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
Silicic acid leakage from the Southern Ocean: Opposing effects of nutrient uptake and oceanic circulation
(Geophysical Research Letters. vol. 34, n° 0094-8276, pp. n/a-n/a, 16/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Statistical process control in assessing production and dissolution rates of biogenic silica in marine environments
This paper provides pieces of advice on the practices of quality assurance and quality control in assessing production and dissolution rates of biogenic silica in marine waters with stable isotope techniques. The objective is to make a rigorous contribution to the interpretation of 30Si isotopic measurements including modelling and uncertainty analyses. The results are illustrated with real data taken from Beucher et al. [Beucher, C., Tréguer, P., Corvaisier, R., Hapette, A/-M., Elskens, M., 2004a. Production and dissolution of biogenic silica in a coastal ecosystem of western Europe. Marine Ecology Progress Series 267:57-69.]. Prior to the flux rate assessment, there are a number of analytical considerations required for screening between optimal and defective experimental conditions. Three indexes were proposed to check the relevance of underlying assumptions when dealing with 30Si tracer enrichment and dilution techniques. Afterwards for extracting rate values from measurements, it is necessary to postulate a model, and if required an optimization method. Various models and formulae were compared for their precision and accuracy. It was shown that oversimplified models risk bias when their underlying assumptions are violated, but overly complex models can misinterpret part of the random noise as relevant processes. Therefore, none of the solutions can a priori be rejected, but each should statistically be assessed with hypothesis testing. A weighted least squares regression strategy combining an analysis of the standardized residuals and cost function (sum of the weighted least squares residuals) was used to select optimal solution subsets corresponding to a given data set, i.e. the solution that uses the most relevant processes and which was tested for the presence of outliers (observations or measurements with undue influence in the flux rate assessment).
(Marine Chemistry. vol. 106, n° 0304-4203, pp. 272-286, 11/07/2007)
VUB, LEMAR, IRD, IFREMER, UBO EPE, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Messinian event in the black sea: Evidence of a Messinian erosional surface
In 1975, sediment cores from leg DSDP 42b (sites 380A and 381) revealed a thin sediment layer in the Black Sea basin which points to a shallow water environment at the Miocene-Pliocene boundary. With these facts and in the wake of hypothesis of the Messinian Salinity Crisis (MSC), it was proposed that the Black Sea, like the Mediterranean Sea, suffered a desiccation period at the end of the Messinian (Hsü, K.J. and Giovanoli, F., 1979. Messinian event in the Black Sea. Palaeogeography, Palaeoclimatology, Palaeoecology, 29: 75–93). Whereas the main topics of the MSC in the Mediterranean Sea is now widely accepted, the lack of evidence for a Messinian erosional surface in the Black Sea left the debate about the Messinian desiccation of this basin open until today. The analysis of high resolution multi-channel seismic data acquired during the BlaSON surveys brings important new elements for this scientific debate: (1) Down the slope offshore the Bosporus, we show a clear erosional surface correlated to the top of the Upper Miocene shallow water environment unit of site DSDP 381. The overlying Lower Zanclean unit inevitably dates this erosional surface of the Messinian event. (2) A wide intra-Pontian erosional surface (IPU) is evidenced on the Romanian shelf. The IPU is characterized by a sharp decrease in the incision rate from outer (deep canyons) to inner shelf (superficial incisions network). According to the most recent Paratethyan and Mediterranean stratigraphic scale correlations, the IPU erosional surface is considered as the analogue to the Messinian erosional surface described down the slope offshore the Bosporus. In addition to recently discovered inland erosional signature, the wide regional erosional surface we underline on the Western Black Sea margins validates the Black Sea Messinian desiccation hypothesis. We also demonstrate that evaporative draw-down of the Black Sea implied the installation in the basin of a negative hydrologic budget during the Messinian. The lack of a major Messinian Danube canyon on the Romanian shelf supports the hypothesis of a Messinian Danube trapped in the Dacic basin. However, the presence of a Messinian superficial incision network connecting the location of the modern Danube delta to deeply incised Messinian canyons on the outer shelf (IPU) makes the hypothesis of a Danube reaching the partially desiccated Black Sea still possible.
(Marine Geology. vol. 244, n° 0025-3227, pp. 142-165, 05/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GM, IFREMER, LDO, INSU - CNRS, UBO EPE, CNRS
Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O2, Mn, Fe and N-Species in Experimental Units
Biogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O2, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O2 from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn2+ and then Fe2+, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe2+. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO3−/NO2− rich horizon at the new oxic horizon. Over the experiment period, NO3− concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production.
(Aquatic Geochemistry. vol. 13, n° 1380-6165, pp. 157-172, 04/07/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Late Quaternary co-seismic sedimentation in the Sea of Marmara's deep basins
The deep, northern, part of the Sea of Marmara (northwestern Turkey) is composed of several aligned, actively subsiding, basins, which are the direct structural and morphological expression of the North Anatolian Fault's northern branch. The last 20 kyr of their sedimentary fill (lacustrine before 12 kyr BP) have been investigated through giant piston coring onboard R/V MARION-DUFRESNE (MARMACORE Cruise, 2001) and chirp subbottom profiler onboard R/V ATALANTE during MARMARASCARPS Cruise (2002). Especially during the lacustrine stage, the infilling of the deep basins (Tekirda?, Central, Kumburgaz, and Çinarcic Basins; up to 1250 m depth) was dominated by turbidites (with coarse mixed siliciclastic and bioclastic basal part), intercalated in “hemipelagic-type” finegrained calcareous and slightly siliceous clays. Often – especially in the thickest ones – the turbidites show strong segregation and a sharp boundary between coarse part and suspendedload part. In the Central Basin, 8 m of a unique sedimentary event include a 5 to 8m-thick “homogenite” well imaged on seismic profiles. The latter is interpreted as related to a major – possibly triggered - tsunami effect, as described in the Eastern Mediterranean by Kastens and Cita (1981). In the marine (Holocene) upper part of the sedimentary fill, repeated to-and-from structures, affecting silt or fine sand, are evidencing seiche-like effects and, thus, earthquake triggering. Detailed correlations between two deep coring sites (1250 and 1200 m) indicate more than 100 % overthickening in the deepest one; this implies specific processes of distribution of terrigenous input by dense hyperpycnal currents (high kinetic energy, seiche effect, complex reflections on steep slopes). The peculiar sedimentary infilling of the Sea of Marmara's Central Basin (and, by extrapolation, of the whole set) is tentatively interpreted as a direct consequence of the strong seismic activity; the imprint of the latter is more obvious prior to the base of the Holocene, as environmental conditions favoured marginal accumulation (especially on the southern shelf) of large amounts of erosion products available for mass wasting.
(Sedimentary Geology. vol. 199, n° 0037-0738, pp. 69-85, 01/07/2007)
LGCA, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, CNRS, GEOAZUR 6526, IRD, UPMC, UNS, INSU - CNRS, UniCA, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GEOLOGY AND MINING DEPARTMENT, ITÜ, LDEO, CEREGE, IRD, INRA, AMU, CdF (institution), INSU - CNRS, CNRS, IPGP, INSU - CNRS, UPD7, UR, IPG Paris, CNRS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA
A New Morphodynamic Modelling Platform: Application to Characteristic Sandy systems of the Aquitanian Coast, France
Along coasts, waves and wave-induced currents are the main factors of morphological evolution. A morphodynamic model is constructed to take into account tide changes, wind conditions and waves in the computation of the induced currents and morphological evolution. The spectral wave model SWAN, the shallow-water model MARS and a sedimentary module are coupled to create the morphodynamic model. First, we validate the hydrodynamics of the model on two characteristic complex bathymetries: an idealised subtidal crescentic bar and an intertidal ridge and runnel system. The crescentic bar induces wave energy focalisation zones which could give rise to transverse bars. Thus, we investigate the morphology evolution of the intertidal area. Simulations appear to show the formation of inner bars that connect the subtidal bar with the intertidal area
(Journal of Coastal Research. vol. 50, n° 0749-0208, 30/06/2007)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM, DYNECO, IFREMER
Anomalous flow below 2700 m in the EPICA Dome C ice core detected using d18O of atmospheric oxygen measurements
While there are no indications of mixing back to 800 000 years in the EPICA Dome C ice core record, comparison with marine sediment records shows significant differences in the timing and duration of events prior to stage 11 (~430 ka, thousands of years before 1950). A relationship between the isotopic composition of atmospheric oxygen (d18O of O2, noted d18Oatm) and daily northern hemisphere summer insolation has been observed for the youngest four climate cycles. Here we use this relationship with new d18O of O2 measurements to show that anomalous flow in the bottom 500 m of the core distorts the duration of events by up to a factor of 2. By tuning d18Oatm to orbital precession we derive a corrected thinning function and present a revised age scale for the interval corresponding to Marine Isotope Stages 11–20 in the EPICA Dome C ice core. Uncertainty in the phasing of d18Oatm with respect to insolation variations in the precession band limits the accuracy of this new agescale to ±6 kyr (thousand of years). The previously reported ~30 kyr duration of interglacial stage 11 is unchanged. In contrast, the duration of stage 15.1 is reduced by a factor of 2, from 31 to 16 kyr.
(Climate of the Past. vol. 3, n° 1814-9324, pp. 341-353, 21/06/2007)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, LGGE, OSUG, UJF, Grenoble INP, INSU - CNRS, IRSTEA, USMB [Université de Savoie] [Université de Chambéry], CNRS, INSU - CNRS, CNRS, NBI, UCPH, GLACCIOS, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, UNIBE, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEP, UNIBE, OCCR, UNIBE