Variabilité cross-shore d'une plage de poche macrotidale : dynamique saisonnière, profondeur de fermeture, bilan de matière et disponibilité sédimentaire.
L'Observatoire du Domaine Côtier (OSU IUEM) supporte depuis 2003 un suivi topographique des plages de l'Iroise dans le but de détecter un éventuel impact de l'Oscillation Nord Atlantique sur leur variabilité morphologique et sur la mobilité du trait de côte à l'échelle décennale. A ce jour, la série de données topographiques se compose de 107 profils levés en zone intertidale à Porsmilin à une fréquence bi-hebdomadaire de janvier 2003 à juin 2007. L'analyse menée sur 75 d'entre eux (Dehouck, 2006) indique un contrôle des conditions d'agitation sur l'activité saisonnière du profil de la plage qui se traduit par des variations de sa réflectivité (présence de barres intertidales lors de conditions prolongées de beau temps, profil hivernal « plan » de pente homogène). A l'échelle annuelle, une tendance à la progradation du profil moyen vers la mer a été décelée laquelle coïncide avec l'accrétion sédimentaire observée sur la période 2003-2005 dans la zone intertidale. Sur la base de ces observations, un suivi bathymétrique mensuel de la plage a été réalisé sur une période de neuf mois (septembre 2003-mai 2004) dans le but : (1) d'explorer la topographie de la zone subtidale en vertu de la continuité des échelles spatiales entre les domaines intertidaux et subtidaux. Les zones de forte variabilité sédimentaire sont associées à la dynamique d'un couple de barres subtidales, la première relativement active et ample (jusqu'à 0,5 m) est localisée à proximité des niveaux de basse mer de vives eaux (-1 m C.M.), la seconde (-2 m C.M.) est ponctuelle et de très faible amplitude. La barre subtidale interne est le principal lieu de stockage sédimentaire entre les zones subtidale et intertidale, son activité étant fortement liée aux processus d'accrétion observés en zone intertidale (barre intertidale de swash sur les niveaux de pleine mer de mortes eaux à l'échelle événementielle, progradation du profil de la plage à l'échelle annuelle). Néanmoins, le temps d'ajustement des formes subtidales à la variabilité des forçages hydrodynamiques reste difficile à évaluer sachant que les morphologies observées résultent souvent de l'action d'une somme d'événements morphogènes. 2) de déterminer la profondeur de fermeture du profil de plage, profondeur limite de mobilité des sédiments. De manière relativement analogue aux études antérieures traitant de la profondeur de fermeture (par ex. Ruessink et al., 2003), celle-ci est associée à la profondeur limite d'activité des barres subtidales qui se situe autour de -3 m C.M. pour la période d'observation. Elle correspond également à la transition entre le prisme sédimentaire de la plage et la zone d'affleurement du substratum rocheux. Bien que cette valeur établie à -3 m puisse paraître faible au regard des travaux antérieurs et du contexte hydrodynamique agité au large, elle est adaptée aux conditions d'agitation moyennes rencontrées sur la plupart des plages d'Iroise (Hs < 0,5 m) du fait de leur position semi-abritée. En outre, la formule d'Hallermeier (1981) classiquement employée pour estimer la profondeur de fermeture a été testée (-2,6 m C.M.) et validée à partir de données simulées d'états de mer au large de la plage. Ce résultat est pour le moins atypique en raison de l'absence, à notre connaissance, de cas de validation de cette formule théorique pour des plages de poche. 3) de quantifier les échanges sédimentaires dans le profil. Alors que chaque bilan de matière individualisé traduit l'intensité relative des conditions hydrodynamiques entre deux levés consécutifs (respectivement positif/négatif lors des périodes de beau temps/mer forte), le bilan sédimentaire global est positif. Cet apport de sédiments dans le système cross-shore est aussi bien susceptible d'exprimer une variabilité longitudinale des barres subtidales (uniquement observées en 2D) que résulter d'un transport local de sédiments sableux depuis d'autres plages de l'anse de Bertheaume à la faveur des courants de marée et des fortes houles. Ces résultats motivent la réalisation de cartographies en 3D de la plage (projet Dyn3D région Bretagne, UMR6538 Domaines océaniques) et des formations superficielles sous-marines pouvant constituer le disponible sédimentaire impliqué dans l'évolution pluri-annuelle de la plage de Porsmilin.
(24/10/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, Ensieta, LETG - Brest, LETG, UNICAEN, NU, UA, EPHE, PSL, UBO EPE, UR2, CNRS, IGARUN, UN, BIAF, UA
Derivation of asymptotic two-dimensional time-dependent equations for ocean wave propagation
A general method for the derivation of asymptotic nonlinear shallow water and deep water models is presented. Starting from a general dimensionless version of the water-wave equations, we reduce the problem to a system of two equations on the surface elevation and the velocity potential at the free surface. These equations involve a Dirichlet-Neumann operator and we show that all the asymptotic models can be recovered by a simple asymptotic expansion of this operator, in function of the shallowness parameter (shallow water limit) or the steepness parameter (deep water limit). Based on this method, a new two-dimensional fully dispersive model for small wave steepness is also derived, which extends to uneven bottom the approach developed by Matsuno \cite{matsuno3} and Choi \cite{choi}. This model is still valid in shallow water but with less precision than what can be achieved with Green-Naghdi model, when fully nonlinear waves are considered. The combination, or the coupling, of the new fully dispersive equations with the fully nonlinear shallow water Green-Naghdi equations represents a relevant model for describing ocean wave propagation from deep to shallow waters.
(06/10/2007)
IMB, UB, Bordeaux INP, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
The origin of the 1500-year climate cycles in Holocene North-Atlantic records
Since the first suggestion of 1500-year cycles in the advance and retreat of glaciers (Denton and Karlen, 1973), many studies have uncovered evidence of repeated climate oscillations of 2500, 1500, and 1000 years. During last glacial period, natural climate cycles of 1500 years appear to be persistent (Bond and Lotti, 1995) and remarkably regular (Mayewski et al., 1997; Rahmstorf, 2003), yet the origin of this pacing during the Holocene remains a mystery (Rahmstorf, 2003), making it one of the outstanding puzzles of climate variability. Solar variability is often considered likely to be responsible for such cyclicities, but the evidence for solar forcing is difficult to evaluate within available data series due to the shortcomings of conventional time-series analyses. However, the wavelets analysis method is appropriate when considering non-stationary variability. Here we show by the use of wavelets analysis that it is possible to distinguish solar forcing of 1000- and 2500- year oscillations from oceanic forcing of 1500-year cycles. Using this method, the relative contribution of solar-related and ocean-related climate influences can be distinguished throughout the 10 000 yr Holocene intervals since the last ice age. These results reveal that the 1500-year climate cycles are linked with the oceanic circulation and not with variations in solar output as previously argued (Bond et al., 2001). In this light, previously studied marine sediment (Bianchi and McCave, 1999; Chapman and Shackleton, 2000; Giraudeau et al., 2000), ice core (O'Brien et al., 1995; Vonmoos et al., 2006) and dust records (Jackson et al., 2005) can be seen to contain the evidence of combined forcing mechanisms, whose relative influences varied during the course of the Holocene. Circum-Atlantic climate records cannot be explained exclusively by solar forcing, but require changes in ocean circulation, as suggested previously (Broecker et al., 2001; McManus et al., 1999).
(Climate of the Past. vol. 3, n° 1814-9324, pp. 569-575, 01/10/2007)
LGGE, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, INSU - CNRS, CNRS, PBDS, INSU - CNRS, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, EDYTEM, USMB [Université de Savoie] [Université de Chambéry], CNRS, Fédération OSUG, WHOI, M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS
Impact of flood events on the transport of terrestrial organic matter to the ocean: A study of the Têt River (SW France) using the BIT index
(Organic Geochemistry. vol. 38, n° 0146-6380, pp. 1593-1606, 01/10/2007)
NIOZ, CEFREM, UPVD, INSU - CNRS, CNRS, NIOZ, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Potential imprint of Spörer and Maunder solar minima on coral skeleton carbon isotopes
Despite being routinely measured with d18O in corals, records of d13C are seldom presented in paleoceanographic studies. Numerous factors influence the biologically-mediated fractionation of carbon isotopes during skeletogenesis, resulting in a complex behaviour of this tracer. Here we present a long coral d13C series, dating back to 1475, and showing the potential impact of solar activity. We note a significant decrease of d13C during the Spörer and Maunder minima. We argue for a change in metabolical processes, from an autotrophical pattern to a more heterotrophical mode of feeding guided by change in sun irradiation. In addition, nearby volcanic eruptions might also have an impact and cause d13C shifts, depending on the dust load associated with the eruption. Following previous work, we advocate the systematic presentation of d13C vs. d18O plots in coral studies to identify whether isotopic fractionation might have been affected by change in mode of feeding.
(Climate of the Past Discussions [Climate of the Past Preprints]. vol. 3, n° 1814-9340, pp. 1021-1035, 28/09/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, PALEOTROPIQUE
Coupling of carbon, nitrogen and oxygen cycles in sediments from a Mediterranean lagoon: a seasonal perspective
Experimental data and simulations were used to investigate the seasonal coupling between carbon, nitrogen and oxygen cycles in marine sediments from a eutrophic shallow lagoon in the Mediterranean Sea area. A negative seasonal correlation was observed between oxygen consumption and coupled nitrification–denitrification rates in surface sediments. Elevated values of oxygen consumption rates were reached during warm periods (up to 87.7 mmol m–2 d–1) whereas nitrification and denitrification rates remained close to the lowest rates reported for coastal sediments (values around 0.021 to 0.35 mmol N m–2 d–1 for nitrification and 0.014 to 0.045 mmol N m–2 d–1 for denitrification). A steady-state diagenetic model closely represented the seasonal negative correlation of oxygen uptake, coupled nitrification–denitrification rates, the vertical distribution patterns of pore water oxygen and the solid phase distribution of organic carbon when nitrification inhibition by sulfide was included. Simulation adjusted to field data also highlighted the importance of oxygen penetration depth in the seasonal variation of nitrification. The modelling indicated that anaerobic metabolism was the most significant pathway (65 to 80%) during organic matter mineralization with a clear seasonal increase during warm periods. These warm periods were also characterized by the higher benthic demand of oxygen mostly used to re-oxidize the by-products from anaerobic reactions (from 57 to 82%), the other part being used for carbon mineralization.
(Marine Ecology Progress Series. vol. 346, n° 0171-8630, pp. 45-59, 27/09/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, OCEANIS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LMGEM, CNRS, NIOO-KNAW, IPG Paris, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GU, M2C, UNICAEN, NU, INSU - CNRS, UNIROUEN, NU, CNRS
Facilitation in plant communities: the past, the present, and the future
Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plantplant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms. Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers. Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plantplant interactions in the design and interpretation of studies from the fields of restoration and global change ecology. Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists.
(Journal of Ecology. vol. 96, n° 0022-0477, pp. 070908024102002, 06/09/2007)
URJC, UdeC, UR EMGR, CEMAGREF, University of Pennsylvania, SFIRC, UMR AMAP, Cirad, INRA, UM, CNRS, IRD [Occitanie], CTFC, BioGeCo, INRA, UB, ECODIV, UNIROUEN, NU, LECA, UJF, USMB [Université de Savoie] [Université de Chambéry], CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Contourite drift construction influenced by capture of Mediterranean Outflow Water deep-sea current by the Portimão submarine canyon (Gulf of Cadiz, South Portugal)
The margin of the Gulf of Cadiz is swept by the deep current formed by the Mediterranean Outflow Water (MOW) flowing from the Mediterranean to the Atlantic. On the northern margin of the Gulf (Algarve Margin, South Portugal), the MOW intensity is low and fine-grained contourite drifts are built up with an alongslope development. From new sedimentological data, this study emphasizes the presence of two types of contourite drifts separated only by a deep submarine canyon incising the slope with a north-south orientation (Portimão Canyon). High-resolution seismic and bathymetry interpretation shows that on the eastern side of the canyon, the MOW forms a thick and large detached drift (Albufeira Drift) prograding toward both north and west, as shown in seismic profiles, with a high sedimentation rate. On this side of the canyon, the MOW intensity is high enough to erode the slope forming a moat channel (Alvarez Cabral). On the western side of Portimão Canyon, the MOW energy is lower, preventing moat channel erosion. Only flat and thin drift develops (Portimão and Lagos Drifts) with slow aggradation and a low sedimentation rate. This difference in drift development is due to the presence of the canyon which generates an important change in hydrodynamic of the MOW, confirmed by temperature-density measurements showing that MOW flows down Portimão Canyon. The canyon is responsible for the deviation of the direction of the MOW as it partly catches the deep-sea current flowing westward (i.e. capture phenomenon). It creates, thus, a decrease of the flow energy, competency and capacity between the east and west sides of the canyon. Through this phenomenon of MOW deep-sea current capture, the canyon constitutes a morphologic feature generating an important change in the contourite deposition pattern.
(Marine Geology. vol. 242, n° 0025-3227, pp. 247-260, 17/08/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Influence of the parasite worm Polydora sp. on the behaviour of the oyster Crassostrea gigas: a study of the respiratory impact and associated oxidative stress
(Marine Biology. vol. 152, n° 0025-3162, pp. 329-338, 01/08/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LMBA, UBS, UBO EPE, CNRS
Cadmium and lead interaction with diatom surfaces: A combined thermodynamic and kinetic approach
This work is devoted to the physico-chemical study of cadmium and lead interaction with diatom water interfaces for two marine planktonic (Thalassiosira weissflogii, TW; Skeletonema costatum, SC) and two freshwater periphytic species (Achnanthidium minutissimum, AMIN; Navicula minima, NMIN) by combining adsorption measurements with surface complexation modeling. Adsorption kinetics was studied as a function of pH and initial metal concentration in sodium nitrate solution and in culture media. Kinetic data were consistent with a two-step mechanism in which the loss of a water molecule from the inner coordination sphere of the metal is rate limiting. Reversible adsorption experiments, with 3 h of exposure to metal, were performed as a function of pH (2 9), metal concentration in solution (10?9 10?3 M), and ionic strength (10?3 1.0 M). While the shape of pH-dependent adsorption edge is similar among all four diatom species, the constant-pH adsorption isotherm and maximal binding capacities differ. Measurements of electrophoretic mobilities (?) revealed negative surface potential for AMIN diatom, however, the absolute value of ? decreases with increase of [Pb2+]aq suggesting the metal adsorption on negative surface sites. These observations allowed us to construct a surface complexation model (SCM) for cadmium and lead binding by diatom surfaces that postulates the Constant Capacitance of the electric double layer and considers Cd and Pb complexation with mainly carboxylic and, partially, silanol groups. In the full range of investigated Cd concentration, the SCM is able to describe the concentration of adsorbed metal as a function of [Cd2+]aq without implying the presence of high affinity, low abundance sites, that are typically used to model the metal interactions with natural multi-component organic substances. At the same time, Cd fast initial reaction requires the presence of ?highly reactive sites? those concentration represents only 2.5 3% of the total amount of carboxylic sites. For reversible adsorption experiments, the dominating carboxylic groups, whose concentration is allowed to vary within the uncertainty of experimental acid base titrations, are sufficient to reproduce the metal adsorption isotherms. Results of this study strongly suggest that laboratory experiments performed in a wide range of metal to biomass ratios, represent robust and relatively simple method for assessing the distribution of metals between aqueous solution and planktonic and periphytic biomass in natural settings.
(Geochimica et Cosmochimica Acta. vol. 71, n° 0016-7037, pp. 3698-3716, 01/08/2007)
LMTG, UT3, Comue de Toulouse, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS