Jean-Francois Bourillet

Tidal deposition systems of China's continental shelf, with special reference to the eastern Bohai Sea

Abstract The continental shelf of China has undergone a large-scale marine transgression. Tidal currents have become the main active agent and have had a profound influence on the deposition and geomorphology of the shelf. The continental shelf of China has five modern tidal deposition systems: (1) the eastern part of the Yellow Sea, (2) the eastern part of the Bohai Sea, (3) off the mouth of Yangtze River; (4) to the west of Taiwan, and (5) the Strait of Qiongzhou. When the tidal current speed is above 150 cm/s, erosion dominates and reciprocating currents form deep scour furrows. When the tidal current speed is 50 to 150 cm/s, deposition is dominant, and tidal currents form tidal shoals (i.e., tidal sand ridges and tidal sand sheets) whose margins are near an absolute ellipticity value for the M2 tidal component of 0.4. When the absolute M2 value is less than 0.4, a reciprocating current is strong enough to form tidal sand ridges. When the absolute value is above 0.4, a rotating current increases so as to form a tidal sand sheet. Tidal deposition material came mainly from tidal erosion and locally redeposition of Late Pleistocene sediments, and from the neighbouring sea bottom. The tidal deposition system in the eastern Bohai Sea is a typical example. It consists of the Laotieshan Channel scour furrow, the Liaodong Shoal sand ridges and the Bozhong Shoal sand sheet. The maximum tidal current speed in the Laotieshan Channel where strong erosion occurs, is up to 250 cm/s. In the Liaodong Shoal, the maximum tidal current speed is 64–115 cm/s and the absolute ellipticity value of the M2 tidal component is less than 0.4. In the Bozhong Shoal, the maximum tidal speed is 58–79 cm/s and the absolute ellipticity value of the M2 tidal component is above 0.4. The small-scale bedforms surveyed in detail in the area show that the sand ridges and sand sheet are still under the action of tidal current processes at the present time. Based on the interpretation of tidal currents, bedforms and sediment thickness, it is inferred that the general direction of sediment transport within the study area is NNW from the Laotieshan Channel to the Liaodong and Bozhong Shoals. The small-scale bedforms in the detailed survey area indicate a local net sediment transport from south to north.

Deglacial laminated facies on the NW European continental margin: The hydrographic significance of British‐Irish Ice Sheet deglaciation and Fleuve Manche paleoriver discharges

We have compiled results obtained from four high sedimentation rate hemipelagic sequences from the Celtic sector of the NW European margin (NE Atlantic) to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles. We focus on periods characteristic of deglacial transitions. We adopt a multiproxy sedimentological, geochemical, and micropaleontological approach, applying a sampling resolution down to ten microns for specific intervals. The investigation demonstrates the relationships between the Bay of Biscay hydrography and the glacial/deglacial history of both the proximal British-Irish Ice Sheet (BIIS) and the western European continent. We identify recurrent phases of laminae deposition concurrent with major BIIS deglacial episodes in all the studied cores. Evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale. We discuss their impact at a global scale considering the present and past key location of the Bay of Biscay versus the Atlantic Meridional Overturning Circulation (AMOC).

Regional assessment of mass failure events in the Baie des Anges, Mediterranean Sea

Abstract Pliocene and Quaternary surficial sediments of the Baie des Anges are commonly affected by instability. Laboratory analyses of sediment samples provide information about the physical properties of these sediments including consolidation state and mechanical behaviour. Some of these physical properties, specifically undrained shear strength values ( S u ), have been calibrated by a few in-situ measurements. The results show that sediments deposited on the slope and on the shelf are mainly homogeneous silty clays, whereas those deposited on the floor of canyons are coarser (possible grain-flow deposits). By using S u measurements we have classified the silty clays into four geotechnical types and constructed a geotechnical map of undrained slope stability analysis. Infinite slope analysis provides an assessment of the possible instabilities under both drained and undrained conditions. A regional hazard offshore map is presented including areas sensitive to earthquake-induced sliding. Two potential types of failure are likely to occur in the bay: shallow failures, which are low-volume slides involving underconsolidated near-surface sediments, and deeper failures, which may occur on the flanks of the ridges, and may be preferentially triggered by earthquakes. These sliding processes involve a large volume of normally consolidated or overconsolidated sediments and may evolve into turbidity currents (such as the 1979 Nice slide, even if it was not an earthquake-induced slide) channelled by the Var or the Paillon canyons for more than 100 km. Our study indicates that future development projects within the Baie des Anges must address offshore hazards.

Understanding continent‐ocean sediment transfer

Submarine canyons are narrow but deep submarine valleys that extend for hundreds of meters. They represent the most impressive structures that shape the present morphology of passive continental margins. They can occur off the mouth of rivers: the Tagus, Zaire, Amazon, and Orinoco in the Atlantic; the Indus in the Indian Ocean; and the Var, Rhone, and Ebro in the Mediterranean. Some are at times disconnected from any stream mouth such as the Nazare canyon, off Portugal, despite the fact that it is close to the coast. Some were connected to a river mouth during lowstands of sea level, such as the Wilmington canyon in the northwest Atlantic, or the Blackmud canyon in the northeast Atlantic.

Stratigraphy and sedimentary environments during the Late Quaternary, in the Eastern Bohai Sea (North China Platform)

Abstract The Bohai Sea is a shallow sea at the east coast of China. It communicates with the Yellow Sea through a narrow strait. During and since the Late Pleistocene, the Bohai Sea has been a filled subsiding basin submitted to successive regressive-transgressive cycles. In the East Bohai Sea, a number of finger-shaped ridges are present near the strait, where tidal currents are dominant. Very-high resolution seismic reflection data have been interpreted relative to sedimentological data provided by a formerly drilled borehole. Seven seismic units are identified and correlated with the following events: units U7-U6 with the Bohai relative sea level rise (65,000–53,000 yr B.P.) and a relative sea level fall (53,000–39,000 yr B.P.); unit U5 with the Xianxian relative sea level rise (39,000–22,000 yr B.P.); unit U4 with the period of the late Wurm glaciation (22,000–15,000 yr B.P.); units U3-U2 with the transitional period from the end of the Pleistocene to the beginning of the Holocene (15,000–9000 yr B.P.); and unit U1 with the Holocene (Huanghua) marine intrusion in the Bohai Sea (since 9000 yr B.P.). In the Bohai Sea, formation of the tidal ridges is probably due to a substantial increase in tidal-current velocities during the late Holocene rise of sea level. Constriction due to the strait amplifies the tidal process, so that the sediments of the late Holocene transgressive sequence have been remodeled as tidal ridge.

L'éventail profond de la marge celtique: stratigraphie et évolution sédimentaire

Seismic data collected on the Celtic Fan during Sedifan cruises (European Program Enam II), stratigraphically correlated to DSDP site 400 (leg 48) show that fan deposition was initiated during the Early Miocene. It progressively extended towards the middle of the Bay of Biscay during the Plio-Quaternary. The fan includes 3 main seismic units with several channel/levee bodies disturbed by mass-movement deposits. Depocenters shifted several times, and the present-day morphology ultimately results from a last shift of detritic inputs to the Whittard System.

Surconsolidation apparente et pression osmotique dans un sédiment marin

This paper concerns the study of the over-consolidation state of a marine sediment extracted from the continental slope in the Gulf of Guinea. Special attention was devoted to the physicochemical phenomena. The study was carried out 1) at a microscopic level using the theory of the diffuse double layer [6, 8, 12] and 2) at a macroscopic level using an elastoplastic model considering the suction as a stress state variable [1]. These models have provided a physical explanation as well as an adequate description of the phenomenon of over-consolidation.

Quaternary sedimentary regime on the Berthois Spur (Bay of Biscay)

A study of cores from the Celtic Margin shows that high terrigenous accumulation rates occurred at the end of isotopic stage 3 and during isotopic stage 2. These high rates prevailed during rapid changes from relatively warm conditions (Älesund interstadial) to maximum glacial condition. The input of terrigenous material may have culminated with the blockage of the North Sea by an ice sheet and the diversion of NW European flumes toward the Celtic Margin. The uppermost sediments are in a state of overconsolidation as the consequence of erosional processes following the episod of high sediment supply.

Integrating Prior Knowledge and Locally Varying Parameters with Moving-GeoStatistics: Methodology and Application to Bathymetric Mapping

The paper aims at presenting an innovative methodology, called M-GS (M-GeoStatistics), which is fully dedicated to the local optimization of parameters involved in variogram-based models. M-GS considers the structural and computational parameters as a set of dependant parameters to be spatially optimized. The optimization process, which may be guided by objective or subjective criteria, is carried out during a M-structural analysis phase that leads to a set of spatially variable structural and computational parameters. The methodology is applied for bathymetry mapping. The availability of accurate seafloor estimates is essential for numerous oceanographic projects, including hydrographic, oceanographic and biological models, sedimentary processes, etc. Seafloor usually presents strong non stationarity and complex structures, such as small channels with varying orientations, spatially varying measurements errors, local heterogeneities for coastal areas, or deep canyons within general gentle slope for continental margins. The adequacy of the M-GS methodology in this framework is illustrated and compared with classical estimates for the Marenne-Oleron coast (West of France). Moreover such methodology could be used to input different local structures into a general model in the aim of a regional synthesis.

Cold-Water Coral Habitats in Submarine Canyons of the Bay of Biscay

The topographical and hydrological complexity of submarine canyons, coupled with high substratum heterogeneity, make them ideal environments for cold-water coral (CWC) habitats. These habitats, including reefs, are thought to provide important functions for many organisms. The canyons incising the continental slope of the Bay of Biscay have distinct morphological differences from the north to the south. CWCs have been reported from this basin in the late 19th century; however, little is known about their present-day distribution, diversity and environmental drivers in the canyons. In this study, the characteristics and distribution of CWC habitats in the submarine canyons of the Bay of Biscay are investigated. Twenty-four canyons and three locations between adjacent canyons were sampled using a Remotely Operated Vehicle (ROV) or a towed camera system. Acquired images were annotated for habitat type (using the CoralFISH classification system), substrate cover and coral identification. Furthermore, the influence of hydrological factors and geomorphology on the CWC distribution was investigated. Eleven coral habitats, formed by 62 morphotypes of scleractinians, gorgonians, antipatharians and seapens, inhabiting hard and/or soft substrate, were observed. The distribution patterns were heterogenous at regional and local scales; the south Bay of Biscay and the southeastern flank favored soft substrate habitats. Biogenic and hard substrate habitats supported higher coral diversities than soft substrate habitats and had similar species compositions. A higher coral species turnover characterized soft substrate habitats. Substrate type was the most important driver of the patterns in both distribution and composition. Observations of coral reefs on steeper areas in the canyons and coral rubble on flatter areas on the interfluve/upper slope, support the hypothesis that canyons serve as refuges, being less accessible to trawling, although natural causes may also contribute to the explanation of this distribution pattern. The results of this study fed into a proposal of a Natura 2000 network in the Bay of Biscay where management plans are rare.