Depositional Processes on the continental slope of Ross Sea (Antarctica) during the Cenozoic in relation with the ice sheet and sub bottom currents dynamics

The southeastern Ross Sea outer shelf and slope are key areas to study the interactions between oceanic and glacial dynamics on a high latitude continental margin. I present a multidisciplinary study with the integration of geological, geophysical and oceanographic dataset, derived from the seismic database SDLS (Antarctic Seismic Data Library System), as well as from new acquisitions, in order to reconstruct the slope and rise processes of the Eastern Ross Sea and to propose a depositional model of the margin evolution. As part of the PNRA (National Antarctic Research Program) - WHISPERS project, during the XII Antarctic expedition of the research vessel OGS Explora (January-March 2017) new data were acquired and subsequently interpreted for this thesis: multibeam bathymetric profiles, single-channel seismic profiles, sub-bottom chirp profiles, XBT (Expandable Bathy-Thermograph) data and ADCP (Acoustic Doppler Current Profiler) data, on the easternmost edge of the Ross Sea (generally covered by sea ice). During the Cenozoic, the continental slope of the Eastern Ross Sea was affected by several changes, caused by erosion and deposition made by glaciers (ice streams) and bottom currents, in response to sea level and climate changes. The main facies, discordances and sequences in the entire study area, allow recognizing three important phases of evolution of the continental margin: • Pre-Miocene - Lower Miocene: the study area is influenced by glaciers and ice streams coming from west-south-west, presumably connected to the EAIS (East Antarctic Ice Sheet) dynamics; • Middle-Upper Miocene: in the study area there is a transition to a greater contribution of glaciers coming from the south, presumably connected to the WAIS (West Antarctic Ice Sheet) dynamic; • Lower Pliocene - current time: the sediment deposition by ice streams becomes more confined, the over deepening of the continental shelf and the construction of trough-mouth fans mark a transition to a colder regime. The most investigated area covers the outer shelf and the upper-medium continental slope, in the southern and eastern part of the Ross Sea. The area is located between the Houtz and the Hayes Bank, at the mouth of the Whales Deep Basin and includes the Ross Sea, Whispers, Explora and Shackleton Canyons. The oceanographic dataset measured in 2017 suggest that currently, in this area, Antarctic Bottom Water formation occurs because of mixing between the cold and dense Ross Sea Bottom Water and the relatively warm Circumpolar Deep Water (which encroaches the continental shelf). The analysis of geological and geophysical data allowed to identify and interpret different types of structures, including gullies, canyons, channels, mounds, ridges, slide scars, iceberg scours and mega-scale glacial lineations, useful to reconstruct the presence of preferential route for melt waters flow and possible trajectories used by bottom currents. The analysis and correlation of the seismic units from the shelf to the slope, completed with the DSDP 271 and IODP U1522 information, suggest that the continental margin underwent at least 4 main episodes of ice shelf advance and retreat, since 2 Ma (possibly starting from 0.65 Ma). These episodes are highlighted by erosional surfaces, chaotic facies sequences, small-scale incisions and channels, both on the shelf and the slope. Locally, prograding wedges are observed up to the continental shelf edge. The thickness and the distribution of those units vary, suggesting a relocation through time of the main sediments depocenters presumably indicative of changes in the mechanisms at the origin of sediment supply. Along the slope, in front of the central part of the Whales Deep Basin, the thickness of the most recent seismic unit is maximum and the current shelf edge presents a convex shape. The slope gradient is 5-10° degrees and the multibeam and the seismic sections highlight the presence of slide scars (on the upper slope) and a lobed-shaped deposit (on the lower slope). This geomorphological arrangement is interpreted as the expression of glacial deposit, resulting from an ice stream activity that occurred in front of the central part of the Whales Deep Basin, and was affected by gravitational instability after its deposition along the slope. In the slope areas located in front of both lateral extremities of the Whale Deep basin, the thickness of the most recent seismic unit is minimal and the current shelf edge presents a concave shape. In those areas, instead dominated by erosion, the slope gradient is >10° and the multibeam reveals more incised structures on the current seabed, with well-developed gullies at the shelf edge. This area, mostly dominated by erosion, can be expression of different processes that prevented the accumulation or removed sediments: preferential route for the melt water, discharged into the sea during the ice retreat that flowed along the Whales Deep at the end of the LGM; and/or corridors for the cold and dense waters formed in the Ross Sea discharge.