Variabilité des données environnementales (exemple des sédiments marins)

Le travail présenté dans ce mémoire de thèse a pour objectif d optimiser l étude des sédiments marins dans un objectif d assurance qualité. Pour illustrer cette approche, je me suis focalisé sur la fraction fine du sédiment. En sédimentologie, la fraction fine est la fraction du sédiment dont la taille des particules est inférieure à une dimension donnée (2, 10, 20, 40, 50, 63,80 m). Son pourcentage est un paramètre fondamental pour de nombreuses études des sédiments puisqu on l utilise aussi bien en géochimie, en géophysique, en pédologie, en archéologie. Aujourd hui, deux approches se font concurrence et complémentaire dans les laboratoires pour déterminer le taux de fraction fine. Le tamisage considéré comme une méthode ancienne mais qui a fait ses preuves et la diffraction laser, méthode plus récente. Chacune d entre elle est encensée ou vilipendée par de nombreux techniciens et/ou chercheurs. Afin de mieux comprendre ce casus belli entre les sédimentologues, j ai, dans une première étape, remis en question toutes les étapes d obtention du taux de fraction fine afin d identifier les sources d approximation et d erreurs qui entachent éventuellement le résultat obtenu. Ce travail a été réalisé à la fois pour la méthode par tamisage humide et pour la méthode par diffraction laser. Dans une seconde étape, j ai cherché a mettre en évidence toutes les approximations qui sont réalisées lors de la chaîne qui relie l élaboration du plan de prélèvements en mer à l interprétation. Ainsi, l influence du schéma de prélèvement, du système de prélèvement, la variabilité spatiale à différentes échelles (du m à la centaine de m) ont été étudiées. Pour plus d universalité, plusieurs faciès sédimentaires ont été étudiés sur deux sites aux caractéristiques environnementales très différentes. La lagune de Oualidia a été étudiée dans le cadre de la coopération Franco-Marocaine (MA/07/179) et la baie de Seine orientale dans le cadre du programme "Colmatage" financé par le GIP "Seine Aval". Les résultats obtenus vont permettre d optimiser la structuration des bases de données sédimentologiques mises en place en précisant l importance de certains champs qui jusqu à présent ne sont jamais renseignés. Seule la prise en compte de ces informations complémentaires permet une interprétation de la meilleure Qualité possible. Ainsi, par exemple, l étude de la variabilité temporelle peut être optimisée lorsqu on peut faire la différence entre les variabilités naturelles, celles que l on recherche, et les variabilités, prises au sens large, liées à la mesure .The work presented in this thesis aims to optimize the study of marine sediments in an objective quality assurance. To illustrate this approach, I focused on the fine fraction of sediment. In sedimentology, the fine fraction is the fraction of the sediment particles whose size is less than a given size (2, 10, 20, 40, 50, 63, 80 m). His percentage is a fundamental parameter for many studies of sediments because the used both in geochemistry, geophysics, soil science, archeology .Today, two approaches are competing and complementary in laboratories to determine the rate of fine fraction. Sieving considered an old method but which has proven itself and the laser diffraction which is a new method. Each one is praised or vilified by many technicians and / or researchers. To better understand this "casus belli" between sedimentologists, I, first questioned all the steps of obtaining the rate of fine fraction to identify sources of approximation errors and possibly taint the result. This work was carried out for both the method by wet sieving and laser diffraction method. In a second step, I tried to highlight all approximations that are made in the chain that links the development of offshore sampling plan for interpretation. Thus, the influence of sampling scheme, sampling system, the spatial variability at different scales (from meters to hundred meters) have been studied. For universality, several sedimentary facies were studied at two sites with very different environmental characteristics. Oualidia lagoon has been studied in the framework of Franco-Moroccan cooperation (MA/07/179) and the eastern Bay of Seine in the program "Colmatage" funded by the GIP "Seine Aval". The results will help to optimize the structure of sedimentological databases implemented by specifying the importance of certain fields that until now were never informed. Only the inclusion of this additional information allows a better interpretation of the "Quality" as possible. For example, the study of temporal variability can be optimized when you can make the difference between natural variability, those that are looking for, and variability, in a broad sense, related to "measure".PERPIGNAN-BU Sciences (661362101) / SudocSudocFranceF

Compound incised-valley characterization by high-resolution seismics in a wave-dominated setting: Example of the Aude and Orb rivers, Languedoc inner shelf, Gulf of Lion, France

International audienceA branch of a time-calibrated late Quaternary compound incised valley complex is investigated using high- and very-high-resolution seismic data. The incised valley system is confined on the inner shelf, and entrenched parallel to the shore in unconsolidated Pliocene deposits. The infilling of the incised valley system comprises three sigmoidal bodies dipping progressively downstream representing depositional sequences. The lowermost sequences are less well preserved at their downstream extremity and the whole system is both aggrading and prograding. Older Pleistocene/Late Quaternary sequences could be preserved under the coastal plain. Individual sequences are closely similar to the classic model of a microtidal incised valley fill. Nevertheless, the central estuary/bay basin muds are seen to interfinger locally with high-energy deposits that represent potential reservoirs. The properties (prograding and aggrading architecture and occurrence of high energy deposits) and preservation of these compound incised valley fill deposits are attributed to general (glacio-eustatic cycles) and local (atmospheric and oceanic regime and proximity of the hinge line) conditions. Data acquisition strategy is a determining factor to interpret such systems

High resolution stratigraphy and evolution of the Rhône delta plain during Postglacial time, from subsurface drilling data bank

International audienceThis study is based upon a large set of core drilling data (lithology and well logs) of the Rhône delta plain extracted from the French geological survey (BRGM) databank. The main goals are to set up a sequence stratigraphy model of the postglacial sedimentary bodies of the Rhône delta plain and to evaluate the relative importance and influence of the main controlling factors and processes (subsidence, eustasy, sediment yield, climate, anthropic influence) that led to their formation.First, a detailed study (60 drill holes with tenth meters spacing) was performed on a restricted area (Saint Ferréol lobe, 1.5 km2). Correlation between well logs data and lithological data allows to identify the main sediment lithofacies and key surfaces, and to set up a detailed stratigraphic framework. Postglacial deposits are subdivided into a lower member of coastal plain and backbarrier deposits and an upper member of marine deposits (coastal barrier environments).Following, the analysis of 160 wells distributed on the whole delta plain depicts the large scale architecture of postglacial deposits. In the upper delta plain, fluviatile coastal plain and palustral deposits prevail. In the lower delta plain, the deposits are made of a lower part of coastal plain deposits organized into four units (T1–T4) stacked in a retrograding pattern. This lower member is truncated by an erosional surface progressively deepening seaward. Above, an upper member of marine deposits thickening seaward and coarsening upward is organized into five units (P1–P5) arranged in a prograding pattern.The core drilling data have been correlated with high resolution seismic data on the adjacent shelf and are interpreted in sequence stratigraphy terms. The retrogradational units (T1–T4), with coastal plain deposits, are correlative of shelf units and constitute together trangressive parasequences deposited during phases of slowing down of postglacial sea level rise. The upper marine units (P1–P5), arranged in a prograding pattern, represent parasequences of the Highstand Systems Tract which have prograded since the period of reduced rate of sea level rise at the end of the Holocene. During this period, the evolution of the sedimentary Rhône system was controlled by major changes of the hydrologic regime. Periods of increased sediment discharge, aggradation of the floodplain and progradation of the coastline are correlated with the development of the deltaic lobes of the Rhône d'Ulmet (unit P3) and of the Rhône du Bras de Fer (unit P4). Unit P5 correlates with adjacent prograding shelf unit that developed seaward of the present Rhône river mouth.Comparison of sea level history between the Eastern part of the Rhône delta plain and the rocky coast to the east of Marseille suggests that tecto-subsident movements account for part of the changes in relative sea-level. Moreover, the truncation of transgressive and progradation surfaces, the unevenness of the MFS, the dilatation of sedimentary record and deformations of key surfaces are also in favor of differential subsidence

Grandes séquences holocènes et discontinuités sédimentaires dans le delta du Rhône

This article presents a synthesis of radiocarbon dates obtained on charcoals and subfossil woods in more than twenty sections in alluvial infill from little torrents in the Southern Alps. This synthesis points out that the period 14500 -0 cal BP is bi-partite. From 14500 to 7000 cal BP alluvial accumulation tendency is dominant. Sedimentation rates are low from 14500 à 11 700 cal BP, higher from 11 700 to 9000 cal BP, and highest from 9000 to 7500 cal BP. These data evidence humid conditions during the period 14 500 -7000 cal BP. Sedimentation process stops around 7000 cal BP and the following millennia are poorly documented. An important incision occurs before (or at the beginning of the Little Ace Age.La quantification des séquences sédimentaires holocènes du delta du Rhône est obtenue par calcul d'accumulation verticale, par mesure de la progradation horizontale de la plaine deltaïque et estimation du volume total accumulé. Nous utilisons pour cela un ensemble de données : profils sismiques haute résolution réalisés sur la plate-forme interne, carottes sur la plaine deltaïque et cartographie géomorphologique à haute résolution chronologique des cordons littoraux. Les variations du stockage sédimentaire sont fortement contraintes par les forçages géomorphologiques (position des embouchures, variation du niveau marin relatif). Le calcul des volumes effectué sous-estime fortement le stockage sédimentaire si l 'on se réfère aux apports actuels. Cela est probablement lié au trop faible nombre de données carottées à terre, à l'incertaine corrélation des faciès sismiques et à la difficulté d'estimer la porosité des sédiments dans la colonne sédimentaire. La période 2800/2525 cal BP semble cependant clairement identifiée comme période de rapide stockage et progradation deltaïque et pourrait correspondre à la détérioration climatique du premier Age du fer identifiée ailleurs dans le bassin versant et en Europe. Cette tendance, moins marquée, se prolongera durant le second âge du Fer. L 'apparent fonctionnement antithétique du bassin versant durancien amont avec les apports sur le delta du Rhône est à noter, mais le caractère non exhaustif de la quantification de l 'accumulation sédimentaire sur le plateau continental interne laisse à l 'état d 'hypothèse cette observation.Vella Claude, Fleury Thomas Jules, Gensous Bernard, Labaune Caroline, Tesson Michel. Grandes séquences holocènes et discontinuités sédimentaires dans le delta du Rhône. In: Collection EDYTEM. Cahiers de géographie, numéro 6, 2008. Du climat à l’homme, dynamique holocène de l’environnement dans le Jura et les Alpes. pp. 155-166

Seismic stratigraphy of the Deglacial deposits of the Rhône prodelta and of the adjacent shelf

In order to achieve a synthesis of the stratigraphic organization of the Deglacial deposits of the inner/middle shelf in front of the Rhône delta plain, we merged high resolution (Mini-sparker and Sparker), and very high resolution (chirp and mud-penetrator) seismic data into a single seismic database. Thus, the merged seismic database improves the lateral correlation between eastern and western parts, separated by the Rhône Incised Valley deposits. As a result the interpretation of seismic units in relation to local and global environmental changes was refined. The Deglacial deposits rest on a basal erosional discontinuity capping a complex of Pleistocene prograding wedges. The identified units make up Transgressive and Highstand Systems Tracts, and are bounded by flooding surfaces. The main flooding surfaces are (1) the transgressive surface (D200) which forms the lower boundary of the Deglacial deposits and (2) the maximum flooding surface which forms the boundary between the Transgressive and Highstand Systems Tracts. Regarding the data set the study area is divided into three parts depending on the stacking pattern and main control factors. In the western area the units present an aggradational stacking pattern and the rate of sediment supply and dynamic conditions seems coupled with glacio-eustacy role on sedimentary units building and evolution. In the central area the units present an overall backstepping pattern mainly controlled by glacio-eustacy. In the eastern area the units present an aggradational stacking pattern and both the glacio-eustacy and rate of sediment supply have an important role. The lower parasequence of the Transgressive Systems Tract (U200) is due to the reworking of Würmian terraces. Above, the parasequence U300 is a transgressive body formed during a rapid sea-level rise. The two upper parasequences are interpreted as ancestral coastal systems with a backstepping pattern. The first coastal system (U400) is due to a decrease in the rate of sea-level rise that occurred during the Younger Dryas. The second coastal system (U500) may be due to a similar event or to an increased rate of sediment supply during a constant sea-level rise. Finally the Highstand Systems Tract is composed of three units: (i) unit U601, located to the West and interpreted as a subaqueous delta, (ii, iii) units U600 and U610, located to the East and interpreted as prograding deltaic lobes

Phase relationship between sea level and abrupt climate change

Direct traces of past sea levels are based on the elevation of old coral reefs at times of sea-level highstands. However, these measurements are discontinuous and cannot be easily correlated with climate records from ice cores. In this study we show a new approach to recognizing the imprint of sea level changes in continuous sediment records taken from the continental slope at locations that were continuously submerged, even during periods of sea-level lowstand. By using a sediment core precisely synchronized with Greenland ice cores, we were able to recognize major floods of the Mediterranean continental shelf over the past 270 kyr. During the last glacial period five flooding events were observed at the onset of the warmest Greenland interstadials. Consistent correspondence between warm climate episodes and eustatic sea level rises shows that these global flooding events were generated by pronounced melting of the Northern Hemisphere ice-sheets, due to rapid intensification of Atlantic Meridional Overturning Circulation. The method described in this study opens a new perspective for inter hemispheric synchronization of marine climate records if applied in other continental margins from the Southern Hemisphere or the equatorial regions

The Ziway–Shala lake basin system, Main Ethiopian Rift: Influence of volcanism, tectonics, and climatic forcing on basin formation and sedimentation

International audienc