ANTARCTIC GLACIAL CHRONOLOGY REFLECTED IN THE OLIGOCENE THROUGH PLIOCENE SEDIMENTARY SECTION IN THE ROSS SEA
BALSHAW-BIDDLE, KATHERINE M.
Doctor of Philosophy
Over 24% of the ice surface area covering East and West Antarctica drains into the Ross Sea and supplies the Ross Ice Shelf. The dominant proportion of this ice originates from the West Antarctic subcontinent. Thus, any change in the glacial regime in West Antarctica, and to a lesser extent East Antarctica, may well be preserved in the thick sequence of sediments underlying the Ross Sea. The Oligocene through Pliocene portion of this glacial marine section is studied using depositional models derived from the abundant piston core data in the region. The sedimentologic evidence from the Ross Sea glacial section is combined with evidence from the literature to establish a glacial chronology for the East and West Antarctic subcontinents. The West Antarctic archipelago was completely glaciated by late Oligocene time. This condition is indicated by the youngest occurrence of stripped soil-minerals in upper Oligocene marine sediments in the central Ross Sea, which are immediately overlain by sediments of the same age containing the oldest IRD in the region. Ice shelves then began to develop around these islands in the late Oligocene and early Miocene, as evidenced by fluctuations in the percentages of IRD (ice-rafted debris) and by the occurrences of zones containing visually massive pebbly mudstones within the Ross Sea section. These ice shelves coalesced to form an ice sheet over West Antarctica, including the Ross Sea, in the middle Miocene. IRD percentages in the Ross Sea glacial section increased rapidly at this time and remained high through the late Miocene and early Pliocene as the Ross Ice Sheet developed and expanded. The initial coalescing of the West Antarctic ice sheet probably corresponds to Vail and Hardenbols' (1979) eustatic sea-level drop at 14 myB.P., in the mid Miocene. Renewed growth and expansion of the Ross Ice Shelf is indicated in the latest Miocene and early Pliocene Ross Sea glacial section by high percentages of IRD (20-40%), deposition of a basal till, and finally by erosion of a portion of the Pliocene glacial section over nearly the entire Ross Sea continental shelf. This ice shelf expansion correlates with the eustatic sea level drop reported at 6.6 myB.P., in the late Miocene. It also corresponds to climatic changes in southern South America and New Zealand and to changes in oceanic circulation patterns. Development of an ice sheet over West Antarctica in the middle Miocene (14 to 10 myB.P.) requires an earlier data of formation for the East Antarctica ice sheet. This is supported by new evidence from subglacial volcanism (Stump et al., 1980). These data suggest that the continental East Antarctic ice sheet formed at 29 myB.P. causing the major eustatic sea level drop reported for that time. The majority of the data reported from southern ocean DSDP drill sites supports a glacial build up in the middle Miocene. If the new dates of ice sheet formation are accepted then all of this DSDP data is related to the development of the West Antarctic ice sheet, not the East Antarctic ice sheet, as generally assumed.