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Because of the paucity of exposed rock the direct physical record of Antarctic Cenozoic glacial history has become known only recently and then largely from off-shore shelf basins through seismic surveys and drilling. The number of holes has been small and largely confined to three areas (McMurdo Sound, Prydz Bay and Antarctic Peninsula), but even in McMurdo Sound, where Oligocene and early Miocene strata are well-cored, the Late Cenozoic is poorly known and dated. The latest Antarctic geological drilling program, ANDRILL, successfully cored a 1285m-long record of climate history spanning the last 13 m.y. from sub-sea floor sediment beneath the McMurdo Ice Shelf (MIS), using drilling systems specially developed for operating through ice shelves. The cores provide the most complete Antarctic record to date of ice sheet and climate fluctuations for this period of Earth’s history. The >60 cycles of advance and retreat of the grounded ice margin preserved in the AND1B record the evolution of the Antarctic ice sheet since a profound global cooling step in deep sea oxygen isotope records ~14 m.y. ago. A feature of particular interest is a ~90m-thick interval of diatomite deposited during the warm Pliocene, and representing an extended period (~200,000 years) of locally open water, high phytoplankton productivity and retreat of the glaciers on land. Historical overview The remarkable Late Cenozoic record of glacial history in the Ross Embayment recovered in late 2006 by the ANDRILL-MIS Project is the culmination of work begun over three decades ago to document and understand the more recent glacial history of Antarctica by drilling close to the margin. Ironically, although the last 3 million years of Earth’s climate is often said to be the best studied interval of the Cenozoic, the contribution of Antarctic ice volume changes is the most poorly understood. Until this most recent hole was drilled the middle Cenozoic record was better known from several holes in both the McMurdo region and Prydz Bay (Table 1). This is because in general the older strata were exposed closer to the coast through basin uplift, where younger strata had been removed by Neogene erosion, but also because in offshore basins glacial debris from the last glacial advance prevented penetration and recovery of the older record from gravity coring (Anderson, 1999). Plainly deep drilling technology was required from an appropriate location. Here we summarise the history and rationale behind this successful outcome. By the early 1970’s the International Geophysical Year (1956-58) had already resulted in a vast increase in knowledge of rock types, ages and history of the continent itself (Bushnell and Craddock, 1970), but the history of its ice sheet was still a mystery. This changed in early 1973 with the voyages of the Glomar Challenger to the South Indian Ocean and Ross Sea (Leg 28; Hayes, Frakes et al., 1975) and the Tasman Sea and Southwest Pacific Ocean (Leg 29; Kennett, Houtz et al., 1974). The first of these cruises showed that the Antarctic ice sheet, which had previously been seen as a Quaternary feature, had been in existence since at least the Oligocene. The second yielded the first oxygen isotope measurements on deep-sea calcareous microfossils, providing the first evidence of dramatic ocean cooling and global ice volume increase at the Eocene-Oligocene boundary (Shackleton and Kennett, 1974). The 1970’s was also a period in which detailed paleontological and chronological studies of cores from continuously deposited deep-sea sediments displaced the long held view of four major Quaternary glaciations (Flint, 1971), showing much more frequent cycles of climate, ice volume and sea level in the late Quaternary (Hays et al. 1976) in response to variations in the Earth’s orbital parameters calculated by Milankovitch. How far back in time these should be evident was not clear, but there seemed no reason why they should not have influenced the Antarctic ice sheet from the time of its inception. While deep ocean sediments were useful for their continuous record of past ocean chemistry, providing an ice volume-temperature signal, they could not provide information on the extent of ice or regional climate in the high latitudes. This could only come out of sediment cores from the Antarctic margin, where the direct influence of ice advance and retreat (and perhaps also sea level fall and rise) could be obtained. In the early 1970’s two drilling platforms were available – the Glomar Challenger operated by the Deep Sea Drilling Project and a land-based system put together for the Dry Valley Drilling Project, an initiative by the United States, Japan and New Zealand to explore the late Cenozoic history of the McMurdo Dry Valleys (Smith, 1981). Over the next three decades both of these platforms (and their successors) ran in parallel (Table 1, Figure 1) with varying degrees of success. The ship-based system had the advantage that it could be deployed at a number of places around the Antarctic margin, but was limited by ice conditions and poor recovery of glacimarine sediments from near shore shelf basins. Nevertheless, the cores taken have provided useful constraints on the Cenozoic history of the Antarctic ice margin and climate from the Ross Sea, Prydz Bay and Antarctic Peninsula sectors (Table 1). The land-based system, once adapted for sea-ice, had the advantage that it could yield long and continuous core with near-complete recovery. However, it was rather cumbersome and required an ice platform that was firmly tied to land. Although the system has been improved through the MSSTS-1, CIROS and Cape Roberts Projects (Table 1), continuing to exploit the fast-ice rim around McMurdo Sound as a drilling platform, attractive drilling targets beneath fast sea ice elsewhere on the Antarctic margin have yet to be identified. The results from both ship-based and sea-ice-based drilling have provided a framework for the Cenozoic history of the Antarctic ice sheet (Kennett and Warnke, 1992, 1993; Barrett, 1996), but more detail has become known for Oligocene and early Miocene times, especially from the 1500 m of strata cored off Cape Roberts (Barrett, 2007; Naish et al., 2001), than in subsequent times. The first results of the shift from sea iceto shelf ice-based drilling reported below add a great deal to the late Cenozoic story. The ANDRILL McMurdo Ice Shelf Project The aim of the MIS Project was to obtain a continuous sediment core through approximately 1,200 meters (m) of Neogene (~0-10 Ma) glacimarine and volcanic sediment that had accumulated in the Windless Bight region of the McMurdo Ice Shelf (Fig. 2A). The present-day MIS forms the northwest part of Ross Ice Shelf where it has been pinned by Ross Island for the last ~10 ka (McKay et al., 2007), and is nourished by ice sourced locally and from the East Antarctic Ice Sheet (EAIS) outlet glaciers in the southern Transantarctic Mountains (TAM). The drillsite was situated above a flexural moat basin formed in response to Quaternary volcanic loading of the crust by Ross Island, superimposed on regional subsidence associated with Neogene extension of the Terror Rift (Horgan et al., 2005; Naish et al., 2006; Fig. 2B). Between 29 October and the 26 of December, 2006, a single 1284.87 m-deep, drill core (AND-1B) was recovered from the bathymetric and depocentral axis of the moat in 943 m of water from an ice shelf platform. The drilling technology employed a sea-riser system in a similar fashion to the Cape Roberts Project (CRP), but utilised a combination of soft sediment coring in upper soft sediments and continuous wire line diamond-bit coring. Innovative new technology, in the form of a hotwater drill and over-reamer, was used to make an access hole through 85 m of ice and to keep the riser free during drilling operations. The MIS project has two key scientific objectives. 1. Provide new knowledge on the Late Neogene behaviour and variability of the Ross Ice Shelf/Ice Sheet (RIS) and the West Antarctic Ice Sheet (WAIS), and their influence on global climate, sea-level and ocean circulation. 2. Provide new knowledge on the Neogene tectonic evolution of the West Antarctic Rift System (WAR), Transantarctic Mountains (TAM) and associated volcanism. A key outcome of the project is to provide age control for, and determine the environmental significance of, seismic reflectors that have been mapped regionally within the Victoria Land Basin (VLB) (Henrys et al., 2007; Fielding et al., 2007) in order to assess the regional impact of global climatic and local tectonic events. A second key outcome of the project will be to use paleoclimatic proxies and boundary conditions to help constrain numerical climate and dynamical ice sheet models. This paper presents an overview of the MIS Project, the details are reported Initial Results volume (Naish et al., 2007). Tectonic and stratigraphic setting Ross Island lies at the southern end of the Victoria Land Basin (VLB), a ~ 350km-long, half-graben, hinged on its western side at the Transantarctic Mountain (TAM) front (Fig. 2). Major rifting in the VLB has occurred since the latest Eocene, perhaps having been initiated in the Cretaceous (Cooper and Davey, 1985; Brancolini et al., 1995), and has accommodated up to 10 km of sediment. A new rift history, based on the Cape Roberts Project drill cores linked to a new regional seismic stratigraphic framework (Fielding et al., 2007; Henrys et al., 2007) indicates that crustal stretching during the Oligocene synrift phase produced rapid subsidence, followed by thermally-controlled slower subsidence in the Early Miocene. Renewed rifting within the centre of the VLB beginning in the Late Miocene has continued through to present day. This forms the Terror Rift (Cooper et al., 1987), and is associated with alkalic igneous intrusions and extrusive volcanism (e.g. Beaufort Island and Ross Island). Quaternary loading of the crust by the Ross

Late Cenozoic Climate History of the Ross Embayment from the AND-1B Drill Hole: Culmination of Three Decades of Antarctic Margin Drilling