South Atlantic intermediate water advances into the North-east Atlantic with reduced Atlantic meridional overturning circulation during the last glacial period

The Nd isotopic composition (epsilon Nd) of seawater and cold-water coral (CWC) samples from the Gulf of Cadiz and the Alboran Sea, at a depth of 280-827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cadiz over the past 40 ka. epsilon Nd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cadiz reveals relatively constant values (-8.6 to -9.0 and -9.5 to -10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; epsilon Nd approximate to -9.4) and Mediterranean Sea Water (MSW; epsilon Nd approximate to -9.9). In contrast, glacial epsilon Nd values for CWC collected at thermocline depth (550-827 m) in the southern Gulf of Cadiz display a higher average value (-8.90.4) compared to the present-day value (-11.70.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic epsilon Nd values (approximate to -8) at 19, 23-24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW epsilon Nd values remained stable during the last glacial period, it is suggested that these radiogenic epsilon Nd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin

Restitution de l’hydrologie de l’Atlantique Nord-Est et de la Méditerranée occidentale depuis la dernière période glaciaire à partir de la composition isotopique du néodyme mesurée dans l’eau de mer et les coraux d’eau froide

: The purpose of this thesis is to constrain the hydrology of the North-East Atlantic and western Mediterranean Sea since the last glacial period from neodymium isotopic composition (εNd) measured on seawater, cold water corals and foraminifera. In particular, hydrological changes of intermediate water masses (LIW, AAIW, MSW, mid-subtropical and subpolar gyre water) have been studied as their role on salt budget in North Atlantic and ultimately on AMOC are currently poorly constrained. This work has been conducted at times of major and abrupt hydrological changes that occurred during rapid climatic variations of the last glacial period (Heinrich and Dansgaard-Oeschger events) and during the last sapropel deposit (S1) in eastern Mediterranean Sea. In a first step, we have improved the spatial distribution of water masses εNd values in North-east Atlantic and Alboran Sea, what is an absolute prerequisite in order to track past hydrological changes in these areas with εNd proxy. Next, we have highlighted a major change of the western Mediterranean circulation pattern during the sapropel S1 deposit, which is marked south of Sardinia by a strong reduction of eastern-sourced water masses (LIW) in favor of western-sourced water masses (WIW). This hydrological change as well as those occurring in Mediterranean Sea since the last glacial period was not associated with strong modifications of εNd values in Alboran and Balearic Sea, suggesting a stability of Nd isotopic signature of MOW over the time. This has highlighted, from an εNd record obtained on cold water corals in the Gulf of Cadiz, an enhanced contribution of more radiogenic AAIW and therefore a stronger northward penetration in North Atlantic at times of reduced AMOC linked to iceberg discharges from Northern Hemisphere ice sheets.L’objectif de cette thèse est d’apporter de nouvelles contraintes sur l’hydrologie de l’Atlantique Nord-Est et de la Méditerranée occidentale depuis la dernière période glaciaire à partir de l’analyse de la composition isotopique du Nd (εNd) dans des échantillons d’eau de mer ainsi que des coraux profonds et des foraminifères, prélevés dans des carottes sédimentaires marines. Les changements de l’hydrologie des masses d’eau intermédiaire (LIW, MSW, AAIW et masses d’eau intermédiaire des gyres subtropical et subpolaire) ont été plus particulièrement étudiés car leur rôle sur les transferts de sels en Atlantique Nord et in fine sur l’AMOC est actuellement mal contraint. Ce travail a été mené au cours des périodes de changements hydrologiques majeurs et abrupts de l’océan qui se sont produits lors des variations climatiques rapides de la dernière période glaciaire (événements d’Heinrich et cycles de Dansgaard-Oeschger) et lors du dépôt du Sapropel S1 en Méditerranée orientale. Dans un premier volet, nous avons amélioré la couverture spatiale des valeurs d’εNd des masses d’eau de l’Atlantique Nord-Est et de la mer d’Alboran, préalable indispensable pour restituer l’hydrologie passée de ces régions avec le traceur εNd. Nous avons ensuite mis en évidence un changement majeur du schéma de circulation de la Méditerranée occidentale durant la période de dépôt du sapropel S1, marquée par une forte réduction des masses d’eau de la Méditerranée orientale (LIW) au sud de la Sardaigne au profit de celles provenant du Golfe du Lion (WIW). Ce changement hydrologique ainsi que ceux qui s’opèrent en Méditerranée depuis la dernière période glaciaire ne sont pas associés à de fortes modifications des valeurs d’εNd de la LIW de la mer d’Alboran et de la mer des Baléares, suggérant une stabilité de la signature isotopique en Nd de la MOW au cours du temps. Ceci a permis, à partir d’un enregistrement d’εNd obtenus sur des coraux profonds du Golfe de Cadix, de mettre en évidence une contribution plus importante de l’AAIW plus radiogénique et donc une pénétration plus marquée en Atlantique Nord de cette masse d’eau lors des périodes de fortes réductions de l’AMOC, liées à la déstabilisation des calottes de glace de l’Hémisphère Nord

Restoration of the North-east Atlantic and Mediterranean Sea hydrology from the neodymium isotopes since the last glacial period

L’objectif de cette thèse est d’apporter de nouvelles contraintes sur l’hydrologie de l’Atlantique Nord-Est et de la Méditerranée occidentale depuis la dernière période glaciaire à partir de l’analyse de la composition isotopique du Nd (εNd) dans des échantillons d’eau de mer ainsi que des coraux profonds et des foraminifères, prélevés dans des carottes sédimentaires marines. Les changements de l’hydrologie des masses d’eau intermédiaire (LIW, MSW, AAIW et masses d’eau intermédiaire des gyres subtropical et subpolaire) ont été plus particulièrement étudiés car leur rôle sur les transferts de sels en Atlantique Nord et in fine sur l’AMOC est actuellement mal contraint. Ce travail a été mené au cours des périodes de changements hydrologiques majeurs et abrupts de l’océan qui se sont produits lors des variations climatiques rapides de la dernière période glaciaire (événements d’Heinrich et cycles de Dansgaard-Oeschger) et lors du dépôt du Sapropel S1 en Méditerranée orientale. Dans un premier volet, nous avons amélioré la couverture spatiale des valeurs d’εNd des masses d’eau de l’Atlantique Nord-Est et de la mer d’Alboran, préalable indispensable pour restituer l’hydrologie passée de ces régions avec le traceur εNd. Nous avons ensuite mis en évidence un changement majeur du schéma de circulation de la Méditerranée occidentale durant la période de dépôt du sapropel S1, marquée par une forte réduction des masses d’eau de la Méditerranée orientale (LIW) au sud de la Sardaigne au profit de celles provenant du Golfe du Lion (WIW). Ce changement hydrologique ainsi que ceux qui s’opèrent en Méditerranée depuis la dernière période glaciaire ne sont pas associés à de fortes modifications des valeurs d’εNd de la LIW de la mer d’Alboran et de la mer des Baléares, suggérant une stabilité de la signature isotopique en Nd de la MOW au cours du temps. Ceci a permis, à partir d’un enregistrement d’εNd obtenus sur des coraux profonds du Golfe de Cadix, de mettre en évidence une contribution plus importante de l’AAIW plus radiogénique et donc une pénétration plus marquée en Atlantique Nord de cette masse d’eau lors des périodes de fortes réductions de l’AMOC, liées à la déstabilisation des calottes de glace de l’Hémisphère Nord.: The purpose of this thesis is to constrain the hydrology of the North-East Atlantic and western Mediterranean Sea since the last glacial period from neodymium isotopic composition (εNd) measured on seawater, cold water corals and foraminifera. In particular, hydrological changes of intermediate water masses (LIW, AAIW, MSW, mid-subtropical and subpolar gyre water) have been studied as their role on salt budget in North Atlantic and ultimately on AMOC are currently poorly constrained. This work has been conducted at times of major and abrupt hydrological changes that occurred during rapid climatic variations of the last glacial period (Heinrich and Dansgaard-Oeschger events) and during the last sapropel deposit (S1) in eastern Mediterranean Sea. In a first step, we have improved the spatial distribution of water masses εNd values in North-east Atlantic and Alboran Sea, what is an absolute prerequisite in order to track past hydrological changes in these areas with εNd proxy. Next, we have highlighted a major change of the western Mediterranean circulation pattern during the sapropel S1 deposit, which is marked south of Sardinia by a strong reduction of eastern-sourced water masses (LIW) in favor of western-sourced water masses (WIW). This hydrological change as well as those occurring in Mediterranean Sea since the last glacial period was not associated with strong modifications of εNd values in Alboran and Balearic Sea, suggesting a stability of Nd isotopic signature of MOW over the time. This has highlighted, from an εNd record obtained on cold water corals in the Gulf of Cadiz, an enhanced contribution of more radiogenic AAIW and therefore a stronger northward penetration in North Atlantic at times of reduced AMOC linked to iceberg discharges from Northern Hemisphere ice sheets

Reconstruction of Ocean Circulation Based on Neodymium Isotopic Composition: Potential Limitations and Application to the Mid-Pleistocene Transition

International audienc

Holocene shifts in sub-surface water circulation of the North-East Atlantic inferred from Nd isotopic composition in cold-water corals

International audienceVariations of the Sub-Polar Gyre (SPG) and the Sub-Tropical Gyre (STG) circulation during the Holocene are believed to be related to regional and global climate over this time period. To improve our understanding of these phenomena we provide new constraints on variations in surface circulation patterns using neodymium isotopes (εNd) on precisely U-Th dated coral fragments of L. pertusa. The fragments were retrieved from two sediment cores taken from cold-water coral (CWC) mounds at ~ 127–134 m water depth in the Mingulay Reef Complex located on the Western British continental shelf. The results have been combined with εNd analyzed on seawater samples from two stations located on the continental shelf and margin in order to establish whether εNd is a reliable proxy of the ocean circulation variations and notably of the relative contribution of water originating from the SPG and STG. εNd values in CWCs from the Mingulay Reef Complex range from −14.5 ± 0.4 to −11.8 ± 0.3, highlighting two major variations. Unradiogenic εNd values (−14.5 ± 0.4) indicate a higher contribution of water from the SPG around 2.8 ka. Conversely, more radiogenic values at 3.4 ka (−11.8 ± 0.3) point to a declining SPG strength, accompanied by stronger northward penetration of STG water along the western European margin transported by the Shelf Edge Current (SEC) and/or cooler and fresher waters from the interior Seas. The eastward extension of the SPG at 2.8 ka is associated with lower 14C reservoir age (200 yrs) compared to periods associated with a higher contribution of STG waters. This indicates that 14C reservoir ages are mainly a function of vertical mixing of the sub-surface of the ocean. As stronger vertical ventilation is not associated with a higher proportion of local radiogenic surface water, we hypothesize it could represent vertical ventilation in the North-Eastern Atlantic. Active SPG is associated with a better ventilation of the water masses within the SPG and warmer climatic conditions in Northern Europe and in the Eastern Norwegian Sea linked to an intensification of the surface limb of the AMOC

New insights into hydrological exchange between the South China Sea and the Western Pacific Ocean based on the Nd isotopic composition of seawater

International audienc

The spatial and temporal distribution of neodymium isotopic composition within the Rockall Trough

International audienc

Eastern Atlantic deep-water circulation inferred from neodymium and carbon isotopic compositions over the past 1200 thousand years

International audienceMid-Pleistocene transition (MPT; 1200 to 800 thousand years, ky) is marked by an intensification and a shift of glacial-interglacial cycle from 41 ky to 100 ky that is not directly supported by solar insolation. These changes can be explained by combined effects of the stabilization of ice sheets associated with regolith removal withatmospheric CO2 reduction caused by active biological pumping in the Southern Ocean. Significant changes in ocean circulation was also suggested.To improve the spatiotemporal coverage of past ocean circulation records, we analysed Nd isotopic ratios recorded in foraminiferal authigenic fractions and epibenthic foraminiferal stable isotopes on two cores: MD03-2705 (18◦06’N, 21◦09’W, 3085 m) in the eastern tropical Atlantic where the reconstruction based on Nd isotopes is absent for this timescale and ODP1085 (29◦22’S, 13◦59’E, 1713 m) that is located at the present boundary between a northern source water (NSW, North Atlantic Deep Water) and a southern source water (SSW, Antarctic Bottom Water).Sequential cleaning and SEM observation of foraminiferal tests of MD03-2705 indicate that Nd isotopic ratios of authigenic phases reflect seawater values despite the fact that the core is located under Saharan dust plume. Reconstructed seawater Nd values of MD03-2705 ranges between -12.7 and -10.3 with more radiogenic values during glacial periods, suggesting an increase in SSW proportion. The glacial-interglacial eNd amplitude is smaller for the MPT than for the post-MPT. Epibenthic d13C values of ODP1085 varies from -0.6 to 0.9h with a clear positive shift at MIS 13 that is not observed for core MD03-2705. These results suggest the reorganisation of Atlantic meridional overturning circulation since the MPT. The analysis of eNd for core ODP1085 is in progress.We will discuss variability in relative proportion between SSW and NSW in relation to carbon cycle by combining reconstructed seawater eNd and d13C values from the two cores with available record

Eastern Atlantic deep-water circulation inferred from neodymium and carbon isotopic compositions over the past 1200 thousand years

International audienceMid-Pleistocene transition (MPT; 1200 to 800 thousand years, ky) is marked by an intensification and a shift of glacial-interglacial cycle from 41 ky to 100 ky that is not directly supported by solar insolation. These changes can be explained by combined effects of the stabilization of ice sheets associated with regolith removal withatmospheric CO2 reduction caused by active biological pumping in the Southern Ocean. Significant changes in ocean circulation was also suggested.To improve the spatiotemporal coverage of past ocean circulation records, we analysed Nd isotopic ratios recorded in foraminiferal authigenic fractions and epibenthic foraminiferal stable isotopes on two cores: MD03-2705 (18◦06’N, 21◦09’W, 3085 m) in the eastern tropical Atlantic where the reconstruction based on Nd isotopes is absent for this timescale and ODP1085 (29◦22’S, 13◦59’E, 1713 m) that is located at the present boundary between a northern source water (NSW, North Atlantic Deep Water) and a southern source water (SSW, Antarctic Bottom Water).Sequential cleaning and SEM observation of foraminiferal tests of MD03-2705 indicate that Nd isotopic ratios of authigenic phases reflect seawater values despite the fact that the core is located under Saharan dust plume. Reconstructed seawater Nd values of MD03-2705 ranges between -12.7 and -10.3 with more radiogenic values during glacial periods, suggesting an increase in SSW proportion. The glacial-interglacial eNd amplitude is smaller for the MPT than for the post-MPT. Epibenthic d13C values of ODP1085 varies from -0.6 to 0.9h with a clear positive shift at MIS 13 that is not observed for core MD03-2705. These results suggest the reorganisation of Atlantic meridional overturning circulation since the MPT. The analysis of eNd for core ODP1085 is in progress.We will discuss variability in relative proportion between SSW and NSW in relation to carbon cycle by combining reconstructed seawater eNd and d13C values from the two cores with available record

Foraminiferal εNd in the deep north-western subtropical Pacific Ocean: Tracing changes in weathering input over the last 30,000 years

International audienc