Sea‐Level and Frontal Controls on Diatom Deposition During the Mid‐Pleistocene Transition, Southern Cape Basin

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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.