Reconstruction of late pleistocene east antarctic ice sheet behavior through stratigraphic analysis of marine sediment cores from the ross sea, antarctica

The study of Grounding Zone Wedges (GZWs), i.e. glacigenic structures that form on the seabed at the base of grounded ice sheets, provides insightful information on past and recent climatic variations. These deposits, in fact, represent the direct evidence of the interplay between sea level changes and ice sheets dynamics, in terms of grounding line variations. Terra Nova Bay, an indentation of the Victoria Land coast facing towards the Western Ross Sea, offshore the Italian Antarctic base “Mario Zucchelli Station”, represents a distinctive and efficient viewpoint for investigating the grounding line behavior through time, and several studies concerning the East Antarctic Ice Sheet (EAIS) evolution from the Last Glacial Maximum (LGM, 26.5-19 ka BP) to the present time have been carried out throughout the past decades regarding this area. The recent discovery of a GZW documenting a possible advance of the Antarctic Ice Sheet (AIS) during the cold phase Antarctic Cold Reversal (ACR, 14.7-13 ka BP) on the Western flank of the Drygalski basin has renewed the interest concerning this sector of the Ross Sea, which holds great potential for better understanding Late Pleistocene and Holocene glacial history. This Ph.D. thesis focuses on the multi-proxy analyses of marine sediment cores collected from various settings within the Drygalski basin, involving a transect crossing the GZW located on the western flank of the trough, along with samples taken from the deepest sector of the basin. Information about the seafloor morphology and sub-surface is available through multibeam bathymetry and seismic reflection profiles acquired in the study area. Sediment lithology, texture, and composition, as derived through visual observation, grain size analyses, X-ray fluorescence and bulk geochemical analyses, coupled with radiocarbon dates and sea-ice biomarker analyses, allowed to depict details regarding the recentmost shifts involving the EAIS and Terra Nova Bay glaciers. The examined gravity cores show various sediment facies reflecting different environments accordingly to the sampling location. Overall, glacial till facies, related to the accumulation of material at the grounding line, overlayed by GZW-aggradation phases have been found in correspondence of the targeted GZW. Seaward, in a proximal position facing the GZW foreset, deposits related to the settling of meltwater plumites, debris flows and progressively finer material under the floating ice shelf were found. A ubiquitous and relatively coarse deposit generated in open marine and polynya conditions drapes over the basin seafloor in all the sampled locations. Punctual observations on the cores have been extended to a relevant portion of the sedimentary basin through the analysis of seismic reflection profiles. The analyses of sedimentary facies combined with seafloor morphological analyses allowed to gather information about the different phases that shaped the evolution of the study area, in terms of interaction between the cryosphere, the climate system and the oceanic realm. The first phase shows the grounding line position of the ice sheet in correspondence of the cored GZW, possibly during a post-LGM climatic oscillation. GZW-accretion is underlined by the deposition of till units, while sub-ice shelf conditions are expressed through related deposits showing a progressive increment of the distance between the sampling sites and the sediment source, represented by the grounding line. Hence, the second phase involves the progressive retreat of the ice sheet grounding line towards its modern position, followed by the instauration of a polynya environment in the Terra Nova Bay area. Chronological constrain of the glacial advance and retreat involves the radiocarbon dating of bulk acid insoluble organic matter (AIOM), which entails higher reliability when performed on marine sediments rather than glacial and glaci-marine deposits. The dated intervals within till deposits show calibrated ages spanning from pre-LGM to LGM, while the calibrated ages obtained from glaci-marine facies mostly postdate the LGM. Older dates obtained from glacio-influenced deposits probably involve old organic carbon, which remains stored in the material that has been reworked by glacial activity and provides ages which are older than expected. Conversely, ages obtained from marine sediments, deposited during the Holocene, are less affected by the influence of old organic carbon. The scarce preservation potential of the sedimentary succession linked to the grounded ice retreat in the study site, pointed out by the discontinuous distribution of such deposits as evidenced through the seismic profiles, is possibly due to the action of bottom currents entraining the finer sediment fraction. For this reason, the preservation of relatively expanded sediment packet is mainly confined to sheltered depressions. The results obtained through this Thesis suggest that the multi-disciplinary approach adopted for the study of glaci-marine sedimentary sequences is effective in outlining the connection between ice sheet/ice shelf- related deposits and climatic events, highlighting the changes occurred during the Quaternary.