Description: How greenhouse forcing affects ice sheets and sea level during peak interglacial times is an unresolved question of great societal importance. Here we examine the development of glacial-interglacial sequence motifs in drill cores from the Pacific margin of Antarctica and explore how the West Antarctic Ice Sheet impacted sedimentary systems under different climate states, including during the Pliocene Climatic Optimum (PCO) ~4.5-4.0 Ma. Hundreds of meters of Pliocene sediments were recovered during IODP Expedition 379 in two drill sites within the Resolution Drift. Site U1533 was positioned near a submarine channel originating from the Amundsen Sea continental slope, and Site U1532 was located on the western upper flank of the hemipelagic drift. Abundance and type of sediment transport structures in the cores change gradually in lower Pliocene strata in both drill sites in concert with changes in sedimentation rate. Pliocene strata are characterized with a repetitive facies stacking pattern composed of greenish gray clast-bearing mud with a biosiliceous component, interbedded with dark brownish gray laminated silty clay. The greenish gray clast-bearing muds are tentatively interpreted as “interglacials”, however, many greenish grey units have sharp upward transitions into “glacial” laminated mud and the units are irregularly spaced with depth. Analysis of the red-green channel (a*) in shipboard reflectance spectroscopy and colorimetry (RSC) data for Site U1532 on the shipboard age model demonstrates the variable orbital cyclicity of the facies assemblage. Sequence motifs, which are further explored in half-core magnetic susceptibility and XRF core-scanner data, highlight how ice growth and decay, downslope sediment transport, and the position and intensity of ocean currents affected the depositional systems. The ultimate goal of this work is to combine facies interpretations with the detrital sedimentology and other data sets to assess the behavior of the West Antarctic Ice Sheet in a warmer than present climate scenario.

Description: The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50°E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.

Description: International Ocean Discovery Program (IODP) Expedition 382 in the Scotia Sea’s Iceberg Alley recovered among the most continuous and highest resolution stratigraphic records in the Southern Ocean near Antarctica spanning the last 3.3 Myr. Sites drilled in Dove Basin (U1536/U1537) have well‐resolved magnetostratigraphy and a strong imprint of orbital forcing in their lithostratigraphy. All magnetic reversals of the last 3.3 Myr are identified, providing a robust age model independent of orbital tuning. During the Pleistocene, alternation of terrigenous versus diatomaceous facies shows power in the eccentricity and obliquity frequencies comparable to the amplitude modulation of benthic δ18O records. This suggests that variations in Dove Basin lithostratigraphy during the Pleistocene reflect a similar history as globally integrated ice volume at these frequencies. However, power in the precession frequencies over the entire ∼3.3 Myr record does not match the amplitude modulation of benthic δ18O records, suggesting Dove Basin contains a unique record at these frequencies. Comparing the position of magnetic reversals relative to local facies changes in Dove Basin and the same magnetic reversals relative to benthic δ18O at North Atlantic IODP Site U1308, we demonstrate Dove Basin facies change at different times than benthic δ18O during intervals between ∼3 and 1 Ma. These differences are consistent with precession phase shifts and suggest climate signals with a Southern Hemisphere summer insolation phase were recorded around Antarctica. If Dove Basin lithology reflects local Antarctic ice volume changes, these signals could represent ice sheet precession‐paced variations not captured in benthic δ18O during the 41‐kyr world.

Description: The paleoclimate and ecosystem variability in Africa during the Plio/Pleistocene has received considerable attention due to its potential links to hominid evolution. However, the reconstruction of this variability hinges critically upon highly temporally resolved proxy data from continuous, well-dated sediment archives. In light of these requirements we use a new XRF core-scanning record from International Ocean Discovery Program (IODP) Site U1478 off the Limpopo River mouth (Mozambique Channel, SW Indian Ocean) spanning the past c. 4 Ma to identify the climate variability in SE Africa. Our results show that the elemental distribution in the Site U1478 cores is mainly controlled by the rate of terrigenous input and – to a lesser extent – by bottom-current transport and post-depositional processes such as propagation of paleoredox boundaries and diagenesis across some intervals. The log(Ti/Ca) ratio, which is used as a tracer of terrigenous sediment input, shows quasi-cyclical variability across the entire record that closely matches the periods of orbital parameters. However, the cyclical behaviour of the log(Ti/Ca) signal varies through time, with the uppermost 106 m of the sequence (0–1.07 Ma) displaying a mix of precession and obliquity signals, the intervals 106–223 m (1.07–2.80 Ma) and 240–257 m (3.68–4.05 Ma) being dominated by precession, and the interval 223–240 m (2.80–3.68 Ma) being controlled by eccentricity. To refine the available chronology for Site U1478, which is based on shipboard biostratigraphic and paleomagnetic data, we have tuned the log(Ti/Ca) record to the LR04 benthic oxygen isotope record, summer insolation at 25° S, and orbital eccentricity depending on the dominant cyclicities in the XRF dataset across individual time intervals. The resulting chronology enables us to evaluate the XRF data as well as the previously available shipboard sedimentological and geochemical datasets within a regional and global climatic context. This allows the connection of a c. 7-m-thick contourite deposit and a prominent paleoredox boundary to hydroclimate and ocean-circulation changes during the early Pleistocene and across the Mid-Pleistocene Transition, respectively. Moreover, a decoupling of the log(Ti/Ca) and the log(Ti/K) records, with the latter indicating the degree of sediment weathering, from 3.2 to 2.8 Ma points to an increased delivery of highly weathered sediments to Site U1478. We attribute this to temporarily wetter and warmer conditions in the catchment of the Limpopo River and/or a change in the sediment source, perhaps associated with the tectonically driven enlargement of the Zambezi River catchment during the late Pliocene/early Pleistocene.

Description: Efforts to understand long-term Indian Ocean dynamics and land-sea linkages in southeast Africa during periods of significant global and regional climate change have been inhibited by a lack of high-resolution climate records, particularly during the Plio-Pleistocene. Here we present new biomarker and pollen records from International Ocean Discovery Program (IODP) Site U1478, located at the Upper Agulhas Confluence near the Limpopo River mouth, to establish environmental conditions at the southeast African margin between 4 and 1.8 Ma and address this spatiotemporal gap. Compound-specific hydrogen isotopes of terrestrial leaf waxes (δDwax) and TEX86, using marine archaeal lipids, document hydroclimate variability and sea surface temperature (SST), respectively, permitting an onshore-offshore climate comparison. The U1478 records establish the Limpopo catchment response to the switch in Indonesian Throughflow source waters, the mid-Pliocene Warm Period, and intensification of Northern Hemisphere glaciations at ~2.7 Ma. Broad coherence between the δDwax and SST records supports a linkage between Indian Ocean temperatures and southeast African hydroclimate. We hypothesize that additional mechanisms including Indian Ocean cross-basin SST gradients (ΔSST) and high latitude glaciation acted as hydroclimate controls during the Plio-Pleistocene. We use ΔSST to evaluate ocean-atmosphere patterns similar to the Indian Ocean Dipole (IOD) and establish generally wetter conditions in the region associated with positive IOD-like phases. Additionally, an obliquity signal evident in the δDwax record indicates that glacial-interglacial variability likely influenced the tropical rain belt position and also controlled rainfall. Hydroclimate and environmental conditions across the Plio-Pleistocene in southeast Africa may have important implications for regional hominin evolution.

Description: The marine-based West Antarctic Ice Sheet (WAIS) is considered the most unstable part of the Antarctic Ice Sheet, with particular vulnerability in the Amundsen Sea sector where glaciers are melting at an alarming rate. Far-field sea-level data and ice-sheet models have pointed towards at least one major WAIS disintegration during the Late Quaternary, but direct evidence for past collapse(s) from ice-proximal geological archives remains elusive. In order to facilitate geochemical and mineralogical tracing of the two most important glaciers draining into the Amundsen Sea, i.e. Pine Island Glacier (PIG) and Thwaites Glacier (TG), we here provide the first multi-proxy provenance analysis of 26 seafloor surface sediment samples from Pine Island Bay. Our data show that the fingerprints of detritus delivered by PIG and TG are clearly distinct near the ice-shelf fronts of both ice-stream systems for all grain sizes and proxies investigated. Glacial detritus delivered by PIG is characterised by low εNd values (~−9), high 87Sr/86Sr ratios (~0.728), low smectite content (〈10%), and hornblende and biotite grains with Late Permian to Jurassic (170–270 Ma) cooling ages. In contrast, glacigenic detritus delivered by TG is characterised by higher εNd values (~−4), lower 87Sr/86Sr ratios (0.714), higher smectite (20%) and kaolinite content (37%), biotite and hornblende grains with 40Ar/39Ar cooling ages of 〈40 Ma and ~115 Ma, and high content of mafic mineral. The geochemical and mineralogical fingerprints for PIG and TG reported here provide novel insights into sub-ice geology and allow us to trace both drainage systems in the geological past, under environmental conditions more similar to those envisioned in the next 50 to 100  years.

Description: In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Here we analyse a sedimentary record from the Agulhas Plateau, located close to the modern position of the subtropical front and use alkenones and coccolith assemblages to reconstruct oceanographic conditions over the past 300,000 years. We identify a strong glacial-interglacial variability in sea surface temperature and productivity associated with subtropical front migration over the Agulhas Plateau, as well as shorter-term high frequency variability aligned with variations in high latitude insolation. Alkenone and coccolith abundances, in combination with diatom and organic carbon records indicate high glacial export productivity. We conclude that the biological pump was more efficient and strengthened during glacial periods, which could partly account for the reported reduction in atmospheric carbon dioxide concentrations.

Description: Palaeoceanographic evidence suggests that glacial periods of the Mid to Late Pleistocene were characterized by markedly different global ocean circulation patterns to modern; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. Whilst proxy records and modelling efforts continue to clarify this picture, an evidence alluding to the origin of this phenomenon remains elusive. Because of this, our understanding of the sequence of events leading to global glacial conditions remains incomplete. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50°E) systematically preceded deep-water mass reorganizations by 1-2 thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. This, in concert with increased sea-ice cover, may have enabled positive buoyancy anomalies to effectively escape into the ‘upper’ Atlantic overturning circulation limb, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. Furthermore, we observe a distinct obliquity pacing of Antarctic iceberg melt both preceding and following the Mid-Pleistocene Transition, become obscured during this interval. With new and existing data we investigate the evolution of orbital forcing at the Agulhas Plateau, considering the implications for ‘Southern Escape’ of freshwater as a key feedback in the transition to the ‘100-kyr world’.

Description: The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian-Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature ( and ) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time-series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid-Pleistocene transition (MPT, ca. 1.2?0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid-Pleistocene Interim State (MPIS, MIS 23?12) and reached its northernmost position during MIS 34?24 and MIS 10. Comparison to a Globorotalia menardii-derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO2 ventilation seen during the MPIS.