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Antarctic Intermediate Water (AAIW) is a water mass originating in the Southern Ocean characterised by its low salinity. The properties of the salinity minimum layer that characterise AAIW in the CMIP6 UKESM1 model and its response to different climate change scenarios are investigated. In UKESM1, the depth of the salinity minimum shoals by 116 m in the SSP5-8.5 run compared to the control run by 2080–2100. The salinity minimum also gets warmer (+ 1.9 °C) and lighter (− 0.4 kg/m3) and surface properties where the salinity minimum outcrops warm, freshen and lighten in all scenarios. In spite of these expected changes in properties, the location where the salinity minimum outcrops does not change in any of the future scenarios. The stability of the outcrop location of the salinity minimum is linked to the relative stability of the position of the Antarctic Circumpolar Current (ACC) in UKESM1. The position of the ACC does not follow the maximum wind stress trend, which intensifies and shifts poleward under radiative forcing. Changes in surface buoyancy fluxes in the region are consistent with the changes in hydrographic properties observed at depth on the salinity minimum mentioned above. However, transformation rates at the density corresponding to the salinity minimum outcrop remain constant in all scenarios. Stability in transformation rates at that density is due to the haline and thermal contributions counteracting one another. This analysis identifies two features (outcrop location, transformation rate) associated with the salinity minimum defining AAIW that show remarkable stability in an otherwise changing world. The effect of model resolution and other parameterisations on these findings have yet to be evaluated.

International audience; The evolution of intermediate circulation in the northern Indian Ocean since the last deglaciation has been reconstructed from two marine cores located at intermediate depths off the southern tip of India (MD77‐191) and in the northern Bay of Bengal (BoB) (MD77‐176). Benthic foraminiferal δ13C, seawater carbonate ion concentration ([CO32−]) estimated from the Sr/Ca, and paleotemperature reconstructed on the basis of the Mg/Li of aragonite benthic species Hoeglundina elegans were used to trace the evolution of past intermediate‐deepwater masses and to constrain ocean‐atmosphere exchanges during the two‐stage increase in atmospheric CO2 across the last deglaciation. The intermediate water [CO32−] was mainly affected by changes in the ocean alkalinity inventory, associated with the modulation of atmospheric CO2 on glacial‐interglacial timescales. Higher benthic foraminiferal δ13C, depleted [CO32−], and decreased benthic‐planktonic 14C age offsets at intermediate water depths suggest a release of deep‐sea CO2 to the atmosphere through the Antarctic Intermediate Water (AAIW) in the Southern Ocean during the 17–15.2 and 12.6–10.5 cal kyr BP time intervals. In addition, the decreased H. elegans Mg/Li record seems to reflect an increased contribution of cold water mass during the 17–15.2 and 12.6–11.9 cal kyr BP intervals and throughout the Holocene. In contrast, two warm events occurred in the 15–13.3 and 11–10.3 cal kyr BP time intervals. During the late Holocene, a decrease in the intermediate water [CO32−] indicates a contribution to atmospheric CO2 rise since 8 cal kyr BP, due to the depleted global ocean alkalinity and/or the variations in surface productivity (at least for MD77‐191).

Antarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (epsilon(Nd)) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium-enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water, the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic epsilon(Nd) reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial epsilon(Nd) variability, can be transported to shallow layers during active AMOC and modulates epsilon(Nd) in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of epsilon(Nd) reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.

This Ph.D.-Thesis aims to analyse the intermediate water mass circulation changes in the Atlantic Ocean in response to major perturbations of the AMOC during the last deglaciation. The approach combined the use of benthic foraminiferal based Cdw as a proxy for past nutrient saturation, Mg/Ca to reconstruct past Intermediate Water Temperatures, d13C as a proxy for nutrient saturation and ventilation and mixed planktonic foraminiferal εNd to reconstruct past water mass distribution in the tropical Atlantic. Ziel dieser Dissertation ist es, den Einfluss von Änderungen in der atlantischen Tiefenwasserbildung (AMOC) auf die Zirkulation von atlantischen Zwischenwassermassen während des Übergangs (Termination) aus dem letzten Glazial in die heutige Warmphase, das Holozän, zu untersuchen. Dazu werden folgende geochemische Werkzeuge genutzt: Cdw zur Rekonstruktion der Verteilung von Nährstoffen im Wasser, das Mg/Ca-Verhältnis in Foraminiferenschalen um Temperaturen im Zwischenwasser (IWT) zu rekonstruieren, d13C als Anzeiger für Änderungen in der Nährstoffkonzentration und in der Ventilation sowie εNd um die Verteilung von verschiedenen Wassermassen zu analysieren.

This dissertation assesses the changing dominance of high-latitude intermediate waters and its impact on the equatorial nutrient-budget. The modern Equatorial Pacific Intermediate Water (EqPIW) is fed by three end-member components: Southern Ocean Intermediate Water (SOIW), Pacific Deep Water (PDW) and, by a smaller proportion, North Pacific Intermediate Water (NPIW). This modern configuration of end-members in the EqPIW results in low productivity of siliceous phytoplankton in the Eastern Equatorial Pacific (EEP) today as SOIW is depleted in silicic acid compared to other nutrients. However, there is growing debate over whether SOIW was capable of stimulating glacial equatorial productivity. Furthermore, recent studies point towards a change in the lateral and vertical extent of both SOIW and NPIW during glacials, impacting the supply of nutrients to the EEP. Most upper ocean water mass reconstructions are based on planktonic foraminifera tests. Different foraminiferal species preferentially dwell in distinct water depths and thus, the calcitic tests of these species can be used to infer past climate conditions. This thesis assesses equatorial foraminiferal ACDs to identify a species suitable to trace nutrient-inflow of extra-tropical intermediate water masses. Using this determined species, this thesis then reconstructs the effect of variable nutrient injections from extra-tropical water masses on the equatorial Pacific upwelling waters using benthic and planktonic foraminiferal carbon isotopes (d13C). It was shown that d13C records from the Bering Sea (as an indicator for glacial NPIW (GNPIW)), the eastern tropical North Pacific and the EqPIW exhibit a similar temporal evolution during MIS 2. These results indicate increased GNPIW ventilation during glacials that spreads southward towards the eastern tropical North Pacific. During peak glacials the southward expansion of GNPIW was at a maximum and extended into the equatorial Pacific. Together with newly published evidence for a shallower penetration of relatively nutrient-depleted SOIW during glacials, these results point towards repeated episodes of reduced southern-sourced nutrient-injections into EqPIW during peak glacials. In contrast, the enhanced ventilation of nutrient-elevated GNPIW resulted in a comparatively increased nutrient contribution to the EqPIW. This intensified GNPIW nutrient-inflow possibly relaxed the nutrient limitation in the EEP, stimulating primary productivity in the EEP during peak MIS 2. As a consequence, the invigorated glacial biological pump would have sequestered more carbon dioxide (CO2) from the atmosphere into the ocean. And thus, in summary, this thesis has contributed important new insights into the role of the dynamics of the EEP in driving the glacial reduction in atmospheric CO2 concentrations

International audience; An enhanced Altimetry Gravest Empirical Mode (AGEM), including both adiabatic and diabatic trends, is developed for the Antarctic Circumpolar Current (ACC) south of Africa using updated hydrographic CTD sections, Argo data, and satellite altimetry. This AGEM has improved accuracy compared to traditional climatologies and other proxy methods. The AGEM for the Atlantic Southern Ocean offers an ideal technique to investigate the thermohaline variability over the past two decades in a key region for water mass exchanges and transformation. In order to assess and attribute changes in the hydrography of the region, we separate the changes into adiabatic and diabatic components. Integrated over the upper 2000 dbar of the ACC south of Africa, results show mean adiabatic changes of 0.16 ± 0.11°C decade−1 and 0.006 ± 0.014 decade−1, and diabatic differences of −0.044 ± 0.13°C decade−1 and −0.01 ± 0.017 decade−1 for temperature and salinity, respectively. The trends of the resultant AGEM, that include both adiabatic and diabatic variability (termed AD-AGEM), show a significant increase in the heat content of the upper 2000 dbar of the ACC with a mean warming of 0.12 ± 0.087°C decade−1. This study focuses on the Antarctic Intermediate Water (AAIW) mass where negative diabatic trends dominate positive adiabatic differences in the Subantarctic Zone (SAZ), with results indicating a cooling (−0.17°C decade−1) and freshening (−0.032 decade−1) of AAIW in this area, whereas south of the SAZ positive adiabatic and diabatic trends together create a cumulative warming (0.31°C decade−1) and salinification (0.014 decade−1) of AAIW.

Glacial/interglacial changes in Southern Ocean's air-sea gas exchange have been considered as important mechanisms contributing to the glacial/interglacial variability in atmospheric CO2. Hence, understanding past variability in Southern Ocean intermediate to deep water chemistry and circulation is fundamental to constrain the role of these processes on modulating glacial/interglacial changes in the global carbon cycle. Our study focused on the glacial/interglacial variability in the vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW). We compared carbon and oxygen isotope records from epibenthic foraminifera of sediment cores bathed in modern AAIW and Upper Circumpolar Deep Water (UCDW; 943-2066 m water depth) to monitor changes in water mass circulation spanning the past 350,000 years. We propose that pronounced freshwater input by melting sea ice into the glacial AAIW significantly hampered the downward expansion of southwest Pacific AAIW, consistent with climate model results for the Last Glacial Maximum. This process led to a pronounced upward displacement of the AAIW-UCDW interface during colder climate conditions and therefore to an expansion of the glacial carbon pool.

On glacial/interglacial timescales, Southern Ocean air-sea gas exchange is considered to be an important factor, driving the variability of atmospheric CO2 concentrations. To understand the role of oceanic variability in the global carbon cycle, it is necessary to reconstruct changes in deep- and intermediate-water circulation and chemistry of Southern Ocean water masses. In this context,we used a transect of sediment cores to reconstruct ventilation and circulation changes of the South Pacific. The data show the presence of very old and CO2 enriched waters between 2000 and 4300 m water depth. Because the deep-water rejuvenation is parallel to the trend in atmospheric CO2, we propose that the deep Pacific Ocean significantly contributet to the deglacial rise in atmospheric CO2. Furthermore, we were able to show that the intermediate-water off New Zealand was significantly shallower during glacial stages, furthermore expanding the volume of the glacial carbon pool.

Reconstructions on Antarctic ice cores revealed pronounced, millennial-scale variabilities in atmospheric CO2 over the past 800,000 years (e.g. Lüthi et al., 2008; Monnin et al., 2001; Petit et al., 1999; Raynaud et al., 2005; Siegenthaler et al., 2005). Despite these variabilities are known for several decades, the mechanisms, driving these patterns are still not fully resolved. As the ocean contains up to 60 times more carbon than the entire atmosphere, it is considered to be a major driver of the atmospheric CO2 levels (Broecker, 1982): Storing CO2 during glacials, releasing it during deglaciations. Because changes in the global thermohaline circulation are thought to operate on glacial/interglacial timescales, it has been suggested that during glacials, the deep ocean was separated from surface-waters and therefore from the atmosphere by enhanced stratification, resulting in the pronounced accumulation of CO2 and nutrients in the lower levels of the water column. These waters, isolated for millennia, then surfaced during interglacial periods and released their load of ancient CO2. This hypothesis is strongly supported by the record of atmospheric radiocarbon activities (Δ14C; Reimer et al., 2013) in which a telltale drop in Δ14C is shown during the interval of enhanced increase in atmospheric CO2. This drop cannot be explained by the atmospheric formation of 14C and is therefore indicative for the release of an old and hence 14C-depleted and CO2-enriched reservoir (e.g. the deep ocean) to the atmosphere. This release can therefore explain both records (atmospheric CO2 and Δ14C). Indeed, several records from the Atlantic and Pacific Oceans point to the existence of this carbon pool in glacial deep-waters below 2000 m. However, the spatial extent (vertical and lateral) and particularly the pathways of upwelled waters during the interglacial remain elusive and even contradictory. The aim of this thesis is to improve the knowledge of changes in South Pacific circulation and ventilation over different glacial to interglacial time-scales. The three manuscripts that form the backbone of this thesis are used to: 1) constrain the spatial extent and upwelling pathways of the glacial carbon pool in the South Pacific (0 – 30,000 years); 2) reconstruct boundary shifts between intermediate-waters and the underlying carbon pool in Circumpolar Deep Water over the last 350,000 years; 3) analyze changes in the South Pacific Gyre’s thermocline during the past 200,000 years. The focus of the first manuscript (Chapter 3) lies on the transition from the last glacial to the current interglacial. The Δ14C-reconstructions on a water mass transect of seven sediment cores from the New Zealand Margin and the East Pacific Rise identify a pool of radiocarbon depleted waters between ~2000 and ~4500 m in the glacial counterpart of Pacific Deep Water. The 14C-depletion of this body of water is up to five times higher than in the modern South Pacific and reaches extreme apparent ventilation ages of ~8000 years. Despite the first deep water rejuvenation begins as early as ~21,000 years B.P., the main signal of rejuvenation and outgassing parallels the rise in atmospheric CO2. The vertical extent of southwest Pacific Antarctic Intermediate Water (AAIW) over the last four glacial/interglacial cycles is analyzed in the second manuscript (Chapter 4). Stable isotope records (δ13C and δ18O) from epibenthic foraminifera of sediment cores bathed in AAIW and Upper Circumpolar Deep Water (UCDW) indicate a shoaling of AAIW during glacial periods. Further support for these findings arises from model reconstructions using the CCSM3-climate model. Throughout glacial maxima, pronounced input of freshwater by melting sea ice into the AAIW significantly increased its buoyancy and hampered its downward expansion. Hence, the upward displacement of the AAIW-UCDW boundary led to an expansion of the glacial carbon pool identified in Chapter 3. In the third manuscript (Chapter 5) the evolution of Southern Ocean Intermediate Waters (SOIWs) is analyzed, using the Mg/Ca paleothermometry on surface- and deep-dwelling species of planktic (Emiliani, 1991) foraminifera. The results suggest opposing glacial subsurface conditions during the LGM and MIS 6 with colder-than-Holocene conditions during the former and warmer-than-LGM conditions during the latter interval. Because of the importance of SOIWs for the ventilation of the South Pacific Gyre (SPG), the results of Chapter 5 reveal the relevance of Southern Ocean subsurface processes on the transfer of climatic signals from higher to lower latitudes via the SPG. Ultimately, this thesis contributes to the broader understanding of ventilation and circulation changes in the Pacific Sector of the Southern Ocean. The combination of various proxies reveals the highly dynamic processes that affect the Southern Ocean on glacial/interglacial timescales. The results do not only constrain the vertical extent of the glacial carbon pool for the first time, but they also facilitate its integration in the global context of glacial circulation. Furthermore, the reconstructions shown in this study might help to improve model simulations that are used to both, reconstruct and predict changes in the global climate., Untersuchungen an antarktischen Eiskernen enthüllten gravierende Schwankungen im atmosphärischen CO2-Gehalt während der letzten 800.000 Jahre (Lüthi et al., 2008; Monnin et al., 2001; Petit et al., 1999; Raynaud et al., 2005; Siegenthaler et al., 2005). Obwohl schon seit Jahrzehnten bekannt, sind die Mechanismen, die diese atmosphärischen Muster bedingen, bis heute nicht vollständig verstanden. Die Tatsache, dass der Ozean etwa 60 mal mehr Kohlenstoff enthält als die gesamte Atmosphäre, macht ihn zu einem wahrscheinlichen Taktgeber für Veränderungen im atmosphärischen CO2-Haushalt (Broecker, 1982) mit ozeanischer Aufnahme von CO2 während Glazialen und Abgabe während Deglazialen. Da Veränderungen in der globalen thermohalinen Zirkulation auf glazialen/interglazialen Zeitskalen ablaufen, wird angenommen, dass der tiefe Ozean während Glazialen durch verstärkte Stratifizierung von Oberflächenwassern abgeschnitten war. Diese Trennung resultierte in der gesteigerten Anreicherung von CO2 und Nährstoffen in der unteren Wassersäule. Diese Wassermassen, welche für Jahrtausende von der Oberfläche abgeschnitten waren, trieben während der Interglazialen auf und setzten das in ihnen gespeicherte alte CO2 frei. Gestützt wird diese Hypothese durch den Verlauf der atmosphärischen Radiokohlenstoffaktivitäten (Δ14C; Reimer et al., 2013). Dieser Verlauf zeigt einen eindeutigen Abfall des atmosphärischen Δ14C im selben Zeitintervall, in dem das atmosphärische CO2 am stärksten ansteigt. Da dieser Abfall nicht mit der atmosphärischen Produktion von 14C erklärt werden kann, deutet der Verlauf auf die Freisetzung eines alten und deshalb 14C-abgereicherten aber CO2-angereicherten Reservoirs (z.B. der tiefe Ozean) hin. Diese Freisetzung führte der Atmosphäre große Mengen an 14C-armen CO2 zu und kann somit beide Verläufe (CO2 und Δ14C) erklären. Die Existenz dieses ozeanischen Kohlenstoffreservoirs wird in der Tat durch mehrere Untersuchungen im tiefen Atlantischen und Pazifischen Ozean unterhalb von ~2000 m bestätigt. Dessen ungeachtet sind die räumliche Ausdehnung dieses Reservoirs (lateral und vertikal), aber auch die Routen des aufgetriebenen Wassers während der Interglaziale weiterhin unklar, seine Rekonstruktionen teils sogar widersprüchlich. Das Ziel dieser Dissertation ist es, das Wissen über Veränderungen der Ventilation und Zirkulation des Südpazifiks auf unterschiedlichen glazialen/interglazialen Zeitskalen zu verbessern. Die drei Manuskripte, die den Kern dieser Arbeit bilden, befassen sich mit: 1) der Eingrenzung der räumlichen Ausdehnung des glazialen Kohlenstoffreservoirs sowie seiner Routen während des Interglazials im Südpazifik während der letzten 30.000 Jahre; 2) der Rekonstruktion der Grenzschichten von Zwischenwasser und dem unterliegenden Kohlenstoffreservoir im Zirkumpolaren Tiefenwasser über die letzten 350.000 Jahre; 3) der Analyse von Veränderungen der Thermoklinen der Südpazifischen Gyre während der letzten 200.000 Jahre. Der Fokus des ersten Manuskriptes (Kapitel 3) liegt auf dem Übergang des letzten Glazials zum jetzigen Interglazial. Untersuchungen von Δ14C an einem Wassermassentransekt aus sieben Sedimentkernen vor Neuseeland und am Ostpazifischen Rücken belegen ein Reservoir von radiokohlenstoffarmen Wasser zwischen ~2000 und ~4500 m Wassertiefe im glazialen Gegenstück des Pazifischen Tiefenwassers. Mit Ventilationsaltern von ca. 8000 Jahren war die glaziale Abreicherung des 14C-Gehaltes in etwa fünfmal stärker als im heutigen Südpazifik. Obwohl die Erneuerung des Tiefenwassers bereits vor ~21.000 Jahren einsetzte, verläuft das Hauptsignal der Erneuerung parallel zum atmosphärischen CO2-Anstieg. Die vertikale Ausdehnung des Antarktischen Zwischenwassers (AAIW) wird im zweiten Manuskript (Kapitel 4) über den Zeitraum der letzten vier glazialen/interglazialen Zyklen rekonstruiert. Untersuchungen von stabilen Isotopen (δ13C und δ18O) an epibenthischen Foraminiferen aus Sedimentkernen des heutigen AAIWs und Oberen Zirkumpolarentiefenwassers (UCDW) zeigen eine Verflachung des AAIWs während glazialer Perioden. Bestätigt wird dieses Ergebnis zusätzlich durch Modellrekonstruktionen mit Hilfe des CCSM3-Klimamodells. Während glazialer Maxima erhöhte die verstärkte Zuführung von Süßwasser durch schmelzendes Meereis den Auftrieb von AAIW, was eine Abnahme der Tiefenausdehnung dieser Wassermasse zur Folge hatte. Als Resultat führte die Verflachung der AAIW-UCDW Grenzschicht zu einer Ausdehnung des glazialen Kohlenstoffreservoirs, wie es in Kapitel 3 identifiziert wurde. Die Evolution Südozeanischer Zwischenwasser (SOIW) wird im dritten Manuskript (Kapitel 5) mit Hilfe der Mg/Ca-Paläothermometrie untersucht. Diese Untersuchung, durchgeführt an oberflächen- und tieflebenden planktischen Foraminiferen, liefert gegenläufige Trends für das Letzte Glaziale Maximum (LGM) und das Marine Isotopen Stadium (MIS) 6. Die Bedingungen im tieferen Wasser während des LGMs waren kälter als das Holozän, während sie in MIS 6 deutlich wärmer als während des LGMs waren. Aufgrund der Bedeutung von SOIW für die Ventilation der Südpazifischen Gyre (SPG), zeigen die Resultate aus Kapitel 5 die Relevanz von Prozessen des tieferen Südozeanes auf den Transport von Klimasignalen aus den höheren Breiten in die niederen Breiten via der SPG. Letztendlich wird diese Dissertation zum besseren Verständnis der Ventilation und Zirkulation im pazifischen Sektor des Südozeans beitragen. Die Kombination von unterschiedlichen Methoden enthüllt die hochdynamischen Prozesse, die den Südozean auf glazialen/interglazialen Zeitskalen prägen. Diese Resultate grenzen nicht nur zum ersten Mal die vertikale Ausdehnung des glazialen Kohlenstoffreservoirs ein, sondern sie bringen dieses auch in den globalen Zusammenhang der glazialen Zirkulation. Weiterhin besitzen die hier gezeigten Rekonstruktionen das Potential, Computermodelle, welche für Rekonstruktionen und Vorhersagen des globalen Klimas genutzt werden, zu optimieren.

The Antarctic Intermediate Water (AAIW) is a key player in global-scale oceanic overturning processes and an important conduit for heat, fresh water, and carbon transport. The AAIW past variability is poorly understood mainly due to the lack of sedimentary archives at intermediate water depths. We present records of benthic stable isotopes from sediments retrieved with the seafloor drill rig MARUM-MeBo at 956m water depth off northern Chile (GeoB15016, 27 degrees 29.48S, 71 degrees 07.58W) that extend back to 970ka. The sediments at this site are presently deposited at the boundary between AAIW and Pacific Deep Water (PDW). For previous peak interglacials, our results reveal similar benthic C-13 values at site GeoB15016 and of a newly generated stack of benthic C-13 from various deep Pacific cores representing the average PDW. This suggests, unlike today, the absence of AAIW at the site and the presence of nearly pure PDW. In contrast, more positive C-13 values at site GeoB15016 compared to the stack imply a considerable AAIW contribution during cold phases of interglacials and especially during glacials. Besides, we used three short sediment cores to reconstruct benthic C-13 values from the AAIW core during the last glacial and found a C-13 signature similar to today's. Assuming that this was the case also for the past 970 kyr, we demonstrate that sea level changes and latitudinal migrations of the AAIW formation site can only account for about 50% of the full range of past C-13 increases at site GeoB15016 during cold periods. Other processes that could explain the remaining of the positive C-13 anomalies are increases in glacial AAIW production and/or deeper convection of the AAIW with respect to preceding interglacials.

Antarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean (SO) to the low-latitude surface ocean and therefore have an important control on low-latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate-producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pCO(2). This mechanism is known as the silicic acid leakage hypothesis (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage 4 (similar to 60-70ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development.

The "World Ocean Circulation experiment" (WOCE) program in the 1990s has produced the equivalent of 300 years of subsurface floats trajectories within the Antarctic Intermediate Water (AAIW) at 800dbar in the South Atlantic ocean, and several hydrological transects. In order to compute the western boundary circulation, the flow is vertically extrapolated by referencing geostrophic hydrological profiles to float speeds. Several transects normal to the continental slope and another one parallel to it off the coast give transports of the water masses with a good transversal and longitudinal resolutions. Floats at 800dbar sampled the Intermediate Western Boundary Current (IWBC) particularly well, and enable one to analyse it from 28°S at the bifurcation of the subtropical gyre return current in the South Atlantic to 2°S. Meridional and transversal structures of the IWBC are described along with the mesoscale activity in the vicinity of the western boundary. We observe two regions (28°S-21°S and 15°S-5°S) where the 50km width current at 800m follows a relatively regular continental slope. In-between, the bathymetry is perturbed and high values of eddy kinetic energy are found. Southward counter currents are detected and their origins and fates are determined thanks to the lagrangian measurements. Local perturbations of the flow in relation to the bathymetry are analysed using the floats, such as those related to capes and then entries/exits of the boundary current., Au cours du programme "World Ocean Circulation Experiment" (WOCE) des années 1990, 100 flotteurs acoustiques MARVOR ont produit environ 300 ans de trajectoires au sein de l'eau Antarctique Intermédiaire (AAIW) à 800dbar dans l'Atlantique Sud-Ouest et plusieurs radiales hydrologiques ont été réalisées, s'ajoutant à la quinzaine déjà existante. L'utilisation des données flotteurs pour référencer les vitesses géostrophiques permet d'extrapoler l'écoulement sur la verticale et apporte une description globale de la circulation dans la région du bord ouest. Les radiales perpendiculaires et parallèles au talus continental fournissent des transports des masses d'eau avec une bonne résolution transversale et longitudinale. Le courant de bord ouest intermédiaire (IWBC), particulièrement bien échantillonné par les flotteurs à 800dbar est analysé depuis la bifurcation du courant de retour du gyre subtropical sud Atlantique à 28°S jusqu'à 2°S. Les structures méridionale et transversale de l'IWBC sont décrites ainsi que l'activité de méso échelle associée au régime de bord ouest. Deux régions sont distinguées (28°S-21°S et 15°S-5°S) où le courant peu perturbé à 800m longe un talus continental relativement régulier sur une largeur de 50km. On trouve entre ces deux régions un relief agité et de fortes valeurs d'énergie cinétique turbulente. Des contre-courants vers le sud sont détectés et les mesures lagrangiennes permettent de déterminer leurs origine et devenir. La forte densité de flotteurs permet l'analyse de phénomènes locaux en relation avec la bathymétrie tels que les effets de caps et les entrées et sorties des flotteurs du courant de bord.

Over the last 5 million years, the global climate system has evolved toward a colder mean state, marked by large‐amplitude oscillations in continental ice volume. Equatorward expansion of polar waters and strengthening temperature gradients have been detected. However, the response of the mid latitudes and high latitudes of the Southern Hemisphere is not well documented, despite the potential importance for climate feedbacks including sea ice distribution and low‐high latitude heat transport. Here we reconstruct the Pliocene‐Pleistocene history of both sea surface and Antarctic Intermediate Water (AAIW) temperatures on orbital time scales from Deep Sea Drilling Project Site 593 in the Tasman Sea, southwest Pacific. We confirm overall Pliocene‐Pleistocene cooling trends in both the surface ocean and AAIW, although the patterns are complex. The Pliocene is warmer than modern, but our data suggest an equatorward displacement of the subtropical front relative to present and a poleward displacement of the subantarctic front of the Antarctic Circumpolar Current (ACC). Two main intervals of cooling, from ~3 Ma and ~1.5 Ma, are coeval with cooling and ice sheet expansion noted elsewhere and suggest that equatorward expansion of polar water masses also characterized the southwest Pacific through the Pliocene‐Pleistocene. However, the observed trends in sea surface temperature and AAIW temperature are not identical despite an underlying link to the ACC, and intervals of unusual surface ocean warmth (~2 Ma) and large‐amplitude variability in AAIW temperatures (from ~1 Ma) highlight complex interactions between equatorward displacements of fronts associated with the ACC and/or varying poleward heat transport from the subtropics., Key Points Reconstructed Tasman Sea surface and Antarctic Intermediate Water temperaturesLong‐term cooling trends from ~3.0 to 2.6 Ma and from 1.5 Ma to presentComplex subtropical front displacement and subantarctic cooling trends since Pliocene