Jean-Louis Turon

Deglacial warming of the northeastern Atlantic Ocean : Correlation with the paleoclimatic evolution of the European continent

Abstract Isotopic, micropaleontologic and pollen analyses of deep-sea cores from the Bay of Biscay and the northeastern Atlantic Ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent European continent. Temperatures were at least as warm as those of today in the Bay of Biscay between 13 300 and 11 000 B.P. coinciding with the combined Bolling/Allerod warm continental events. A major spread of polar water occurred between 11 000 and 10 000 B.P. During this event which coincides with the Younger Dryas continental cold event, marine temperatures were almost as low as those of the last glacial maximum. The final deglacial warming of the norteastern Atlatntic Ocean occurred during the following 3000 yr.

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.

Millennial‐scale iceberg discharges in the Irminger Basin during the Last Glacial Period: Relationship with the Heinrich events and environmental settings

High-resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the Irminger Basin. The last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from Iceland and corresponding to millennial-scale instabilities of the coastal ice sheets and ice shelves in the Nordic area. A comparison with midlatitude sediment cores shows that ice-rafted material corresponding to the Heinrich events was deposited synchronously from 40° to 60°N. There are thus two oscillating systems: every 5–10 kyr massive iceberg armadas are released from large continental ice caps, whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1.2–3.8 kyr. At the time of the Heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

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.

Organic-walled dinoflagellate cysts: palynological tracers of sea-surface conditions in middle to high latitude marine environments

Abstract A reference data base of dinoflagellate cyst assemblages was developed from the palynological analysesof surface sediment samples collected at ∼450 sites from middle to high latitudes of the North Atlantic and adjacent basins. The data set is representative of a wide range of sea-surface conditions with respect to sea-ice cover (0–12 months/year), temperature (up to 25°C), salinity (20–36 psu), and seasonality. The analyses reveal relatively high dinoflagellate cyst concentrations especially along the continental margins (up to 105 cysts/cm3). The distribution of species and assemblages show close relationships with the salinity, the temperature of the warmest and coldest months, and the seasonal duration of the sea-ice cover. On these grounds, transfer functions using the best analogue method were developed in order to reconstruct the following sea-surface parameters with a reasonable degree of accuracy, i.e. within the range of interannual variations: temperature in February (± 1.2°C) and August (± 16°C), salinity (± 0.7 psu) and sea-ice cover extent (± 1.1 months/year). The transfer functions based on dinoflagellate cysts are original and yield paleoceanographic and paleoclimatic data complementary to those produced currently on the basis of transfer function using planktonic foraminifera.

Hydrographic changes of the Southern Ocean (southeast Indian Sector) Over the last 230 kyr

Hydrographical changes of the southern Indian Ocean over the last 230 kyr, is reconstructed using a 17-m-long sediment core (MD 88 770; 46°01′S 96°28′E, 3290m). The oxygen and carbon isotopic composition of planktonic (N. pachyderma sinistra and G. bulloides) and benthic (Cibicidoides wuellerstorfi, Epistominella exigua, and Melonis barleeanum) foraminifera have been analysed. Changes in sea surface temperatures (SST) are calculated using diatom and foraminiferal transfer functions. A new core top calibration for the Southern Ocean allows an extension of the method developed in the North Atlantic to estimate paleosalinities (Duplessy et al., 1991). The age scale is built using accelerator mass spectrometry (AMS) 14C dating of N. pachyderma s. for the last 35 kyr, and an astronomical age scale beyond. Changes in surface temperature and salinity clearly lead (by 3 to 7 kyr) deep water variations. Thus changes in deep water circulation are not the cause of the early response of the surface Southern Ocean to climatic changes. We suggest that the early warming and cooling of the Southern Ocean result from at least two processes acting in different orbital bands and latitudes: (1) seasonality modulated by obliquity affects the high-latitude ocean surface albedo (sea ice coverage) and heat transfer to and from the atmosphere; (2) low-latitude insolation modulated by precession influences directly the atmosphere dynamic and related precipitation/ evaporation changes, which may significantly change heat transfer to the high southern latitudes, through their control on latitudinal distribution of the major frontal zones and on the conditions of intermediate and deep water formation.

Land–sea correlations for the last glaciation inferred from a pollen and dinocyst record from the portuguese margin

Pollen and dinoflagellate cyst assemblages from Core SU 81-18 recovered off Portugal (37°46′N, 10°11′W; 3135-m water depth) have been used to document the short-term environmental changes that occurred in southwest Europe since 25,000 yr B.P. The relationship between the oceanic and continental environments has been further examined by the use of other marine proxies (coarse sedimentary fraction, foraminifera) and by comparison with proximal land pollen records. Heinrich 2 (H2) and Heinrich 1 (H1) events were the most extreme parts of the highly variable last glacial period, with the maximum extension of dry steppe on land and the occurrence of cool and dilute waters at the core site. Our study shows that H1 and H2 are divided in two distinct phases: one with Neogloboquadrina pachyderma left coiling associated with the maximum input of ice rafted debris, reflecting the in situ release of icebergs and the occurrence of cool and dilute seawater at the core site; the other with dinoflagellate cysts of subpolar affinity, Bitectatodinium tepikiense, reflecting a seasonal control marked by warm summer SST and cold winter SST.

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.

COULD DEEP SUBANTARCTIC CONVECTION FEED THE WORLD DEEP BASINS DURING THE LAST GLACIAL MAXIMUM

Simple box model calculations are used to simulate the oceanic circulation during the last glacial maximum (LGM). These experiments show that the main features of the δ13C and Δ14C distributions and of the lysocline depth may be explained by a circulation pattern very different from the modem one. Intermediate and upper deep waters were produced in the North Atlantic Ocean, whereas deep waters of Subantarctic Mode type, forming at the northern edge of the Subantarctic convergence, invaded the main oceanic basins. The Southern Ocean, mainly self ventilated, had a reduced deep component that flew southward along the East Pacific Ridge and the Australian west cost. The thermodynamic fractionation that occurs during air-sea exchange has only contributed slightly to the glacial deep δ13C distribution through surface water temperature variations.