Congratulations to Clay Tabor who successfully defended his dissertation on December 4, 2015.

Advisor: Chris Poulsen


During the Quaternary, Earth experienced a series of large glaciations. The pacing of these glacial cycles is often attributed to orbitally controlled high-latitude summer insolation, with greater insolation causing more ice sheet retreat and vice versa. However, this simple relationship is not linearly reflected in ice-volume records. In the early Pleistocene (2.6-0.8 Ma), glacial cycles oscillate mainly at the frequency of obliquity (~41 kyr-1) while summer insolation varies most strongly at the frequency of precession (~21 kyr-1). Using an Earth system model, I show that the combination of vegetation and sea-ice feedbacks, seasonal offset of precession forcing, and orbital cycle duration differences amplify the ice-volume response to cycles of obliquity relative to precession and help explain the 41 kyr glacial cycles of the early Pleistocene.

Another enigma of Quaternary climate is the mid-Pleistocene transition, which terms the shift from 41 to 100 kyr glacial cycles between 1.2 and 0.7 Ma with little change in external forcing. The "regolith hypothesis" provides a potential explanation for this transition. It posits that glacial cycles gradually eroded preexisting high-latitude regolith, causing a change in ice sheet response to orbital forcing. Here, I simulate the ice sheet responses to a removal of regolith. My results provide support for the regolith hypothesis; only with reduced basal sliding does the 100 kyr ice-volume signal of the late Pleistocene (0.8-0 Ma) appear in the simulated ice-volume cycles.