GEOCHRONOLOGICAL EVIDENCE SUPPORTING ANTARCTIC DEGLACIATION 3 MILLION YEARS AGO

THE response of the Antarctic ice sheets to increased global temperatures is an important unresolved issue in the assessment of future climate change. In particular, considerable controversy exists as to whether the East Antarctic ice sheet suffered extensive deglaciation during the mid-Pliocene epoch (approximately 3 Myr ago), when temperatures were only slightly warmer than today. Although the ice sheet is widely assumed to have existed in something like its present form for the past 14 Myr (ref. 1), marine diatoms eroded from the Antarctic interior have been found in glacial till deposits high in the Transantarctic Mountains2,3, and have been biostratigraphically dated at approximately 3 Myr before present. This age has been disputed4 because it implies marine deposition in the Antarctic interior, and hence substantial deglaciation, at a time when other evidence has been marshalled for the persistence of cold, polar conditions4. Here we report K-Ar and 40Ar/39Ar ages for a volcanic ash bed in diatom-bearing glaciomarine strata cored in Ferrar Fiord (East Antarctica) by the CIROS-2 drill-holes, which confirm the age of the diatoms at approximately 3 Myr, and hence also confirm the mid-Pliocene deglaciation.