Reduced West Antarctic Ice Sheet Size During Prominent Quaternary Interglacials Constrained by Iceberg‐Rafted Debris Provenance in the Amundsen Sea

Abstract The stability of the West Antarctic Ice Sheet (WAIS) remains a major uncertainty in sea‐level projections, and geologic records provide important constraints on its past behavior. We present 40 Ar/ 39 Ar ages of biotite and hornblende grains from iceberg‐rafted debris (IRD) in two Amundsen Sea sediment cores (PC493 and PS58/254), located south and north of the southern boundary of the Antarctic Circumpolar Current (sbACC), respectively. Over the past 850,000 years, PC493 records a stable IRD provenance dominated by eastern Amundsen Sea sources, consistent with transport by the westward‐flowing Antarctic Coastal Current. In contrast, PS58/254 shows temporally variable sources, with southeastern Amundsen Sea inputs during glacials and increased Marie Byrd Land contributions during interglacials, indicating latitudinal sbACC shifts. The absence of East Antarctic IRD is consistent with no open‐marine connection between the Weddell and Amundsen Seas, while abundant <10 Ma grains during Marine Isotope Stages 5, 11, and 15 imply reduced WAIS elevation.