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Earth’s climate system has experienced significant changes throughout its history. Tropical, glacial and interglacial conditions have persisted across the globe during the Tertiary, and the present interglacial period with strong seasonality and episodes of warming and cooling has existed for the last several thousand years. However, rapid warming during the last century has highlighted the importance of understanding past climate behaviour, in order to predict the likely effects of such warming on our future climate. This has led to the use of proxies, such as sediment cores, tree rings, ice and carbonates, which provide high resolution palaeoclimate archives. Studies using a variety of proxies from around the globe have developed a network of site specific climate information, offering insights of climate fluctuations on a millennial scale, and identifying changing oceanic conditions as a major influence on global climate change. Coral skeletons are particularly sensitive recorders of ambient seawater conditions as they record fluctuations of oxygen and carbon in their aragonitic skeletons, which are caused by perturbations of sea surface temperature (SST) and sea surface salinity (SSS). Therefore, tropical oceans themselves provide a crucial record of climate change. Many of the coupled atmospheric-oceanographic processes that drive the global climate system occur in the Indo-Pacific, making it a particularly important region for data collection. Porites corals from the Indo-Pacific have been studied in detail and have revealed high resolution SST and SSS records, yet comprehensive investigations using other coral species are minimal. This study expands upon the current understanding of coral proxies by investigating three coral species of different ages, and provides new insights into the palaeoclimate history of the Indo-Pacific. Goniastrea retiformis, Platygyra pini and Platygyra lamellina corals have been retrieved from Timor Leste in the Indo-Pacific, and analysed using stable and radiogenic isotopic techniques. Sclerochronology was used to determine that the 4.5 year old, modern G. retiformis coral was living from 2006-mid 2010. Uranium-thorium (U/Th) analysis provided a mid Holocene age of 4.5 ± 0.092 (2σ) ka for the fossil P. pini. Uranium-lead (U/Pb) techniques were used to obtain a 2.7 ± 0.34 (2σ) Ma age for the fossil P. lamellina. A rigorous diagenetic screening process of X-ray diffraction (XRD), petrographic analysis of thin sections, and scanning electron microscopy (SEM) has been carried out to ascertain the extent of preservation in each coral. X-ray images allow annual density bands to be located and unique micromilling paths were developed for the three samples. This enabled us to obtain the first δ¹⁸O and δ¹³C stable isotope results for a G. retiformis coral, and the first fossil coral results for the Platygyra genus. Cross spectral analysis has been used to verify the periodicity of seasonal fluctuations visible in the data, and confirmed that these coral species are suitable for use as climate proxies. G. retiformis is an abundant reef coral distributed throughout the tropical Pacific, and has a robust skeletal configuration making it suitable for use as a climate proxy. The 4.5 year long record has revealed that δ¹⁸O and δ¹³C values range from -4.62‰ ± 0.03‰ (2σ) to -6.11‰ ± 0.02‰ (2σ), and 1.57‰ to -2.53‰, with means of -5.35‰ and -1.02‰, respectively. These values are in agreement with the average δ¹⁸O range of -5.1 to -5.6‰ in modern corals throughout the Indo-Pacific, predominantly of the Porites genus. Statistical analysis of the isotopic data has revealed a quasi-biennial signal in G. retiformis, typical of the temporal interaction between El Niño Southern Oscillation (ENSO) and the South Asian monsoon. Results have also been compared to the Southern Oscillation Index (SOI) in order to assess potential relationships between coral isotopes and ENSO. This has shown an accurate record of El Niño and La Niña events for the first half of the ~ 4.5 year record, from 2006 to mid 2008. Climate processes such as local rainfall and the Indian Ocean Dipole (IOD) are also reflected in the record, to varying extents. The use of Platygyra corals as climate proxies is limited to three investigations using modern samples, despite the fact that this genus has a much broader latitudinal range than the extensively studied Porites genus. This thesis provides the first examination of fossil samples, and provides analysis of both pristine and altered specimens. The 4.5 ka P. pini coral has allowed investigation of the palaeoclimate record in a sample that has been exposed to diagenesis. XRD, petrographic and SEM analyses have revealed widespread secondary aragonite growth, dissolution and secondary calcite within the coral, which would have occurred due to marine and freshwater diagenesis. However, primary growth textures have been retained in some areas of the coral. δ¹⁸O and δ¹³C values range from -3.47‰ ± 0.03‰ (2σ) to -5.45‰ ± 0.02‰ (2σ), and 3.43‰ to -0.49‰, with means of -4.82‰ and 1.12‰, respectively. These values are significantly more positive that the mean δ¹⁸O of four Platygyra corals from the Pacific region, which range from -4.82‰ to -5.10‰. There are two reasonable explanations for this. Secondary aragonite, which is detected throughout the milled section of coral, causes positive shifts in coral δ¹⁸O, an effect which is likely to have altered the geochemical record of this particular coral. But the fact that SST in the southern Indo Pacific Warm Pool (IPWP) during the mid Holocene were 1.2° C cooler than present must also be acknowledged, as cooler temperatures also impart a shift toward higher δ¹⁸O, due to equilibrium fractionation processes. SST reconstruction using the temperature dependence equation from a modern Platygyra coral reveals an SST range of 18.5-27.6° C, and mean of 24.7° C. The present day mean annual SST of 28° C suggests that mid Holocene temperatures less than 26.8° C are unrealistically cool, highlighting the fact that isotopic fractionation during diagenesis has affected coral chemistry. However, the presence of interannual periodicity indicates that ENSO was operating, and the magnitude of isotopic fluctuation through the 10 year record is similar to that found in modern and other Holocene corals. We suggest that although absolute isotopic and SST values are unreliable, prohibiting the extraction of high resolution climate records, insights into the behaviour of broad scale, seasonal and interannual climate processes may still be obtained. A strong annual periodicity has been detected when analysing the stable isotopic values recorded in the 2.7 Ma P. lamellina coral. This indicates that seasonal SST fluctuations were the dominant influence on this coral. Ranges of -4.67‰ ± 0.03‰ (2σ) to -5.48‰ ± 0.02‰ (2σ) and 0.88‰ to -1.12‰ for δ¹⁸O and δ¹³C, respectively, are similar to modern Platygyra coral results, suggesting that this coral has been preserved in pristine condition. Palaeo-SSTs have been reconstructed using a modern Platygyra temperature dependence equation, providing a range of 24-27.7° C and a mean of 25.9° C. Foraminiferal data from sediment cores in the greater Indo-Pacific suggest that mean annual SSTs at this time were ~ 2-3° C cooler than present. The coral record I present supports this statement, providing new insights into our understanding of tropical palaeoclimates. This coral has been entrained within a turbidite deposit on the sea floor that has subsequently been uplifted during the emergence of Timor, with U/Pb dating allowing further constraints of the stratigraphic age of the deposit. Tectonic narrowing is postulated to have caused major changes to the Indonesian Throughflow (ITF) from 4-2 Ma, and been a driver of major global climate change beginning in the late Pliocene. This means that fossil Platygyra corals Timor Leste could provide unique time slices of information about this important time in global climate history. This study confirms that G. retiformis, P. pini and P. lamellina corals are excellent candidates for further, detailed investigations. They provide the opportunity to develop new coral proxies which are both abundant throughout the tropics and distributed over a wide latitudinal range. Their prevalence in both modern and fossil reefs means that once modern samples of each species have been calibrated against modern SST and SSS, these corals will provide reliable, quantitative palaeoclimate proxies, with potential for data capture throughout the Indo-Pacific and mid latitudes. Geochemical coral archives are a crucial tool in the study of climate processes, and we believe that these species are ideally suited to enhancing and refining our current understanding of earth’s climate system.

The Southern Ocean (SO) is a critical component in the global ocean conveyor. As the only conduit linking the Atlantic, Indian and Pacific Oceans, as well as an important region of upwelling and water mass formation, it is thought to have played a key role in modulating Earth’s past climate. Changes in the circulation of SO deep and bottom waters over the last 1.5 million years are investigated using stable carbon isotope $δ^{13}C$ measurements made on the tests of the benthic foraminfer Cibicidoides ($δ^{13}C_{b}$), and the rare earth element concentrations and Neodymium isotope ($ɛ_{Nd}$) values of marine sediments and their authigenic ferromanganese coatings. Being a proxy for past seawater nutrient contents, $δ^{13}C_{b}$ provides important insights into both past ocean circulation and the potential storage of remineralised organic carbon within the deep ocean, while simultaneously providing information on the past ventilation state of the deep ocean interior. As seawater $ɛ_{Nd}$ remains unaffected by biological fractionation or air-sea exchange processes, reconstructions of past deep and bottom water $ɛ_{Nd}$ provides a tool with which to study past changes in the circulation and mixing of these water masses. A suite of previously published late Holocene (0-6 ka) and Last Glacial Maximum (LGM; 18-24 ka) $δ^{13}C_{b}$ data are used alongside newly acquired $δ^{13}C_{b}$ data from the Amundsen Sea in the eastern Pacific sector of the SO to investigate past changes in the pattern of circum-Antarctic seawater carbon isotope composition. The $δ^{13}C$ signature of deep and bottom waters was much more heterogenous during the LGM than the late Holocene, with negative $δ^{13}C$ excursions occurring within the Atlantic and Indian sectors of the SO below c. 2-3 km water depth. Some of this negative $δ^{13}C$ signal was advected through the SO to the Pacific sector, but this appears to have been restricted by bathymetric barriers within the SO. New $δ^{13}C_{b}$ data spanning the last 800 ka from the Amundsen Sea are presented and suggest differing modes of bottom water formation in the Atlantic vs Pacific sectors of the SO during glacial periods of the last 800 ka. An authigenic $ɛ_{Nd}$ record measured on sediments from a core located in the deep Indian Ocean is used to investigate the palaeocirculation history of modified Circumpolar Deep Water (mCDW) within the Indian Ocean during the last 1.5 million years. Shifts towards more radiogenic $ɛ_{Nd}$ values during glacial periods are interpreted as reflecting a decreased entrainment of deep waters sourced in the North Atlantic (Northern Component Water, NCW) within CDW, which led to a reduced advection of an unradiogenic $ɛ_{Nd}$ NCW signal to the core site. $ɛ_{Nd}$ and REE measurements made on sediments from two cores located on the Pacific-Antarctic Ridge in the western Pacific sector of the SO (to the north of the Ross Sea Embayment) are used to reconstruct the bottom water palaeocirculation in this region across the last 540 ka. The proportion and $ɛ_{Nd}$ signature of Ross Sea Bottom Water (RSBW) bathing these core sites has fluctuated throughout the last 540 ka. These fluctuations suggest the rate and location of bottom water formation within the Ross Sea, and the supply of terrigenous material with radiogenic $ɛ_{Nd}$ values with which to isotopically `labelled' RSBW, may have changed in the past.

Speleothems (cave calcite) provide many different proxy indicators for palaeoclimatic changes during the Quaternary era. Generally, the occurrence of growing speleothems is a strong proxy for global palaeoclimate, both geographically and through time. A database of speleothem U-Th ages shows some evidence for an early transition from the penultimate glacial to the last interglacial at -140 Ka BP. The database suggests an age of 63 Ka BP for the maximum cold period within isotope stage 4. Isotope stage 3 contains three periods of increased number of growing speleothems, at 40,50 and 56 Ka BP mainly in low latitude caves. The start of growth, growth rate, oxygen isotopes, carbon isotopes and luminescence intensity in two speleothems from southern Ireland provide high-resolution records of rapid palaeoclimate changes in that region during the Late Glacial. Several of these palaeoclimate proxies appear to be linked. The Younger Dryas (YO) cold event is dated at between 12.5 and 11.4 Ka BP in both speleothem records. The dominant agent of palaeoclimatic variation during the YD is probably changes in North Atlantic ocean circulation. A Microsoft Excel spreadsheet is developed as a means of rapidly converting between 14C and calendar years and vice versa.

At the boundary between the Paleocene and Eocene epochs (ca. 56 Ma) a significant global warming event, termed the Paleocene-Eocene Thermal Maximum (PETM), occurred. Records of this event are characterised by a negative carbon isotope excursion (CIE) which has been associated with the release of thousands of petagrams of isotopically light carbon into the ocean-atmosphere system, initiating changes in the carbon cycle, the climate system, ocean chemistry and the marine and continental ecosystems. The amount of isotopically light carbon that was required to cause the event, its source and the rapidity of its release are, however, are still debated. This study uses δ13CTOC, δ13Cn-alkane, δ13CCARB and palynological data to evaluate the PETM CIE in terms of the magnitude of the CIE in both continental and marine settings, rapidity of release and drawdown of carbon, and mobilisation of different organic matter (OM) pools as a response to the climate change. The sections studied span a continental to marine transect in northern Spain. This represents the first organic geochemical study of these PETM sections, one of the first comparisons of CIE magnitude between continental and marine sections within the same sediment routing system, and one of the first comparisons of the same OM proxies within different depositional environments. The data suggest that different OM pools were mobilised in response to the PETM, with reworking of older material, soil residence times, and contemporaneous vegetation all contributing. CIE profile shapes predominantly suggest a rapid onset and recovery from the event. The magnitude of the CIE was also assessed. The current resolution of the data suggests that the differences between continental and marine CIE magnitudes could be minimal within a single sediment routing system, perhaps establishing a realistic CIE magnitude for the PETM, for use in future modelling scenarios.