257 research outputs found
SinemurianâPliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Available online 12 December 2016The biostratigraphy of Sinemurian to lower Toarcian calcareous nannofossils has been investigated in the Sancerre-Couy core (Paris Basin), which contains a mixed assemblage of species with affinities to the northern and southern areas of the peri-tethyan realm, thus allowing for the use and calibration of the Mediterranean Province (Italy/S France) and NW Europe (UK) biozonation schemes. This study is based on semi-quantitative analyses of the calcareous nannofossil assemblage performed on 145 samples and the recorded bioevents are calibrated to the NW European Ammonite Zonation and to a new organic carbon isotope curve based on 385 data points. The main bioevents, i.e. the first occurrences of Parhabdolithus liasicus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Similiscutum cruciulus sensu lato, Lotharingius hauffii, Crepidolithus cavus and Lotharingius sigillatus as well as the last occurrence of Parhabdolithus robustus, have been identified. However, we show that a large number of standard biostratigraphic markers show inconsistent occurrences at the base and top of their range, possibly accounting for some of the significant discrepancies observed between the different domains. In addition to the nine main bioevents used for the biozonation of the core, we document an additional 50 distinct bioevents, evaluate their reliability and discuss their potential significance by comparison to previous studies. A total of five significant negative organic carbon isotope excursions are identified and defined in the Paris Basin including the well-documented SinemurianâPliensbachian boundary event. One positive excursion is further defined in the Pliensbachian interval. Our calibration of high-resolution calcareous nannofossil biostratigraphy to ammonite biostratigraphy and organic carbon isotopes represents a new stratigraphic reference for the Lower Jurassic series
Climatic and palaeoceanographic changes during the Pliensbachian (Early Jurassic) 2 inferred from clay mineralogy and stable isotope (C-O) geochemistry (NW Europe)
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Available online 17 January 2017The Early Jurassic was broadly a greenhouse climate period that was punctuated by short
warm and cold climatic events, positive and negative excursions of carbon isotopes, and
episodes of enhanced organic matter burial. Clay minerals from Pliensbachian sediments
recovered from two boreholes in the Paris Basin, are used here as proxies of detrital supplies,
runoff conditions, and palaeoceanographic changes. The combined use of these minerals with
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stable isotope data (C-O) from bulk carbonates and organic matter allows palaeoclimatic
reconstructions to be refined for the Pliensbachian. Kaolinite/illite ratio is discussed as a
reliable proxy of the hydrological cycle and runoff from landmasses. Three periods of
enhanced runoff are recognised within the Pliensbachian. The first one at the SinemurianPliensbachian
transition shows a significant increase of kaolinite concomitant with the
negative carbon isotope excursion at the so-called Sinemurian Pliensbachian Boundary Event
(SPBE). The Early/Late Pliensbachian transition was also characterised by more humid
conditions. This warm interval is associated with a major change in oceanic circulation during
the Davoei Zone, likely triggered by sea-level rise; the newly created palaeogeography,
notably the flooding of the London-Brabant Massif, allowed boreal detrital supplies, including
kaolinite and chlorite, to be exported to the Paris Basin. The last event of enhanced runoff
occurred during the late Pliensbachian (Subdonosus Subzone of the Margaritatus Zone),
which occurred also during a warm period, favouring organic matter production and
preservation. Our study highlights the major role of the London Brabant Massif in influencing
oceanic circulation of the NW European area, as a topographic barrier (emerged lands) during
periods of lowstand sea-level and its flooding during period of high sea-level. This massif was
the unique source of smectite in the Paris Basin. Two episodes of smectite-rich sedimentation
(âsmectite eventsâ), coincide with regressive intervals, indicating emersion of the London
Brabant Massif and thus suggesting that an amplitude of sea-level change high enough to be
linked to glacio-eustasy. This mechanism is consistent with sedimentological and
geochemical evidences of continental ice growth notably during the Latest Pliensbachian
(Spinatum Zone), and possibly during the Early Pliensbachian (late Jamesoni/early Ibex
Zones).The study was supported by the âAgence Nationale pour la Gestion des DĂ©chets Radioactifsâ (AndraââFrench National Radioactive Waste Management Agency)
THE CHANGING ECONOMIC SPATIAL STRUCTURE OF EUROPE
Many theoretical and practical works aim at describing the spatial structure of Europe, where spatial relations have undergone continuous change. This article gives an overview of models describing the spatial structure of Europe. Their diversity is highlighted by listing of these models, without any claim to completeness. Our study aims at describing the economic spatial structure of Europe with bi-dimensional regression analysis based on the gravitational model. With the help of the gravity model, we get a spatial image of the spatial structure of Europe. With these images, we can justify the appropriateness of the models based on different methodological backgrounds by comparing them with our results. Our goal is not to create and show a new model that overwrites the existing ones, but rather to contribute to understanding the European spatial structure through a new methodological approach
Drivers for Rift Valley fever emergence in Mayotte: A Bayesian modelling approach
Rift Valley fever (RVF) is a major zoonotic and arboviral hemorrhagic fever. The conditions leading to RVF epidemics are still unclear, and the relative role of climatic and anthropogenic factors may vary between ecosystems. Here, we estimate the most likely scenario that led to RVF emergence on the island of Mayotte, following the 2006â2007 African epidemic. We developed the first mathematical model for RVF that accounts for climate, animal imports and livestock susceptibility, which is fitted to a 12-years dataset. RVF emergence was found to be triggered by the import of infectious animals, whilst transmissibility was approximated as a linear or exponential function of vegetation density. Model forecasts indicated a very low probability of virus endemicity in 2017, and therefore of re-emergence in a closed system (i.e. without import of infected animals). However, the very high proportion of naive animals reached in 2016 implies that the island remains vulnerable to the import of infectious animals. We recommend reinforcing surveillance in livestock, should RVF be reported is neighbouring territories. Our model should be tested elsewhere, with ecosystem-specific data
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