DIATOM CONSTRAINTS ON THE POSITION OF THE ANTARCTIC POLAR FRONT IN THE MIDDLE PART OF THE PLIOCENE

The relative percentages of diatom taxa in 5 deep-sea cores (DSDP 266, ODP 699A, ODP 747A, ODP 751A, and Eltanin Core 50-28) from the Southe rn Ocean are determined for an interval centered on 3.1 to 2.9 Ma in t he middle part of the Pliocene. This climatically warm interval, which is being studied by the PRISM Project of the U.S. Geological Survey, coincides. with a proposed interval of major deglaciation of East Anta rctica. The maximum southerly position of the Antarctic Polar Front be tween 3.1 and 3.0 Ma is inferred from these diatom studies, the presen ce of calcareous nannofossils in the sediments, and sedimentologic and micropaleontologic information from the literature. It is suggested t hat the Antarctic Polar Front may have migrated by as much as 6 degree s of latitude further to the south in the southeastern Atlantic and In dian Oceans during this Pliocene warm interval but probably lay close to its present day position in the southwest Atlantic and Drake Passag e. Summer sea surface temperatures are inferred to have been no more t han 3 degrees-4 degrees C warmer than present at latitudes between 55 degrees and 60 degrees S.