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Prior to ~1 million years ago (Ma), variations in global ice volume were dominated by changes in obliquity; however, the role of precession remains unresolved. Using a record of North Atlantic ice rafting spanning the past 1.7 million years, we find that the onset of ice rafting within a given glacial cycle (reflecting ice sheet expansion) consistently occurred during times of decreasing obliquity whereas mass ice wasting (ablation) events were consistently tied to minima in precession. Furthermore, our results suggest that the ubiquitous association between precession-driven mass wasting events and glacial termination is a distinct feature of the mid to late Pleistocene. Before then (increasing), obliquity alone was sufficient to end a glacial cycle, before losing its dominant grip on deglaciation with the southward extension of Northern Hemisphere ice sheets since ~1 Ma.

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 ( K' 37 U and 86 TEX ) 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 CO 2 ventilation seen during the MPIS.CARTAGENA-SIERRA ET AL.

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.

Before ~1M years ago, variations in global ice volume were dominated by changes in obliquity but the role of precession remains unresolved. Using a record of North Atlantic ice rafting spanning the last 1.7Myr, we find that the onset of ice rafting within a given glacial cycle (reflecting ice sheet expansion) consistently occurred during times of decreasing obliquity, while mass ice wasting (ablation) events were consistently tied to minima in precession. Furthermore, our results suggest that the ubiquitous association between precession-driven mass wasting events and glacial termination is a unique feature of the mid/late Pleistocene. Before then, (increasing) obliquity alone was sufficient to end a glacial cycle, before losing its dominant grip on deglaciation with the southward extension of northern hemisphere ice sheets since ~1Ma.

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<p>The coupled ocean-atmosphere circulation of the Indian Ocean Dipole (IOD) controls monsoon rainfall in eastern Africa and southeast Asia at seasonal to decadal time-scale. In years when the dipole is particularly active, it can lead to catastrophic floods and droughts. A growing body of evidence suggests that IOD variability influenced the continental hydroclimate also at longer timescales in the past and thus may have affected human evolution.  However, long-term continuous high-resolution well-dated records have so far been unavailable to test this hypothesis. In 2016, long-term continuous deep-sea sediment cores have been recovered from the Davie Ridge in the Mozambique Channel during Expedition 361 ‘Southern African Climates’ as part of the International Ocean Discovery Program (IODP).</p><p>Here, we present a more than seven million-year-long multi-proxy record of Mozambique Channel Throughflow (MCT), which is tightly coupled to IOD variability; defined here as the zonal sea surface temperature gradient (ΔSST) between the Indo-Pacific warm pool (IPWP) and the Arabian Sea. We show that the MCT was relatively weak and steady until 2.1 million years ago (Ma), when it started to significantly accelerate with progressively increasing glacial-interglacial amplitude, culminating in high flow speeds from 0.8 Ma onwards. The invigoration of MCT activity coincided with increasing zonal ΔSST, which fuels the atmospheric Walker Cell circulation along the tropical Indian Ocean.  Our results demonstrate that the overall intensification of the Indian Ocean Walker Cell amplified the coupled ocean-atmosphere Indian Ocean zonal circulation at orbital time-scales, which agrees with the heightened glacial continental aridity recorded in other eastern African climate proxy records. We argue that the corresponding progressively drier glacials alternated with relative humid interglacials, providing the climatic-environmental setting –varying at seasonal to orbital timescales- for speciation and global expansion of our genus <em>Homo</em> after 2.1 Ma.</p>