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Discoasters are the remains of an enigmatic group of nannoplankton, whose last representatives disappeared globally at 1.89 Ma. The one million years prior to their extinction has been analysed in high time resolution and a global database has been developed from ten sites in the Atlantic, Indian and Pacific Oceans. It has been shown that the variability in Discoaster abundance at low latitudes cannot be attributed solely to changes in sea-surface temperature; discoasters were demonstrated to be suppressed at upwelling sites. In combination with global satellite images of phytoplankton blooms (Lewis, 1989) and maps of modern sea-surface temperatures (CLIMAP, 1981), Discoaster abundance variations were interpreted in terms of the interplay between productivity pressure and sea-surface temperature. Discoasters thrived in the Indian Ocean at Site 709 (4°S) in warm waters with little productivity pressure, whereas Discoaster abundance was suppressed at Sites 677 (l°N) and 662 (l°S) in the Pacific and Atlantic, which although located in warm waters, were affected by upwelling. In addition to the reduced Discoaster abundances associated with upwelling, sea-surface temperature gradients are a significant factor when comparing high and low latitude sites. A marked drop in Discoaster abundance is apparent between 41°N and 56°N (i.e., between Sites 607 and 552). Relative abundances of species in the Discoaster assemblage revealed clearly their contrasting environmental preferences; D. brouweri was produced under a wide range of conditions, favouring warm, low productivity waters; D. asymmetricus and D. tamalis were produced in higher abundances relative to D. brouweri during cooler, low productivity episodes; D. pentaradiatus showed highest absolute abundance in warm, low productivity waters, although increasing in relative abundance at higher latitudes; D. surculus increased in relative abundance in high productivity regions and in cooler waters. At all sites, though most markedly at higher latitudes, Discoaster abundances declined after 2.4 Ma, when glaciation began in the North Atlantic.

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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.

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The Ediacaran in Uruguay preserves a unique record of deposits generated during the assembly of the palaeocontinent Gondwana and concurrent with major changes in the atmosphere and oceans, and the rise of animal life. Recent studies have suggested that the deep oceans remained anoxic and highly ferruginous throughout the Ediacaran and possibly into the Cambrian. Unfortunately, acceptance of this idea has been hindered by the virtual absence of iron formations (IF). Detailed studies of Ediacaran IF in Uruguay confirm that ferruginous conditions dominated the pre-Gaskiers (~580 Ma), and interestingly, they also extended well into the upper Ediacaran before complete ocean ventilation occurred. Significantly, a simple twolayer stratified system that argues for an oxygenated surface layer overlying a suboxic zone is proposed. The association of negative 13C excursions in Neoproterozoic carbonates and large-scale glaciations has become a tempting explanation for the short-term perturbation of the global carbon cycle. Not surprisingly, negative 13C shifts in Ediacaran-aged carbonates from Uruguay have been interpreted as recording post-Gaskiers glacial events. New highresolution 13C-chemostratigraphy of carbonates shows negative fractionations in deep facies with a progressive rise towards shallow-water settings, and suggests a deposition across a stratified ocean. Furthermore, 87Sr/86Sr chemostratigraphy coupled with radiometric data allowed a more precise chronostratigraphy, which supports an age of ~600-575 Ma for the unit, and suggests a deposition concurrent with the Gaskiers glaciation. Notwithstanding whether associated 13C variations in shallow water facies were produced by glacially-related conditions or by the dynamic of the basin itself remains unresolved. Although these conclusions are particularly valid for these deposits, they carry important implications for the understanding of other negative 13C excursions recorded in the Precambrian. Finally, bilaterian burrows occur in Gaskiers age glaciomarine rocks in Uruguay implying that these are the oldest definite animal tracks yet reported. Crucially, our new discovery unites the palaeontological and molecular data pertaining to the origin of bilaterians, and brings the origin of animals firmly into the interval of the Neoproterozoic glaciations. It also implies that ancestral bilaterians likely evolved first in relatively shallow seas, and only colonized the deep-sea floor once sufficient bottom water oxygenation had taken place.

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