Oceanographic dynamics and the end of the last interglacial in the subpolar North Atlantic

The last interglacial interval was terminated by the inception of a long, progressive glaciation that is attributed to astronomically influenced changes in the seasonal distribution of sunlight over the earth. However, the feedbacks, internal dynamics, and global teleconnections associated with declining northern summer insolation remain incompletely understood. Here we show that a crucial early step in glacial inception involves the weakening of the subpolar gyre (SPG) circulation of the North Atlantic Ocean. Detailed new records of microfossil foraminifera abundance and stable isotope ratios in deep sea sediments from Ocean Drilling Program site 984 south of Iceland reveal repeated, progressive cold water-mass expansions into subpolar latitudes during the last peak interglacial interval, marine isotope substage 5e. These movements are expressed as a sequence of progressively extensive southward advances and subsequent retreats of a hydrographic boundary that may have been analogous to the modern Arctic front, and associated with rapid changes in the strength of the SPG. This persistent millennial-scale oceanographic oscillation accompanied a long-term cooling trend at a time of slowly declining northern summer insolation, providing an early link in the propagation of those insolation changes globally, and resulting in a rapid transition from extensive regional warmth to the dramatic instability of the subsequent ∼100 ka

Enregistrements des variations climatiques et reconstitution des paléoenvironnements depuis le Dernier Maximum Glaciaire dans le Loch Sunart (Nord-Ouest Ecosse)

Les fjords ou lochs sur la côte nord-ouest de l'Ecosse, préservent une importante séquence sédimentaire qui représente une archive du climat depuis le Dernier Maximum Glaciaire. La sismique THR acquise dans quatre fjords écossais Lochs Sunart, Creran, Spelve et Buie, révèle le dépôt de quatre faciès acoustiques interprétés en terme de dynamique des glaciers. Sur le soubassement basal une séquence sédimentaire de 20 à 60 m d'épaisseur s est accumulée depuis 17 000 ans BP. La séquence sismique a été corrélée aux sédiments de la carotte Calypso MD04-2833 prélevée dans le Loch Sunart. L analyse de ces sédiments a permis de retracer l histoire des changements environnementaux et climatiques décrits dans l'Atlantique Nord. L analyse granulométrique indique la variation des vitesses des courants profonds en rapport avec les changements globaux de la circulation océanique de l Atlantique Nord. L analyse palynologique discerne la période du réchauffement holocène et identifie les phases de refroidissement pendant cette période. Les foraminifères benthiques enregistrent la diminution de la température de l eau en réponse à la variation climatique holocène globale et la diminution de la température due aux facteurs locaux reliés à la fonte des glaciers. Ainsi, deux pulses de fonte peuvent être discernés, qui correspondent respectivement à la mwp-Ia et mwp-Ib décrits dans l océan Atlantique. Les données sismiques couplées aux analyses des foraminifères, des pollens et de la granulométrie sont en mesure de donner une reconstitution précise et continue du paléoenvironnement et du paléoclimat dans les deux domaines, marin et continental dans la zone étudiée depuis le Dernier Maximum Glaciaire jusqu à nos jours.Fjords or Lochs on the northwest coast of Scotland contain an important sedimentary sequence that records climatic variations since the Last Glacial Maximum. Very high-resolution sub-bottom seismic profiles were obtained from four Scottish Lochs Sunart, Creran, Spelve and Buie. Four acoustic facies were recognised and interpreted in terms of glacial dynamic. The seismic profiles revealed igneous and metamorphic basement covered by a 20 to 60 m thick sediment sequence accumulated since 17 000 yr BP. This sequence has been correlated to analyses obtained from the long Calypso core MD04-2833 acquired in Loch Sunart. These analyses allowed us to recognise the environmental changes and cooling events well described in the North Atlantic. Grain-size data indicate changes in the deep-water speed associated with global shift in the thermohaline circulation. Pollen analysis, have yielded information on the Holocene warming and significant cooling events during this period. Marine benthic foraminifera have also allowed recognition of climatic variations during the Holocene as they record changes in the water temperatures related to the cooling events. Moreover, it is possible to discriminate local water temperature decreases due to ice-melting processes. Thus, two meltwater pulses can be discerned, which respectively correspond to the mwp-Ia and mwp-Ib. Seismic data combined with foraminiferal, pollen and grain-size analyses are able to give an accurate and continuous record of palaeoenvironment and climate shift since the Last Glacial Maximum to present day.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

A multiproxy palaeoenvironmental reconstruction of Loch Sunart (NW 1 Scotland) since the Last Glacial Maximum

International audienceLoch Sunart is located on the northwest coast of Scotland and contains a sedimentary sequence that records Late Glacial to Holocene climatic variations. A 12 m core MD04-2833 was acquired in the main basin of the loch sampling this sequence. We present the palaeoenvironmental data and palaeoclimatical scenario, based on a multiproxy approach, using pollen concentrations, sortable silt variation, lithic fraction and marine benthic foraminifera assemblages. These analyses allow the identification of major climate fluctuations such as cooling events. Global temperature decreases are discriminated from local water temperature decreases due to ice-melting processes by the presence of Elphidium subarcticum and the assemblage of Cassidulina obtusa and Haynesina germanica. Two meltwater pulses (MWP) are distinguished, which respectively correspond to the MWP-Ia (15,500–13,000 cal yr BP) and MWP-Ib (12,200–10,100 cal yr BP). After the maximum water stratification occurred at 7,500 cal yr BP, full marine conditions were establishedaround 6,000 cal yr BP, which correspond to the highest relative sea-level reached in the loch

Multiproxy reconstruction of palaeoenvironmental evolution since the Last Glacial Maximum in a fjord environment : Loch Sunart (NW Scotland).

International audienc

Nouvelle lecture des phases de la dernière déglaciation dans le Loch Sunart (Nord-Ouest Ecosse) basée sur des traceurs géochimiques

International audienc

Nouvelle lecture des phases de la dernière déglaciation dans le Loch Sunart (Nord-Ouest Ecosse) basée sur des traceurs géochimiques

International audienc

Organic matter isotopes reveal enhanced rainfall activity in Northwestern Mediterranean borderland during warm substages of the last 200 kyr

International audienceIn this study we use bulk sediment organic matter stable isotopes (δ13Corg and δ15N) to examine the nature and timing of preserved organic matter in borehole PRGL1-4 from Northwestern Mediterranean Sea. This region is known as a transitional zone between the North Atlantic atmospheric circulation and the North African monsoon system. In the Gulf of Lion (SE France), increased inputs of organic matter from sediment-laden rivers occurred during warm substages of the last 200 kyr (MIS 5e, 5c, and 5a; MIS 6d, and 6b). Increased terrestrial organic matter are inferred from organic carbon isotopic composition (δ13Corg), and are interpreted as resulting from enhanced rainfall over the Rhone river catchment area. Such increase in terrestrial organic carbon induced enhancement of the primary productivity leading to the formation of local oxygen minimum zone as demonstrated by ∼3‰ values in δ15N. Comparison with regional paleohydrological records from the northern Mediterranean borderlands reveals the regional character of these pluvial events. Taking advantage of the location of PRGL1-4 borehole, out of Mediterranean cyclogenesis area, we suggest that the pluvial events recorded during warm substages of the last 200 kyr occurred in response to enhanced passage of North Atlantic atmospheric perturbation over the Gulf of Lion catchment area (especially over Rhone river watershed which represent 80% of the GoL catchment area), a scenario possibly similar to that encountered today during negative- NAO like conditions. At a regional scale, our data suggests that high rainfall events over the Gulf of Lion catchement area and the Rhone watershed occurred at the time of North African summer monsoon and the sapropel deposition in the Mediterranean basin, thus highlighting a close coupling between mid- (North Atlantic) and low-latitudes (monsoon) climate systems. Importantly, our geochemical evidence from the Gulf of Lion support an extra-Mediterranean source for the regional pluvial events described in many paleoclimatic records from the northern Mediterrean borderlands during warm substages of the last climate cycles. Consequently, we suggest that this region as a whole could provide, in addition to the river runoff from the North African sector, the necessary conditions for the sapropel deposition

Evaluating climatically-driven changes in a Scottish sea loch (fjord) over the last 20,000 years

International audienc

Nouvelle lecture des phases de la dernière déglaciation dans le Loch Sunart (Nord-Ouest Ecosse) basée sur des traceurs géochimiques

International audienc