Leah J. LeVay

The Indian‐Atlantic Ocean Gateway modulates interocean exchange and influences large‐scale ocean circulation, including the Atlantic Meridional Overturning Circulation. Site U1479 (Cape Basin; IODP Expedition 361) lies at the Benguela‐Agulhas transition and records interactions among the Indian, Atlantic, and Southern Ocean basins during the Mid‐Pleistocene Transition (MPT). We use diatom abundance, ecological groupings, and sediment geochemistry to evaluate the roles of sea‐level change, frontal migrations, and water‐mass variability on diatom deposition in the Cape Basin over the MPT. Diatom and geochemical records reveal four diatom phases (DP1‐4) in biogenic silica deposition across the MPT. DP1 (1050–917 ka) shows glacial‐interglacial cyclicity, with productivity linked to frontal positions. DP2 (917–740 ka) exhibits sustained diatom productivity during a broader interval of circulation and nutrient reorganization in the early MPT, likely driven by upwelling of Antarctic Intermediate Water and Subantarctic Mode Water via an expanded Benguela system. Principal Component Analysis indicates that productivity gradients and sea‐level–modulated coastal proximity are the primary controls on assemblage variability. DP3 (760–620 ka) records declining productivity as fronts migrated southward. DP4 (<620 ka) is characterized by sediments devoid of diatoms, reflecting silica retention south of the Polar Front and increased influence of the Agulhas Current. Elemental ratios record terrigenous inputs, hydroclimate variability, and shifts between calcareous and siliceous production. Glacial lowstands exposed the Agulhas Bank, reducing Agulhas influence and enhancing Benguela upwelling. Together, sea‐level changes, frontal migrations, and current reorganizations regulated the delivery of Southern Ocean‐sourced nutrients and silica availability, thereby controlling diatom production and burial across the MPT.

The Indian-Atlantic Ocean Gateway significantly modulates thermohaline circulation and impacts the Atlantic Meridional Overturning Circulation (AMOC). Site U1479 in the Cape Basin, recovered during International Ocean Discovery Program Expedition 361, lies at the transition between the Benguela and Agulhas currents, recording interactions between the Indian, Atlantic, and Southern oceans during major climate transitions. We use absolute diatom abundance (ADA), diatom assemblage groups, and high-resolution biogenic silica and sediment geochemistry to evaluate sea-level change, ocean frontal migrations, and water-mass variability in the southeastern Cape Basin from 1,050 to 440 ka. Diatom reconstructions reveal a three-step shift in abundance, assemblage, and preservation across the Mid-Pleistocene Transition (MPT). From Marine Isotope Stage 23 to 12, the record captures evolving southern sourced water transport and upwelling, alongside a southward expansion of the Benguela Upwelling Zone. The data are consistent with paleoclimate evidence for glacial equatorward displacements of Southern Ocean fronts by roughly 5 to 7° latitude during the MPT, altering Agulhas Leakage, with significant implications for the AMOC. Elemental geochemistry, together with ADA and assemblages, also suggests periods when the exposure of the Agulhas Bank likely influenced interactions between the Agulhas and Benguela current systems, enhancing productivity in the southern Cape Basin. Overall, these records highlight a coupled system in which sea level, frontal position, and ocean current dynamics jointly control nutrient delivery and ecosystem structure. Siliceous microfossil evidence of changes in circulation and primary productivity in the Cape Basin significantly refines the timing and mechanisms of Southern Ocean reorganization during the MPT.

Vast amounts of carbon are stored beneath the seafloor in the form of methane hydrate. Hydrate is stable at moderate pressure and low temperature at a depth extending several hundred meters beneath the seafloor to the base of gas hydrate stability (BGHS) often marked by bottom simulating reflections (BSRs) in seismic profiles. However, data from logging‐while‐drilling and coring during Integrated Ocean Discovery Program Expeditions 372 and 375 offshore New Zealand identified hydrate ∼60 m beneath the BSR. This hydrate appears to be dissociating over thousands of years following a gradual temperature increase from sediment burial modulated by changes in bottom‐water temperature and sea‐level fluctuations. Slow hydrate dissociation significantly buffers the release of methane and therefore, carbon through glacial cycles. Dissociating hydrate beneath the BGHS may also increase estimated global budgets of methane stored in hydrate.

By studying deep‐sea drilled records from the North Atlantic Ocean, several magnetic instabilities of short duration, such as the Iceland Basin (188 ka), the Björn (1,255 ka) and the Gardar (1,460 ka) excursions, were discovered. These records have contributed to our understanding of Earth's magnetic field and are the foundation of the Geomagnetic Instability Time Scale (GITS) in the Quaternary. Here, we present the magnetostratigraphy from Sites U1555 (0 to ∼2.7 Ma) and U1563 (0 to ∼5.2 Ma) drilled during the International Ocean Discovery Program Expedition 395C on the eastern side of the modern Mid‐Atlantic Ridge (∼60°N, 20–30°W). Shipboard paleomagnetic and microfossil data provided a preliminary age model, extending the regional record to 3.4 Ma. The Virtual Geomagnetic Pole latitudes from archive halves, corroborated with data from discrete samples, were used to build a high‐resolution magnetostratigraphy, which contained the expected Brunhes and Matuyama Chrons and their respective Subchrons. We also identified most of the magnetic events reported in the GITS, including the less well‐documented ones, such as Osaka, Kamitzukara, Huckleberry Ridge, Reunion, Gardar, Halawa and L4 events. The high‐resolution magnetostratigraphy from Sites U1555 and U1563 is compared with two previous legacy sites and contributes toward an increasingly robust GITS, expanding its use as a correlation and dating tool.

North Atlantic Deep Water (NADW), the return flow component of the Atlantic Meridional Overturning Circulation (AMOC), is a major inter-hemispheric ocean water mass with strong climate effects but the evolution of its source components on million-year timescales is poorly known. Today, two major NADW components that flow southward over volcanic ridges to the east and west of Iceland are associated with distinct contourite drift systems that are forming off the coast of Greenland and on the eastern flank of the Reykjanes (mid-Atlantic) Ridge. Here we provide direct records of the early history of this drift sedimentation based on cores collected during International Ocean Discovery Programme (IODP) Expeditions 395C and 395. We find rapid acceleration of drift deposition linked to the eastern component of NADW, known as Iceland–Scotland Overflow Water at 3.6 million years ago (Ma). In contrast, the Denmark Strait Overflow Water feeding the western Eirik Drift has been persistent since the Late Miocene. These observations constrain the long-term evolution of the two NADW components, revealing their contrasting independent histories and allowing their links with climatic events such as Northern Hemisphere cooling at 3.6 Ma, to be assessed.