter within the stratigraphic record of the Urumaco Trough, in the central area of the Falcon Basin. Due to its lithological characteristics and stratigraphic position, this formation is of great interest regarding the basin's petroliferous systems. The evaluation of various inorganic and organic geochemical parameters indicates that the organic matter is primarily of marine origin, deposited in a marine carbonate environment typical of reefal systems, under oxic-to-dysoxic conditions. The low variability in the TOC concentrations and in the distributions of the biomarkers extracted from the samples suggests that the paleoenvironmental conditions and the organic-matter supply remained approximately constant throughout the sedimentation of this unit. The Pedregoso type-II organic matter (marine origin) and initial organic richness value (∼1.8%) suggest that this unit has probably generated hydrocarbons within the Urumaco Trough. However, present-day thermal maturity parameters reveal that the Pedregoso organic matter is overmature (dry gas window), indicating that this unit is only capable to generate gas. In addition, the geothermal gradient, maturity parameters, and the maximum paleotemperature estimated in this study suggest that the Pedregoso Formation reached a maximum burial depth the ∼6.5 km, consistent with the value obtained from data of stratigraphic thickness in the Urumaco Trough. This implies that the thermal anomaly that affected the basin during the Late Eocene–Early Miocene did not reach the central part of the basin, and therefore, the organic matter maturation in this unit is due to the sedimentary burial.

The biomarkers retene, cadalene, tetrahydroretene and isohexylalkylnaphthalene have been identified in the extracts of Silurian–Devonian clastic sediments from southern Tunisia. The occurrence of retene is unexpected as it is generally considered to be a diagenetic product of the conifer constituent abietic acid; however, conifers evolved only after the Devonian. The retene shows a strong correlation with the abundant terrestrial signal from bryophyte-derived cryptospores in the palynofacies, suggesting that it derives from a terrestrial source. Accordingly, we propose that either abietic acid synthesis evolved prior to the appearance of conifers or retene is a diagenetic product of precursors other than abietic acid, which were synthesized by the earliest land plants. Without excluding the first possibility, it appears likely that retene derives here from the diagenesis of compounds with a kaurane-type skeleton, which were abundantly produced by the early Palaeozoic bryophytes.

The colonization of land required new strategies for safe gamete/diaspore dispersal, and to cope with desiccation, harmful radiation, fire and gravity. Accordingly, the morphology, behaviour and physiology of the organisms changed. Here, we explore to what extent physiological adaptations, reflected in the molecular content of the sediments, add to our understanding of the terrestrialization. Many compounds considered characteristic of land organisms do not provide valuable information from the fossil record since (1) they were not preserved; (2) they occur or correspond to substances that evolved prior to the terrestrialization (e.g. cutan vs. algaenan, cellulose); or (3) they have been changed diagenetically and/or catagenetically. The latter leads to geo(macro)molecules without a chemical fingerprint relating them to their original bio(macro)molecules despite, sometimes, excellent morphological preservation of the organic remains. Nevertheless, some molecular markers and their stable isotopes provide independent information on the terrestrialization process. The odd predominance of n-alkane surface waxes is a feature already apparent in early land plants and could, with caution, be used as such. Furthermore, fossil terpenoids and their derivatives are valuable for reconstructing the evolution of major plant groups. The radiation of the phenylpropanoid pathway with for example, sporopollenin and lignin seems to be closely related to the evolution of land plants. As for other disciplines occupied with unravelling past life and environment, organic geochemistry relies heavily on the paradigm that the present provides a key to the past. For organic geochemistry, the biosynthetic pathways of living organisms provide such a key. The biosynthetic differences between organisms provide insight into the evolution of biosynthetic pathways and this can be applied to, and calibrated against, the fossil record. Evolution also implies adaptation and, by linking species ecology to their biochemistry, the adaptive value of the biosynthetic pathways and the biomolecules produced may be resolved. This biosynthetic link to environment can also be applied to the past. For instance, the notion that oxygen is required in steroid and non-hopanoid triterpenoid synthesis implies that analysis of steroids in ancient sediments may help to unravel the early evolution of the atmosphere (Summons et al. 2006). Analogous to the oxygenation of the Earth’s atmosphere, terrestrialization also required major adaptations of the terrestrializing organisms to the living conditions on land. The conquest of land required the development or strengthening of supportive structures such as skeletons, stems and roots to withstand Earth’s gravity without the support of water and to resist wind. It also demanded water-saving strategies such as arthropod and plant cuticles, vertebrate skin and cork to survive low humidity environments and sometimes fire, as well as the development of water-conducting tissues in plants (roots, tracheids). Finally, the higher exposure to harmful radiation required sunscreens such as pigments, aromatic and other substances as UV filters. Whereas the endoand exoskeletal adaptations are intrinsically based on the formation of large biopolymers, water saving and UV protection may be also realized by means of smaller molecules. Our working hypothesis is that the major changes in biosynthetic pathways which accompanied the terrestrialization of organisms are reflected in the molecular composition of the organic matter present in Palaeozoic sedimentary rocks. For instance, a minor change in the biosynthesis of sporopollenin may have led to lignin. In return, we can expect the changes in composition of the organic matter (OM) in Palaeozoic rocks to help determine following evolutionary trends. The question being asked is to what extent is our working hypothesis valid and what does this provide us with to understand terrestrialization? We restrict ourselves to primary producers and arthropods since they have a rich fossil record, both organic geochemically and as microand macroscopic remains. We exclude other terrestrial (heterotrophic) life forms (worms, snails, vertebrates, etc.) since they do not usually leave a chemically characteristic signature in the sediments. From: Vecoli, M., Clement, G. & Meyer-Berthaud, B. (eds) The Terrestrialization Process: Modelling Complex Interactions at the Biosphere–Geosphere Interface. Geological Society, London, Special Publications, 339, 11–36. DOI: 10.1144/SP339.3 0305-8719/10/$15.00 # The Geological Society of London 2010. Organic matter analysis: lipids and

International audience; Cored sediments from the Pigmy Basin, northern Gulf of Mexico, were analyzed in order to better constrain late deglacial and early Holocene paleoenvironmental and sedimentary changes in response to North American climate evolution. Mineralogical and geochemical proxies indicate the succession of two sedimentary regimes: dominantly detrital during the deglaciation (15-12.9 cal ka BP) whereas biogenic contribution relatively increased later on during the Younger Dryas and early Holocene (12.9 and 10 cal ka BP). Geochemical data reveal that the deglacial record mainly reflects variations of terrigenous supply via the Mississippi River rather than modifications of redox conditions in the basin. Specific variations of almost all the parameters measured in this paper are synchronous with the main deglacial meltwater episode (Meltwater Spike) described or modeled in previous marine or continental studies. During this episode, most parameters display ''stair-step-like''-pattern variations highlighting three successive steps within the main meltwater flow. Variations in grain-size and clay mineral assemblage recorded in the Pigmy Basin indicate that the erosional regime was very strong on land during the first part of the Meltwater Spike, and then milder, inducing more subtle modifications in the sedimentary regime in this part of the Gulf. Specific geochemical and mineralogical signatures (notably, clay minerals and trace metal geochemistry) pinpoint a dominant origin from NW North America for detrital particles reflecting meltwater outflow from the southwestern Laurentide Ice Sheet (LIS) margin during the most intense freshwater discharge. The observed decrease of the sedimentation rate from about 200 to 25 cm/ka at ca 12.9 ka evidenced a drastic decrease of erosional processes during late phase of discharge, consistently with the hypotheses of major reduction of meltwater flow. The major modification at 12.9 cal ka BP is interpreted to result from both modifications of the main Mississippi fluvial regime due to eastward and northward rerouting of meltwater flow at the onset of the Younger Dryas, and the increase of sea-surface temperature linked to insolation. Finally, slight grain-size modifications suggest that some freshwater discharges may have episodically reached the Gulf of Mexico after the Younger Dryas reflecting possible small adjustments of the postglacial hydrological regime.

The early Cretaceous Bernissart lake deposits (Mons Basin, Belgium) constitute an important historical site in the field of palaeontology. In this study, we focus on the organic matter (OM) content of the 50 m-long borehole core, "Ber 3", drilled at Bernissart. Organic data (Rock-Eval, palynofacies, C/N ratios) and carbon-isotope measurements on bulk OM and selected, isolated wood fragments were collected. The OM distribution exhibits two total organic carbon (TOC) cycles (12 m and 38 m-thick, respectively) that are stacked in a lower frequency cyclic pattern. A general increase of in situ aquatic production and preservation of OM through time is evidenced within the freshwater masses. A spectral analysis of a high-resolution gamma-ray record demonstrates that the sedimentation was controlled by orbitally-induced climatic changes. Precession, obliquity and short- and long-term eccentricity cycles are recognized. The 12 m-thick and the 38 m-thick TOC cycles fit well with a 100 kyr short-term and a 400 kyr long-term eccentricity cycle, respectively. Long-term organic fluctuations are interpreted as resulting from orbitally-induced fluctuating lake levels through time. Carbon-isotope evolution of bulk OM is thought to reflect firstly two local patterns of sedimentation: (1) the changing contribution of in situ amorphous organic matter versus terrestrial OM, and (2), the possible influence of enhanced productivity. The negative δ13CWOOD trend recorded at Bernissart seems equivalent to published coeval δ13CWOOD and δ13CCARB in the Upper Barremian to Lower Aptian. This trend noted at Bernissart is thus regarded as reflecting changes of the carbon-isotope ratios of atmospheric CO2. Previous presented palynological data suggest a "middle" Barremian to Lowermost Aptian stratigraphic range for the succession at Bernissart. According to this new dataset, we refine the age-assignment for Bernissart: the duration of deposition is now estimated to range between 0.55 and 2.2 myr in the Upper Barremian to Lowermost Aptian.

Palynological and palynofacies analyses combined with organic carbon isotope measurements have been performed on terminal Ordovician through Silurian clastic sediments from the North African margin of Gondwana (southern Tunisia). A first carbon isotopic signal (δ13Corg) from Gondwanan Silurian sedimentary sequences is presented, showing interesting correlation with existing coeval isotopic curves from other areas. Changes in lithology, palynofacies characteristics, palynomorph diversity, carbon isotope developments, and organic geochemistry parameters appear to be all causally linked, and to reflect changes in palaeoceanographic conditions. In particular, the detailed chronostratigraphic correlation and the observed similarities in patterns of carbon isotopic developments and (palyno- and litho-) facies changes through the study section has permitted to identify the supposedly global earliest Wenlock (“Ireviken Event”) and late Ludlow (“Lau Event”) isotopic excursions for the first time in high-latitude Gondwana. This confirms that these excursions were linked to global changes in the oceanic system. The present results suggest that from Rhuddanian to early Wenlock times, an extended period of black shale deposition occurred over the North African Gondwanan margin, progressively transgressing from basin palaeodepressions to basin palaeohighs. Palynofacies and organic geochemistry support a coastal upwelling-promoted productivity increase during this interval, associated to a decrease in diversity of the marine microplanktonic communities. The earliest Wenlock strong positive isotopic shift appears associated with this protracted period of massive black shale deposition, and thus of organic carbon burial, on continental platforms located in high-latitude settings. This could well explain the apparent paradox between excessive carbonate deposition simultaneous to a carbon isotopic shift towards lighter values observed in low palaeolatitude localities (Laurentia and Baltica). the strong, distinct isotope excursion occurring in late Ludlow times is possibly linked to the well know Lau Event. Evidence for extensive organic carbon burial is lacking to explain this strong excursion. Some significant changes in the marine palynomorph communities are recorded in connection to the increase in stable isotope values (δ13Corg). Additionally, a strong correlation between abundance of terrestrially derived palynomorphs (miospores) and δ13Corg development is recorded which seems to support increased clastic input from a terrestrial source.

The paleoecological interpretation of fossil foraminiferal assemblages depends on an understanding of the ecological processes operating at the present. This study investigates both the quality of organic matter (OM) by elemental analysis as well as the sediment grain size and clay mineralogy to understand their relative influence on distribution and abundance of benthic foraminifera. This study is carried out on 15 samples regularly spaced from the mudflat to the tidal marsh. The results indicate that grain size is the most limiting parameter. Living (stained) benthic foraminiferal density and species richness are both very low within coarser sediments. OM is the second limiting factor. The density of foraminifera is the lowest and the species richness is the highest with the lowest organic carbon (Corg) contents and C/N 12, the density is high and the species richness medium. A high smectite proportion within the clay-size fraction seems to favor the development of Miliammina fusca. Trochammina inflata and Jadammina macrescens are both favored by an increase of organic carbon proportion but Trochammina inflata preferentially feeds on algal-derived OM when compared with Jadammina macrescens.

This paper is a synthesis of the use of selected trace elements as proxies for reconstruction of paleoproductivity and paleoredox conditions. Many of the trace elements considered here show variations in oxidation state and solubility as a function of the redox status of the depositional environment. Redox-sensitive trace metals tend to be more soluble under oxidizing conditions and less soluble under reducing conditions, resulting in authigenic enrichments in oxygen-depleted sedimentary facies. This behavior makes U, V and Mo, and to a lesser extent certain other trace metals such as Cr and Co, useful as paleoredox proxies. Some redox-sensitive elements are delivered to the sediment mainly in association with organic matter (Ni, Cu, Zn, Cd) and they may be retained within the sediment in association with pyrite, after organic matter decay in reducing sediment. This particularity confers to Ni and Cu a good value as proxies for organic C sinking flux (frequently referred to as productivity). Elements with only one oxidation state such as Ba and P are classically used to assess paleoproductivity levels but they suffer from the fact that they are solubilized under reducing conditions and may be lost from oxygen-deprived sediments. The combined used of U, V and Mo enrichments may allow suboxic environments to be distinguished from anoxic–euxinic ones. Specifically, these elements tend to be much more strongly enriched in anoxic–euxinic environments and to exhibit weaker covariation with TOC than in suboxic environments.

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.

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.

International audience; A high resolution analysis of the clay mineral content of sediments of the Cariaco Basin spanning the last climatic cycle was performed in order to determine how the detrital sedimentation in the basin records climatic oscillations at various timescales. At the scale of glacial interglacial cycles, the clay mineral content records changes in the geographic origin of the detrital supply, from a dominantly kaolinite-and smectite-rich southern source during glacial to a more illite-rich northerly source during interglacials. Though possibly influenced by eustatic variations, these changes mostly reflect the latitudinal fluctuations of the intertropical convergence zone (ITCZ). The Eemian shows an increased contribution from the northern source compared to the Holocene. During MIS3, seasonality modulates the effect of ITCZ position and leads to increased smectite contribution. Fluctuations of the smectite and kaolinite contents during Dansgaard/Oeschger cycles reflect changes of runoff intensity. Enhanced supplies in kaolinite during Heinrich-like events and stadials also suggest remobilizations of sediments deposited on the Unare platform resulting from rapid sea-level variations. The Younger Dryas is characterized by a significantly different clay mineral association reflecting a drastic modification of terrigenous supply. An eolian contribution would explain this peculiar mineralogy, although local contributions cannot be ruled out. A similar event is noted at the MIS6-5 transition pointing for the occurrence of a Younger Dryas like episode during Termination II.

International audience; The fine-grained sediments of the Cariaco Basin, Venezuela, of the last 130 ky, whose deposition history is well characterized, were analyzed geochemically in order to test the validity of sediment bulk geochemistry as an indicator of detrital provenance. Several binary and ternary diagrams as well as the chemical index of alteration (CIA) were tested for their capacity to discriminate the poorly contrasted detrital sources to the Cariaco Basin, and to describe the temporal evolution of the contributions of these different sources. Most of the diagrams tested did not allow a good discrimination of sources or, when sources were well discriminated, did not allow an interpretation of the temporal variations consistent with the known history. A relatively good discrimination of sources and a consistent interpretation of temporal variations were however obtained using Hf vs. Th and La/Yb vs. Gd/Yb binary diagrams, as well as Ti-Zr-Th, Ti-Zr-La, and Lu-Hf-Th ternary diagrams. Compared to the previous studies of the detrital content of the Cariaco Basin sediments, the geochemical approach permitted the recognition of a sediment contribution eroded from the Unare platform and Gulf of Cariaco during rapid sea level oscillations, and the contribution of Saharan eolian particles during the Younger Dryas-Preboreal and MIS6-5 transition. The choice of plotted elements was determined after considering carrier minerals, so that different elements may be informative in different sedimentary contexts. Overall, mineral sorting during transport appears as a major limit to quantitative estimation of the different contributions. In particular mineral sorting leads to the selective enrichment of elements associated with clays (Al, Rb, Th and LREE) in sediments deposited in the basin. Unless the geochemical effect of mineral sorting can be measured, it appears that quantitative provenance analysis should be performed on fractions of similar grain size instead of bulk sediment.

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.

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.

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.