Frédérique Eynaud

The Cenozoic palaeoenvironment of the Arctic Ocean

The history of the Arctic Ocean during the Cenozoic era (0–65 million years ago) is largely unknown from direct evidence. Here we present a Cenozoic palaeoceanographic record constructed from >400 m of sediment core from a recent drilling expedition to the Lomonosov ridge in the Arctic Ocean. Our record shows a palaeoenvironmental transition from a warm ‘greenhouse’ world, during the late Palaeocene and early Eocene epochs, to a colder ‘icehouse’ world influenced by sea ice and icebergs from the middle Eocene epoch to the present. For the most recent ∼14 Myr, we find sedimentation rates of 1–2 cm per thousand years, in stark contrast to the substantially lower rates proposed in earlier studies; this record of the Neogene reveals cooling of the Arctic that was synchronous with the expansion of Greenland ice (∼3.2 Myr ago) and East Antarctic ice (∼14 Myr ago). We find evidence for the first occurrence of ice-rafted debris in the middle Eocene epoch (∼45 Myr ago), some 35 Myr earlier than previously thought; fresh surface waters were present at ∼49 Myr ago, before the onset of ice-rafted debris. Also, the temperatures of surface waters during the Palaeocene/Eocene thermal maximum (∼55 Myr ago) appear to have been substantially warmer than previously estimated. The revised timing of the earliest Arctic cooling events coincides with those from Antarctica, supporting arguments for bipolar symmetry in climate change.

High resolution palynological record off the Iberian margin: direct land-sea correlation for the Last Interglacial complex

Abstract We present high resolution pollen, dinocyst and isotopic data for the Last Interglacial complex from marine core MD952042 (southwestern margin of the Iberian Peninsula; 37°48′N; 10°10′W; 3148 m). Direct land-sea correlation from this core indicates that during this period, North Atlantic sea surface temperatures were in phase with Iberian climate. Our palynological analysis suggests a Younger Dryas-like event at the Marine Isotope Stage (MIS)-6/5 transition. The analysis also indicates that the Eemian spans from the lightest isotopic values of MIS-5e (ca. 126 ky BP) to the heavier isotopic values towards the MIS-5e/5d transition. Therefore, the Eemian is not entirely equivalent to MIS-5e. Pollen analysis identifies four climatic phases of low amplitude during the Eemian. A Mediterranean climate in southwestern Europe is gradually replaced by oceanic conditions. The middle of the Eemian is characterized by an increase in precipitation on the land and ocean, associated with a slight cooling. This seems to be the result of a displacement of the Polar Front as far south as southern Europe during this period. After the Eemian, three relatively short climatic phases on land (Melisey I, St. Germain Ia and Montaigu cold event) occurred contemporaneously with three shifts of sea surface temperatures. The Montaigu event, first identified in terrestrial pollen sequences, is, therefore, also recorded in core MD952042 on the basis of pollen, dinocyst and planktonic isotopic data. Our results also show that the warm periods of MIS-5 are not characterized by similar climatic conditions on land.

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.

Sea-surface distribution of coccolithophores, diatoms, silicoflagellates and dinoflagellates in the South Atlantic Ocean during the late austral summer 1995

The sea-surface distribution of four selected fossilizable phytoplankton groups (coccolithophores, diatoms, silicoflagellates and dinoflagellates) has been studied along a transect from Cape Town (34°S) to South Sandwich Islands (57°S) during the late austral summer. The observed distribution of these groups shows that their biogeographical distribution is significantly constrained by the water masses and associated frontal systems of the Southern Ocean. Coccolithophores are the dominant group and show cell abundances up to 51×103 cells/l down to 57°S. Three restricted areas are marked by particularly high cell densities: the continental shelf of South Africa, the area between the Sub-Tropical Convergence and the Sub-Antarctic Front, and the southern border of the Antarctic Polar Front, where the highest abundances are recorded (>650×103 cells/l). The species composition of the various assemblages representative of the four groups defines distinct biogeographical zones bounded by marked sea-surface temperature gradients. This biogeographical distribution is confirmed by factor analysis of the coccolithophore (5 factors, 85% of the total variance) and diatom and silicoflagellate (7 factors, 87.5% of the total variance) populations. When compared with the distribution pattern of siliceous fossil assemblages in surface sediments, our data show a more accurate coupling between the various water-masses of the South Atlantic Ocean and the living siliceous population.

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.

Quaternary paleoceanography of the central Arctic based on Integrated Ocean Drilling Program Arctic Coring Expedition 302 foraminiferal assemblages

The Integrated Ocean Drilling Program (IODP) Arctic Coring Expedition (ACEX) Hole 4C from the Lomonosov Ridge in the central Arctic Ocean recovered a continuous 18 m record of Quaternary foraminifera yielding evidence for seasonally ice-free interglacials during the Matuyama, progressive development of large glacials during the mid-Pleistocene transition (MPT) �1.2–0.9 Ma, and the onset of high-amplitude 100-ka orbital cycles �500 ka. Foraminiferal preservation in sediments from the Arctic is influenced by primary (sea ice, organic input, and other environmental conditions) and secondary factors (syndepositional, long-term pore water dissolution). Taking these into account, the ACEX 4C record shows distinct maxima in agglutinated foraminiferal abundance corresponding to several interglacials and deglacials between marine isotope stages (MIS) 13–37, and although less precise dating is available for older sediments, these trends appear to continue through the Matuyama. The MPT is characterized by nearly barren intervals during major glacials (MIS 12, 16, and 22–24) and faunal turnover (MIS 12–24). Abundant calcareous planktonic (mainly Neogloboquadrina pachyderma sin.) and benthic foraminifers occur mainly in interglacial intervals during the Brunhes and very rarely in the Matuyama. A distinct faunal transition from calcareous to agglutinated foraminifers 200–300 ka in ACEX 4C is comparable to that found in Arctic sediments from the Lomonosov, Alpha, and Northwind ridges and the Morris Jesup Rise. Down-core disappearance of calcareous taxa is probably related to either reduced sea ice cover prior to the last few 100-ka cycles, pore water dissolution, or both.

European climate optimum and enhanced Greenland melt during the Last Interglacial

The Last Interglacial climatic optimum, ca. 128 ka, is the most recent climate interval signifi cantly warmer than present, providing an analogue (albeit imperfect) for ongoing global warming and the effects of Greenland Ice Sheet (GIS) melting on climate over the coming millennium. While some climate models predict an Atlantic meridional overturning circulation (AMOC) strengthening in response to GIS melting, others simulate weakening, leading to cooling in Europe. Here, we present evidence from new proxy-based paleoclimate and ocean circulation reconstructions that show that the strongest warming in western Europe coincided with maximum GIS meltwater runoff and a weaker AMOC early in the Last Interglacial. By performing a series of climate model sensitivity experiments, including enhanced GIS melting, we were able to simulate this confi guration of the Last Interglacial climate system and infer information on AMOC slowdown and related climate effects. These experiments suggest that GIS melt inhibited deep convection off the southern coast of Greenland, cooling local climate and reducing AMOC by ~24% of its present strength. However, GIS melt did not perturb overturning in the Nordic Seas, leaving heat transport to, and thereby temperatures in, Europe unaffected.

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).

Comparison of the Holocene and Eemian palaeoenvironments in the South Icelandic Basin: dinoflagellate cysts as proxies for the North Atlantic surface circulation

A precise assessment of the hydrological changes in the northern Atlantic Ocean throughout the last climatic cycle stands as one of the key priorities for understanding the mechanisms of global climate change. A high resolution micropalaeontological study of a sediment core (MD95-2015) retrieved from the South Icelandic Basin, allows us to infer patterns of North Atlantic surface hydrological changes during the present (Holocene) and the ultimate (Marine Isotopic Stage 5) Interglacial periods. The downcore distribution of organic-walled dinoflagellate cysts (dinocysts) is used, in conjunction with additional proxies (sediment magnetic susceptibility, CaCO3, stables isotopes and planktic foraminifer assemblages) to identify climatic instabilities of various amplitudes. These events are mostly characterised by prominent changes in relative abundance of the dinocysts Spiniferites mirabilis and Operculodinium centrocarpum, whose maximum values are thought to trace sea-surface temperature peaks at the core site. Two hypsithermal periods are identified on this basis, between 126 and 120 kyr BP and from 9.2 to 5.7 cal kyr BP (∼8–5 14C kyr BP), respectively. Some discrepancies between the micropalaeontological tracers used are discussed here in the light of their qualitative and quantitative (transfer functions) ecological interpretation.

African humid periods triggered the reactivation of a large river system in Western Sahara.

The Sahara experienced several humid episodes during the late Quaternary, associated with the development of vast fluvial networks and enhanced freshwater delivery to the surrounding ocean margins. In particular, marine sediment records off Western Sahara indicate deposition of river-borne material at those times, implying sustained fluvial discharges along the West African margin. Today, however, no major river exists in this area; therefore, the origin of these sediments remains unclear. Here, using orbital radar satellite imagery, we present geomorphological data that reveal the existence of a large buried paleodrainage network on the Mauritanian coast. On the basis of evidence from the literature, we propose that reactivation of this major paleoriver during past humid periods contributed to the delivery of sediments to the Tropical Atlantic margin. This finding provides new insights for the interpretation of terrigenous sediment records off Western Africa, with important implications for our understanding of the paleohydrological history of the Sahara.