Sediments recovered from the outer continental shelf of the central Ross Sea provide important constraints on past Antarctic ice sheet variability and oceanographic conditions. International Ocean Discovery Program (IODP) Hole U1522A recovered upper Miocene to Pleistocene sediments, offering an opportunity to explore paleoenvironmental changes during intervals of variable ice sheet extent following extreme warmth of the Miocene Climatic Optimum (~16.9–14.7 Ma) and cooling during the Middle Miocene Climate Transition and Mi-3 glaciation (~13.9-13.8 Ma).

Here, we present foraminiferal assemblage data from the late Miocene to early Pliocene (~11–5 Ma), integrated with counts of diatoms, radiolarians, sponge spicules, echinoderms, and pollen, as well as sedimentary components such as glauconite and coal. A total of 248 samples were analyzed from the 700-350 m interval. Foraminifera are mostly rare, with maximum abundances between ~50-150 specimens, and exhibit pronounced variability, characterized by repeated intervals of gradual increases followed by abrupt decreases. Intervals of elevated foraminiferal abundance coincide with high diatom and radiolarian counts, consistent with more open-marine conditions. In contrast, abrupt decreases in foraminifera, often dominated by Globocassidulina subglobosa and Nonionella iridea, are accompanied by reductions in siliceous microfossils, although diatoms and radiolarians often remain present, suggesting sub-ice shelf conditions. Near-absence of foraminifera is interpreted as intervals of carbonate dissolution or dilution by high sedimentation rates.

Foraminiferal assemblages can be broadly divided into three intervals. From ~700-620 m, Miliammina co-dominated with G. subglobosa, Eponides/Alabaminella/Epistominella, and N. iridea, along with high biogenic counts suggests relatively warm, productive and open-marine conditions. Between 620-500 m, Miliammina disappears and assemblages are dominated by G. subglobosa, N. iridea, and Ammoelphidiella uniforamina. Two potential ice-grounding or winnowing events occur during this interval, indicated by the near-absence of microfossils and increased glauconite. This assemblage indicates a transition to sub-ice shelf setting under cooler conditions. From 500-400 m, A. uniforamina declines and assemblages are dominated by G. subglobosa, a proposed proxy of Circumpolar Deep Water (CDW), together with Cibicides, Uvigerina, and Ehrenbergina, taxa associated with increased bottom current activity. Elevated diatom and radiolarian abundances during this interval suggest renewed open-marine influence, potentially linked to incursion of warmer CDW prior to the development of RSU3 at ~400 m. These results document repeated shifts between open-marine, sub-ice shelf, and grounded ice-sheet conditions during the late Miocene.