Northward Shift of South Pacific Polar Frontal System Across the Mid‐Pleistocene Transition: A 1.4 Myr Continuous Diatom Paleoenvironmental Record From the Pacific Sector of the Antarctic Circumpolar Current

Abstract The Mid‐Pleistocene Transition (MPT) marks a major shift in the pacing of Earth's climate changes from 40 to 100 thousand years. The Polar Frontal Zone (PFZ), which controls upwelling and productivity in the Southern Ocean, migrates latitudinally during major climate transitions. Data about past PFZ behavior in the Pacific Sector of the Southern Ocean are sparse. Here, we present new diatom and geochemical records from International Ocean Drilling Program (IODP) Site U1539, located just south of the modern Subantarctic Front (SAF) in the Pacific Sector of the Southern Ocean. At Site U1539, a long‐term shift in the diatom assemblage across the MPT indicates a northward shift of the PFZ around 770 ka. The early‐MPT assemblage is dominated by Thalassiothrix antarctica mats interpreted to reflect the position of the SAF. This assemblage transitions to a more diverse assemblage post‐MPT. The post‐MPT assemblage reveals cyclic increases in the relative abundance of Eucampia antarctica during glacial periods, suggesting potential sensitivity to dust‐derived nutrients. Throughout the 1.4 Myr record, summer sea surface temperature (SSST) and biogenic opal content vary with the global benthic oxygen isotope stack, indicating sensitivity to orbitally‐paced glacial‐interglacial change. South Pacific glacial SSSTs gradually cool across the MPT, with increased siliceous productivity and cooler SSSTs during glacials and reduced siliceous productivity and warmer SSSTs during interglacials. Reduced absolute diatom abundance during Marine Isotope Stages 21–15 (∼860–560 ka) indicates persistent sea ice during lukewarm interglacials, suggesting that decreased ventilation preconditioned the Southern Ocean to store more carbon in the late Pleistocene.