WARM WATER INCURSIONS ONTO THE ROSS SEA SHELF OF ANTARCTICA DURING THE PLIO-PLEISTOCENE BASED ON FORAMINIFERA FROM IODP SITES U1523, U1522, AND U1521

The West Antarctic Ice Sheet (WAIS) is currently thinning and retreating because shifting ocean currents are transporting warmer waters to the ice shelf margin, which could lead to a collapse of the ice sheet and global sea level rise. International Ocean Discovery Program (IODP) Expedition 374 sailed to the Ross Sea in 2018 to study the history of the WAIS over the last 20 million years. Previous geologic drilling projects into Ross Sea sediments that record the history of the WAIS, as well as modeling studies, show considerable variability of ice-sheet extent during the Neogene and Quaternary including ice sheet collapse during times of extreme warmth.

In this study we reconstruct paleoenvironments in the Ross Sea and assess model simulations that show warming waters in the Southern Ocean led to the loss of Antarctic ice in the past. Site U1523 (water depth 828 m) is a key site located close to the shelf break and therefore sensitive to warm water incursions from modified Circumpolar Deep Water (mCDW) onto the Ross Sea shelf when the Antarctic Slope Current weakens with a changing climate. Shelf sites U1522 (558 m) and U1521 (562 m) provide perspective for events closer to the Ross Ice Shelf. Age models were created using biochronology and magnetostratigraphic chronology done during Expedition 374. Multiple incursions of subpolar or temperate planktic foraminifera taxa occurred during the latest Pliocene and early Pleistocene prior to ~1.8 Ma at Site U1523 indicating times of warmer than present conditions and less ice in the Ross Sea. High abundances of foraminifera in the mid-to-late Pleistocene associated with Marine Isotope Stage (MIS) 31, MIS 11, and MIS 5e might also indicate a reduced ice shelf and relatively warmer conditions. The interval of abundant foraminifera around MIS 31 (MIS 37 to 21) suggests multiple warmer interglacials during the Mid-Pleistocene Transition (MPT). A change in benthic foraminiferal assemblages and a large increase in foraminiferal fragments after the MPT (~800 ka) indicate stronger currents at the seafloor and perhaps corrosive waters, suggesting a major change in water masses entering (mCDW) or exiting the Ross Sea (AABW) since the MPT.