Dynamique hydrogéomorphologique et diversité végétale dans le delta du Rhône (France) de -10 000 ans à demain

International audienceLe delta du Rhône représente aujourd’hui une vaste plaine de niveau de base dont la surface est estimée à 1742 km2(fig. 1). Ancrée entre les deux bras du Rhône, la « Grande Camargue » n’en constitue à peine que la moitié (850 km2).La présente étude vise à retracer l’histoire hydrogéomorphologique et végétale du « grand delta » à l’échelle del’Holocène1.Dans le delta du Rhône, les écosystèmes végétaux sont très bien connus. La diversité végétale y est élevée malgrél’absence de relief : plus de 1000 espèces recensées parmi les plantes à fleurs pour un total de 4700 espèces habitant laFrance. La végétation actuelle est fortement dépendante non seulement des paramètres climatiques globaux mais aussidu milieu local, en particulier de la teneur en sel des eaux de surface et de sub-surface, et des actions humaines2.L’évolution de la plaine deltaïque, dont les principaux prismes sédimentaires ont été édifiés en moins de 6000 ans, dansun contexte de fluctuations plus ou moins marquées du niveau de la mer, des apports alluvionnaires etd’occupation/utilisation des sols, a forcément eu une incidence sur la dynamique végétale et la biodiversité en général.Il nous a donc paru utile de reprendre les étapes de l’édification du delta du Rhône, à la lumière des travaux dequelques-uns de nos prédécesseurs3, enrichis et complétés par les données stratigraphiques acquises plus récemment, etsurtout grâce aux jalons chronologiques apportés par les trouvailles archéologiques et les datations 14C, qui souventavaient fait défaut dans les études antérieures

Le déluge et la Crise messinienne

National audienceLa Crise messinienne débuta il y a 5,971 Millions d’années (Ma) pour s’achever 500 000 ans plus tard. L’eau, qu’elle soit salée, saumâtre ou douce, y a tenu un rôle déterminant. La fin de crise s’est soldée par un déluge d’une violence inouïe tirant sa source dansl’ampleur même de la crise, à savoir l’évaporation quasi-totale de la Mer Méditerranée. Depuis 14 Ma, la Méditerranée est un appendice de l’Océan Atlantique. La tectonique a fermé successivement trois corridors pour n’en laisser qu’un seul au sud du Rif (figure 1a-b). Les stades de la Crise messinienne sont bien datés par les microfossiles marins, le paléomagnétisme, la cyclostratigraphie

Messinian erosional and salinity crises: View from the Provence Basin (Gulf of Lions, Western Mediterranean)

International audienceThough the late Miocene “Messinian Salinity Crisis” has been intensely researched along the circum-Mediterranean basins, few studies have focused on the central part of the Mediterranean Basin and, especially, the pre-salt deposits. To improve our knowledge of the Messinian events, it is imperative to better understand this domain. In this study, we provide a more complete understanding of this central domain in the Provence Basin. We were able to recognize: a) thick marine detrital series (up to 1000 m) derived from the Messinian subaerial erosion which is partly prolongated in the distal part by b) a thick unit of deep marine deposits (up to 800 m) prior to the evaporites; c) a thick presumed alternation of detritals and evaporites (1500 m) below the mobile halite; and d) a two-step transgression at the end of the Messinian. Spatially, we document the eroded shelf to the deep basin (and from the western to the eastern parts of the Gulf of Lions), and temporally, we extend the interpretations from the early deposition of detritic sediments to the final sea-level rise. The results provide a new basis for discussion not only for the development of the Messinian Salinity Crisis but also for the reconstruction of the subsidence history of the Provence Basin

Climat et niveau de l’océan Atlantique restitués en Espagne du Sud-Ouest de 6,3 à 5,2 Ma. Implications sur la Crise messinienne en Méditerranée

The Mio-Pliocene succession of Andalusia on the Atlantic coast (Guadalquivir Basin) is known as one of the former stratotype candidate for the Andalusian Stage, proposed during the seventies as the last stage of the Miocene. Its type section is located in Carmona, east of Seville. Our investigation includes the drilling of three cored boreholes, which provide bio- and magnetic-stratigraphic data in complement to preexisting industrial information, and a high-resolution palynological analysis (pollen grains, spores and dinoflagellate cysts). The pollen flora and its climatic quantification provide the mean to correlate the section to the oxygen isotope curve from the Montemayor-1 borehole, located about 80 km to theWest of Carmona. The variations in the ratio between dinoflagellate cysts and pollen grains are used to identify high and low oceanic levels, consistently with a recent paleobathymetric reconstruction based on foraminifera: the two lowest levels being successively marked by the deposit of a littoral calcarenite (the Calizza Tosca Formation) then by a subaerial erosive episode. Based on the correlation with the Montemayor-1 regional drilling, the two major lowerings in oceanic level observed at Carmona are linked with the two episodes of the Messinian Crisis. The interlocking position of the high-energy sandstone deposits inscribed in the Messinian valley leads to an assessment of a drop in the global oceanic level of about 114m at the beginning of the paroxysm of the Messinian Crisis, amplitude to be moderated with respect to the potential effect of isostatic readjustements due to the Messinian Crisis. Comparisons are discussed with the amplitude of the Messinian Erosional Surface in theWest-Alboran Basin which potentially remained suspended and fed with Atlantic waters during the height of the crisis and isolated from the rest of the almost totally dried Mediterranean Basin. The quantified climate constructed from the pollen records confirms that dry conditions existed before the Messinian Crisis in Southern Mediterranean latitudes including the Atlantic side, making the Mediterranean Sea climatically predisposed to desiccation. Atlantic sea-level variations observed in the Guadalquivir region and measured at Carmona suggest that global glacio-eustatism somewhat facilitated the onset and completion of the Messinian Crisis in the Mediterranean Basin. At last, this work allows to discriminate two regional erosive events: the first one, dated at 5.60 Ma, of fluvial origin in relation with global eustasy; the second one, submarine, occurred just before 5.33 Ma, and referred to the strain exerted by the Guadalquivir olistostrome.La succession mio-pliocène d’Andalousie atlantique (bassin du Guadalquivir) est connue comme ancienne candidate pour le stratotype de l’Andalousien, proposé comme dernier étage du Miocène dans les années 70. Sa coupe-type est située à Carmona, à l’est de Séville. Nos travaux comprennent notamment la réalisation de trois forages carottés où furent acquises des données bio- et magnéto-stratigraphiques complémentaires aux informations connues des recherches industrielles et, surtout, une analyse palynologique (grains de pollen et kystes de dinoflagellés) à haute résolution. La flore pollinique et la quantification climatique autorisent des corrélations climatostratigraphiques avec la courbe d18O du forage Montemayor-1 implanté près de Huelva. Les variations du rapport entre kystes de dinoflagellés et grains de pollen permettent d’identifier les hauts et bas niveaux océaniques en accord avec une récente estimation paléobathymétrique basée sur les foraminifères, les deux plus bas niveaux étant successivement marqués par le dépôt d’une calcarénite littorale (Formation de la Caliza Tosca) puis par un épisode érosif subaérien. Après corrélation avec le forage Montemayor-1, ces deux baisses du niveau océanique sont respectivement mises en correspondance avec les deux temps de la Crise messinienne. L’emboîtement des dépôts gréseux à haute énergie inscrits dans la vallée messinienne conduit à évaluer une chute du niveau atlantique d’environ 114m lors du paroxysme de la Crise messinienne, amplitude à modérer toutefois compte tenu du possible effet des rebonds isostatiques liés à la Crise messinienne. Des comparaisons sont discutées avec l’ampleur de l’érosion messinienne dans le bassin ouest-Alboran potentiellement resté suspendu et alimenté en eaux atlantiques pendant le paroxysme de la crise et isolé du reste du bassin méditerranéen quasi-totalement asséché. Les restitutions quantifiées du climat comparées entre la province atlantique et le domaine méditerranéen confirment les conditions xériques des latitudes sud-méditerranéennes s.l. et ainsi la prédisposition climatique de la Méditerranée à la dessiccation. Les variations du niveau atlantique observées dans la région du Guadalquivir et mesurées à Carmona suggèrent que le glacio-eustatisme global a quelque peu contribué au déclenchement et à l’achèvement de la Crise messinienne en Méditerranée. Ce travail permet enfin de discriminer deux événements érosifs régionaux : le premier à 5,60 Ma, d’origine fluviatile en relation avec l’eustatisme global, le second intervenu peu avant 5,33 Ma, sous-marin, lié aux contraintes exercées par l’olistostrome du Guadalquivir.CNRS-INSU, Total and Elf-Aquitaine companie

The Pliocene succession of Lyon Metropolis (SE France): an overfill of a Messinian incised-valley

The Pliocene ria, a narrow seaway running up the Rhône Valley, has been mapped for a while by field geologists. Only much later, after the DSDP Leg 13 in 1970, a consensus was reached that this unique geological feature of the Rhône Valley was created by the major Mediterranean sea-level drop associated with the Messinian Salinity Crisis, followed by a sudden sea-level rise caused by the breach of the Strait of Gibraltar and the invasion of the Mediterranean Basin by the Atlantic waters. At the regional level of the Lyon Metropolis in the upper Rhône Valley, main issues were however remaining about the course and depth of the Messinian valley, and about the valley fill, namely where and how do the Pliocene marine strata of the Rhône Valley pass to the continental deposits of the Bresse Basin to the north? These are key-questions in that the Plio-Pleistocene makes up a large fraction of the basement that holds up a large city, not free from potential geological hazards and subject to problems of groundwater management, high-cost tunneling projects, etc. Our survey reviews first the historical researches − descriptions of the outcrops and fossil assemblages. It is followed by an unprecedented analysis and correlation of a thousand boreholes, which makes it possible to physically link and reconcile ancient local observations. Sections across the Messinian valley reveal a proper canyon morphology for the segments that cut the crystalline basement. The magnitude of the incision has been calculated as 335 m to a minimum, three hundred kilometers away from the river mouth. Three major depositional systems are distinguished for the Pliocene − Lower Pleistocene succession. The valley that initially ran much farther north of Lyon was occupied in the Zanclean by a series of pounded lakes, dammed by transverse local alluvial fans, filled by minor Gilbert-type deltas, and repeatedly flooded by marine ingressions. The valley wings were then encroached during the Piacenzian by a major, Alps-rooted alluvial sheet. At the level of Lyon, the fluvial deposits were deflected to the north (Sables de Trévoux) and to the south (Alluvions jaunes) as a dichotomy. The succession was then capped by a gravel-dominated fluvio-glacial fan (Alluvions jaunes sommitales) at the Plio-Pleistocene transition. It spread out from the north-east to the south, and intersected the previous valley overfill due to the shift to the north, i.e., South Jura, of the feeder stream. The depositional and current elevations of the marine-influenced episodes, i.e. marker bands that punctuate the regional Neogene succession, are used to bring out successive uplift and subsidence phases of the region. Finally, we tentatively link the major shifts in the depositional patterns of the Late Neogene succession in the Lyon area to major changes in the thrust belt activity, exhumation story, and outset of glaciers in the western Alps

THE PIONEERING CONTRIBUTIONS AND LEGACY OF WALDO H. ZAGWIJN (1928-2018)

Professor Dr. Waldo H. Zagwijn passed away on 26 June, 2018, in his ninetieth year. His thesis research (Zagwijn, 1960) stimulated palynological investigations into the Pliocene and early Quaternary, and caused a fundamental shift in our under-standing of the transition from the warm and relatively stable climate of the late Neogene to the extreme glacial-interglacial oscilla-tions of the early Quaternary. Distinct in several aspects from the usual methodologies of that time, Zagwijn’s approach can be summarized as: focusing on establishing the botanical identification of pollen grains, following the guiding approach of his supervisor F. Flor-schütz, using botanical nomenclature to highlight these identifications and clarifying relationships between botany and the ear-lier artificial pollen nomenclature; comprehensive and continuous attention to plant macroremains (fruits, seeds, etc.; Zagwijn, 1990) as a mean of integrating classical palaeobotany with modern palynology; palaeoenvironmental interpretations informed by a deep understanding of modern plant ecology and associations; leading investigations in the field, and paying special attention to stratigraphic correlations between (onshore and offshore) boreholes and exposed sections in quarries, using a range of available proxies including heavy mineral analysis. Waldo H. Zagwijn was a visionary in realizing how modern palynology can be used for vegetational and climatic reconstructions in the Neogene, and indeed as far back as Paleogene times

High resolution climate and vegetation simulations of the Mid-Pliocene, a model-data comparison over western Europe and the Mediterranean region

International audienceHere we perform a detailed comparison between climate model results and climate reconstructions in western Europe and the Mediterranean area for the mid-Piacenzian warm interval (ca 3 Myr ago) of the Late Pliocene epoch. This region is particularly well suited for such a comparison as several quantitative climate estimates from local pollen records are available. They show evidence for temperatures significantly warmer than today over the whole area, mean annual precipitation higher in northwestern Europe and equivalent to modern values in its southwestern part. To improve our comparison, we have performed high resolution simulations of the mid-Piacenzian climate using the LMDz atmospheric general circulation model (AGCM) with a stretched grid which allows a finer resolution over Europe. In a first step, we applied the PRISM2 (Pliocene Research, Interpretation, and Synoptic Mapping) boundary conditions except that we used modern terrestrial vegetation. Second, we simulated the vegetation for this period by forcing the ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems) dynamic global vegetation model (DGVM) with the climatic outputs from the AGCM. We then supplied this simulated terrestrial vegetation cover as an additional boundary condition in a second AGCM run. This gives us the opportunity to investigate the model's sensitivity to the simulated vegetation changes in a global warming context

The Messinian Salinity Crisis in the Dacic Basin (SW Romania) and early Zanclean Mediterranean-Eastern Paratethys high sea-level connection

International audienceNew field observations and fossil analyses complete and clarify the strong impact of the Mediterranean sea-level changes linked to the peak of the Messinian Salinity Crisis on the Dacic Basin in southwestern Romania. In addition to the Gilbert-type fan delta already evidenced along the Danube River in the area of Turnu Severin, a new Gilbert-type fan delta is described northward. Early Zanclean bottomset beds are evidenced and dated based on nannofossils at the junction of the two coalescing Gilbert-type fan deltas. A clear sedimentological, morphological and chronologic differentiation is established in the area between the Carpathians Late Miocene piedmont alluvial fans and the early Zanclean Gilbert-type fan deltas. The early Zanclean age of the Hinova clays, where the bottomset beds of the Gilbert-type fan deltas are mostly developed, is confirmed by the occurrence of nannofossil markers of Subzone NN12b and a Bosphorian mollusk macrofauna. Early Zanclean inflow of Mediterranean marine waters into the Dacic Basin is also supported by the record of planktonic foraminifers. In the Dacic Basin, the Messinian Salinity Crisis resulted in the cutting of the Iron Gates by a Carpathians river. Fluvial erosion also affected the residual Pannonian Basin and probably catched the paleo-Tisza River which contributed to the erosion of the Iron Gates and to the fluvial drainage of the partly desiccated Dacic Basin. Arguments are reinforced in favor of a marine gateway between the Mediterranean and Dacic Basin through the Balkans before and after the Messinian Salinity Crisis