The palynology of ODP site 1165, Prydz Bay, East Antarctica: A record of Miocene glacial advance and retreat

ODP Site 1165 recovered around 1000 m of Early to Late Miocene mixed hemi-pelagic sediments from the Wild Drift, offshore Prydz Bay. Samples from between the bottom of the hole and 110 m below sea floor yielded distinctive palynomorph assemblages, ranging in age from 22.2 Ma at the base to 8 Ma at 110 mbsf. Many assemblages contain large numbers of reworked terrestrial pollen and spores varying in age from Devonian to Early Eocene. In situ palynomorph assemblages are dominated by the acritarch Leiosphaeridia. Other components of the assemblages are dinoflagellate cysts, prasinophyte algae and acanthomorph acritarchs. Comparison with modern Arctic assemblages suggests that the presence of these forms implies significant fresh water input, resulting in complex, stratified water masses unlike the situation in Prydz Bay today. Based on the palynomorph assemblages, which are alternately dominated by glacially derived reworked terrestrial miospores and in situ leiospheres typical of ice edges, the interval between the bottom of the hole and 310 mbsf (22.2–15 Ma) can be divided into three intervals. Samples from the base of each interval are dominated by high percentages of reworked terrestrial miospores and are interpreted as representing a period of time dominated by increased glacial flow down the Lambert Glacier and an expanded Amery Ice Shelf. Samples from the upper part of each interval are characterised by low percentages of reworked terrestrial miospores and generally high percentages of leiospheres. This is thought to represent a period of time of low glacial flow and retreat of the Amery Ice Shelf. At the same time an increase in the minor components of the marine palynomorph assemblage is recorded, possibly in response to an overall climatic amelioration onto which the three glacial cycles are superimposed. Overlaying these glacial cycles, between 310 and 99.1 mbsf (15–8 Ma), the assemblages are completely dominated by reworked material, including an Eocene dinoflagellate assemblage previously recoded on the continental shelf in ODP drill-hole 1166. Leiospheres or other in situ marine palynomorphs are either absent or only make up a very small percentage of the assemblage. This is interpreted as representing an interval where the climatic amelioration recorded during the underlying three cycles has been sharply reversed and the Amery Ice Shelf has expanded to the point where it is grounded across the continental shelf, eroding the Eocene material present there. The age of this climatic reversal (15 Ma) coincides with the well-known shift in the oxygen isotope curve thought to be associated with the development of permanent ice sheets on Antarctica.