<p>Between ~800 and 430 thousand years ago lukewarm interglacials were characterized by lower atmospheric CO₂ levels and colder Antarctic temperatures than subsequent interglacials. The Southern Ocean is thought to have played a crucial role, but associated ocean circulation changes remain poorly constrained, at least in part, due to the scarcity of proxy data. By using a novel 2D laser ablation technique, we here provide the first orbital-resolution Southern Ocean seawater Pb isotope records over the past 800 thousand years from a ferromanganese crust located at mid-depth (~1.6 km water depth) on Antarctica’s Pacific margin. Our results reveal systematically higher ²⁰⁸Pb/²⁰⁶Pb ratios during lukewarm interglacials than during more recent interglacials while ²⁰⁶Pb/²⁰⁴Pb ratios remained similar, suggesting reduced vertical deep-water mixing in the Southern Ocean during lukewarm interglacials. By enhancing deep-sea carbon sequestration and thereby lowering atmospheric CO₂, strengthened deep Southern Ocean stratification likely imposed critical impacts on the lukewarm interglacial climates.</p>