Role of astronomical forcing in shaping the 41-kyr glacial cycles before the Middle Pleistocene Transition

Abstract Glacial cycles during the Early Pleistocene (EP) are characterized by a dominant 41-kyr periodicity and amplitudes smaller than those of glacial cycles with ~ 100-kyr periodicity during the Late Pleistocene (LP). However, it remains unclear how the 41-kyr glacial cycles during EP respond to Earth’s astronomical forcings. Here we employ a three-dimensional ice-sheet model to simulate the glacial cycles at ~ 1.6–1.2 Ma and analyse the phase angle of precession and obliquity at each deglaciation. We show that each deglaciation occurs at every other precession minimum, when obliquity is large. This behaviour is explained by a threshold mechanism determined by ice-sheet size and astronomical forcings. The lead-lag relationship between precession and obliquity controls the length of each glacial/interglacial period. The large amplitudes of obliquity and eccentricity during this period help to establish robust 41-kyr glacial cycles. These findings support the combined role of astronomical forcings common for both EP and LP.