Biomarker assemblages associated with the Eocene-Oligocene transition in lacustrine deposits of the Rennes Basin (France)

International audienceThe Eocene/Oligocene transition is one of the main climatic events to which more recent climate changes are often compared. Dated back to 34 Ma, it is remarkable because the Earth evolved from a greenhouse to icehouse state, permanent ice sheets appearing during the Late Eocene. In marine setting, it is marked by an increase in detrital input associated with an increase of primary productivity that resulted from well mixing of water masses (Peihzen et al., 2001). In terrestrial setting, this cooling led to major changes in animal and plant communities. For instance, palynology data show a vegetation shift from wet to dry taxa (Gregory and Chase, 1992) coincident with a large mammal turnover, the well-known "Grande coupure" (Stelhin, 1909). Although this transition is well documented in marine settings, the lack of data on lands - due to limited available outcrops - does not allow appreciating precisely its impacts on terrestrial ecosystems. Here, we have analysed the sediment infill of the Rennes Basin that consists of 300 m of organic-rich lacustrine deposits (laminated and massive) that encompass the Middle Eocene to Oligocene series, including the Eocene-Oligocene transition, the whole fully cored (CINERGY project, BRGM). About 100 samples were collected through the core enabling us to precisely decipher environmental changes that occurred during the studied interval and to focus on the Eocene/Oligocene transition. All samples were subjected to Rock-Eval and EA-irMS to measure bulk δ13C. Four over the 100 samples were analysed for their molecular content so far. For a preliminary test, we have chosen representative facies (a laminated and a massive Upper Eocene sample and a laminated and a massive Lower Oligocene sample). Lipids were extracted with an Automated Solvant Extractor (ASE200®, Dionex) using DCM:MeOH (9:1). The total extract was later separated into neutrals, acidic and polar compounds. The neutral compounds were further separated into aliphatic, aromatic, ethers and esters, ketones and alcohols. After addition of 5α-cholestane, each fraction was then analysed by Gas Chromatography-Mass spectrometry on a Trace GC Ultra gas chromatograph coupled to a TSQ Quantum XLS mass spectrometer equipped with an AS 3000 autosampler. The analysis of the different fractions of each sample revealed a high diversity of compounds that could partly be related to their source organisms or inform on the depositional environment. Hydrocarbon fraction is dominated by a series of n-alkanes that display a strong odd-over-even predominance, indicative of a good preservation. The dominance of long-chain homologues shows a strong higher plants contribution. This is confirmed by the presence of abundant and diversified higher plant pentacyclic triterpenes. These comprise angiosperms biomarkers such as oleanane, ursane and lupane, friedelin, onocerane I and diagenetic by-products (des-A-tritrepenes and aromatic derivatives). Fernene-type compounds could reveal an input from pteridophyts whereas tricyclic diterpenes indicate the presence of conifers. An algal influence is evidenced, for example, by the presence of 4-methylsteranes in the four samples. C27 to C32 hopanoids with α,β, β,β and β,α configurations show a substantial bacterial contribution. This is confirmed by high levels of hopane ketones in some beds. The comparison of angiosperm-derived biomarkers with gymnosperm-derived biomarkers indicates a stronger contribution of angiosperms in Upper Eocene samples than in Lower Oligocene samples (triterpenoids, aromatic, and rearranged-triterpenoids). This result is in agreement with palynological data (Bourdillon et al., 2012) and with the well-known cooling that is coeval with the Eocene/Oligocene transition. Reversely, the comparison between the two facies (massive and laminated) does not exhibit significant differences in the molecular assemblage. Based on this preliminary inventory of biomarkers and the richness in various biomarkers, we will proceed with the identification and quantitation on the remaining samples that cover the whole section. These preliminary results are also encouraging in the perspective of performing compound-specific isotopic analyses in order to depict, at high temporal resolution, the climatic evolution and the associated changes in plant communities during the Late Eocene and Early Oligocene and, more specifically, at the Eocene/Oligocene transition

Mineralogical and isotopic record of biotic and abiotic diagenesis of the Callovian-Oxfordian clayey formation of Bure (France)

International audienceThe Callovian-Oxfordian (COx) clayey unit is being studied in the Eastern part of the Paris Basin at depths between 400 and 500 m depth to assess of its suitability for nuclear waste disposal. The present study combines new mineralogical and isotopic data to describe the sedimentary history of the COx unit. Petrologic study provided evidence of the following diagenetic mineral sequence: (1) framboidal pyrite and micritic calcite, (2) iron-rich euhedral carbonates (ankerite, sideroplesite) and glauconite (3) limpid calcite and dolomite and celestite infilling residual porosity in bioclasts and cracks, (4) chalcedony, (5) quartz/calcite. Pyrite in bioturbations shows a wide range of δ34S (−38‰ to +34.5‰), providing evidence of bacterial sulphate reduction processes in changing sedimentation conditions. The most negative values (−38‰ to −22‰), measured in the lower part of the COx unit indicate precipitation of pyrite in a marine environment with a continuous sulphate supply. The most positive pyrite δ34S values (−14‰ up to +34.5‰) in the upper part of the COx unit indicate pyrite precipitation in a closed system. Celestite δ34S values reflect the last evolutionary stage of the system when bacterial activity ended; however its deposition cannot be possible without sulphate supply due to carbonate bioclast dissolution. The 87Sr/86Sr ratio of celestite (0.706872-0.707040) is consistent with deposition from Jurassic marine-derived waters. Carbon and oxygen isotopic compositions of bulk calcite and dolomite are consistent with marine carbonates. Siderite, only present in the maximum clay zone, has chemical composition and δ18O consistent with a marine environment. Its δ13C is however lower than those of marine carbonates, suggesting a contribution of 13C-depleted carbon from degradation of organic matter. δ18O values of diagenetic chalcedony range between +27‰ and +31‰, suggesting precipitation from marine-derived pore waters. Late calcite crosscutting a vein filled with chalcedony and celestite, and late euhedral quartz in a limestone from the top of the formation have lower δ18O values (not, vert, similar+19‰), suggesting that they precipitated from meteoric fluids, isotopically close to present-day pore waters of the formation. Finally, the study illustrates the transition from very active, biotic diagenesis to abiotic diagenesis. This transition appears to be driven by compaction of the sediment, which inhibited movement of bacterial cells by reduction of porosity and pore sizes, rather than a lack of inorganic carbon or sulphates

Assessing anthropogenic pressures on groundwater using stable OH isotopes: perspectives and issues

International audienceLarge developments of isotope hydrogeology were done and well-established techniques mainly applying stableisotopes of the water molecule (hydrogen and oxygen) are now used largely to trace water provenance but alsorecharge processes. New methods allow the use of non-traditional isotopes (metals, compound specific stable iso-tope analysis CSIA. . . ) to trace anthropogenic pressures in surface- and groundwater. Groundwater contaminationin large industrial sites may come from several origins such as leakage from tanks during the production processof chemical products, liquid storage tanks, solid end product or past accumulated product in soil which is releasedover the time. The understanding of the origin and the further evolution of the chemical contamination in ground-water in an industrial site issued from past or current industrial activities is essential for the industrial companiesregarding their environmental policies. The objective of this study was to use with an innovative way the stableisotopes of the water molecule as a low cost tool to trace pollutant plumes in groundwater and help to a bettermanagement of contaminated industrial sites.We present data on stable isotopes O and H in an European region where electrochemistry plants occur. Forconfidentiality purposes, the sites remain anonymous. Present day industrial activities have a direct impact on thegroundwater over the site and migration of the contaminant(s) plume out of the site is supposed.We first characterize the natural groundwater background through the O-H characterization of surface water, lakes,thermal waters and regional shallow aquifers. High and low altitude recharge can be demonstrated in the area.Secondly, we used the stable isotope of the water molecule to trace over the site the impact of the Cl-rich liquormanufacturing process. Large deuterium enrichment was evidenced in the groundwater and the high values canbe related to a direct contamination of the groundwater through leakage of Cl-rich liquor with aD values up to400h ̇After technical operations to improve sealing of the liquid storage tanks, no evidence ofD enrichment canbe shown in present day groundwater. All values are along the global meteoric water line even for wells previouslyH-enriched, reflecting the absence of new leakage from tanks or during the manufacturing process over the site.Therefore our study shows that the stable isotopes of the water molecule can also be successfully used in aninnovative way to trace pollutant plumes in groundwate

L’Optimum Climatique de l’Eocène Moyen dans le Bassin de Paris. Enregistrement d’un événement hyperthermal en domaine littoral

International audienc

L'Optimum climatique de l'Eocène Moyen dans le Bassin de Paris. Enregistrement d'un événement hyperthermal en domaine littoral

International audienceL’Optimum Climatique de l’Eocène Moyen (MECO), événement hyperthermal rapide (50ka) et global de l’Eocène moyen (40Ma), est notamment caractérisé par une augmentation de la température des eaux de fond et de surface de 4 à 6°C en domaine océanique. Les causes de ce réchauffement demeurent inexpliquées mais il est associé à une augmentation de la pCO2 atmosphérique. Ces caractéristiques font du MECO un analogue du réchauffement actuel. Le MECO est bien documenté en milieu océanique, mais l’analyse des foraminifères océaniques ne permet d’obtenir que des données climatiques moyennées et ne permet pas un suivi à très haute résolution des variations pluriannuelles de températures et du gradient saisonnier de température. C’est pourquoi cette étude se focalise sur le Bassin de Paris, présentant (1) un enregistrement sédimentaire important et varié en domaine marin proximal à l’Eocène moyen et (2) une grande biodiversité et une qualité de préservation paléontologique remarquable. Des études ont montré que le MECO serait bien enregistré dans la stratigraphie du Bassin de Paris, à la base de l’Auversien et sur l’île de Wight, mais sa localisation exacte dans les archives sédimentaires reste à déterminer. Le but de cette étude est donc (1) de localiser précisément le MECO par une approche chimiostratigraphique (δ18O, δ13C) dans les dépôts littoraux du Bassin de Paris, (2) de déterminer les caractéristiques climatiques (T°moyenne annuelle et gradient saisonnier) de cet événement par l’analyse isotopique des coquilles de mollusques et (3) de déterminer la réponse sédimentaire associée à ce réchauffement en contexte de bassin intracratonique. Pour cela, nous menons une approche couplée terrain et laboratoire avec l’étude des affleurements et l’échantillonnage de coquilles mollusques dans les sables marins de l’Auversien, dans une zone allant de la région du Vexin au Tardenois. Les résultats préliminaires confirment l’enregistrement d’un réchauffement au sein de la formation des Sables d’Auvers, à la base de l’Auversien

Water exchange, mixing and transient storage between a saturated karstic conduit and the surrounding aquifer: groundwater flow modeling and inputs from stable water isotopes

International audienceWater exchanges between a karstic conduit and the surrounding aquifer are driven by hydraulic head gradient at the interface between these two domains. The case-study presented in this paper investigates the impact of the geometry and interface conditions around a conduit on the spatial distribution of these exchanges. Isotopic (δ18O and δD), discharge and water head measurements were conducted at the resurgences of a karst system with a strong allogenic recharge component (Val d’Orléans, France), to estimate the amounts of water exchanged and the mixings between a saturated karstic conduit and the surrounding aquifer. The spatio-temporal variability of the observed exchanges was explored using a 2D coupled continuum-conduit flow model under saturated conditions (Feflow®).The inputs from the water heads and stable water isotopes in the groundwater flow model suggest that the amounts of water flowing from the aquifer are significant if the conduit flow discharges are less than the conduit flow capacity. This condition creates a spatial distribution of exchanges from upstream where the aquifer feeds the conduit (recharge area) to downstream where the conduit reaches its maximum discharge capacity and can feed the aquifer (discharge area). In the intermediate transport zone no exchange between the two domains takes place that brings a new criterion to delineate the vulnerable zones to surface water.On average, 4% of the water comes from the local recharge, 80% is recent river water and 16% is old river water. During the November 2008 flood, both isotopic signatures and model suggest that exchanges fluctuate around this steady state, limited when the river water level increases and intensified when the river water level decreases. The existence of old water from the river suggests a transient storage at the aquifer/conduit interface that can be considered as an underground hyporheic zone

Compound-specific δ 18O of triterpenols: rationale, first results and issues

International audienceThe D18OLE project aimsat developing the determination of δ18O values of triterpenols in order to reconstruct pastatmospheric O2(O2atm) δ18O values and evaluate changes in the Dole Effect over the Cenozoic. Cultivation of Panicum miliaceumwith varying H2O and CO2δ18O valuesshowed that oxygen in miliacin, a C3-oxygenated pentacyclic triterpene,derives from respired O2, in agreement with biogeochemical observationsand consistant with modeled isotope fractionation factor. δ18O values of triterpenols preserved in the fossil record should thus allow estimating the long-term evolution of O2atmδ18O values.The first challenge consists in affordinga robust determination of sterols and triterpenols δ18O values by GC-Py-irMS. The low O/C ratio for these compounds constitutes an analytical challenge. First results acquired on miliacin and friedelin display large peaks by gas chromatography, preventingacceptable reproducibility. A second issue is the current lack of international lipids standards with known δ18O values.The fractionation factor between O2atmand O in triterpenols will be determinedby cultivating vascular plants and algae under controlled atmosphere with known O2δ18O values. Finally, δ18O values of triterpenols will be determined in a set of lignites and lacustrinesediments that cover two major climatic crisis of the Cenozoic : the Paleocene-Eocene Thermal Maximum and the Eocene/Oligcene boundary