On the mid-Pleistocene transition to 100-kyr glacial cycles and the asymmetry between glaciation and deglaciation times

A mechanism is proposed for the mid-Pleistocene transition from a dominant periodicity of 41 kyr to 100 kyr in glacial oscillations. The same mechanism is shown to also explain the asymmetry between the long glaciation and short deglaciation phases of each cycle since that transition versus the symmetry of the 41-kyr oscillations prior to the transition. These features arise naturally within the framework of the sea-ice switch glacial cycle mechanism of Gildor and Tziperman [2000] as a result of the gradual cooling of the deep ocean during the Pleistocene. This cooling results in a change of the relation between atmospheric temperature and the rates of accumulation and ablation of continentals ice sheets. It is this latter change that leads to the activation of the sea-ice switch and therefore to the initiation of the 100-kyr oscillations. The gradual glaciation and rapid deglaciations during these oscillations occur because the mean value of the ice sheet ablation is not far from the maximum rate of snow accumulation during warm periods. This proximity of mean ablation and maximum accumulation rates is shown to be also a consequence of the mid-Pleistocene gradual cooling of the deep ocean.