Middle Pleistocene West Antarctic Ice Sheet variability in the Ross Sea inferred from diatom assemblages

Hole U1524A from IODP Expedition 374 provides a new record of sedimentary cycles on the Ross Sea continental slope over the past 1.5 Ma. A robust stratigraphic framework for the upper 51.13 m of Hole U1524A is established using diatom biostratigraphy, paleomagnetic data, volcanic ash layers, and geochemical (Zr/Rb) records. On the continental slope, opal cannot be directly used as a proxy for productivity, as it is more strongly influenced by reworking processes. Further reconstruction of surface ocean conditions based on diatom assemblages reveals a sedimentary pattern characterized by gradual WAIS expansion during glacial periods, with extensive sediment transport to the lower continental slope, and by enhanced ASC and southward CDW intrusion during interglacials, causing substantial ice-shelf melting and IRD input. Changes in surface ocean conditions began around ~1 Ma. Prior to the early MPT (~1.5-1.0 Ma), glacial-interglacial variability was relatively mild and open ocean conditions prevailed. Subsequently, long-term cooling led to progressively greater sea ice extent and longer sea ice seasons even during interglacials, likely driven by declining atmospheric CO₂. This promoted longer sea ice seasons, enhanced upper-ocean stratification, increased carbon storage, and expansion of the Antarctic Ice Sheet. Following the MPT (post ~0.7 Ma), glacial-interglacial cycles were characterized by stronger oscillations. Diatom records from U1524 also provide evidence supporting WAIS retreat after the MPT. These findings highlight the critical role of surface ocean environmental changes and oceanic forcing in regulating Antarctic ice sheet dynamics and carbon storage, with implications for future ice-sheet stability in a warming climate.