A physical record of Antarctic Bottom Water (AABW) outflow in the Ross Sea from the late Pliocene (3.3 Ma) through present

Antarctic Bottom Water (AABW) is the coldest (-1°C), deepest (> 2000 m), and most extensive water mass in the modern ocean, and it plays a key role in global meridional overturning circulation. In the Ross Sea, during the last 40 years, AABW has warmed and freshened mostly due to the interaction with the West Antarctic Ice Sheet (WAIS), currently retreating due to the wind-driven oceanic currents that transport warm waters toward the ice margin. To understand the evolution of the WAIS during warmer and transitional climates of the Neogene and the Quaternary, IODP Expedition 374 drilled a transect of five sites from the shelf to the deep sea in the eastern Ross Sea. This work is focused on Site U1524 (-74.0507°, -173.6335°; 2394 m water depth), located on the continental rise, ~120 km seaward of the Ross Sea continental edge, on the southeastern levee of Hillary Canyon, a 40 km wide by 500 m deep submarine canyon-channel system and one of the largest conduits for AABW outflow. We suggest that deposits on the Hillary Canyon levee record the lateral overspill of turbidity currents triggered by dense shelf water cascading from the shelf edge down to the abyssal ocean and, thus, represent a record of pulsed AABW outflow. The ~270 m thick Plio-Pleistocene succession is a combination of hemipelagic and turbiditic mud, coarse silt to very fine sand beds, and diatom-bearing mud layers. Our sedimentological analysis includes the count of >3,300 thin (1.5 mm average thickness) turbidite beds, and the grain size analysis of 200 samples, from both mud-dominated intervals and the sand beds. This data will provide information to help constrain turbidity current parameters such as flow height, volume, and potentially the flux of the outflow. The overall frequency of the thin turbidite beds decreases through time, coincidently with WAIS expansion following the Mid Pliocene warm period (MPWP). This climatic event is the most recent interval in the geological record where atmospheric CO₂ concentrations were above 400 ppm and are associated with higher temperatures and sea level than today. The sedimentary record from the Hillary Canyon levee will help us to understand the occurrence and magnitude of turbidity currents and their relation to AAWB outflow in the context of ice sheet dynamics over the past three million years.