The Potential Role of Regolith in the Mid-Pleistocene Transition

Abstract:

The mid-Pleistocene transition (MPT) occurred between 1.2 and 0.7 Ma with a shift to ~100-kyr glacial cycles and ~50 m greater sea level variability. This transition appears to suggest an increased ice-volume response to the 100-kyr^-1 frequency periodicity of eccentricity; however, insolation forcing from eccentricity decreased during this period. Further, proxy records show no significant corresponding decrease in mean CO₂. A possible explanation for the MPT invokes removal of North American regolith. In theory, the gradual exposure of crystalline bedrock by multiple cycles of ice advance and retreat provides a higher friction substrate, which leads to thicker ice sheets. These thicker ice sheets then require greater insolation forcing, potentially produced by a combination of high obliquity and eccentricity / precession, to retreat. Here we test this hypothesis using an Earth system model with dynamic atmosphere, vegetation, and ice components. Using idealized, transient orbits of obliquity and precession, we compare the ice-volume responses under several basal sliding scenarios. Our preliminary results show increased ice-volume spectral power at the frequency of precession relative to obliquity and a larger ice-volume response when basal sliding is low, lending support to the reduced regolith hypothesis. These differences in ice response are due, in part, to changes in the temperature and circulation feedbacks as the ice sheets become bigger and thicker.