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 }.

The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S / The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S

The Antarctic Intermediate Water (AAIW) plays an important role on controlling globally climatic change. Numbers of researchers attempted to discuss whether AAIW across the equator or notduring glacial- interglacial transitions. In this study, we analyzed Mg/Ca and £UREE/Ca on benthic foraminiferal species, Cibicidoides wuellerstorfi, from core MD052922C, which retrieved from western equatorial Pacific region, for revealing the long-term variations of Paleohydrology in intermediate water depths of tropical Pacific. Our evaluated results reveal that the Mg/Ca ratios ranging from 1.27 to 2.04 mmol/mol, and ranging between 2.2 and 3.2¢J when calculating into paleotemoeratures.Highest Mg/Ca can be observed in MIS 6. This finding implies that the present AAIW mean temperature is colder than glacials, and the present AAIW is well-ventilated than that in glacials.Higher £UREEs/Ca values are usually well correlated with old and poor ventilated water masses. In our record, higher £UREEs/Ca values are observed at MIS 5 impling that the intermediate depth water mass of the western tropical Pacific is poor ventilated when comparing to glacials.

A transect of cores ranging from 798 m to 1585 m water depth in the South Atlantic Ocean document the relative intermediate water mass nutrient geometry and stable isotopic variability of AAIW during the Younger Dryas cooling event. The data reveal concurrent delta^13 C and delta^18 O excursions of 0.59 ppt and 0.37 ppt within the core of Antarctic Intermediate Water (AAIW) centered at 11,381 calendar years before present based on radiometric age control. A portion of the delta^1 3C variability (0.22 ppt) can be explained by a shift in thermodynamic equilibrium concurrent with a drop in temperature of 1.8°C at the locus of AAIW formation. The remaining 0.37 ppt increase in delta^13 C most likely resulted from increased wind velocities, and a greater coupling between the ocean and the atmosphere at the locus of AAIW formation (increased efficiency of the thermodynamic process). Deepwater coral mounds are aggregates of corals, other organisms, their skeletal remains, and sediments that occur on the seafloor of the world’s oceans. In the Straits of Florida, these features have been referred to as lithoherms. We use digital, side-scan sonar data collected from the submarine NR-1 from an 10.9 km^2 area at ~650 m water depth to characterize quantitatively aspects of the morphology of 216. Their lengths, widths, heights, areas, orientations and concentration on the seafloor have been determined. Analysis indicates that the outlines of relatively small to medium sized lithoherms can be effectively described with a piriform function. This shape is less applicable to the largest lithoherms because they are aggregates of smaller lithoherms. Nearly all of the lithoherms studied have axes parallel to the northward flowing Florida Current, and the heads of 80 percent of these features face into the current. The shape and orientation of the lithoherms, and evidence of megaripples and scouring in the sonar data suggest that these features are formed by a unidirectional current. Following an extensive investigation of over 200 lithoherms via side-scan sonar imagery and direct observation, we have developed a qualitative model for the formation of the lithoherm type of deep-water coral mounds in the Straits of Florida. Lithoherm formation can be characterized by four main stages of development: nucleating, juvenile, mature singular, and fused. Fused lithoherms can form via transverse and/or longitudinal accretion, however, transverse accretion at the head of the mound is likely the most efficient mechanism. A comparison of lithoherm spatial relationship to local bathymetry agrees with previous observations of deep-water coral mound formations along the levied margins of density flow scour channels.

The objective of this dissertation is to apply radiogenic isotopes extracted from marine sediments to investigate aspects of global climate change over the past 25 kyr, especially ocean and atmospheric circulation, continental aridity, and hydrology. By focusing on the geochemical records from marine sediments and authigenic precipitates preserved in these sediments, I aim to better understand climate forcing and feedback mechanisms, which are critical to models of climate change. Firstly, I have investigated the dynamics of the Intertropical Convergence Zone (ITCZ) over the past 25 kyr in the eastern equatorial Pacific by fingerprinting dust provenance using radiogenic isotopes (Nd, Sr, Pb) and trace elements (Fe, Si, Ba) in the detrital fraction of marine sediments along a transect across the equator at 110ºW. Results from this study suggest no glacial-Holocene difference in the mean position of the ITCZ, but a more northerly, possibly stronger, deglacial ITCZ. Secondly, I have applied Nd isotope ratios from authigenic precipitates extracted from marine sediments and those from fish debris to trace past intermediate water circulation changes on glacial-interglacial and millennial timescales. The authigenic Nd isotope record from the Florida Straits suggests a reduced circulation of Antarctic Intermediate Water (AAIW) into the tropical North Atlantic during the Younger Dryas (YD) and Heinrich 1 (H1) events, associated with a significant reduction in AMOC. However, in the Southern Caribbean, apparent deviations in the Nd isotopic compositions between the acid-reductive leachate and the fish debris suggest that the leachate method is not reliable at this location and that it needs to be tested in more detail in various oceanic settings. In the Southern Caribbean, the fish debris Nd isotope results suggest a two-step recovery of the upper North Atlantic Deep Water during the last deglaciation. Comparing our new fish debris Nd isotope data to authigenic Nd isotope data for sediments from the Florida Straits and the Demarara Rise, we propose that glacial and deglacial AAIW does not penetrate beyond the lower depth limit of modern AAIW in the tropical North Atlantic. Both studies suggest a tight connection between Atlantic intermediate water circulation variability and high-latitude North Atlantic climate change.

The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S / The South Atlantic has a relevant role on the AMOC variability as it includes two main conduits of its upper-ocean return flow: the NBUC and the IWBC that carry, mainly, the SACW and the AAIW and are originated from the bifurcation of the SEC. One of the hypotheses of this work is that analyzing the bifurcation variability it is possible to get an index of the AMOC changes. Another hypothesis is that in a global warming scenario, changes in the hydrological cycle would drive modifications in the water masses that are part of the AMOC, and thus, contribute to its variability. Four global model results were used, with different forcing and spatial resolution. Results show that changes in the bifurcation are linked to modications in the currents both caused by variations in the wind stress curl. Good correlations were found between the SEC bifurcation at the surface and the AMOC. The NBUC seems to be the link between them. Shallowing of the SACW core is related to an increase of the salinity on neutral surfaces. The AAIW is occupying less space in the water column due to an increasing of the salinity in the neutral surfaces at 11°S, while the opposite happens at 27°S

The relatively understudied intermediate waters of the world have been implicated as an important part of the global ocean circulation. This thesis discusses the intermediate waters of the Pacific over space and time. Initially, by using geochemical tracers to look at the present distribution, sources and mixing of the water masses. Secondly, by using oxygen and carbon isotopes from sediment cores to study changes in Antarctic Intermediate Waters (AAIW) over the late Quaternary in the north Tasman Sea. The sediment cores also provide sedimentological data on the hemipelagic sedimentation in the Capricorn Channel in the southern Great Barrier Reef as well information on changes in the East Australian surface current (EAC) over the last glacial-interglacial transition. [A more extended Abstract can be found in the files]