Variations in Earth’s orbit pace global ice-volume and sea-level changes, but the variability in the response for different sectors of the Antarctic Ice Sheet to orbitally forced climate change remains unclear. Here we present geological records of iceberg-rafted debris and other proxies from locations adjacent to the West Antarctic Ice Sheet (WAIS) with comparisons to an existing East Antarctic Ice Sheet (EAIS) record over the time interval ~3.3–2.3 million years ago. Iceberg calving events from the WAIS recorded in Ross Sea sediment cores show a linear response to orbital forcing at timescales corresponding to obliquity (~40,000 years) and precession (~23,000–19,000 years) modulated by eccentricity (~100,000 years). This contrasts with an existing record adjacent to the EAIS, which does not contain obliquity pacing. Combined with ice-sheet model sensitivity tests, the geological data show that the WAIS is highly dynamic and responsive to oceanic melt driven by changes in Southern Ocean circulation, together with atmospheric forcing through variations in local insolation. Conversely, the EAIS appears less responsive to oceanic forcing, despite being the dominant source of meltwater to the global ocean during the mid-Pliocene. Our results imply a substantial role for atmospheric warming on mid-Pliocene sea-level from both WAIS and EAIS. The West Antarctic Ice Sheet responded to different natural forcing mechanisms than the East Antarctic Ice Sheet through the mid-Pliocene due to a greater sensitivity to oceanic feedbacks, according to iceberg-rafted debris records and ice-sheet modelling experiments.

Drill cores from the Antarctic continental shelf are essential for directly constraining changes in past Antarctic Ice Sheet extent. Here, we provide a sedimentary facies analysis of drill cores from International Ocean Discovery Program (IODP) Site U1521 in the Ross Sea, which reveals a unique, detailed snapshot of Antarctic Ice Sheet evolution between ca. 18 Ma and 13 Ma. We identify distinct depositional packages, each of which contains facies successions that are reflective of past baseline shifts in the presence or absence of marine-terminating ice sheets on the outermost Ross Sea continental shelf. The oldest depositional package (>18 Ma) contains massive diamictites stacked through aggradation and deposited in a deep, actively subsiding basin that restricted marine ice sheet expansion on the outer continental shelf. A slowdown in tectonic subsidence after 17.8 Ma led to the deposition of progradational massive diamictites with thin mudstone beds/laminae, as several large marine-based ice sheet advances expanded onto the mid- to outer continental shelf between 17.8 Ma and 17.4 Ma. Between 17.2 Ma and 15.95 Ma, packages of interbedded diamictite and diatom-rich mudstone were deposited during a phase of highly variable Antarctic Ice Sheet extent and volume. This included periods of Antarctic Ice Sheet advance near the outer shelf during the early Miocene Climate Optimum (MCO)-despite this being a wellknown period of peak global warmth between ca. 17.0 Ma and 14.6 Ma. Conversely, there were periods of peak warmth within the MCO during which diatom-rich mudstones with little to no ice-rafted debris were deposited, which indicates that the Antarctic Ice Sheet was greatly reduced in extent and had retreated to a smaller terrestrial-terminating ice sheet, most notably between 16.3 Ma and 15.95 Ma. Post-14.2 Ma, diamictites and diatomites contain unambiguous evidence of subglacial shearing in the core and provide the first direct, well-dated evidence of highly erosive marine ice sheets on the outermost continental shelf during the onset of the Middle Miocene Climate Transition (MMCT; 14.2-13.6 Ma). Although global climate forcings and feedbacks influenced Antarctic Ice Sheet advances and retreats during the MCO and MMCT, we propose that this response was nonlinear and heavily influenced by regional feedbacks related to the shoaling of the continental shelf due to reduced subsidence, sediment infilling, and local sea-level changes that directly influenced oceanic influences on melting at the Antarctic Ice Sheet margin. Although intervals of diatom-rich muds and diatomite indicating open-marine interglacial conditions still occurred during (and following) the MMCT, repeated advances of marine-based ice sheets since that time have resulted in widespread erosion and overdeepening in the inner Ross Sea, which has greatly enhanced sensitivity to marine ice sheet instability since 14.2 Ma.

Growing evidence suggests that portions of the Antarctic Ice Sheet (AIS) could cross a tipping point over the next decades due to global warming. The Mid-Pliocene Warm Period (mPWP, 3.3-3 Ma, +2°C) is regarded as one possible geologic analog to the climate of the near future, and paleo-sea level during mPWP interglacials indicates that portions of the AIS were lost at that time. However, due to a lack of ice-proximal data, the timing, magnitude and trigger mechanisms of AIS retreats remain unconstrained. Here, we focus on the Ross Sea, where the IODP Exp. 374 Site U1523 recovered the first Antarctic Plio-Pleistocene record from a current-controlled sediment drift in an environment evolving from ice-proximal to open marine over time. U1523 is located where intrusions of warm deep water and outflows of cold water occur today, controlled mainly by the strength and route of the Antarctic Slope Current. To constrain the relative influence of oceanic currents and AIS dynamics on sediment erosion, transport and deposition across the Plio-Pleistocene transition (3.3-2.6 Ma), we integrate grain size, morpho-mineralogical, magnetic fabric analysis and geophysical logs from site U1523 with the multi-channel seismic line IT94-127A. We complement our dataset with a closeby box core (PNRA ODYSSEA exp., box core 08), that can be regarded as a present day analogue. Here, we present our morpho-mineralogical results on the box core and some specific intervals of the mPWP from site U1523. In particular, we perform single mineral Raman spectroscopy which, together with the entire suite of minerals and their relative abundance, highlight the different depositional environments and the source of the detritus, identifying local vs distant and magmatic vs metamorphic sources. Furthermore, we use the geophysical logs to perform rock physics correlation and we tie them to the seismic line, allowing the analysis to be extrapolated along the shelf.

AbstractEarth’s obliquity and eccentricity cycles are strongly imprinted on Earth’s climate and widely used to measure geological time. However, the record of these imprints on the oxygen isotope record in deep-sea benthic foraminifera (δ18Ob) shows contradictory signals that violate isotopic principles and cause controversy over climate-ice sheet interactions. Here, we present a δ18Ob record of high fidelity from International Ocean Drilling Program (IODP) Site U1406 in the northwest Atlantic Ocean. We compare our record to other records for the time interval between 28 and 20 million years ago, when Earth was warmer than today, and only Antarctic ice sheets existed. The imprint of eccentricity on δ18Ob is remarkably consistent globally whereas the obliquity signal is inconsistent between sites, indicating that eccentricity was the primary pacemaker of land ice volume. The larger eccentricity-paced early Antarctic ice ages were vulnerable to rapid termination. These findings imply that the self-stabilizing hysteresis effects of large land-based early Antarctic ice sheets were strong enough to maintain ice growth despite consecutive insolation-induced polar warming episodes. However, rapid ice age terminations indicate that resistance to melting was weaker than simulated by numerical models and regularly overpowered, sometimes abruptly.

Through the Cenozoic (66-0 Ma), the dominant mode of ocean surface circulation in the Southern Ocean transitioned from two large subpolar gyres to circumpolar circulation with a strong Antarctic Circumpolar Current (ACC) and complex ocean frontal system. Recent investigations in the southern Indian and Pacific oceans show warm Oligocene surface water conditions with weak frontal systems that started to strengthen and migrate northwards during the late Oligocene. However, due to the paucity of sedimentary records and regional challenges with traditional proxy methods, questions remain about the southern Atlantic oceanographic transition from gyral to circumpolar circulation, with associated development of frontal systems and sea ice cover in the Weddell Sea. Our ability to reconstruct past Southern Ocean surface circulation and the dynamic latitudinal positions of the frontal systems has improved over the past decade. Specifically, increased understanding of the modern ecologic affinity of organic-walled dinoflagellate cyst (dinocyst) assemblages from the Southern Ocean has improved reconstructions of distinct past oceanographic conditions (sea surface temperature, salinity, nutrients, and sea ice) using downcore assemblages from marine sediment records. Here we present new late Oligocene to latest Miocene (similar to 26-5 Ma) dinocyst assemblage data from marine sediment cores in the southwestern Atlantic Ocean (International Ocean Discovery Program (IODP) Site U1536, Ocean Drilling Program (ODP) Site 696 and piston cores from Maurice Ewing Bank). We compare these to previously published latest Eocene-latest Miocene (similar to 37-5 Ma) dinocyst assemblage records and sea surface temperature (SST) reconstructions available from the SW Atlantic Ocean in order to reveal oceanographic changes as the Southern Ocean gateways widen and deepen. The observed dinocyst assemblage changes across the latitudes suggest a progressive retraction of the subpolar gyre and southward migration of the subtropical gyre in the Oligocene-early Miocene, with strengthening of frontal systems and progressive cooling since the middle Miocene (similar to 14 Ma). Our data are in line with the timing of the removal of bathymetric and geographic obstructions in the Drake Passage and Tasmanian Gateway regions, which enhanced deep-water throughflow that broke down gyral circulation into the Antarctic circumpolar flow. Although the geographic and temporal coverage of the data is relatively limited, they provide a first insight into the surface oceanographic evolution of the late Cenozoic southern Atlantic Ocean.

The role played by ocean circulation in major transitions in Earth's climate is debated. Here, we investigate the physical evolution of the Deep Western Boundary Current (DWBC) in the western North Atlantic Ocean through the late Eocene-to-mid Oligocene (35-26 Ma) using terrigenous grain size and geochemistry records of marine sediment cores. Our records cover the most pivotal transition in Cenozoic climate history, the Eocene-Oligocene Transition (EOT; similar to 33.7 Ma), when Earth first became sufficiently cool to sustain large ice sheets on Antarctica. To assess changes in deep-water circulation in the northwest Atlantic across the EOT, we assembled sortable silt (10-63 mu m) grain-size and Nd, Hf, and Pb radiogenic isotope records at two Integrated Ocean Drilling Program (IODP) drill sites on the Newfoundland ridges (Sites U1406 and U1411). These records reveal an overall gradual increase in sortable silt abundance (SS%) at both sites with no change in sediment provenance. We interpret a steady, long-term invigoration of the DWBC, likely driven by deepening of the Greenland-Scotland Ridge and resultant enhanced inflow of waters sourced from deep-water production sites in the Nordic Seas to the North Atlantic Ocean. Our results do not support abrupt and widespread invigoration of bottom current activity in the North Atlantic synchronous with accelerated cooling and Antarctic ice growth at the EOT. Instead, our records suggest that the DWBC started to intensify before this pivotal event in Cenozoic climate history (at similar to 35 Ma) and then further strengthened gradually across the EOT (similar to 34 Ma) and through the early-to-mid Oligocene (similar to 34-26 Ma). New terrigenous grain size and provenance records from Integrated Ocean Drilling Program Sites U1406 and U1411, Newfoundland ridges contourite drifts, North Atlantic Sortable silt abundance data reveals long-term coarsening at both sites over the studied interval (35-26 Ma) Deep North Atlantic circulation intensified prior to the Eocene-Oligocene Transition and continued to intensify into the mid-Oligocene

International Ocean Discovery Program (IODP) Expedition 379 to the Amundsen Sea margin of West Antarctica recovered drill cores at two sites spanning the Latest Miocene-Holocene interval with the aim of reconstructing past West Antarctic Ice Sheet dynamics. The recovered Plio-/Pleistocene sediment sequences offer an opportunity to apply and test different dating approaches in an Antarctic deep-sea drift setting, where the records are nearly continuous and unaffected by scouring of icebergs or grounded ice. Here, through palaeomagnetic analysis of continuous u -channel samples and application of X-ray fluorescence (XRF) scanning, we revise the IODP Exp. 379 Site U1533 age model for the uppermost Pliocene and Pleistocene composite interval (0.0-2.9 Ma). We first refine the magnetostratigraphic age model with high -resolution u -channel analysis and interpreted directional data. Consistent with shipboard results, all major geomagnetic polarity chrons and subchrons are identified in the Pleistocene section. The new high -resolution u -channel dataset also allows us to identify a geomagnetic polarity excursion at -884 ka (interpreted as the Kamikatsura excursion) and another excursion at -2734 ka with confidence (potentially the Porcupine excursion). Based on the improved polarity stratigraphy, we then develop two new highly resolved age models for Site U1533 using: (i) barium enrichment cycles identified in XRF scanning data, and (ii) geomagnetic relative palaeointensity (RPI). In our first age model, we correlate cyclic variations in sedimentary barium enrichment, inferred to represent changes in export productivity, to glacial- interglacial cycles of the Lisiecki and Raymo (2005) benthic foraminiferal oxygen isotope (delta 18O) stack (LR04). Nearly all Pleistocene Marine Isotope Stages (MIS) are interpreted to be present in the barium enrichment record of Site U1533, assuming simultaneous changes in Antarctic sea -ice extent/local export productivity and global oxygen isotope stratigraphy. We then construct the second, independent age model using the Plio-/Pleistocene RPI record developed for Site U1533, which represents the longest (nearly) continuous RPI record currently available for the Antarctic margin. Comparison of the two, independently derived age models shows a variable offset, on average +/- 12 kyr, with the RPI-based ages consistently older than the barium -based ages in the interval from 1.9 to 2.9 Ma and then consistently younger from 0.0 to 1.9 Ma. We interpret these offsets to result from a combination of lock -in depth effects in the younger interval (due to the relatively low sedimentation rates at this site, -2 cm/kyr), temporal offsets between global delta 18O changes in the deep ocean and productivity response on the Antarctic margin, and/or systematic miscorrelation in the construction of the two age models. Finally, we construct a hybrid age model for the Pleistocene section of Site U1533 by combining a mixture of RPI- and barium -based age tie points that are deemed to be robust. The Site U1533 RPI record is then used, together with other Southern Ocean RPI records, to construct an Antarctic RPI stack (designated as 'ANT -1600') for the interval 0.0-1.6 Ma. Although sedimentation rates at two-thirds of the sites selected for the stack are lower than 10 cm/

The European Petrophysics Consortium (EPC) is part of the ECORD Science Operator (ESO). EPC comprises the University of Leicester and Géosciences Montpellier and provides petrophysics staff scientists and petrophysicists, as well as expertise in downhole logging and core petrophysics programmes. The EPC has dedicated equipment for core logging and discrete measurements and is involved in data calibration, quality control, evaluation and interpretation of these data. The EPC is also involved in post-expedition activities, the preparation of upcoming expeditions, capability development, and training for IODP MSP expeditions and other key activities, including education and training.Over the past pandemic years, EPC has been active within expeditions and the community. EPC recognizes the importance of scientific drilling to palaeoclimate studies amongst other key topics, which is also reflected in our new science and operations roadmap: i) hired new staff members with a paleoclimate background; ii) developed a system for knowledge exchange between petrophysics and climate scientists, for instance via the ECORD summer schools; iii) renewed focus to include the development of measurement protocols and data analysis techniques to better serve the IODP community.EPC also has a website (http://www.le.ac.uk/epc) and will host the next ECORD Summer School Downhole Logging for IODP Science in person in Leicester in summer 2023. *European Petrophysics Consortium Team Members:Sarah Davies, Simon Draper, Tim van Peer, Andrew McIntyre, Marisa Rydzy (University of Leicester).Philippe Pezard, Johanna Lofi, Erwan Le Ber, Laurent Brun (University of Montpellier).

The late Oligocene (~27.8–23 My ago) offers an opportunity to study past climate variability under high-CO 2 , warmer-than-present and the unipolar (Antarctic) glaciated state. Here, we present new high-resolution geochemical records from exquisitely well-preserved benthic foraminifera for the late Oligocene, an interval for which Antarctic ice-sheet size and stability are debated. Our records indicate four obliquity-paced glacial-interglacial cycles with ice-volume changes of up to ~70% of the modern Antarctic ice-sheet. The amplitude of ice-volume change during these late Oligocene glacial-interglacial cycles is comparable to that of the late Pliocene and early Pleistocene. Ice-volume estimates for interglacials are small enough to be accommodated by a land-based Antarctic ice-sheet but, for three of the four glacials studied, our calculations imply that ice sheets likely advanced beyond the Antarctic coastline onto the shelves. Our findings suggest an Antarctic ice-sheet vulnerable to melting driven by both bottom-up (ocean) and top-down (atmospheric) warming under late Oligocene warmer-than-present climate conditions.

Contribution of melting Antarctic Ice Sheets (AIS) to rising sea level remains one of the least quantified inputs to predictive models for the future.To improve these estimates, International Ocean Discovery Program (IODP) Expedition 374 cored five sites in the Ross Sea, Antarctica to examine the stability of the AIS to past intervals of global warmth.Site U1523 proved difficult to core due to the presence of gravel lags and Accepted Version 01/04/2022 indurated intervals; thus, we cored 3 holes with overlapping stratigraphy at that site t to recover a more complete stratigraphic section.Given these challenges, no attempt was made to create a composite depth scale or stratigraphic splice during the expedition.Here we use a combination of physical property data (primarily magnetic susceptibility and natural gamma radiation), X-ray fluorescence core scanning, and visual core description to construct a core composite depth below seafloor (CCSF) to the base of Hole U1523B.This composite depth scale is discontinuous due to challenging coring conditions and variable core recovery, although there are several intervals of reasonably good stratigraphic continuity between 0 and 26 m CCSF and 82 and 96 m CCSF.We also created a stratigraphic splice from 0 to 93.95 m CCSF, although the splice is only continuous to 15.82 m CCSF.Additionally, we mapped the off-splice interval of Core U1523E-1H to the composite depth scale over several intervals with significant core disturbance by stretching and squeezing to obtain a best fit.Development of the composite depth scale and stratigraphic splice will improve post-cruise research results by allowing scientists to compare samples from different holes on the same depth scale.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Early to Middle Miocene sea-level oscillations of approximately 40–60 m estimated from far-field records 1 – 3 are interpreted to reflect the loss of virtually all East Antarctic ice during peak warmth 2 . This contrasts with ice-sheet model experiments suggesting most terrestrial ice in East Antarctica was retained even during the warmest intervals of the Middle Miocene 4 , 5 . Data and model outputs can be reconciled if a large West Antarctic Ice Sheet (WAIS) existed and expanded across most of the outer continental shelf during the Early Miocene, accounting for maximum ice-sheet volumes. Here we provide the earliest geological evidence proving large WAIS expansions occurred during the Early Miocene (~17.72–17.40 Ma). Geochemical and petrographic data show glacimarine sediments recovered at International Ocean Discovery Program (IODP) Site U1521 in the central Ross Sea derive from West Antarctica, requiring the presence of a WAIS covering most of the Ross Sea continental shelf. Seismic, lithological and palynological data reveal the intermittent proximity of grounded ice to Site U1521. The erosion rate calculated from this sediment package greatly exceeds the long-term mean, implying rapid erosion of West Antarctica. This interval therefore captures a key step in the genesis of a marine-based WAIS and a tipping point in Antarctic ice-sheet evolution.

Hydrogenetic ferromanganese crusts are considered a faithful record of the isotopic composition of seawater influenced by weathering processes of continental masses. Given their ubiquitous presence in all oceans of the planet at depths of 400-7000 meters, they form one of the most well-distributed and accessible records of water-mass mixing and climate. However, their slow accumulation rate and poor age constraints have to date limited their use to explore 100 ka paleoclimatic phenomena. Here it is shown how the Pb isotope signature and major element content of a Fe-Mn crust from the north-east Atlantic responded to changes in the intensity and geographic extent of monsoonal rainfall over West Africa, as controlled by climatic precession during the Paleocene. The studied high-spatial resolution (4 mu m) laser-ablation multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS) Pb isotope data is a nearly 2 order of magnitude improvement in spatial and temporal resolution compared to micro-drill subsamples. The record demonstrates cyclicity of the (206)pb/(204)pb and (208, 207)pb/Pb-206 ratios at the scale of single Fe-Mn oxide laminae, in conjunction with variations in the Fe/Mn ratio, Al, Si and Ti content. Time-frequency analysis and astronomical tuning of the Pb isotope data demonstrates the imprint of climatic precession (similar to 20 ka) modulated by eccentricity (similar to 100 and 405 ka), yielding growth rates of 1.5-3.5 mm/Ma consistent with previous chemostratigraphic age models. In this context, boreal summer at the perihelion causes stronger insolation over West Africa, resulting in more intense and geographically extended monsoonal rainfalls compared to aphelion boreal summer conditions. This, in turn, influences the balance between the weathering endmembers feeding the north-east Atlantic basin. These results provide a new approach for calibrating Fe-Mn crust records to astronomical solutions, and allow their isotopic and chemical archive to be exploited with an improved temporal resolution of 1000-5000 years. (C) 2020 Copyright British Geological Survey (c) UKRI 2020. Published by Elsevier B.V.

<p>Variations in solar insolation exert a fundamental control on the high-latitude climate–cryosphere system. Controversy, however, exists about the relative importance of orbital eccentricity versus axial tilt (obliquity) in driving pre-Quaternary Antarctic ice sheet variability. This problem is particularly acute during the late Oligocene-to-early Miocene interval (Oligo-Miocene, ~27-21 Ma), because several benthic foraminiferal oxygen isotopes (δ<sup>18</sup>O) records show strong pacing by obliquity, while others primarily show eccentricity pacing. The differences in orbital pacing are impossible to reconcile with the globally congruent imprint of ice volume on benthic δ<sup>18</sup>O on orbital time scales. Here we present a new astronomically tuned δ<sup>18</sup>O record generated at Integrated Ocean Drilling Program (IODP) Site U1406 (north-western Atlantic Ocean), a key area in modern-day thermohaline circulation. Clear imprints of both obliquity and eccentricity on the δ<sup>18</sup>O record are observed at Site U1406 throughout the study interval, irrespective of changes in sedimentation rate. The eccentricity variations at Site U1406 are remarkably similar to those seen in all other δ<sup>18</sup>O records, suggesting that eccentricity exerts a strong control on the high-latitude climate–cryosphere system via the modulation of the precession cycle. In contrast, the δ<sup>18</sup>O sensitivity to obliquity is globally variable, suggesting the influence of temperature in different bottom-water masses.</p>

R.M. McKay, L. De Santis, D.K. Kulhanek, J.L. Ash, F. Beny, I.M. Browne, G. Cortese, I.M. Cordeiro de Sousa, J.P. Dodd, O.M. Esper, J.A. Gales, D.M. Harwood, S. Ishino, B.A. Keisling, S. Kim, S. Kim, J.S. Laberg, R.M. Leckie, J. Müller, M.O. Patterson, B.W. Romans, O.E. Romero, F. Sangiorgi, O. Seki, A.E. Shevenell, S.M. Singh, S.T. Sugisaki, T. van de Flierdt, T.E. van Peer, W. Xiao, and Z. Xiong2

P-wave velocity data were measured on undisturbed section halves using pairs of piezoelectric transducers mounted in bayonets that are inserted into soft sediment along the JRSO-defined y-axis and/or z-axis. Report includes P-wave velocity in y and/or z direction, bayonet separation, traveltime between transducers, and first arrival picks.

Magnetic susceptibility was measured on whole-round sections (and rarely section halves) on the Whole-Round Multisensor Logger (WRMSL) and/or Special Task Multisensor Logger (STMSL) using a Bartington MS2 meter and a 90 mm or 80 mm MS2C loop. As volume of the sample is not controlled for this experiment, susceptibility units are recorded in instrument units and are not volume-corrected.