Abstract

During the Late Pleistocene, the climate system experienced a persistent shift in interglacial intensity during the Mid-Brunhes Transition (~425 ka), with post-MBT interglacials exhibiting higher atmospheric CO₂ and warmer Antarctic and global ocean temperatures than earlier “lukewarm” interglacials. Whether this systematic difference reflects changes in ice-sheet configuration remains uncertain owing to ambiguities in δ¹⁸O-based reconstructions and persistent model–data discrepancies. Here we integrate Antarctic ice-core temperature and δ¹⁸O records, the benthic δ¹⁸O stack, marine sediment constraints, and isotope-enabled climate model simulations to reconcile the systematic contrasts between pre- and post-MBT interglacials. Simulations forced solely by orbital parameters and greenhouse-gas concentrations systematically underestimate the observed isotopic and thermal contrasts, including global benthic δ¹⁸O variability, implying a substantial ice-volume deficit during lukewarm interglacials if temperature effects are held constant. Consistent agreement across Antarctic temperature and isotopic signals, deep-ocean temperature, and global benthic δ¹⁸O stack records is achieved only when a higher Antarctic ice-sheet surface elevation is prescribed. Using a marine biogeochemical general circulation model, we further show that increased Antarctic ice-sheet elevation strengthens Antarctic Bottom Water formation and enhances Southern Ocean stratification, thereby suppressing deglacial deep-ocean carbon release and limiting interglacial atmospheric CO₂ levels. These results identify Antarctic ice-sheet elevation as a necessary physical constraint on lukewarm interglacials and suggest that the interval spanning Termination 5 and Marine Isotope Stage 11c marks a threshold shift in the coupled ice–ocean–carbon system.