Gérard Auffret

Changes in sea surface hydrology associated with Heinrich event 4 in the North Atlantic Ocean between 40° and 60°N

Abstract The changes in distribution of sea surface temperature and salinity in the North Atlantic between 40 and 60°N were reconstructed for the time interval between 40 and 30 kyr BP, which includes the large iceberg discharge event associated with the deposition of Heinrich layer 4. We found that the meltwater input during deposition of Heinrich layer 4 resulted in a 1–2 kyr temperature decrease of about 2°C and a salinity decrease in the range of 1.5‰–3.5‰ between 40 and 50°N. Sites above 50°N did not experience significant salinity variations. A much larger area was affected by the reduction in sea surface temperature. The amplitude of the sea surface temperature shift was, however, much smaller than the atmospheric temperature changes over Greenland at GISP and GRIP sites.

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.

Rapid post-bloom resuspension in the northeastern Atlantic

The importance of particulate matter input on the seafloor following spring bloom and of resuspension processes in middle and high latitudes of the north Atlantic ocean has been recently emphasized. However the possibility of the coupling between both processes until now, has not been reported. To study the fate of the particulate matter after its arrival on the bottom, a deep-sea observatory has been launched in the NEA (Nuclear Energy Agency) area of the northeastern Atlantic abyssal hill province. Time series of botton currents, near bottom particle concentration measurements and time lapse photographs were obtained from March to July 1988, while particulate matter fluxes were recorded with a sequential sediment trap. The most interesting results are the observation of a series of events starting with the arrival on the bottom of 5–10 mm aggregates following the spring bloom, their disaggregation within 2 weeks immediately followed by a rise of the near-bottom particle concentration, concomitant with the passage of a bottom current eddy. These observations illustrate how rapid disaggregation rates may lead to resuspension following high particulate matter input.

Identification of Heinrich Layers in core KS 01 North-Eastern Atlantic (46 °N, 17 °W), implications for their origin

Abstract Sediments of the North Atlantic contain between 40 ° and 55 °N a series of layers rich in ice-rafted material (Heinrich layers, HLs). In core KS01 from the northeastern Atlantic (46 °00.2′N, 17 ° 12.1′W) we identified HL1 to 5. Important parts of these layers are abnormally dense. In order to unravel the depositional history of the HLs and the environmental conditions which have allowed such induration, we investigated the 18O and 13C contents of planktonic foraminifers and studied the bulk physical and lithological properties of the layers. The detailed investigation of HL1, HL2 and HL4 shows that, at this site, drastic changes in the surface water and at the sediment-water interface occurred during the HL deposition. We distinguished two phases. The first phase, which includes two steps, is characterised by glacial conditions and high productivity (not reflected by planktonic foraminifers). During the second step the detrital carbonate supply increased and organic carbon content maxima occur in these HLs. The second phase is characterised by an important decrease of the surface water salinity and an important reduction of the productivity. The diagenetic precipitation of calcite within microburrows in relation with anoxic and microbial mineralisation of organic matter may have contributed to the induration of the layers.

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.

Late Quaternary sedimentation patterns on the Meriadzek Terrace, Bay of Biscay (ESSCAMP 02 core: 47°N 9°W)

Abstract Foraminiferal assemblage changes, size and mineralogy of lithic grains, oxygen isotopes, CaCO 3 , and dolomite content were studied along a 9-m-long core from the Meriadzek Terrace to gain insight into climate, productivity, and sediment source changes at this part of the Northeast Atlantic margin during the late Quaternary. An age model has been generated on the basis of radiocarbon dating, downcore foraminiferal assemblages, and δ 18 O values. High sedimentation rates at this site allow very detailed records for the last glacial period down to late isotopic stage 3. Sea surface temperature (SST) inferred from the foraminiferal assemblages, δ 18 O curve, and the temperature estimation by the SIMMAX analog technique reveal three distinctive periods during isotopic stage 2, with late stage 2 (15.3–13.4 ka) being the coldest period of the last 26 ka. A northward retreat of the polar front at 13.4 ka based on the SST record coincides with the strongest peak of primary productivity as indicated by the foraminiferal fluxes. Levels rich in large lithic grains (LLG) interpreted as ice-rafted debris (IRD) correspond to periods of low SST and are coeval with Heinrich layers 1, 2 and 3. However, the hematite-stained quartz found in the detrital fraction and the scarce dolomite and detrital carbonate content in our core point to an Iceland and/or Fenno-Scandian rather than a Laurentian or Greenland origin of this material.

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.

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.