Reconstructing the dynamics of the West Antarctic Ice Sheet since the last glacial (MIS2) by studying the sector from the Glomar Challenger inner continental shelf to the slope and rise area (East Ross Sea, Antarctica): a multidisciplinary approach

The objective of this PhD thesis is to add more data to try to better constrain the history of the Glomar Challenger Basin and the slope and rise area from the last glacial to the present. This work has been conducted in the frame of two projects: ● STREAM (Late Quaternary evolution of the ocean - ice sheet interactions: the record from the Ross Sea continental margin (Antarctica)); ● ANTIPODE (Onset of Antarctic Ice Sheet vulnerability to ocean conditions). The PhD thesis considered geophysical data and gravity cores/box cores collected during several PNRA expeditions (Programma Nazionale di Ricerca in Antartide). Geophysical data were used to provide the geomorphological context and information about sedimentary dynamics. A multidisciplinary approach has been adopted on sediment cores and box cores. A series of radiocarbon ages (AIOM) were already available: 4 for the innermost shelf gravity core, 13 for the dataset of the slope and rise. 23 new radiocarbon ages were planned, but unfortunately the chosen samples were not analysed for reasons beyond my control. The results allowed to identify 4 sedimentary facies and 2 sub-facies in the gravity cores collected in the Glomar Challenger Basin: 1) stiff diamicton: identified under an unconformity in the inner continental shelf cores, interpreted as a subglacial deposition. One AIOM radiocarbon age dates this facies as pre-LGM (34,3 cal kyr); 2) soft diamicton, recovered in all the cores and interpreted as a subglacial deposit. Radiocarbon age (AIOM) obtained from core GC12 is 35,1 cal kyr (pre-LGM); 3) glaciomarine diamicton, characterised by abundant clasts and laminated intervals, was recovered in the inner/middle shelf and interpreted as a sub-ice shelf facies. The radiocarbon age suggests an age around the beginning of the LGM (26,3 cal kyr); 4) divided in two sub-facies 4a) inner shelf: silt with very low content of sand and clasts. 4b) middle/outer shelf: sandy silt with a high content of sand and clasts. Both were interpreted as seasonal open marine for the presence of agglutinated foraminifera and biomarkers. The absence of some facies may be related to erosive events. Radiocarbon ages of core GC12 may suggest a hiatus. The presence of reworked organic matter cannot be excluded. These results will be supported in future by radiometric analyses. Regarding the slope and rise area, three facies were identified: 1) laminated clayey silt, deposited during the glacial period. Radiocarbon ages (AIOM) range from 27,1 kyr to 23,3 cal kyr BP; 2) massive sandy silt with clasts interpreted as glacigenic material (deglacial phase) between 20,8 to 13,6 cal kyr BP; 3) sandy silt with high content of clasts dropped by icebergs (seasonal open marine facies) deposited during the Holocene (from 11,4 cal kyr BP to the present). The results obtained allow to highlight the ice sheet evolution since the last glacial to the present: during the LGM, the ice sheet advanced across the continental shelf and stiff and soft diamicton were deposited. A perennial ice cover, probably an ice shelf, leads to deposit the laminated facies in the slope and rise area. When the ice sheet retreated from the shelf edge, glacigenic material was deposited in the slope and rise area, while a sub ice shelf facies was deposited on the continental shelf when the ice sheet decoupled from the sea floor. Seasonal open marine conditions were established in the Holocene.