Central Ross Sea cryosphere and ocean variability during the early and middle Miocene: palynology from IODP 374 Site U1521

The fate of the Antarctic Ice Sheet (AIS) under climate warming remains highly uncertain. Geological evidence from the warm Miocene Climatic Optimum (MCO, ~17–14.7 Ma) are still limited to few circum-Antarctic records, but they generally suggest a substantial AIS retreat and reduced or absent sea-ice. Yet the magnitude and temporal variability of the cryosphere dynamics within the generally warm MCO remain poorly constrained. Here, we present marine and terrestrial palynological results from sediments drilled at Site U1521 on the outer shelf of the central Ross Sea during International Ocean Discovery Program (IODP) Expedition 374 spanning portions of the interval ~ 17.3 to ~ 14.5 Ma. Dinoflagellate cysts and other aquatic palynomorphs, including brackish-water algae, indicate phases of progressive cryosphere melting, related to marine-terminated ice to fully open-ocean conditions, punctuated by ephemeral ice re-advances including both sea-ice and land-ice during the interval 16.3 to 15.8 Ma. Terrestrial palynomorphs reflect a generally warming trend, but alternating cooling and warming conditions on the hinterland, closely coupled to ice and ocean dynamics. Despite the fluctuations, persistent vegetation implies a vigorous hydrological cycle, consistent with enhanced moisture delivery under a generally warm climate. In addition, we conducted marine palynological analyses at unprecedented millennial-scale resolution in two intervals of the MCO (~ 16.2 Ma and ~ 16 Ma) where such resolution could be achieved. During the first interval dinoflagellate cysts suggest stepwise changes towards more open-ocean conditions interrupted by transient cooling events (with sea-ice presence) and generally high productivity. In contrast, the interval ~ 16 Ma is characterized by persistent open-ocean condition and low productivity.