Originally issued as the author's thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2006. / "February 2006". "Doctoral dissertation." "Department of origin: Geology and Geophysics." "Joint Program in Oceanography/Applied Ocean Science and Engineering"--Cover. Bibliography: p. 117-126.

Bibliography: pages 69-77. / An analysis of the Holocene ostracod fauna of the Agulhas Bank has been carried out on seventy-three surficial sediment samples. Sixty-six species of Ostracoda have been recorded, of which fifty-nine species are accounted for in forty genera and the remaining seven species are of indeterminate classification. The species are described and their distribution and ecology is given. An. analysis of the sedimentology, as well as an oceanographic analysis of the bottom water on the Agulhas Bank, has provided environmental parameters for each sediment sample location, enabling relationships to be described between ostracod faunas and environmental conditions. Quantitative factor analysis has been carried out on the twenty-four most abundant species, generating seven factor associations relating ostracod assemblages to a set of environmental parameters. The independent variables analyzed were the temperature, salinity and dissolved-oxygen content of the bottom water, as well as the sand content of the sediment. Contour maps of these variables have been drawn up using SADCO data for the oceanographic variables, and the sediment samples to calculate the sand content. The overall oceanography of the Agulhas Bank has been analyzed by relating the environmental parameters generated at each location to the water masses present on the shelf, and to the oceanic currents affecting them. Finally, the seven factor associations generated have been related directly to the substrate types, the water masses, and the currents present on the Agulhas Bank.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998. / Includes bibliographical references (leaves 93-97). / by Mary Ann Schlegal. / M.S.

This thesis uses the paleoceancirculation proxy ²³¹Pa/²³⁰Th, coupled with water mass tracers δ¹³C and εNd, to reconstruct circulation histories for southern source waters masses in the South Atlantic, in addition to North Atlantic Deep Water (NADW) far from its source, for the last ~25 kyrs. Downcore ²³¹Pa/²³⁰Th records from a suite of cores along a depth transect in the Argentine Basin show distinct differences with depth, suggesting that ²³¹Pa/²³⁰Th ratios in sediments are reflective of conditions in only the bottom most waters. This indicates the importance of consideration of changes in water mass distribution when interpreting ²³¹Pa/²³⁰Th records. Opal and particle flux data from these cores show little correlation with ²³¹Pa/²³⁰Th values meaning that changes in ²³¹Pa/²³⁰Th cannot be explained by a local composition or particle flux effect and are instead likely to be reflecting changes in circulation. A core bathed by Antarctic Intermediate Water (AAIW) throughout the last 25 kyrs (GeoB 2107, 1045 m), has relatively high ²³¹Pa/²³⁰Th values (0.075) during the Holocene and distincly lower values (0.055) at the LGM suggesting faster AAIW transport during the last glacial. At greater depths, ²³¹Pa/²³⁰Th, δ¹³C and εNd data in core GeoB 2109 (2504 m) indicate a change in both circulation and water mass distribution on glacial-interglacial timescales, with moderate flow of Antarctic Bottom Water (AABW) at the LGM being replaced by more vigorous flow of NADW during the Holocene. On millenial timescales, ²³¹Pa/²³⁰Th values in deep cores GeoB 2109 and GeoB 2112 (4010 m) indicate enhanced production of AABW during northern hemisphere stadials, when variations in ²³¹Pa/²³⁰Th records are of opposite sign between hemispheres, supporting a possible bipolar seesaw relationship in deep water formation between hemispheres. These data indicate that the ²³¹Pa/²³⁰Th proxy can be used to reconstruct past flow rates of multiple water masses in the Argentine Basin and provide evidence that southern source water masses play a dynamic counterpart to NADW formation on abrupt as well as glacial-interglacial timescales.

The last two glacial terminations represent the most recent, and best documented, periods of Earth warming in the geological record. During these terminations atmospheric CO\textsubscript{2 }rose by approximately 100 ppm and global mean temperatures increased by 4-6\textsuperscript{o}C. Whilst the driver for these deglaciations ultimately derives from changes in the insolation forcing at the edge of the atmosphere, feedbacks within the Earth\textquoteright s climate system act to amplify these small external forcings tipping the Earth from a cold glacial climate state to a warm interglacial climate state. A key question in Quaternary climate science is understanding which feedbacks are important in regulating global climate on glacial-interglacial timescales. On this topic, the Southern Ocean has long been considered to be an important player in regulating atmospheric CO\textsubscript{2 } on glacial-interglacial timescales. This thesis investigates some of the hypothesised drivers of changes in atmospheric CO\textsubscript{2 } on glacial-interglacial timescales by generating high-resolution multi-proxy records from the Southern Ocean spanning the last two glacial terminations. In particular, I focus on changes in the structure, circulation and biological productivity within the sub-Antarctic zone. A change in the deep ocean density structure has been hypothesised to have resulted in the release of CO\textsubscript{2 } from the deep ocean. Centennial records from the sub-Antarctic are used to reconstruct deep and intermediate water density for the first time. I demonstrate that timing of the major breakdown in the density gradient of the ocean significantly lagged the breakdown in the chemical gradient, suggesting that changes in the deep ocean density structure were not the major driver of the deglacial rise in atmospheric CO\textsubscript{2 }. Changes in the density structure of the Southern Ocean likely had significant implications for global circulation. In particular, the flow of low salinity water through the Drake Passage is thought to be important in setting the strength and geometry of Atlantic Overturning Circulation. Drake Passage through-flow speed was reconstructed from two sites in the central and northern margins of the Antarctic Circumpolar Current downstream of Drake Passage. These records suggest a very different structure of Antarctic Circumpolar flow through Drake Passage during glacial periods, and evidence significant changes in ocean temperature as a result of pronounced reductions in Drake Passage through-flow. The strength of the biological pump has long been identified as an important player in regulating atmospheric CO\textsubscript{2 }. In particular, a strong glacial increase in sub-Antarctic productivity has been observed at open ocean sites in the South Atlantic and Indian Ocean. However, the glacial-interglacial changes in productivity in sub-Antarctic shelf settings are less well-documented. The new high-resolution records presented here from the sub-Antarctic southwest Atlantic suggest a significant change in the CaCO\textsubscript{3}:C\textsubscript{org} ratio which likely has implications for the surface ocean\textquoteright s ability to uptake CO\textsubscript{2 }.

Marine sediments are a storehouse of the geochemical, biological, and physical changes in the ocean over thousands to millions of years. Intensive study of the Atlantic Ocean has well constrained the role of this basin in global climate change, but the vast Pacific Ocean, deeper and more corrosive to carbonate, has remained more elusive. This thesis leverages a new suite of sediment cores collected on the Juan de Fuca Ridge in the East Subarctic Pacific Ocean (~45˚N, 135˚W) to better understand how the paleoceanographic history of this region has evolved over the past 500kyr. In Chapter 1, I developed age models for multiple cores using benthic δ18O and physical properties of sediment as stratigraphic markers. Despite the proximity of the cores (within 50km2), the sedimentation rates varied by an order of magnitude, likely reflecting remobilization of sediment caused by the high relief of the mid-ocean ridge bathymetry. In Chapter 2, I analyzed uranium series disequilibria in the sediment in order to investigate the processes generating the highly variable sedimentation rates. This chapter presents evidence that the particle flux settling through the water column (based on excess 230Th) is relatively constant at six different sites, and the variability in sedimentation rates is largely driven by lateral sediment remobilization along the rough bathymetry of the ridge. Chapter 3, entitled “Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean”, presented high-resolution x-ray fluorescence records of metal diagenesis in response to changing oxygen conditions in the sediment. This study is the first to show strong evidence for low sedimentary oxygen conditions during interglacial periods in the North Pacific, which we suggest may be linked to hydrothermal sulfide deposition. In Chapter 4, I returned to uranium series disequilibria by utilizing 231Pa/230Th records from the Juan de Fuca Ridge to reconstruct productivity in the East Subarctic Pacific Ocean over the last 200kyr. Productivity across much of the Subarctic Pacific is low during glacial periods and high during interglacial periods, which is usually associated with changes in stratification. I investigated several different mechanisms for increasing stratification during glacial periods, and conclude that a combination of surface freshening, weak winds, and reduced subsurface nutrient concentrations likely created the stratification that led to low glacial productivity. Finally, in Chapter 5, “Dust deposition in the East Subarctic Pacific on glacial-interglacial timescales”, I reconstructed the patterns of dust fluxes in the East Subarctic Pacific Ocean over the last 500kyr to assess the climatic effects on the spatial distribution of dust in the North Pacific Ocean. I predict that migration of the westerlies would have caused a shift in dust provenance away from Asian dust and towards higher North American contributions during glacial periods. Although lithogenic endmembers are currently poorly constrained in this region, I present some evidence for variable provenance over time that may be consistent with the influence of the westerlies on dust fluxes in the East Subarctic.

Uranium and thorium isotopes are powerful and sensitive tracers of a wide range of oceanographic and environmental processes. This thesis makes use of these isotopes in deep sea sediments to reconstruct dust fluxes and deep ocean respired carbon storage over the last 350 kyr in the central equatorial Pacific. The paleoenvironmental information obtained through the application of these isotopes as proxies reveals important information about the Earth’s ocean and atmosphere, and their connectivity on millennial and glacial-interglacial timescales. In Chapter 1 of this thesis I introduce the proxies and principals employed in our paleoenvironmental reconstructions. Subsequently, the first section of this thesis explores the use of 230Thxs,0-derived 232Th fluxes as a proxy for aeolian dust deposition at three sites beneath the shifting Pacific Intertropical Convergence Zone (ITCZ). The new records presented here improve upon existing records of tropical Pacific dust fluxes by increasing the temporal resolution ~5 fold and adding almost an order of magnitude more data. Specifically, we reconstruct dust fluxes in two cores from 0-150 ka and from one core from 0-350 ka In addition to substantially improving constraints on tropical dust fluxes this work also utilizes the spatial transect of cores to infer past positions of the ITCZ on glacial-interglacial and millennial timescales. This proxy approach to reconstruct ITCZ position has only been applied and published once previously, in a relatively low-resolution study. Chapter 2, entitled “Large deglacial shifts of the Pacific Intertropical Convergence Zone,” presents evidence that the Pacific ITCZ experienced large shifts in latitudinal position, on millennial timescales during the penultimate deglaciation. The data resolve abrupt shifts in atmospheric circulation associated with deglaciation, in this case Termination II, at the boundary between the full glacial marine isotope stage (MIS) 6 and the peak interglacial MIS 5. These shifts are significant in that they appear to have occurred at the same time as changes in the North Atlantic driven by Heinrich Stadial 11 and may have played an important role in pushing the climate system over the threshold for deglaciation. Indeed, this study is the first to show evidence of a millennial-scale ITCZ response at the time of the Heinrich Event 11 catastrophic iceberg discharge event. Additionally, the data point to the existence of a previously unidentified millennial peak in northern hemisphere dust abundance during the penultimate deglaciation. In Chapter 3, “Climate-related response of dust flux to the central equatorial Pacific over the past 150 kyr,” records of dust flux are used to provide strong evidence for an association between high latitude stadial events and tropical dust fluxes during the last 150 kyr. These high-resolution observations permit the drawing of conclusions about the meridional location of the Pacific ITCZ during six Greenland stadials. As with the shift of the ITCZ during Heinrich Stadial 11, these events were associated with perturbations of the interhemispheric thermal gradient and coincident movement of the ITCZ presents an important constraint on the sensitivity of the tropical atmosphere to high latitude perturbations. The conclusions stemming from the interpretation of geochemical and paleoceanographic data presented in Chapters 2 and 3 are of broad relevance to a variety of geoscience disciplines that seek an understanding of the climate system. For example, these results confirm predictions made by modeling studies about the response of the ITCZ to high latitude climate forcing and provide an important new set of boundary conditions for modeling studies aimed at reconstructing changes in insolation forcing and tropical hydroclimate. The results show that paleo-reconstructions can constrain the magnitude of even abrupt ITCZ movement, demonstrating the potential to relate ITCZ changes to the magnitude of thermal forcing and to investigate thermal and hydrological components of other climate change events, past and future. Additionally, these results help improve understanding of the relationship between atmospheric dust abundance and climate, with implications for planetary albedo and micronutrient fertilization of the oceans. The second portion of this thesis focuses on using authigenic uranium (aU) to reconstruct deep water chemistry with implications for paleocirculation. Chapter 4, “Repeated storage of respired carbon in the equatorial Pacific Ocean over the last three glacial cycles,” presents evidence that the Pacific was a significant reservoir for respired carbon during glacial periods over at least the last 350 kyr. This reconstruction is based on the precipitation of the redox sensitive metal uranium as a proxy for deep water oxygen concentrations. Because any change in oceanic storage of respiratory carbon must be accompanied by corresponding changes in dissolved oxygen concentrations, data reflecting bottom water oxygenation are of value in addressing questions of glacial carbon sequestration. The record reveals periods of deep ocean aU deposition during each of the last three glacial maxima. Export productivity data indicate these intervals are not associated with local productivity increases, indicating episodic precipitation of aU occurs in response to basin-wide decreases in deep water oxygen concentrations. Not only does the aU record show the history of dissolved oxygen concentrations in the central equatorial Pacific, it also provides an opportunity for the reconciliation of records previously interpreted as incompatible with one another and with the storage of respired carbon. Synthesis of existing data suggests the existence of a ’floating’ pool of respired carbon between 2 and 3.5 km depth in the equatorial Pacific. This hypothesis permits the interpretation of existing proxy data reflecting abyssal LGM circulation and carbon storage without invoking a glacial watermass geometry significantly different from present. The new data and conclusions presented in Chapter 4 represent a significant advance in our understanding of where carbon was stored in the ocean during successive glacial periods. The perspective provided by the new aU time series is spatiotemporally unique and constitutes compelling evidence that hypotheses of marine carbon storage developed to explain the last glacial period are equally applicable to previous Pleistocene glacial periods. The three studies presented in this thesis provide strong support for the utility of U and Th series isotopes in paleoenvironmental reconstructions. Not only does this work demonstrate the range of paleoclimatic proxy data that can be obtained from isotopes of U and Th, it also illustrates the value of interpretations derived from their analysis. The records presented here represent a substantial contribution to our knowledge of marine hydroclimate and ocean circulation over the last 350 kyr and motivate additional high resolution paleoclimate work using isotopes of U and Th.

The fossilized remains of the calcite shells of foraminifera comprise one of the most continuous and reliable records of the geologic evolution of climate and ocean chemistry. The trace elemental composition of foraminiferal shells has been shown to systematically respond to seawater properties, providing a way to reconstruct oceanic conditions throughout the last 170 million years. In particular, the boron/calcium ratio of foraminiferal calcite (B/Ca) is an emerging proxy for the seawater carbonate system, which plays a major role in regulating atmospheric CO2 and thus Earth’s climate. In planktic foraminifera, previous culture studies have shown that shell B/Ca increases with seawater pH, which is hypothesized to result from increased incorporation of borate ion (B(OH)4 -) at high pH; increasing pH increases the [B(OH)4 -] of seawater. However, further experiments showed that B/Ca responds to both pH and seawater dissolved inorganic carbon concentration (DIC), leading to the hypothesis that B/Ca is driven by the [B(OH)4 -/DIC] ratio of seawater. Because pH (and thus B(OH)4 -) can be determined via the δ11B composition of foraminiferal calcite, B/Ca therefore may provide an opportunity to determine seawater DIC in the geologic past. The magnesium/calcium ratio (Mg/Ca) of foraminifer shells is a well-established proxy for seawater temperatures, where foraminiferal Mg/Ca increases at greater temperatures. However, foraminifera shell chemistry such as B/Ca and Mg/Ca ratios also depend on the major ion chemistry of seawater. For example, the seawater Mg/Ca ratio (Mg/Casw), which has increased significantly over the last 60 million years, is known to affect the sensitivity of the Mg/Ca proxy to temperature. In addition, the seawater boron concentration ([B]sw) has also increased across the Cenozoic. The dependence of B/Ca proxy relationships on Mg/Casw and [B]sw composition remains unknown. During the Paleogene era (65-34 Ma), Earth’s climate was characterized by a number of rapid warming events termed ”hyperthermals”. Evidence from the sedimentary record suggests that hyperthermals were catalyzed by rapid carbon release and caused widespread ocean acidification and deep-sea deoxygenation. These hyperthermal events present the best geologic analog conditions to anthropogenic climate change, and their study can therefore help to illuminate how the Earth system responds to rapid carbon release and warming. Planktic foraminiferal B/Ca records from the largest hyperthermal event, the Paleocene-Eocene Thermal Maximum (PETM), show a large decrease, which agrees with the theory that ocean acidification should cause B/Ca to decline. However, the decrease is larger than can be reconciled from existing proxy calibrations conducted in modern seawater, begging the question of whether the low Mg/Casw of the Paleogene Ocean affected the sensitivity of B/Ca to the seawater carbonate system. Because there are also a number of outstanding uncertainties regarding the controls on B/Ca- including seawater [Ca] and shell growth rate, light intensity, and phosphate concentration- it is also possible that these factors contributed to the PETM B/Ca excursion. The influence of these additional parameters on B/Ca, as well as the influence of Mg/Casw, needs to be tested in controlled culture experiments. To address these outstanding questions in proxy development, I conducted a series of culture experiments in three living planktic foraminifer species- Orbulina universa, Trilobatus sacculifer, and Globigerinoides ruber (pink). In order to refine our understanding of proxy controls on foraminiferal B/Ca, I investigated how foraminiferal B/Ca is affected by variable light intensity, growth rate (indirectly via seawater [Ca] manipulation), and seawater [B]. Subsequently I tested the influence of low seawater Mg/Ca, analogous to that of the Paleocene ocean, on B/Ca-carbonate chemistry relationships. In Chapters 2 and 3, I detail how my results support the notion that planktic foraminiferal B/Ca in these three symbiont-bearing species is driven by the B(OH)4 -/DIC ratio of seawater and is not compromised by growth rate effects. Furthermore, the sensitivity of B/Ca to B(OH)4 -/DIC is increased under low Mg/Casw in both O. universa and T. sacculifer. In Chapters 2 and 3, I hypothesize that this increased sensitivity is due to decreased cellular pH regulation under low Mg/Casw, leading to a greater sensitivity of the foraminiferal microenvironment’s carbon system to external forcing. I define new culture calibrations that can be applied to records from Paleocene seawater in Chapter 3, and use these calibrations to reconstruct surface ocean DIC and the overall size of the carbon system perturbation across the PETM in Chapter 4. Finally, in Chapter 5, I show how foraminiferal Mg/Ca responds to seawater Mg/Ca and the carbon system from these same experiments, with implications for accounting for carbon system influences on Mg/Ca from early Cenozoic proxy records.

Approximately 6.6 million years ago in the Eastern Equatorial Pacific a large increase in biogenic mass accumulation rates (MAR’s) occurred. This increased level of biogenic mass accumulation persisted until about 4.4 Ma at which time levels returned to those similar to before the transition at 6.6 Ma. The exact nature of the change that facilitated this transition in biogenic MAR’s, however, was not understood. Here we present the results of a study which characterizes the diatom assemblage spanning the depositional transition at 6.6 Ma from sediments taken from ODP Hole 850B. A close inspection of lithology reveals a clear change in lithology from a diatom nannofossil ooze to a nannofossil diatom ooze at 6.6 Ma. This transition is immediately followed by the occurrence of laminated diatom ooze (LDO). Diatom absolute abundance data reveals three levels of productivity associated with pre-transitional, post-transitional, and LDO formational sediments. An increase in the absolute abundance of Thalassionema nitzschioides was the major contributor to the formation of post-transitional, and LDO sediments. The known ecological preferences of this species indicate an overall increase in nutrient availability followed by sporadic changes in nutrient availability. These changes in nutrient availability could be associated with the establishment, or increase in strength, of an upwelling cell and/or the increase in nutrients made available via upwelling due to a shoaling of the thermocline.

Investigaciones paleocenográficas y simulaciones numéricas están mostrando que el Hemisferio Sur desempeña un papel importante, tanto en los cambios de la circulación global como en la definición del estado climático terrestre. Una región clave para la circulación global se encuentra en la región alrededor de Sudáfrica. Aquí, aguas superficiales y de termoclina del Océano Índico son transferidas al Atlántico a través de anillos y de filamentos de la Corriente de Agulhas. En profundidad, la transferencia de agua se produce en la dirección opuesta, con Agua Profunda del Atlántico Norte (North Atlantic Deep Water, NADW), abandonando la cuenca atlántica y entrando en la índica. La entrada de aguas cálidas y saladas de Agulhas al Atlántico Sur se cree que puede influir en el Atlántico Norte a través de perturbaciones en los balances de flotabilidad, siendo crucial para el modo y estabilidad de la Circulación Meridional Atlántica (Meridional Overturning Circulation, MOC). En esta Tesis presento registros de 345.000 años de isótopos estables de foraminíferos planctónicos y bentónicos, Cd/Ca de bentónicos, Mg/Ca de planctónicos, granoclasificación de arcillas (sortable silt) y censos de foraminíferos de un registro combinado de dos testigos sedimentarios recuperados a profundidades medias (~ 2500 m) aguas adentro de Sudáfrica. Los registros permiten discernir la circulación superficial y profunda pasada y evaluar la conexión entre el flujo superficial de aguas índicas hacia el Atlántico y el flujo de aguas profundas atlánticas hacia el Índico. La circulación profunda del Atlántico se cree que era diferente durante el Último Máximo Glacial (Last Glacial Maximum, LGM), con NADW siendo sustituida por un agua intermedia (Glacial North Atlantic Intermediate Water, GNAIW) y con Aguas de Componente Sur (Southern Component Water, SCW) expandiéndose más hacia el norte e invadiendo la cuenca a menores profundidades. Hemos encontrado evidencias de que la circulación profunda durante las partes intermedias de los períodos glaciales MIS 6 y 8 (Estadio Isotópico Marino: Marine Isotope Stages, MIS) era similar a la del LGM. Asimismo, encontramos indicios de que la SCW es una mezcla de un agua superior bien ventilada (Upper Southern Component Water, USCW) e inferior mal ventilada (Lower Southern Component Water, LSCW). Durante las fases iniciales de ambos períodos glaciales, la circulación profunda probablemente no difería substancialmente de la actual. Esto, juntamente con valores de los registros de superficie similares a los presentes sugiere un modo de conexión entre la transferencia de aguas de Agulhas y la MOC similar a la actual. Desde estos valores tipo¬interglacial en los períodos glaciales tempranos, los registros superficiales muestran tendencias crecientes, alcanzando niveles máximos mucho antes de las Terminaciones y colapsando a niveles interglaciales al entrar en los períodos cálidos MIS 5e, 7e y 9c. Estudios previos y modelos numéricos muestran una presencia considerable de aguas de Agulhas en el Atlántico durante las Terminaciones y sugieren que esta entrada de aguas cálidas y saladas podría haber sido fundamental para el restablecimiento de las condiciones interglaciales. Nuestros registros demuestran este postulado ya que la máxima presencia de aguas de Agulhas ocurre en paralelo con la recuperación de niveles interglaciales en los registros profundos. Es posible que el colapso de aguas de Agulhas al comienzo de los interglaciales indique el restablecimiento de un modo moderno de conexión entre el leakage de aguas de Agulhas a través de anillos y la liberación de aguas cálidas y saladas acumuladas en el Índico. Curiosamente, durante MIS 2 y las partes intermedias de MIS 6 y 8, los indicadores de circulación superficial registran una fuerte presencia de aguas de Agulhas a la vez que la SCW dominaba la hidrografía profunda. Esto indicaría que en estos momentos, o bien la fuerte presencia de aguas de Agulhas no generaría una anomalía de flotabilidad en el Atlántico Sur de modo eficiente o bien que otros factores, por ejemplo, la propia climatología del Atlántico Norte, podrían prevalecer sobre la anomalía de salinidad impuesta por las aguas de Agulhas. / Paleocenographic research and numerical model simulations increasingly are emphasizing that the Southern Hemisphere plays an important role, both in global circulation changes and in defining the state of Earth's climate. A key region for the global circulation system is found in the neighbourhood of southern Africa. Here, surface and thermocline Indian Ocean Waters are transferred to the Atlantic by rings and filaments of the Agulhas Current. At depth, the water is transferred in the opposite direction, with North Atlantic Deep Water (NADW) leaving the Atlantic and entering the Indian. The input of warm, salty Agulhas Waters to the South Atlantic is thought to influence the far distant North Atlantic by way of buoyancy perturbation, being crucial for the mode and stability of the Meridional Overturning Circulation (MOC). In this Thesis I present 345,000-year-long records of planktonic and benthic foraminifera stable isotopes, benthic Cd/Ca, planktonic Mg/Ca, sortable silt and foraminiferal census counts from a spliced record of two mid-depth (~2500 m) sediment cores from off South Africa. The records allow deciphering the past surface and deep circulation in the area and assessing the linkage between both, the surface flux of Indian waters towards the Atlantic and the deep flux of Atlantic waters towards the Indian. The Atlantic deep circulation is thought to have been different during the Last Glacial Maximum (LGM), with NADW being replaced by Glacial North Atlantic Intermediate Water (GNAIW) and Southern Component Waters (SCW) spreading further north and bathing the basin to shallower depths. We find evidence that the deep circulation during middle phases of the glacials Marine Isotope Stage (MIS) 6 and 8 was similar to that of the LGM. At the same time, we find indications that SCW was a mixture of an Upper well-ventilated component (USCW) and a Lower poorly-ventilated one (LSCW). During early glacial phases of both, MIS 6 and 8, the deep Atlantic circulation was plausibly not much different from today's. This, in conjunction with values of the surface records similar to present ones, indicates presence of Agulhas Water at our site and suggests a persistence of the modern-type linkage between Agulhas Leakage and MOC. From these interglacial-like values at the initiation of the glacial periods, the surface records display increasing trends reaching maximum values well before glacial Terminations and collapsing to interglacial levels as the warm periods MIS 5e, 7e and 9c are reached. Previous studies and numerical models show a prominent presence of Agulhas Waters in the South Atlantic during glacial Terminations and suggest that this input could have been pivotal for the reestablishment of interglacial climate conditions. Our records demostrate this contention as maximum presence of Agulhas Waters occurs in parallel with the recovery to interglacial levels of the deep circulation records. It is possible that the collapse of Agulhas Waters at the beginning of interglacial periods indicates the reestablishment of a modern-type mode of Agulhas Water Leakage through rings and the release of warm saline waters stored in the Indian Ocean. Intriguingly, during MIS 2 and mid-glacial MIS 6 and 8, the surface circulation proxies record a strong presence of Agulhas Waters while SCW was dominating the deep hydrography. This indicates that at these times either the presence of Agulhas Waters was unable to efficiently generate a buoyancy anomaly in the South Atlantic or that other factors were able to overcome the salt anomaly imposed by the Agulhas Waters, for instance North Atlantic climatology.