OXYGEN AND ORGANIC CARBON FLUXES IN SEDIMENTS OF THE BAY OF BISCAY
The relationship between particulate organic carbon (POC) measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC as a paleo-environmental proxy. Our objective was to compare POC concentrations, POC mineralization rates calculated from O2 consumption and POC burial rates. Benthic O2 distributions were determined in 58 sediment cores collected at different periods at 14 stations in the Bay of Biscay with depths ranging from 140 m to 2,800 m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM). O2 fluxes showed that the fast-decaying OM flux was much higher than buried POC. Sedimentary POC burial was calculated from mass accumulation rate and the POC content measured at the top of sediments. The proportion of buried POC relative to total exported POC ranged between 50% and 10%, depending on station location. The most labile fraction of exported POC engenders a steep decrease in the upper 5 mm of vertical O2 profiles. This rupture may be related to the bioturbation-induced mixing depth of fast-decaying carbon. A second class of POC would consist of slow-decaying OM that is mineralized within the upper decimetres of sediment through oxic and anoxic processes. At sites located below 500 m water depth, the fast-decaying carbon did not reach the anoxic sediment. When O2 is not entirely consumed in the presence of the fast pool of OM, the slow-decaying pool may control the O2 penetration depth.
(23/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LGE, UPD7, IPG Paris, LOBB, OOB, UPMC, CNRS, UPMC, INSU - CNRS, CNRS, ATST, UFC, UBFC
LONG-TERM ENVIRONMENTAL CHANGES IN THE GIRONDE ESTUARY
The Gironde estuary is considered the largest South-Western European estuary and covers an area of 625 km2 at high tide. This transition zone between marine and freshwater environments is formed by the junction of the Garonne and the Dordogne rivers. The Gironde is characterized by a strong spatiotemporal variability of environmental parameters, modulating biological response. While most studies deal with the impact of anthropogenic pressures on estuaries, this research aims at understanding and distinguishing the part of variability that can be explained by human local activities from those that is linked to Climate Change. Using data from the ecological monitoring of the Blayais nuclear power plant, this research contributes to better know the estuarine evolution over the last thirty years. This paper presents the chemical, physical and climatic parameters changes by characterizing global and local trends, interannual variability and an eventual periodicity using different data analyses (i.e. linear regression, moving average, and eigenvectors filtering). Then, correlations between environmental parameters were analyzed and the potential influence of climate variability was evaluated with a standardized Principal Component Analysis.
(23/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LOB ARCACHON, UB, INSU - CNRS, LOG, INSU - CNRS, ULCO, CNRS, IRD [Ile-de-France]
The Ordovician caryocaridid phyllocarids (Crustacea): diversity and evolutionary tendencies
An overall look at the Ordovician phyllocarid representatives assigned to the Family Caryocarididae led to the recognition of an unexpected biodiversity during the Tremadocian - Hirnantian time interval. Three new genera are described: Jellicaris n.g. from Australia, Saltericaris n.g. from Bohemia, and Soomicaris n.g. from South Africa, which add to the three previously described taxa, namely Caryocaris, Janviericaris, and Ivocaris; hence the Family Caryocarididae now includes six genera. The stratigraphical range of these genera, with respect to the graptolite zonation, displays that morphological characters vary accordingly, which is considered to reflect evolutionary tendencies which are exposed and discussed herein. Hypothetical phyletic relationships between caryocaridid genera are briefly discussed and are suggestive of two parallel lineages within the Family Caryopcarididae
(Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen. vol. 257, n° 0077-7749, pp. 237-248, 05/08/2010)
LDO, INSU - CNRS, UBO EPE, CNRS, MNHN, UPMC, CNRS, CR2P, MNHN, UPMC, CNRS
Perched lobe formation in the Gulf of Cadiz: Interactions between gravity processes and contour currents (Algarve Margin, Southern Portugal)
The Gulf of Cadiz is swept by the strong saline Mediterranean Outflow Water (MOW). On the Algarve Margin (South Portugal), this current has constructed fine-grained contourite drifts. This margin is dissected by the Portimao Canyon and three short channels that only incise the upper slope, and are absent on a terrace located at mid-slope depths along the Algarve Margin. High-resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace. Coarse-grained lenticular chaotic bodies formed during major relative sea-level lowstands are intercalated within the drift. The lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity-generated deposits that are perched on the middle continental slope.In the Gulf of Cadiz, the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea-level changes during the late Quaternary. During cold periods when sea-level was low, erosion intensified on the continental shelf and the deepest part of MOW was active. Sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid-slope terrace. During warm periods when relative sea-level was high, the supply of sediment from the shelf was shut off and the shallowest part of MOW was more active. Contourite drifts fill the channels and bury the sandy lobes.
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 81-94, 01/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Simulation of the interactions between gravity processes and contour currents on the Algarve Margin (South Portugal) using the stratigraphic forward model Sedsim
The margin of the Gulf of Cadiz is swept by an intermediate current 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 alignment. Recent sedimentological studies emphasize the presence of complex process interactions resulting in the formation of a unique depositional architecture. Alongslope processes related to contour currents generate contourite drift, while downslope processes form canyons and channels aligned on deep faults. This paper uses a combined oceanographic and geological dataset to simulate the different types of interactions between gravity processes and contour currents, which were evidenced on this margin. An extrapolation of the contour current intensity has been used based on the present day velocity field and sea-level fluctuations over the simulated geological time-scale. According to our model results, the construction of the contourite drift is closely linked to contour current velocities and directions, the types of sediments transported and the existing topography. Using modern sedimentological understanding of the area, we have correlated gravity flow's strongest activity to sea level lowstand periods mainly due to a closer connection between canyon's mouth and river or deltaic systems. The simulated gravity flows are initialized at different locations and times on the margin depending on the preserved lobes retrieved from seismic analysis. Their resulting morphological features are identified as perched-lobes with volumes and forms close to the ones observed on Portimão and Lagos Drifts. This study provides a process-based understanding of the construction of contourite system and a physical evaluation of the interactions between gravity flows perpendicular to the slope, and alongslope processes. In addition, it shows the influence of autocyclic factors in the construction of contourite sedimentation, which is important to consider in future sedimentary paleo-reconstruction interpretations.
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 95-109, 01/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Lobe construction and sand/mud segregation by turbidity currents and debris flows on the western Nile deep-sea fan (Eastern Mediterranean)
Based on an unusual data set comprises bathymetric data, backscatter imagery, seismic-reflection and Chirp profiles, and sediment cores, the Late Quaternary lobe at the mouth of the youngest turbidite channel off the western Nile deep-sea fan was investigated. The large-scale construction of the lobe through time and space is mainly controlled by 1) a pre-existing topography inherited from the downslope movement of Messinian evaporites, and 2) the type and nature of gravity flows delivered to the basin floor. The margins of the lobe are defined by high-backscatter acoustic facies that contrasts strongly with the low-backscatter facies from the surrounding abyssal-plain deposits. Within the lobe, low-backscatter facies characterise the main channel-levee systems and lobate bodies immediately beyond the end of the channels. Cores reveal that the high-backscatter facies corresponds to a serie of extensive but thin debris-flow deposits with a fingered margin. These debrites comprise a muddy-sand matrix and dispersed clasts with diameter of 5 to 10 cm. The lower backscatter facies at channel mouths corresponds to alternations of thin sandy turbidites and muddy hemipelagites. Extensive thin debris flows therefore traversed surprisingly low gradients to reach the distal fringes of the lobe complex but are never found in the lobate bodies just beyond the channel mouths. Although the Nile deep-sea fan is considered as a silt/mud-rich accumulation, sand-prone deposits exist within the lobe. This sand/mud segregation results either from the presence of channelized features in the lobe and/or from the hydrodynamic process of particle transport by debris flows and turbulent flows.
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 124-143, 01/08/2010)
GEOAZUR 6526, IRD, UPMC, UNS, INSU - CNRS, UniCA, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GR, UR, INSU - CNRS, CNRS
Variations of the Antarctic Circumpolar Current intensity during the past 500 ka
[1] We examine the past variations of the Antarctic Circumpolar Current (ACC) by using variations in size and abundance of the magnetic grains deposited by this current at a site east the Kerguelen-Crozet plateau. Core MD00-2375G was taken at a midlatitude site during the ANTAUS cruise conducted in 2000 by the French R/V Marion Dufresne. Marine isotope stages (MIS) down to MIS 13 are identified, and an age model is derived from a correlation to the ice core isotopic record obtained from EPICA in Antarctica. Continuous records of abundance and size of the magnetic grains were obtained using environmental magnetism methods. Results show a strong modulation of the ACC flow intensity in response to glacial and interglacial cycles. The low abundance and the small size of magnetic grains indicate that the ACC was weak during warm stages and strong during glacial epochs. A large modulation is also observed at the sub-stage scale during the interglacials. A minimum in concentration and grain size occurs at the onset of MIS 11. At the MIS 6-5, 10-9, and possibly 12-11 transitions, observed variations suggest a mechanism involving both rapid and progressive variations of the ACC flow at deglaciations.
(Geochemistry, Geophysics, Geosystems. vol. 11, pp. n/a-n/a, 01/08/2010)
LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CLIMAG, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 144-159, 01/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, SHOM, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, CLIMAG, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, IFREMER, IFP
The sandy channel-lobe depositional systems in the Gulf of Cadiz: Gravity processes forced by contour current processes
The sedimentation in the Gulf of Cadiz (NE Atlantic Ocean) is significantly controlled by the Mediterranean Outflow Water (MOW). Along its pathway onto the continental slope, the MOW is canalized by contourite channels, some of them feeding gravity sandy channel-lobe depositional systems firstly recognized in previous study [Habgood et al., 2003. Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50(3), 483-510.].Using very high resolution acoustic data and cores, a detailed characterization and a new evolution pattern of these channel-lobe depositional systems is established. Complex internal geometry of the lobes shows several depositional units revealing a polyphase evolution of these systems, with a general progradation punctuated by retrogradation and avulsion phases. A gravity origin controlled by contouritic processes and climatic changes is demonstrated for the feeding and the evolution of these sandy channel-lobe depositional systems. Climate oscillations, via the MOW variations, act as a major forcing of the activity of the channel-lobe depositional systems during the Late Quaternary.
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 110-123, 01/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GM, IFREMER, LFCR, UPPA, CNRS
High-resolution analysis of submarine lobes deposits: Seismic-scale outcrops of the Lauzanier area (SE Alps, France)
The Lauzanier area represents the northernmost extension of the Annot Sandstone series and contains deposits between 650 and 900 m-thick. This basin was active from upper Bartonian or lower Priabonian to early Rupelian. It is composed of two superposed units separated by a major unconformity. The sediment supply is due to channelled flows coming from the south. Flow processes include mass flow to turbidity currents. The size of the particles and the absence of fine-grained sediment suggest a transport over a short distance. The Lower Unit is made of coarse-grained tabular beds interpreted as non-channelled lobe deposits. The Upper Unit is made of massive conglomerates interpreted as the channelled part of lobes. These lobe deposits settle in a tectonically confined basin according to topographic compensation that occurs from bed scale to unit scale. The abrupt progradation between the lower and the upper unit seems related to a major tectonic uplift in the area. This uplift is also suggested by a change in the petrographic nature of the source and an abrupt coarsening of the transported clasts. This field example allows providing high resolution analysis for depositional sedimentary sequences of terminal lobe deposits in a coarse-grained turbidite system. The outcrop analysis shows the lateral evolution of deposits and the system progradation allows a longitudinal analysis of facies evolution by superposing on the same outcrops the channelled lobe system and the non-channelled lobe system. These results of high-resolution outcrop analysis can be extrapolated to results obtained on sedimentary lobes in recent deep-sea turbidite system that are either restricted to cores, or with a lesser resolution (seismic).
(Sedimentary Geology. vol. 229, n° 0037-0738, pp. 160-191, 01/08/2010)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM, AREVA-BU Mines, IFPEN, GR, UR, INSU - CNRS, CNRS, UPPA, GM, IFREMER