Cenozoic palaeoenvironment of the Southern Ocean and East Antarctica : geological and paleontological evidence from the Kerguelen Plateau, Vestfold Hills and Prince Charles Mountains

Southern Ocean and Antarctic sediments from the Kerguelen Plateau, Vestfold Hills and Prince Charles Mountains contain evidence that suggest climatic conditions and ice sheet volume have fluctuated during the Cenozoic. Geological and palaeontological data show that at certain intervals the Antarctic climate was significantly warmer, and the East Antarctic Ice Sheet smaller, than today. Marine fossils are used to date these intervals of ice sheet reduction and to reconstruct the associated climatic conditions. The findings are compared with previous geological and ice sheet computer modeling studies, to help reconstruct Antarctica's climate history and test predictive models for ice sheet response to future global warming. Fluctuating climatic conditions were identified in Quaternary and Pliocene marine sediments from the Southern Kerguelen Plateau. Unconformities formed during intense glacial intervals, when the velocity of the Antarctic Circumpolar Current increased. Deposition occurred during warmer climatic intervals. During the late Pliocene, 3.1 - 2.64 Ma and 3.2 - 3.1 Ma, the summer sea-surface temperature (SST) was 4.5 C0 warmer than today at 62° S. This could have occurred only if the Antarctic Polar Front Zone was either -1200 km further south or the temperature gradient across the associated fronts became significantly shallower. Pliocene (4.5 -4.1 Ma) marine sediments of the S0rsdal Formation (Vestfold Hills) were also deposited when the summer SST was between 1.6° and 3.0 C0 higher than today. When such temperatures are compared to glacial models, an increase in ice sheet volume would be expected due to increased snow accumulation versus ablation. Previous work - suggests that the S0rsdal Formation was deposited when the ice margin was -50 km further inland. This suggests that a lower temperature than that proposed by earlier glacial models is required for ice sheet retreat. These findings may support more recent modeling studies that indicate ice sheet reduction can occur due to minor increases in water temperature. The Pagodroma Group, in the northern Prince Charles Mountains, consists largely of diamict, and to a lesser degree, siltstone and sands deposited in the Lambert Graben during intervals of glacial retreat. In situ and glacially reworked marine diatoms on the Amery Oasis and Fisher Massif suggest that marine conditions occurred >250 km inland from the modem Amery Ice Shelf edge in the Plio-Pleistocene (3.1 - 1 Ma), Pliocene (4.9 - 3.7 Ma) and middle Miocene (12.2 - 11.7 Ma), and >300 km inland in the Miocene (14.2 - 12.5 Ma and 14.2- 6.2 Ma) and Oligocene (-36.6 - 30.2 Ma). Diatoms also provide an age control for three formations in the Pagodroma Group. Glacially reworked diatoms and stratigraphic relationships indicate that the Mt Johnston Formation, on Fisher Massif was deposited sometime between 36.3 - 12.5 Ma. In situ marine diatoms in Middle Miocene (14.2 - 12.5 Ma) siltstone from the Fisher Bench Formation, on Fisher Massif, suggest a summer SST of 3.5° to 5°C. An associated relative mean annual air temperature rise > 15 C°, during ice shelf absence, caused deglaciation of the Lambert Graben, and is consistent with the predicted response from glacial models. The Bardin Bluffs Formation was deposited in the Amery Oasis 3.1 - I Ma. The lithostratigraphy of the formation indicates that the ice sheet volume varied greatly. The Cenozoic geology of the central Menzies Range in the Southern Prince Charles Mountains is described. Glacial landforms and deposits formed more recently than 40 Ma during two climate phases. The first phase was warmer than today, and wet-based glacial conditions existed. Associated deposits consist of glacial diamict, lacustrine siltstone and sand deposited in a terrestrial alpine glacial environment. These deposits may be contemporaneous with the Pagodroma Group; however, age relationships are yet to be determined. The second climate phase was cold and similar to today's, and caused relatively minor glacial erosion and deposition. Previous researchers have found controversial terrestrial higher plant fossils in the Transantarctic Mountains, which indicate a warmer Antarctic climate sometime in the late Neogene. The lack of a similar flora in the Vestfold Hills and Prince Charles Mountains may reflect earlier denudation of this vegetation during ice sheet development. However, biogeogeographical barriers may have prevented re-colonisation from the Transantarctic Mountains to other East Antarctic regions during later episodes of deglaciation in the Lambert Graben catchment identified in this study.