On the mechanism of the Middle Pleistocene Transition.

The phenomenon of the Middle Pleistocene Transition (MPT)-a change of 41 k.y. periodicity of Pleistocene glacial cycles to 100 k.y. ones at about 1.24 Ma-is studied. Some MPT mechanisms proposed earlier by different authors are considered. It is concluded that the resonant MPT mechanism, implying the change in eigen (resonant) frequency of glacial sheets in the Northern Hemisphere due to the change in size of these sheets, is the least contradicting and fits the best the relevant empirical data. With respect to this mechanism, climate variations in the Early Pleistocene, until 1.24 Ma, resonantly responded to 41 k.y. insolation oscillations, which were caused by variations in Earth's axial tilt. After 1.24 Ma, owing to the growth of the global ice sheet volume and the increase in the period of eigen oscillations of the ice sheets, the direct eccentricity insolation signal characterized by the 100 k.y. (not 400 k.y.) periodicity was resonantly strengthened first. Some problems that emerged in the basis of the proposed mechanism are discussed. This mechanism explains the MPT phenomenon, the unsolved '100 k.y. period problem,' and the absence of the 400 k.y. periodicity in the Pleistocene climate oscillations from a common viewpoint. [ABSTRACT FROM AUTHOR]