Multi-scale slope instabilities along the Nile deep-sea fan, Egyptian margin: A general overview
The Nile deep-sea fan (NDSF), turbiditic system reaching a size of about 90,000 km2, has been investigated since 1998 by several geophysical methods (multibeam bathymetry, backscatter imagery, seismic data, 3–5 kHz echo-sounding). The analysis of this important data set evidenced that the NDSF is the locus of numerous multi-scale slope instabilities. Three main types of instabilities have been defined, mainly on the basis of their size or origin. (1) First type of instabilities related to the generalized gravity spreading of the Plio-Quaternary deep-sea fan on Messinian salt layers. This global spreading is accommodated by numerous localized slides. (2) Second type of instabilities correspond to giant mass movements probably triggered either by earthquakes, fluids, or climate and eustatic oscillations. Finally, (3) third type of instabilities correspond either to localized levee liquefactions or to thin-skinned slides on the steep slopes of the Eratosthenes seamount. The deposits generated by these slope movements greatly participate in the building of the NDSF. The characterization of these different instabilities, in a petroleum province as the NDSF, has important implications in terms of risk assessments when considering drilling operations.
(Marine and Petroleum Geology. vol. 26, n° 0264-8172, pp. 633-646, 22/05/2009)
IMAGES, UPVD, LDO, INSU - CNRS, UBO EPE, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, GEOAZUR 6526, IRD, UPMC, UNS, INSU - CNRS, UniCA, CNRS
Tectonic shortening and gravitational spreading in the Gulf of Cadiz accretionary wedge: Observations from multi-beam bathymetry and seismic profiling
The Gulf of Cadiz lies astride the complex plate boundary between Africa and Eurasia west of the Betic-Rif mountain belt. We report on the results of recent bathymetric swathmapping and multi-channel seismic surveys carried out here. The seafloor is marked by contrasting morphological provinces, spanning the SW Iberian and NW Moroccan continental margins, abyssal plains and an elongate, arcuate, accretionary wedge. A wide variety of tectonic and gravitational processes appear to have shaped these structures. Active compressional deformation of the wedge is suggested by folding and thrusting of the frontal sedimentary layers as well as basal duplexing in deeper internal units. There is evidence for simultaneous gravitational spreading occurring upslope. The very shallow mean surface and basal slopes of the accretionary wedge (1° each) indicate a very weak decollement layer, geometrically similar to the Mediterranean Ridge accretionary complex. Locally steep slopes (up to 10°) indicate strongly focused, active deformation and potential gravitational instabilities. The unusual surface morphology of the upper accretionary wedge includes “raft-tectonics” type fissures and abundant sub-circular depressions. Dissolution and/or diapiric processes are proposed to be involved in the formation of these depressions.
(Marine and Petroleum Geology. vol. 26, n° 0264-8172, pp. 647-659, 21/05/2009)
LDO, INSU - CNRS, UBO EPE, CNRS, UEB, UAG, INSU - CNRS, UM, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UAlg, ULISBOA, INETI
Do trematode parasites affect cockle ( Cerastoderma edule) secondary production and elimination?
Digenean trematodes are omnipresent in the cockle Cerastoderma edule, a common coastal bivalve of (semi-)sheltered north-eastern Atlantic coasts. They can use their host as a second intermediate host where they remain in a relatively latent stage as metacercariae. Cockle population dynamics and trematode parasite load were monitored for two years in two sites, Arcachon (France) and Merja Zerga (Morocco) for the cockle cohort of 2005. Individual growth was slightly higher at Arcachon than at Merja Zerga (Von Bertalanffy parameters: K = 1.5 yr−1 in both sites but L∞ = 31.2 mm at Merja Zerga against 38.3 mm at Arcachon). Production during cockle life was twice as high at Merja Zerga (48.5 compared to 22.2 gDW.m−2 at Arcachon). Elimination compensated production at Merja Zerga while elimination was low at Arcachon (8.1 gDW.m−2) due to sediment dynamics which concentrated cockles within the sampling area. P/B was similar in both sites (2.4–2.6 yr−1) while E/B was higher at Merja Zerga (2.2 yr−1 versus 1.0 yr−1). Trematode communities were similar in both sites in terms of species and abundance. During the cockle cohort lifespan, there were 3 to 5 events in the parasite abundance survey that suggested parasite-dependent mortality. At Merja Zerga, trematodes metacercariae did not affect P/B (2.4 yr−1) but increased E/B (+14%). At Arcachon, they also had no effect on P/B (2.6 yr−1) but on the contrary increased E/B (+20%). The threshold of metacercariae abundance beyond which cockles are affected was lower for Echinostomatid trematodes at Merja Zerga, suggesting an interaction with other factors such as temperature which is higher at the southern limit of cockle distribution.
(Journal of the Marine Biological Association of the United Kingdom. vol. 89, n° 0025-3154, pp. 1395, 15/05/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Evolution of the Nile deep-sea turbidite system during the Late Quaternary: influence of climate change on fan sedimentation
This paper presents an overview of the evolution of the Nile deep-sea turbidite system during the last 200 kyr, over a series of glacial to interglacial cycles. Six individual deep-sea fans were identified from an extensive field data set. Each fan comprises a canyon, channel system and terminal lobes. Two of these fan systems were possibly active at the same time, at least during some periods. Large-scale slope failures destroyed channel segments and caused the formation of new submarine fan systems. These slope failures thus played an important role in the overall evolution of the turbidite system. During the last glacial maximum (ca 25 to 14·8 ka) the central and eastern parts of the Nile deep-sea turbidite system were relatively inactive. This inactivity corresponds to a lowstand in sea-level, and a period of arid climate and relatively low sediment discharge from the Nile fluvial system. Rapid accumulation of fluvial flood-derived deposits occurred across the shallower part of the submarine delta during sea-level rise between ca 14·8 and 5 ka. The most recent deep-sea channel–lobe system was very active during this period of rising sea-level, which is also associated with a wetter continental climate and increased sediment and water discharge from the Nile. Increased sediment deposition in shallower water areas led to occasional large-scale slope failure. The Nile deep-sea turbidite system was largely inactive after ca 5 ka. This widespread inactivity is due to retreat of the coastline away from the continental shelf break, and to a more arid continental climate and reduced discharge of sediment from the Nile. The Nile deep-sea turbidite system may be more active during periods of rising and high sea-level associated with wetter climates, than during lowstands, and may rapidly become largely inactive during highstands in sea-level coupled with arid periods. These acute responses to climate change have produced sedimentary/stratigraphic features that diverge from traditional sequence models in their nature and timing. This large-scale sedimentary system responded to monsoon-driven climate change and sea-level change in a system-wide and contemporaneous manner.
(Sedimentology. vol. 56, n° 0037-0746, pp. 2061-2090, 08/05/2009)
GEOAZUR 6526, IRD, UPMC, UNS, INSU - CNRS, UniCA, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, INTECHMER, Cnam, ISMAR, ISMAR, CNR, ETH Zürich
Testing the enemy release hypothesis: trematode parasites in the non-indigenous Manila clam Ruditapes philippinarum
The present study tested the ‘Enemy Release Hypothesis’ (ERH) which states that the success of an introduced species is related to the scarcity of natural enemies in the introduced range compared with the native range. Digeneans are dominant macroparasites of molluscs; therefore, the interaction between R. philippinarum and these parasites was ideal for investigation. A two-year monitoring in Arcachon Bay (SW France) was performed to estimate digenean loads in R. philippinarum and in three infaunal native bivalves (R. decussatus, Paphia aurea, Cerastoderma edule). A laboratory experiment allowed comparison of infection success among these bivalves (except P. aurea) by generalist digenean larvae (Himasthla elongata cercariae). R. philippinarum digenean abundance in Arcachon Bay was much lower than in native bivalves, with values depending on species, sites and time. Similarly, mean digenean species richness per host individual was always lower in R. philippinarum than in sympatric bivalves. A comparison of digenean metacercariae abundance between R. decussatus and C. edule in Mundaka Estuary (Spain) showed that both species had similar digenean loads but that R. decussatus was depleted in digenean species encysting in host tissues (the non-gymnophallid species). Experimental infection confirmed that the two species of the genus Ruditapes (and not R. philippinarum only) were resistant to encysting digeneans, with an infection success 3–5 times lower than that of C. edule. The lack of infection that was observed in the field would therefore be the consequence of a tissue barrier, R. philippinarum epithelium being too tough for cercariae penetration. Concordantly, according to the literature, digenean infection in the native range of R. philippinarum is also low. Consequently, the ERH, as an explanation for R. philippinarum success in Europe, is not totally consistent in the case of digenean trematodes as enemies, R. philippinarum hosting low load of digeneans in its native as well as colonized range.
(Hydrobiologia. vol. 630, n° 0018-8158, pp. 139-148, 03/05/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Deposits related to the failure of the Malpasset Dam in 1959. An analogue for hyperpycnal deposits from jökulhlaups
Sediment cores collected in the Gulf of Fréjus (SE France) contain submarine deposits related to the failure of the Malpasset Dam in 1959. These deposits constitute a dark, sandy terrigeneous layer of 10–40 cm thickness lying above an erosion surface. The deposits are composed of ungraded, non-bioturbated sands and silts displaying no apparent sedimentary structure, but rich in organic matter and rock or shell fragments. During the event, this layer prograded onto the inner continental shelf and froze rapidly. These hyperconcentrated flow deposits are related to an unsteady inertia flow generated by a surge-like flood and bedload-dominated hyperpycnal flow. Rapid freezing on flow sides generated lateral, coarse-grained, levee-shape deposits. The deposits related to the failure of the Malpasset Dam are drastically different from classical suspended-load-dominated hyperpycnites deposited by a steady, flood-generated, hyperpycnal flow. However, they are comparable with present-day deposits on a volcanic, ice-covered margin (Icelandic jökulhlaups), with ancient deposits resulting from the pulsating output of subglacial lakes during a deglaciation, or with Martian landforms resulting from sporadic ice–melt events during early Martian times.
(Marine Geology. vol. 260, n° 0025-3227, pp. 81-89, 01/05/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
A round-the-world tour almost completed: first records of the invasive mussel Musculista senhousia in the north-east Atlantic (southern Bay of Biscay)
The Asian date mussel Musculista senhousia, previously recorded from the Pacific coast of North America, Australasia and the Mediterranean, is reported for the first time from the Atlantic coast of Europe in Arcachon Bay, where it occurs intertidally and to depths of 5 m. It has been present in Arcachon Bay since 2002 and has since been found in 2006 within two further inlets of the Bay of Biscay, Hossegor Lake and the Bidasoa estuary. Their appearance is probably associated with oyster transfer from Thau lagoon in the Mediterranean Sea to Arcachon Bay.
(Marine Biodiversity Records. vol. 2, n° 1755-2672, 01/05/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Brown Muscle Disease and Manila Clam Ruditapes philippinarum Dynamics in Arcachon Bay, France
Brown Muscle Disease (BMD) affects Manila clam Ruditapes philippinarum. It was described for the first time in 2005 in Arcachon Bay, France. The pathology consists in a progressive necrosis of the posterior adductor muscle, valve gaping, clam migration to the sediment surface, and death. This study aims to quantify the prevalence of BMD in the bay and to evaluate the effect of BMD on Manila clam dynamics. The prevalence was assessed on 50 stations spread within Arcachon Bay. About 62% of Manila clam habitat surface was infected by BMD. A survey of buried and surface clams was conducted from November 2006 to March 2008 in Lanton, a site infected by BMD. Modal progression analysis separated confidently cohorts from 2003 to 2005 recruitments. This pathology only affected adult clams (>25 mm, >2 years). For both buried and surface individuals, shell length was significantly correlated with BMD infection. Surface clams had prevalence (67%) higher than buried clams (23%) and showed greater mortality rate after 15 d in running water: 82% against 12% for buried individuals. The final disease index (FDI) and the condition index (CI) were monthly evaluated on 50 clams located at each position in the sediment. CI displayed a significant decrease after BMD's infection from light to severe disease stages. Length data analysis through Bhattacharya's method (FISAT II software) allowed identifying four cohorts. The 2003s cohort enabled to calculate mortality rate that was 39% for 5 months and to estimate that BMD was responsible of 95% of that mortality. The temperature was certainly an important factor in BMD transmission, because cohort dynamics results argued that BMD developed during spring and summer.
(Journal of Shellfish Research. vol. 28, n° 0730-8000, pp. 355-362, 26/04/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Particle fluxes and recent sediment accumulation on the Aquitanian margin of Bay of Biscay
(Continental Shelf Research. vol. 29, n° 0278-4343, pp. 1044-1052, 01/04/2009)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BIAF, UA, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA
Planktic foraminiferal production along an offshore–onshore transect in the south-eastern Bay of Biscay
(Continental Shelf Research. vol. 29, n° 0278-4343, pp. 1123-1135, 01/04/2009)
BIAF, UA, NOC, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, PALEOCEAN, LSCE, UVSQ, INSU - CNRS, CNRS, DRF (CEA), CEA