Marine geology of the western Ross Sea: implications for Antarctic glacial history
Meyers, Nathan Cebren
Anderson, John B.
Master of Arts
The stability of the Antarctic Ice Sheet has long been the subject of debate. Recently, basal till has been identified in the Ross Sea, yielding evidence for the expansion of the Antarctic Ice Sheet and its grounding in the Ross Sea. However, the sedimentologic criteria used to identify basal till may be inconclusive. Therefore, one purpose of this study is to support the validity of these sedimentologic criteria. Another goal is to determine if basal tills are present in the western Ross Sea. Marine sedimentologic processes on high latitude continental shelves are not well known. Therefore, surface sediment distributions can be used to infer marine processes active in the western Ross Sea. Three major sediment types, siliceous ooze and mud, sands, and diamictons were identified. Sands include graded volcanic sands, granitic sands, and calcareous shell hash sands. Diamictons include compound glacial marine sediment, basal till, and mass flow diamictons. Siliceous sediment distribution is controlled both by bottom currents and surface currents. Sands are found in near-coastal or shallow waters, or in areas of high sediment supply. The distribution of surface sediments in the western Ross Sea is controlled by thermohaline bottom currents, by surface currents, by the presence of polynyas, and by sediment gravity flow processes. Basal tills were identified as far north as Coulman Island, and as far south as Ross Island, but no basal tills were found in McMurdo Sound. A petrologic analysis of these basal tills reveals that three petrologic provinces can be recognized in the western Ross Sea and four petrologic provinces can be recognized in the central Ross Sea. The boundaries between petrologic provinces can be correlated with geologic provinces in Victoria Land and Marie Byrd Land. Paleo-ice flow paths are reconstructed based on petrologic province boundaries and bathymetry, and indicate that during the last glacial maximum ice flowed into the Ross Sea from both East and West Antarctica. The petrologic data also indicate that sedimentologic criteria used to identify basal tills are indeed valid. The marine sedimentologic record in the western Ross Sea indicates that the ice sheet which grounded there initially eroded the sea floor, and later deposited basal till except in Mcmurdo Sound where flow was restricted. Sharp contacts between basal till and compound glacial marine sediment indicate that decoupling of the Ice Sheet from its base was rapid; gradational contacts between compound glacial marine sediment and siliceous sediments indicate that ice-shelf retreat was slow allowing siliceous sediments to slowly dominate sedimentation.