Astronomical forcing shaped the timing of early Pleistocene glacial cycles

Abstract

Glacial cycles during the early Pleistocene are characterised by a dominant 41,000-year periodicity and amplitudes smaller than those of glacial cycles with ~100,000-year periodicity during the late Pleistocene. However, it remains unclear how the 41,000-year glacial cycles during the early Pleistocene 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 million years before present and analyse the phase angle of precession and obliquity at deglaciations. We show that each deglaciation occurs at every other precession minimum, and when obliquity is large. The lead-lag relationship between precession and obliquity controls the length of interglacial periods, the shape of the glacial cycle, and the glacial ice-sheet geometry. The large amplitudes of obliquity and eccentricity during this period helped to establish robust 41,000-year glacial cycles. This behaviour is explained by the threshold mechanism determined by ice-sheet size and astronomical forcings.