The Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic (∼183 Ma ago) was characterized by widespread near-synchronous deposition of organic-rich shales in marine settings, as well as perturbations to several isotopic systems. Characteristically, two positive carbon-isotope excursions in a range of materials are separated by an abrupt negative shift. Carbon-isotope profiles from Toarcian fossil wood collected in England and Denmark have previously been shown to exhibit this large drop (∼ −7‰) in δ 13 C values, interpreted as due to an injection of isotopically light CO 2 into the ocean-atmosphere system. However, the global nature of this excursion has been challenged on the basis of carbon-isotope data from nektonic marine molluscs (belemnites), which exhibit heavier than expected carbonisotope values. Here we present new data, principally from fossil wood and bulk carbonate collected at centimetre scale from a hemipelagic section at Peniche, coastal Portugal. This section is low in organic carbon (average TOC = ∼0.5%), and the samples should not have suffered significant diagenetic contamination by organic carbon of marine origin. The carbon-isotope profile based on wood shows two positive excursions separated by a large and abrupt negative excursion, which parallels exactly the profile based on bulk carbonate samples from the same section, albeit with approximately twice the amplitude (∼ −8‰ in wood versus ∼ −3.5‰ in carbonate). These data indicate that the negative carbon-isotope excursion affected the atmosphere and, by implication, the global ocean as well. The difference in amplitude between terrestrial organic and marine carbonate curves can be explained by greater water availability in the terrestrial environment during the negative excursion, for which there is independent evidence from marine osmiumisotope records and, plausibly, changes in atmospheric CO 2 content, for which independent evidence is also available. The Peniche succession is also notable for the occurrence of re-deposited sediments: their lowest occurrence coincides with the base of the negative excursion and their highest occurrence coincides with its top. Thus, slope instability and sediment supply could have been strongly linked to the global environmental perturbation, an association that may misleadingly simulate the effects of sea-level fall.

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International audienceQuantitative analysis of Early Jurassic calcareous nannofossil assemblages from the Peniche section in Portugal have been performed in order to interpret palaeoenvironmental changes occurring in the Lusitanian Basin during the Late Pliensbachian Davoei, Margaritatus and Spinatum Ammonite Zones. Nannofossil data are compared to already published carbon and oxygen stable isotope data, organic matter content (wt\%Total Organic Carbon, TOC), and biomarker analysis. A significant change in calcareous nannofossil assemblages and species diversity at the transition between the Margaritatus and Spinatum Ammonite Zones matches with the pattern shown by geochemical data. This suggests that a profound change in environmental conditions occurred at that time. In the Davoei and Margaritatus Ammonite Zones, in a context of general sea-level rise, the Lusitanian Basin was characterized by water column stratification that favoured the sedimentation and preservation of organic matter. Biomarkers and oxygen isotope trends suggest that stratification of water masses occurred because of low salinity in surface waters. The shallowest part of the water column, characterized by oligotrophic conditions, was inhabited by the (probable) calcareous dinocyst Schizosphaerella spp., while the deep-dweller Crepidolithus crassus flourished in the lower photic-zone layers. In the Spinatum Ammonite Zone, a regressive trend occurred and a salinity increase is inferred on the basis of oxygen isotope values. Water masses were probably less stratified at that time. The upper photic-zone nannofossil assemblages were still dominated by Schizosphaerella spp. whilst, in the deep photic-zone. Mitrolithus jansae (a Mediterranean taxon) replaced C crassus (a taxon with NW-European affinity). This pattern may indicate a change in palaeoceanographic conditions related to surface current circulation. The sea-level fall occurring during the Spinatum Ammonite Zone may have resulted in the partial isolation of the Lusitanian Basin from the NW-Europe basins because of the creation of a threshold. Alternatively, tectonic drowning of North African carbonate platforms in the late Pliensbachian may have promoted better exchanges of nannoplankton between the Lusitanian Basin and the Mediterranean Tethys via the creation of new seaway connections. (C) 2010 Elsevier B.V. All rights reserved

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P R O V A S CALCAREOUS NANNOFOSSILS AND PALYNOMORPHS FROM PLIENSBACHIAN-TOARCIAN BOUNDARY IN LUSITANIAN BASIN, PORTUGALABSTRACT -The Pliensbachian/Toarcian boundary (Lower Jurassic) is well represented in the Lusitanian Basin (Portugal), mainly in Peniche area, where more than 450 m of marls and limestones are exposed. These deposits were chosen as a candidate for the Toarcian Global Stratotype Section and Point (GSSP). Calcareous nannofossil and palynomorph assemblages are described herein, with the aim to contribute to the GSSP definition. Esta assembléia, aliada ao aparente posicionamento das amostras acima da extinção de Crucirhabdus primulus, indica que o limite PliensbaquianoToarciano estaria localizado na parte superior da subzona NJ5b. Os gêneros Schizosphaerella e Lotharingius dominam numericamente a assembléia, também caracterizada pela ocorrência abundante de C. jansae e comum de B. grande, indicadores de forte influência de águas do Tétis. A associação palinológica recuperada se apresenta, de modo geral, com baixo grau de preservação, pouco diversificada, com formas pequenas e domínio de matéria orgânica lenhosa, algo carbonizada. Predominam as formas rimuladas do gênero Corollina, sendo os esporos triletes um componente importante nas associações. Os dinoflagelados são comuns na porção mais inferior da seção. O relativo alto percentual dos rimulados indica um clima quente à época de deposição. O resíduo orgânico rico em material lenhoso indica um forte influxo de material continental, detrítico, invadindo a plataforma carbonática instalada, particularmente a partir do Toarciano. Palavras-chave: nanofósseis calcários,