Sébastien Zaragosi

Initiation of the European deglaciation as recorded in the northwestern Bay of Biscay slope environments (Meriadzek Terrace and Trevelyan Escarpment): a multi-proxy approach

Three cores retrieved on the northwestern slope of the Bay of Biscay are described and discussed in the light of the European last deglaciation history. This integrated sedimentological and micropalaeontological study provides a detailed evolution scheme for the deep and sea-surface conditions of the Bay during the final deglacial step, with a direct link with the continental palaeoenvironments. As early as 15 ka 14C-BP, a European precursor melting event is recorded as a purge of the Channel and Irish Sea palaeoriver systems. ‘Pleni–Heinrich event conditions’ occurred in the Bay of Biscay between 14.4 and 13 ka 14C-BP with a typical Canadian signature only recorded at 14 ka 14C-BP, namely 1 ka later than the first evidence of melting of the British Ice-sheet. Our data demonstrate that, following Heinrich event 2, the Last Glacial Maximum was characterised by a gradual warming accompanied by, at least, two pulses of the North Atlantic Drift. These North Atlantic Drift/heat northward penetrations are supposed to have primarily forced the Heinrich event 1 collapse.

Physiography and recent sediment distribution of the Celtic Deep-Sea Fan, Bay of Biscay

The Celtic Deep-Sea Fan located in the northwestern part of the Bay of Biscay is a middle sized fan with a surface area of more than 30,000 km 2 . The whole system is a mature mud/sand-rich submarine fan on a passive margin. Multi-beam echo sounder data, 3.5 kHz seismic and 12 Kullenberg cores were examined to define the fan morphology, the lithological characteristics, the sedimentary processes and the relationship between the evolution of the fan deposits and the environmental conditions on the Celtic continental shelf. The upper fan is characterised by the presence of two distinct tributary systems: (1) the Whittard system with a large, persistent, slightly sinuous channel, which is linked to the southern end of the Irish Sea River system; and (2) the Shamrock system, with a moderate sized channel, which is linked to the western end of the English Channel River system. The middle and lower fan corresponds to divergent braided secondary channels and associate lobes. Successive lobe elements, without impor- tant relief, were generated during periodic avulsions of middle fan channels. The lithological, palaeontological, and geochemical analyses on cores show the evolution of sedimentation since the last glaciation. During the last lowstand and rise of sea-level frequent low-density turbidity currents were predominant and deposited sediments throughout the whole fan system. They were initiated at the front of a deltaic environment on the Celtic outer-shelf. During the high sea-level conditions, occasional high-density turbidity currents and/or non-cohesive debris flows occur and were responsible for sand deposition in the middle-lower fan. They are derived from reworked sands due to the high- energy conditions on the outer shelf. Thus for the Celtic Fan, the variations of the hydrodynamic conditions on the outer Celtic Shelf seem to be the primary control on facies shift and fan growth. q 2000 Elsevier Science B.V. All rights reserved.

Terrigenous fluxes at the Celtic margin during the last glacial cycle

The sedimentary sections of three cores from the Celtic margin provide high-resolution records of the terrigenous fluxes during the last glacial cycle. A total of 21 14 C AMS dates allow us to define age models with a resolution better than 100 yr during critical periods such as Heinrich events 1 and 2. Maximum sedimentary fluxes occurred at the Meriadzek Terrace site during the Last Glacial Maximum (LGM). Detailed X-ray imagery of core MD95-2002 from the Meriadzek Terrace shows no sedimentary structures suggestive of either deposition from high-density turbidity currents or significant erosion. Two paroxysmal terrigenous flux episodes have been identified. The first occurred after the deposition of Heinrich event 2 Canadian ice-rafted debris (IRD) and includes IRD from European sources. We suggest that the second represents an episode of deposition from turbid plumes, which precedes IRD deposition associated with Heinrich event 1. At the end of marine isotopic stage 2 (MIS 2) and the beginning of MIS 1 the highest fluxes are recorded on the Whittard Ridge where they correspond to deposition from turbidity current overflows. Canadian icebergs have rafted debris at the Celtic margin during Heinrich events 1, 2, 4 and 5. The highresolution records of Heinrich events 1 and 2 show that in both cases the arrival of the Canadian icebergs was preceded by a European ice rafting precursor event, which took place about 1^1.5 kyr before. Two rafting episodes of European IRD also occurred immediately after Heinrich event 2 and just before Heinrich event 1. The terrigenous fluxes recorded in core MD95-2002 during the LGM are the highest reported at hemipelagic sites from the northwestern European margin. The magnitude of the Canadian IRD fluxes at Meriadzek Terrace is similar to those from oceanic sites. = 2002 Elsevier Science B.V. All rights reserved.

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).

Neotectonics of the Owen Fracture Zone (NW Indian Ocean): Structural evolution of an oceanic strike-slip plate boundary

The Owen Fracture Zone is a 800 km-long fault system that accommodates the dextral strike-slip motion between India and Arabia plates. Because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the Early Pliocene, the fault is clearly observed on bathymetric data. It is made up of a series of fault segments separated by releasing and restraining bends, including a major pull-apart basin at latitude 20°N. Some distal turbiditic channels from the Indus deep-sea fan overlap the fault system and are disturbed by its activity, thus providing landmarks to date successive stages of fault activity and structural evolution of the Owen Fracture Zone from Pliocene to Present. We determine the durability of relay structures and the timing of their evolution along the principal displacement zone, from their inception to their extinction. We observe subsidence migration in the 20°N basin, and alternate activation of fault splays in the vicinity of the Qalhat seamount. The present-day Owen Fracture Zone is the latest stage of structural evolution of the 20-Myr-old strike-slip fault system buried under Indus turbiditic deposits whose activity started at the eastern foot of the Owen Ridge when the Gulf of Aden opened. The evolution of the Owen Fracture Zone since 3-6 Myr reflects a steady state plate motion between Arabia and India, such as inferred by kinematics for the last 20 Myr period. The structural evolution of the Owen Fracture Zone since 20 Myr- including fault segments propagation and migration, pull-apart basin opening and extinction - seems to be characterized by a progressive reorganisation of the fault system, and does not require any major kinematics change.

The Sao Tomé deep-sea turbidite system (southern Brazil Basin): Cenozoic seismic stratigraphy and sedimentary processes

The Sao Tom deep-sea turbidite system, elongated parallel to the rise of the south Brazilian continental margin, was first interpreted as a channel-levee system resulting from contour-current activity. Study of new seismic data permits the proposal of a stratigraphy for the system and a new interpretation of depositional processes. Three major depositional units have been recognized that are separated by major erosive discontinuities. The basal unit seems to be Paleocene to lower or middle Eocene, and the second one, subdivided into two subunits, is probably upper Oligocene to middle Miocene. Both units show superimposed north-to-southchannelized turbidite systems, with supply provided directly from a channel network that crosses the upper margin in the north. The third unit is upper Miocene(?) to Pliocene or Quaternary and is still under predominantly gravity processes: turbidite processes in the lower and upper subunits, and major mass-flow processes in the median subunit. The sediment sources are located either in the north or in the south, with sediment provided by major deep-sea channels. The base of the upper subunit is well marked by an erosive discontinuity (late Pliocene or PlioceneQuaternary boundary). Impact of the contour currents is mainly recorded as widespread erosive surfaces (seismic discontinuities) correlated to global hydrological events and transparent or wavy deposits. Because this system contains a significant amount of upper Quaternary sands, it suggests the occurrence of petroleum reservoirs along the rise and the Sao Paulo Plateau in the lower continental slope.

Contourites in the Gulf of Cadiz: a cautionary note on potentially ambiguous indicators of bottom current velocity

Facies associations in cores collected in the deep part of the Gulf of Cadiz, which is under the influence of the lower branch of the Mediterranean Outflow Water, are investigated in terms of the classical contourite model using grain-size analyses and thin sections of indurated sediment. Cores include both low-energy (contourite drift) and high-energy (channel) environments. The thin sections and grain-size distributions show that clayey fine silts and sandy coarse silts are the most common facies associations in the studied contourite sequences, while coarse-grained, gravelly contourites are less common. Grain-size distributions are unimodal in the fine-grained and bi- or trimodal in the coarser-grained contourites. This change in grain-size composition is related both to the partial removal of the fine-grained fraction and to the replenishment of the coarser-grained one. In addition, most of the contacts between individual facies are sharp rather than transitional. This suggests that the contourite sequence is only in part related to changes in bottom current velocity and flow competency, but may also be related to the supply of a coarser terrigeneous particle stock, provided by either increased erosion of indurated mud along the flanks of confined contourite channels (mud clasts), or by increased sediment supply by rivers (quartz grains) and downslope mass transport on the continental shelf and upper slope. The classical contourite facies association may therefore not be solely controlled by current velocity, but may be the product of a variety of depositional histories. The classical contourite depositional sequence should therefore be interpreted with greater care and in the light of the regional sedimentological background. In addition, the wisdom of exclusively using mean or modal particle size for the interpretation of depositional contourite processes is questioned. Instead, it is proposed that the vertical evolution of grain-size populations in the facies successions forming contourite sequences be assessed.

The Columbia Channel-levee system: a fan drift in the southern Brazil Basin

Abstract The Columbia Channel is a turbiditic channel elongated W-E on the rise of the south Brazilian basin (4200 to 5000 m water depth). The whole area is swept by the northward flowing Antarctic Bottom Water. As a consequence, depositional processes have built a fan drift system. This system displays a levee along the northern flank of the channel while no levee occurs on its southern flank due to the Coriolis effect. The levee (400 km in length and 100 to 200 km in width) is bounded to the north by the Vitoria-Trindade Seamounts. It shows, first, a W-E trend parallel to the channel axis and predominantly turbiditic pattern, and then a S-N trend parallel to the rise contours with a predominant contouritic pattern. Its thickness is up to 1000 m. The distribution of sedimentary processes and associated deposits were investigated on the basis of water gun seismic and 3.5 kHz echosounding profiles, and core lithology. On the lower S-N part of the levee, the deposits consist of muddy contourites. On the shallowest part, turbidites that originate from the upper continental margin in the channel and on the southern part of the levee close to the channel, and from the Vitoria-Trindade Seamounts on the northern part of the levee, are interbedded with contouritic muds, and top-truncated silty turbidites. Areas subjected to turbidity current processes show chaotic to well-stratified, high amplitude reflections, in the subsurface, and more or less prolonged echofacies with or without sub-bottom reflectors, at the seabed. Areas subjected to contour currents show, in the subsurface, transparent seismofacies with some discontinuous low amplitude wavy reflections, and, in the surficial deposits, predominant wavy echofacies with sub-bottom relectors, frequently associated with tangential hyperbolae.

New constraints on European glacial freshwater releases to the North Atlantic Ocean

During the late Quaternary, both external and internal forcings have driven major climatic shifts from glacial to interglacial conditions. Nonlinear climatic steps characterized the transitions leading to these extrema, with intermediate excursions particularly well expressed in the dynamics of the Northern Hemisphere cryosphere. Here we document the impact of these dynamics on the north-eastern North Atlantic Ocean, focussing on the 35-10 ka interval. Sea-surface salinities have been reconstructed quantitatively based on two independent methods from core MD95-2002, recovered from the northern Bay of Biscay adjacent to the axis of the Manche paleoriver outlet and thus in connection with proximal European ice sheets and glaciers. Quantitative reconstructions deriving from dinocyst and planktonic foraminiferal analyses have been combined within a robust chronology to assess the amplitude and timing of hydrological changes in this region. Our study evidences strong pulsed freshwater discharges which may have impacted the North Atlantic Meridional Overturning Circulation.

High frequency environmental changes and deposition processes in a 2 kyr-long sedimentological record from the Cap-Breton canyon (Bay of Biscay)

The core MD03-2693 (43°39.258′N; 01°39.805′W; 431 m water depth) was collected on an abandoned meander of the Capbreton Canyon (SE Bay of Biscay), filled over the last millennia by very high sedimentation rates (mean sedimentation rate of 1.2 cm/yr) linked to its specific environmental location and fine-grained clayed sediment decantation from the proximal canyon axis. This archive thus permits to undertake the study of late Holocene regional climatic patterns at a decadal temporal resolution. In the present work, we use data derived from planktonic foraminifera assemblages coupled to a multiproxy approach that associates grain-size measurements, x-ray fluorescence (XRF) elemental analysis and stable oxygen isotope on Globigerina bulloides shells to infer Sea Surface Temperature (SST) and Sea Surface Salinity (SSS) changes over the last two millennia. Signals reconstructed in the Bay of Biscay show significant oscillations that are consistent with well-known temperature anomalies such as the ‘Little Ice Age’ (LIA) and the ‘Medieval Warm Period’ (MWP). It additionally displays strong similarities with other areas in the western temperate and northern North Atlantic Ocean, suggesting a narrow coupling between its main gyre surface systems. Abrupt decrease of SSS together with significant change in terrigenous inputs suggests a change in precipitation regime at the onset of the LIA (around ad 1400). Moreover, superimposed to the relative long-term change in environmental parameter, the core MD03-2693 records rapid and discrete pulses of sand grain–sized material that are correlated with the local history of migration of the mouth of the Adour River.