Marine-terminating West Antarctic Ice Sheet during the latest Oligocene

West Antarctica’s transition to its first extensive glaciation likely post-dated the establishment of the East Antarctic Ice Sheet (EAIS) by more than 10 million years, yet the timing and nature of West Antarctic Ice Sheet (WAIS) development have remained poorly constrained due to the absence of reliable sediment records. Newly recovered sediment sequences drilled with the seafloor drill rig MARUM-MeBo70 during RV Polarstern Expedition PS104 from the eastern Amundsen Sea Embayment were explored using multi-proxy sediment analyses alongside biostratigraphic and isotopic dating methods. These revealed ice-proximal terrigenous and diatomaceous diamictites as well as diatomaceous mudstones indicating the advance of a substantial WAIS to these drill sites already prior to the Oligocene–Miocene transition (~23 million years ago). Coupled climate-ice sheet modelling simulates the growth of a distinct WAIS separated from the EAIS, which expanded far into basins below sea level. Therefore, our new sedimentary data validate these model results and demonstrate that extensive WAIS expansion to the coast occurred several million years earlier than previously thought.