Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 degreesC warmer than at present and atmospheric CO2 concentrations wer e twice as high as today(1), the Antarctic ice sheets may have been unstabl e(2-7). Oxygen isotope records from deep-sea sediment cores suggest that du ring this time fluctuations in global temperatures and high-latitude contin ental ice volumes were influenced by orbital cycles(8-10). But it has hithe rto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets(11). Here we p resent sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the Eas t Antarctic ice sheet directly to orbital cycles during the Oligocene/Mioce ne transition (24.1-23.7 Myr ago). Three rapidly deposited glaci-marine seq uences are constrained to a period of less than 450 kyr by our age model, s uggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence f or a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mil even t(5)).