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International audienceValanginian sedimentary archives display a positive anomaly in the carbon-isotope record which is associated with a crisis in neritic and pelagic carbonate production. This study aims to reconstruct the evolution of palaeoenvironments in the Jura area and the Vocontian Basin during late Berriasian and early Valanginian times, in order to better understand the controlling factors leading to the Valanginian episode of major environmental change. Three sections along a transect through the Jura platform (Switzerland and France) and the Vocontian Basin (France) have been studied. Stratigraphic correlations have been established by combining biostratigra- phy (benthic foraminifera, ammonites and calpionellids), with geochemistry (δ13C trends) and sequence stratigraphy. A change from a rimmed ramp to a swell-dominated ramp morphology is observed through the Berriasian-Valanginian boundary. The disappearance of the barrier appears to have been controlled by large sea-level variations in combination with the arrival of significantly more humid climate conditions. This barrier played a major role in the distribution of geochemical, mineralogical and palynological fluxes towards the basin: during the Berriasian, continental fluxes (detrital particles, nutrients, pollen and spores) were buffered by the platform morphology, whereas in the early Valanginian they were more readily transported towards the basin. The initiation of a major transgression onto lateritic soils, leading to their intense reworking, instigated a fertilization of ocean waters during the earliest Valanginian and with that a change towards heterozoan carbonate production, and subsequently to the demise of the already weakened carbonate platfor

The Valanginian stage (Early Cretaceous) includes an episode of significant environmental changes, which are well defined by a positive δ13C excursion. This globally recorded excursion indicates important perturbations in the carbon cycle, which has tentatively been associated with a pulse in volcanic activity and the formation of the Paraná-Etendeka large igneous province (LIP). Uncertainties in existing age models preclude, however, its positive identification as a trigger of Valanginian environmental changes. Here we report that in Valanginian sediments recovered from a drill core in Wąwał (Polish Basin, Poland), and from outcrops in the Breggia Gorge (Lombardian Basin, southern Switzerland), and Orpierre and Angles (Vocontian Basin, SE France), intervals at or near the onset of the positive δ13C excursion are significantly enriched in mercury (Hg). The persistence of the Hg anomaly in Hg/TOC, Hg/phyllosilicate, and Hg/Fe ratios shows that organic-matter scavenging and/or adsorbtion onto clay minerals or hydrous iron oxides only played a limited role. Volcanic outgassing was most probably the primary source of the Hg enrichments, which demonstrate that an important magmatic pulse triggered the Valanginian environmental perturbations.

During the Phanerozoic, Earth has experienced a number of transient global warming events associated with major carbon cycle perturbations. Paradoxically, many of these extreme greenhouse episodes are preceded or followed by cold climate, perhaps even glacial conditions, as inferred from the occurrence of glendonites in high latitudes. Glendonites are pseudomorphs of ikaite (CaCO 3 •6H 2 O), a hydrated carbonate mineral increasingly stable at low temperatures. Here, we show that methane seepage and oxidation provide an overriding control on Mesozoic glendonite formation (i.e., ikaite fossilization). Geochemical and petrological analyses of 33 Early Jurassic to Early Cretaceous glendonites from five sections in Siberia (Russia) reveal that most of their infilling carbonate phases are reminiscent of methane-derived authigenic carbonates. Bulk glendonites and surrounding sediments exhibit exceptionally high and low carbon isotope values (+20‰ to −45‰ VPDB [Vienna Peedee belemnite]), typical for carbon sources linked to methane generation and oxidation. Gas inclusion data confirm the presence of methane and longer-chain hydrocarbon gases, suggesting a thermogenic source for the methane. Glendonitebearing layers can be traced for hundreds of kilometers, suggesting widespread trapping of methane in the sub-seafloor during the Jurassic. As such, glendonites constitute an unexplored archive for detecting past episodes of methane release and oxidation in polar settings.

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The Early Cretaceous period is characterized by widespread carbonate production in tropical and subtropical epicontinental seas, which was modulated by changes in sea-level, detrital and nutrient fluxes, and the global carbon cycle. As a result, carbonate platforms were sensitive recorders of environmental change, which often anticipated global environmental perturbations. A good example is provided by the northern Tethyan carbonate platform, which is presently preserved in the central European Helvetic Alps. There, the latest early to late Valanginian Weissert episode of global change, which is defined by the first important positive shift in d 13 C records of the Cretaceous, is expressed by a prolonged, stepwise drowning phase. In this contribution, a detailed reconstruction of palaeoenvironmental change before and during the Weissert episode is provided based on three representative sections of the Helvetic platform. The sections are placed along a deepening transect and correlated by means of ammonite and microfossil biostratigraphy, sequence stratigraphy and d 13 C chemostratigraphy. In a first phase of palaeoenvironmental change during the latest Berriasian, photozoan carbonate production was stopped by a major and hitherto undetected drowning episode, which was followed by a phase of renewed carbonate production by heterozoan biota. This phase was linked to major sea-level rise, a change to a more humid climate and strong regional subsidence associated with tectonic block tilting. During the Valanginian, the circulation of nutrient-enriched sea waters prevented a return to oligotrophic conditions and two further drowning episodes occurred, which are both documented by condensed phosphate-rich beds and dated as middle early Valanginian and late Valanginian to early Hauterivian. The exact causes of the threestep deterioration in carbonate production are not established but a link to episodic volcanic activity is likely, eventually related to the formation of the Parana-Etendeka large igneous province.

We investigate herein the lateral and vertical lithological heterogeneities of the Lower/Middle Oxfordian deposits (“Terrain à Chailles” and “Marnes des Eparges” formations) in the north-eastern Paris Basin. This new detailed stratigraphic framework documents the evolution at high resolution of an outer ramp based on regional correlations in order to constrain the evolution of petrological properties between the clayey “Argiles de la Woëvre” Formation and the more calcareous “Marnes et Calcaires à Coraux de Foug Formation. The “Argiles de la Woëvre” Formation is targeted for the deep storage of nuclear waste in north-eastern France. Nine wells are correlated over the “Zone of Interest for Further Research” (ZIRA), defined by the French agency for radioactive waste management (Andra), with a resolution of 0.5–1.0 m. The architecture and the age control of these formations have been refined, revealing that the “Terrain à Chailles” Formation is characterised by a regular slightly inclined sedimentation gently deeping in the SW direction and shows a lithological evolution from silty claystones to an increased occurrence of its calcareous content towards the top (Lower Oxfordian, uppermost mariae and cordatum ammonite zones). The above “Marnes des Eparges” Formation, characterised by claystone limestone alternations, is assigned to the Middle Oxfordian (plicatilis ammonite zone), deposited during a slightly enhanced subsidence phase in the SE part of the basin and documented and associated with onlaps geometries on the more proximal areas. However, this change in geometry does not affect petrological properties over ZIRA, as this is not accompanied by lithological changes. The environmental factors controlling petrological heterogeneities over ZIRA are also discussed. The stepwise increase in the carbonate content and the decrease in the detrital content towards the Lower to Middle Oxfordian deposits was likely triggered by a climate change towards drier conditions, modulated by sea level changes on a ramp morphology. A major condensation phase encompassing most of the Lower Oxfordian cordatum ammonite zone is also highlighted. The occurrence of a maximum regressive surface associated with gentle slope topography is a probable trigger for condensation. Changes in geometries are, however, associated with the activity of the Metz Fault, which potentially had an influence on the subsidence rates of the basin at that time.

<p>Fossil ikaite, preserved as the pseudomorph glendonite, occurs in vast amounts in Jurassic and Cretaceous successions in the high Arctic. Thermodynamics predict that ikaite is only stable at near-freezing temperatures and glendonite is thus widely used as a paleo-indicator of cold climate conditions, conflicting with traditional views of a very warm and equable Mesozoic greenhouse. Here, we show based on a multi-proxy investigation of Jurassic and Cretaceous glendonites from Siberia and Svalbard that this one-dimensional view detracts from their exceedingly complex biogeochemistry. NanoSIMS analyses of a Jurassic glendonite from Siberia produced large C-isotope gradients (> 40‰) over micrometer distances hinting at strong kinetic fractionation that is coupled to the formation of various precipitates, including an inclusion-rich primordial phase with C-isotope values as low as -38‰ that records methane oxidation. In line with previous results from Siberia, all investigated glendonites from the Cretaceous of Svalbard contain methane gas (700 - 2500 ppb/g) with enriched δ<sup>13</sup>C-CH<sub>4</sub> signatures (-44 to -50‰ V-PDB), depleted δ<sup>2</sup>H--CH<sub>4</sub> (-285 to -245‰ V-SMOW), and relatively large proportions of C<sub>2</sub>-C<sub>5</sub> gas. Such values are potentially indicative of thermogenic methane gas sourced from structure II gas hydrates. Organic geochemistry of glendonites from Svalbard shows the presence of abundant hopanes, including bisnorhopanes with a CSIA signature of -41‰, suggesting activity of sulfide oxidizing bacteria possibly also linked to the inclusion of oil droplets. Moreover, exceptionally heavy bulk δ<sup>34</sup>S<sub>cas</sub> values of +46.2‰ clearly link marine ikaite formation in deep time to sulfate-driven anaerobic methane oxidation. Marine ikaite formation and preservation is thus a highly complex process, driven by temperature and (bio)chemical processes in the sea floor, complicating its use as a simple paleoclimate proxy. Regardless, glendonite episodically trapped large amounts of greenhouse gases and stored those for hundreds of millions of years, making this authigenic mineral a potential recorder of past carbon cycle perturbations.</p>