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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 0.3Ma to examine implied changes in the upper oceanographic conditions at the mid-Pleistocene transition (MPT, ca. 1.2–0.8Ma). 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 aGloborotalia 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 CO2ventilation seen during the MPIS.

The exchange of shallow and deep water masses between the Indian Ocean and the Atlantic constitutes an integral inter-ocean link in the global thermohaline circulation. In the gateway south of South Africa long-term changes in deep water flow during the Cenozoic have been initially studied using seismic reflection profiles. But the seismic stratigraphy was poorly constrained and not further resolved within the time period from the late Miocene to present. In particular, there were limited Pliocene records that could be used to investigate the influence of climatic (e.g. Antarctic ice volume) and tectonic (e.g. closure of the Central American seaway) on the deep-water variability. In 2016 the International Ocean Discovery Program (IODP) Expedition 361 (“SAFARI”) recovered complete high-resolution Plio-/Pleistocene sediment sections at six drilling locations on the southeast African margin and in the Indian-Atlantic ocean gateway. Here, we present results from Site U1475 (Agulhas Plateau), a location proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site is located over a sediment drift in 2669 m water depth and comprises a complete carbonate rich (74 – 85%) stratigraphic section of the last ~7 Ma. We edited high-resolution data sets of density, velocity and natural gamma radiation measured at Site U1475 and corrected them to in-situ conditions. Cross correlations show that acoustic impedance contrasts and thus the formation of seismic reflectors are mainly due to density changes that are caused by climate-induced variations in biogenic vs. terrigenous sediment input. The calculated synthetic seismograms show an excellent correlation of drilling results with the site survey seismic field record, provide an accurate traveltime to depth conversion, and allow preliminary age assignments (± 0.3 Ma) based on the shipboard bio- and magnetostratigraphy. The most prominent reflectors are associated with compositional changes related to late Pleistocene glacial/interglacial variability, the middle Pleistocene transition, and the onset of the northern hemisphere glaciation. Furthermore, a peculiar early Pliocene interval (~ 5.3 – 4.0 Ma) bounded by two reflectors is characterized by 3-fold elevated sedimentation rates (> 10 cm/ka) and the occurrence of sediment waves. We argue that this enhanced sediment transport to the Agulhas Plateau was caused by a reorganization of the bottom current circulation pattern due to maximized inflow of NADW. Rhythmic bedding within the Pliocene sediment wave sequence likely reflects the 100-kyr orbital cycle. On the other hand, colour reflectance and natural gamma radiation show highest variability in the precession band. The very regular response of the core logging data to orbital forcing suggests that the shipboard age model can be significantly improved by cyclostratigraphy.